Ee6361 Electrical Devices and Control

M.I.E.T. ENGINEERING COLLEGE (Approved by AICTE and Affiliated to Anna University Chennai) TRICHY – PUDUKKOTTAI ROAD, TIRUCHIRAPPALLI – 620 007

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

COURSE MATERIAL

EE6361 ELECTRICAL DEVICES AND CONTROL

II YEAR - III SEMESTER

UNIT I: INTRODUCTION 1.1 Basic elements

A drive is a combination of various system combined together for the purpose of motion control (or) movement control. Especially the drive which employs electric motors for motion control is known as “Electrical drives.” Parts of Electrical Drives i) Source (ii) Power modulator (iii) Motor (iv) Load (v) Sensing unit (vi) Control unit

1.1.1 Source Generally we have two types electrical supplies and they are single phase and three phase AC supplies. Drives which runs in AC supply for low power, will be given with 1 – phase supply and that for higher power 3 – phase supply will be given. For traction purpose, the drives need DC supply, which is brought out from AC supply after rectification. For portable (low size) drives the supply is given from a battery. Hence for portable drives DC batteries will act of source. 1.1.2 Power modulator It serves many functions:

 Modulate the source type to match the drive type. Also the source parameters will be modulated thereby we can utilise the full efficiency and speed – torque characteristics of the drive.

 During starting, braking and other operations, this power modulator will keep all the operating parameters within the safe value.

 In power modulator rectification, inversion and filtering etc., operation will be carried out.  It will command the drive for necessary operations i.e., Motoring and Braking Some drives may employ more than one of this modulator. It can be classified in to as follows: i) Convertors (ac to dc converters) ii) Inverters (dc to ac) iii) AC voltage controller (ac to ac) iv) DC choppers (dc to dc) v) Cyclo converters (frequency conversion) 1.1.3 Motor Motor is the machine which drives the load. So, motor will have the capability of driving the load, without overheating and without causing any other damages. We have to select a motor such that it has all the characters require by the load. This selection is influenced by various factors, which we are going to see in the later part of this chapter 1.1.4 Sensing unit It is this unit which sense the actual parameters of the motor i.e., voltage, current, speed, temperature of the machine etc., It is doing so, to control the drive operation so that we can safe guard the drive from any major damage. The cause for the damage may be the crossover of operating parameters beyond its maximum valve allowed for the motor (or) load. The sensing unit will give the necessary control signal (depending on the sensed signal) to the control unit, where corresponding action is taken. 1.1.5 Control unit This control unit will control the power modulator, depending upon the control signal received form sensing unit. For this, control unit will have semiconductor converters, its firing circuit, transistors and a microprocessor.

1.2. Types of electric drives The electric drives used in industry may be divided into three types namely

(a) Group drive (b) Individual drive (c) Multi-motor drive

1.2.1 Group drive

In group drive, a single motor drives as number of machines. The motor is mechanically connected to a long shaft. It is also called line shaft drive. The line shaft is fitted with multi- stepped pulleys and belts. The driven machines are connected to these pulleys and belts for their required speed.

In a group drive a large motor capable of taking the load of all machines simultaneously has to be installed. At times it happens that only a few machines are working and the power of motor is not completely utilized.

Advantages:

1. The installation cost and cost of one large motor will be much less than a number of smaller motor totalling the capacity. 2. The efficiency and power factor of a large group drive motor will be higher, provided it is operated fairly near its full load. Disadvantages:

1. The breakdown of a large single- motor causes all the operations to be stopped. 2. If most of the machines are idle then, the main motor will operated on load with less efficiency.

1.2.2 Individual drive

In this drive there is a separate driving motor for each machine. Such a drive is very common in most of the industries. It is also necessary to use individual motors for heavy machinery such as for lifts, canes etc.,

Advantages:

1. The machines can be installed at any desired position. 2. If there is a fault in one motor other machines will not be affected since they are working independently. 1.2.3 Multi motor Drive

In multi motor drives, separate motors are used for operating different parts of the same mechanism, e.g. in case of an overhead crane, difference motors are used for hoisting, long travel motion and cross travel motion. Such drive is also essential in complicated metal- cutting machines tools, paper making machines, rolling mill and similar types of machinery. The use of multi- motor drive is continuously expanding in modern industry. Complete or partial automation of this drive increases reliability and safety of operations.

1.3 Thermal model of electric motor

 During the operation of the motor various losses occur such as copper loss, iron loss and windage loss.

 Due to these losses heat produced inside the machines. 1.3.1Heating curve Consider a homogeneous machine developing heat internally at uniform rate and gives it to the surrounding proportional to temperature rise. W – loss taking place in a m/c in watts 6

G-- Mass of the machine in kg S—Specific heat in watts/sec -- Rise in temperature above ambient temp

F –Final temperature with continuous load A – Area of cooling surface  -- Rate of heat dissipation Heat developed = Heat absorbed+ Heat dissipated W.dt  G.Sd  Adt (W  A )dt  G.Sd      1  dt d  G.S W  A dt d  G.S  W  Aλ  Aλ  dt d       2  G.S  W   Aλ Aλ 

When final temperature is reached, there is no heat absorbed. The heat which is generated is totally dissipated. W.dt A dt 3  λλ F      

W  Aλλ F W 4 F        Aλ

Sub 4 in 2 dt d

G.S     F   Aλ  Integrating both side we get d dt   F   G.S 

 Aλ  Aλ .t  Ln( F  )  K ------ 5 G.S  

At initial condtion

At t=0,  1

0  Ln(F  1 )  K K Ln( ) 6  F  1      

Sub 6 in 5 Aλ .t  Ln( F  )  Ln( F  1 ) G.S

Aλ F  1 .t  G.S F  Aλ  .t G.S F  1 e  F  Aλ  .t G.S F   ( F  1 )e Aλ  .t G.S  F  ( F  1 )e GS  τ  Heating timeconstant Aλ t . τ  F  (F  1 )e

If the machine started from ambient temperature 1  0C

t . τ  F  ( Fe ) t . τ  F (1  e ) Let us consider time period t= -1  F (1  e )

 F (1  0.367)

 .0 632 F Similarly

t  2τ   0.865 F

t  3τ  0.95 F

t  4τ  0.982 F

1.3.2 Cooling curve When the machine switched off from main supply, there is no heat generation and all the heat stored in machine is dissipated to surroundings. Heat generated + Heat sored in body=Heat dissipated to surrounding medium

' W.dt  GSd  Aλ dt   7 ' (Aλ  W)dt  GSd W '   Aλ   ' dt  GSd  Aλ  GS W d  dt 8 '   '    Aλ  Aλ 

As temperature decrease multiple –ve sign in the left hand side

GS  W   ' d   ' dt  Aλ   Aλ  d Aλ' GS W dt ------9 -        ' d   ' dt   W   GS Aλ  Aλ   '    Aλ 

' If F is final temperature drop then at this temperature whatever heat is generated is dissipated. From the equation 1

'' Wdt  A F dt     10 Sub10in 9 d Aλ '  '  dt  GS   F  ' ' Aλ  Ln   F  t  k     11   GS

At t  ,0   0

'  Ln(0  F )  K sub k value in 11 ' ' Aλ '  Ln(  F )  t  Ln(0  F ) GS ' ' ' Aλ  Ln(  F )  Ln(0  F )  t GS ' '   F  Aλ Ln t   '   GS  0  F  ' ' 0  F  Aλ Ln  t  '  GS   F  ' Aλ'  t 0  F GS '  e  F Aλ'  t ' ' GS  F  (0  F )e Aλ'  t ' ' GS  F  (0  F )e

GS '  τ  Cooling Time constant Aλ ' t  ' ' τ  F  (0  F )e

1.4 Classes of duty cycle

The followings are the types of duty as per IS: 4722-1986 “Specification of rotating machinery”.

i) S1: Continuous duty. ii) S2: Short-time duty. iii) S3: Intermittent periodic duty. iv) S4: Intermittent periodic duty with starting. v) S5: Intermittent periodic duty with starting and braking. vi) S6: Continuous duty with intermittent periodic loading. vii) S7: Continuous duty with starting and braking. viii) S8: Continuous duty with periodic speed changes. 1.4.1Continuous duty (duty type S1) On this duty the duration of load is for a sufficiently long time such that all the parts of the motor attain thermal equilibrium. i.e. the motor attains its maximum final steady temperature rise. Examples of drives with continuous duty are continuously running fans, pumps and other equipment which operate for several hours and even days at a time. The simplified load diagram for this duty is horizontal straight line. The continuous rating of a motor may be defined as the load that may be carried by the machine for an indefinite time without the temperature rise of any part exceeding the maximum permissible value.

1.4.2 Short time duty (Duty type S2)

The motor operates at a constant load for some specified time which is then followed by a period of rest. The period for load is so short that the machines cannot reach its thermal equilibrium i.e. steady temperature rise while the period for rest is so long that the motor temperature drops to the ambient temperature. Railway turntable, navigation lock gates are some examples of the drives which operate on short time duty.

The short time rating of a rotor may be defined as its output at which it may be operated for a certain specified time without exceeding the maximum permissible value of temperature rise. The period is so short that the temperature rise of the motor does not reach its final steady value and the period of rest is so long that the motor returns to cold conditions. 11

Standard short time ratings are: 10,30,60 and 90 minutes.

1.4.4 Intermittent periodic Duty (Duty type S3)

On intermittent duty the periods of constant load and rest with machine de-energised alternate. The load periods are too short to allow the motor to reach its final steady state value while periods of rest are also too small to allow the motor to cool down to the ambient temperature. This type of duty cycle is encountered in cranes, lifts and certain metal cutting machine tool drives.

For the evaluation of intensity of heating due to intermittent period loads, use is made duty factor. The duty factor (also called as load factor or cyclic duration factor) is generally defined as the ratio of the heating (working) period to the period of whole cycle.

The intermittent rating of a motor applies to an operating condition during which short time load periods alternate with periods of rest or no load without the motor reaching the thermal equilibrium and without the maximum temperature rising above the maximum permissible value. In this duty the current does not significantly affect the temperature rise.

1.4.5 Intermittent periodic duty with starting (Duty type S4)

This type of duty consists of a sequence of identical duty cycles each consisting of a period of starting, a period of operation at constant load and a rest period, the operating and rest periods are too short to obtain thermal equilibrium one duty.

In this duty the stopping of the motor is obtained either by natural deceleration after disconnections of the electric supply or by means of braking such as mechanical break which does not cause additional heating of windings.

1.4.6 Intermittent periodic duty with starting and braking (Duty type S5)

This type of duty consists of sequence of identical duty cycles each consisting of a period of starting, a period of operation at constant load, a period of braking and a rest period. The operating and rest periods are too short to obtain thermal equilibrium during one duty cycle as shown in figure. In this duty braking is rapid and is carried out by electrical means.

1.4.7Continuous duty with intermittent periodic duty (Duty type S6)

This type of duty consists of a sequence of identical duty cycles each consisting of a period of operation at constant load and period of operation at no load. The machines with excited windings have normal no load voltage excitation during the load period. The operation and no load periods are too short to attain thermal equilibrium during one cycle.

Unless and otherwise specified, the duty cycle is 10 minutes. The recommended duty factors are 15,25,40 and 60 percent.

1.4.8 Continuous duty with starting and braking (duty type S 7)

This type of duty consists of a sequence of identical duty cycles each having of a period of starting, a period of operation at constant load and a period of electric braking. There is no rest or de - energised period.

The duty factor for this duty cycle is 1.

1.4.9 Continuous with periodic speed charges (Duty Type S8)

This type of duty consists of a sequence of identical duty cycles each consisting of a period of operation at constant load corresponding to a determined speed of rotation, followed immediately by a period of operation at another load corresponding to a different speed of operation. The operating period is too short to attain thermal equilibrium during one duty cycle there being no rest and de-energized period.

1.5Selection of power rating

1.5.1 Power rating for drive motors

 Power rating of motors selected to get better economy and reliability.  Motor rating selected based on thermal loading of motor. Since the temperature has direct relationship with output power F- Force in Kg V- Velocity in m/s T- Torque in Kg-m   Efficiency of motor N- Speed in r.p.m The rating of the motor required in case of linear motion is given by F.V F.V P  H.P  KW 75 102 In case of rotary motion, the rating of motor is given by T.N T.N P  H.P  KW 716 975 In case of loads where the torque and speed are known, the output power of load is given by 2π Pout  TN Watts 60 1.5.2 Determination motor rating From the point of view of calculation of motor rating for various duty cycles classified as i) Continuous duty loads ii) Fluctuating loads iii) Short time and intermittent duty Continuous duty

 Maximum continuous power demand of the load is approximated.  After calculation maximum power, then the next commercial range will be selected.  It is also necessary to check the speed – torque requirements at the starting. 14

1.5.3 Equivalent Current Method

 Based on approximation of actual variable current can be replaced by an equivalent Ieq, which produces same losses as actual current.

 Motor loss has two components

1. Constant loss (Pc) 2. Copper loss

 Thus for a fluctuating load consisting of n values of motor currents I1, I2, …, In

For duration’s t1, t2… tn respectively then the equivalent current Ieq is given by

2 2 2 2 (PC  I1 R) t1  (PC  I2R) t 2  .....  (PC  In R) t n Pc+Ieq R = t1  t 2  ....  t n

2 2 2 2 1 tI(1  2 tI2  .....  n n R)tI Pc+ Ieq R- Pc= t1  t 2  ....  t n

2 2 2 2 1 tI(1  2 tI2  .....  n n R)tI Ieq R= t1  t 2  ....  t n

2 2 2 1 tI(1  2 tI2  .....  n n )tI Ieq = t1  t 2  ....  t n If the current varies smoothly over a period T above the equation can be written as

T 1 2 Ieq  I dt T 0

Fig. Load diagram of fluctuating load

1.5.3 Equivalent Torque Method

 After Ieq is determined, a motor with higher current rating from commercially available range is selected.

 When torque is directly proportional to current, the torque is given by

2 2 2 T(1 t1  T2 t 2  .....  Tn n )t Teq = t1  t 2  ....  t n 1.5.3 Equivalent Power Method

 When motor operated at nearly fixed speed, its power directly proportional to torque

2 2 2 P( 1 t1  P2 t 2  .....  Pn t n Peq = t1  t 2  ....  t n Short time duty

 In short time duty, time of motor operation is less than the heating time constant. It will be cooled before operating again.

 If motor with continuous duty power rating Pr is subjected to short time duty then then it

is underutilised and motor temperature below the permissible value p er .

 So the motor can be overloaded by a factor K, then it can reach permissible value p er .

a------with power KPr; b------with power Pr

tr  τ per  ss (1 e )   1 1 ss  tr per  1 e τ

Motor losses for powers Pr and KPr be P1r and P1s respectively. P 1 ss 1s 2   tr   per P1r  1 e τ

 Pc  P1r  Pc  Pcu  Pcu 1 P   cu 

 Pc  α   P   cu  P1r  Pcuα 1    3 2 Kp r  2 P1s  Pc  Pcu  pc  K pcu  p   r 

 Pc 2  P1s  Pcu  K P   cu  P P K 2 4 1s  cu α     Substitute 3 and 4 in 2 P K 2 1 cuα    tr P 1  cuα   τ 1 e 2 α 1 α K    tr  1 e τ  2 α 1 K  α  tr   1 e τ α 1 K  α  tr   1 e τ Intermittent duty

 During a period of operation, if the speed changes in wide limits, its leads to changes in heating and cooling conditions.

 So the previous methods are not ideal for finding equivalent current, torque or power

Fig.Intermittent periodic duty Temperature at the end of working the interval is given by

t t r r   τ τ (1 e r ) e r 1 max  ss   min     

Fall in temperature rise at the end of standstill interval ts will be t s  τ e s 2 min  max      Sub 2 in 1 t t r t r  s  τ  τ (1 e r ) e τs e r max  ss   max  t t t s r r    τ τ τ e s e r (1 e r ) max  max   ss     t t  t  s   r  r       τ τ  τ  s r  1( e   ) (1 e r ) max   ss 

   t t   s   r       τ τ  1 e  s r  ss  3  t        r max  τ r 1 e

For full utilisation of motor

max  ss    t t   s r       τ τ  2  s r  max P1s Pcu (  k ) 1 e    P P ( )1 t ss 1r cu   r  τr 1 e    t t   s r       τ τ  1 e  s r  k 2 ( )1    t  r  τr 1 e    t t   s r       τ τ  1 e  s r  k ( )1    t  r  τr 1 e

1.5.7Thermal overloading When the motor operates heat is produced due to losses (copper, iron and friction) inside the machine and its temperature rises. The heat produced during no load is from iron parts to winding and when loaded heat is flows from winding to surrounding as more heat is seen on winding than winding. The temperature reaches at a steady state when the heat generated becomes equal to heat dissipated into the surrounding medium. This steady temperature depends on power loss, which in turn depends on the output power of the machine. Since the temperature has direct relation with output power, it is termed as thermal loading of the machine

1.6.1,1.6.2&1.6.3:At full load of a 10 HP motor, the temperature rise of a motor is 25˚C after one hour and 40 ˚C after two hours. Find (i) The final temperature rise on full load (ii) Heating time constant of motor (iii) Half hour rating if iron losses which remain constant are 80% of copper losses at full load.

Given data:

Pout=10 H.P, t1=1 hrs, t2= 2 hrs, 1  25C, 2  40C

t1  τ 1  F (1 e )

t 2  τ 2  F (1 e )

t1 t1 1    τ τ τ 1 F (1 e ) 1 e 25 1 e  t 2  t 2   2    2 τ τ 40 τ F (1 e ) 1 e 1 e 1 2   2 e τ  e,u τ  u 25 1 u 1 u  2  40 1 u (1 u) (1 u) 25 1 1   0.6  40 (1 u) 1 u 1 1 u  0.6 1.6 1 u u 11.6 u  0.6 1  e τ  u 1  e τ  0.6 1   Ln0.6 τ 1   0.51082 τ τ 1.9576Hrs       Heating time constant

t 1  (1 e τ ) 1  F  1  25 (1 e 1.19576 )  F  25 (1 0.6)  F  25 0.4  F 62.5 C Final temperatur e rise F     

Wiron=80%, Wcu= on full load

P 80 α  C   0.8 PCu 100 N 1/2  0.5 hrs

PX α 1  N  α P  R 1 e τ P Short time rating X  PR  Full time rating 10H.P

PX 0.8 1  0.5  0.8 10  1 e 19576. P X  7.02 10

PX  26.5H.P

1.6.1 &1.6.2: The enclosure of a 10 kw motor is equivalent to a cylinder of a 70 cm diameter and 100 cm length. The motor weight 500 Kg. Assuming the specific heat is 700 J/Kg/c and that the peripheral surface of enclosure of the alone is capable of heat dissipation of 12.5 w/sq-m/c. Calculate the heating time constant and its final temperature. Efficiency of the motor is 90% [NOV/DEC 2012]

Given data:

Pout= 10kw=10000 w

D=70 cm =0.7m

L=100 cm = 1 m

G= 500 Kg

S=12.5 w/sq-m/c700 J/Kg/c

  12.5 w/sq-m/c700   90%

Losses =I/p power –o/p power

Losses= Pout/efficiency - o/p power

Losses = 10000/0.9 -10000

Losses, W = 1111 Watts.

Area, A = Surface area of cylinder= π. d. l A= π. x0.7x1

A= 2.1991 m2

Heating time constant,

GS 500X700    Aλ 2.1991X12. 5 τ 12732.4 sec/60  212.2 m//60 τ  3.54 Hr W 1111   F Aλ 2.1991X12. 5 40.4 C F  

1.6.6: The heating and cooling time constant of an electric motor are 100 and 150 minutes respectively. The rating of motor is 125 kw. If it is working on duty cycle of 15 minutes on load and 30 minutes on no load. Determine the permissible overloading of the motor. 2 Assume the losses are given by the expression Pc+x pcu and pc/pcu= α =0.4 [NOV/DEC 2012]

Given data:

τ 100 r  τ 150 S  Rating of motor 125kw  t 15minutes r  t 30minutes S  α  0.4 Permissible overloading factor, 22

   t t   s   r       τ τ  1 e  s r  k ( )1    t  r  τ r 1 e

  15 30          100 150    1 e   k 4.0()1 4.0   15   100 1 e .0( 15 )2.0 1 e  k  4.1 .0 15  4.0 1 e 1 .07046 k  4.1  4.0 1 .08607 k  .294 4.0 .256 k  .1602 Permissible overload=k X Rating =1.602X125

Permissible overload=200.25 kw

1.6.7: A constant speed drive operating at a speed of 500 r.p.m has a cyclic loading as given below

200 N-m for 10 minutes

300 N-m for 20 minutes

150 N-m for 20 minutes

No – load for 10 minutes. Estimate power rating of motor.

2 2 2 T(1 t1  T2 t 2 .....  Tn n )t Teq = t1  t 2 ....  t n

2 2 2 (200 10)  (300  20) (150  20) Teq  10 20 2010 2650000 Teq   44166.82 60

Teq  210.16 N  m

Power rating of the motor,

2NTeq Peq  60 2 500 210.16 Peq  60

Peq 11003.95watts

Peq 11kw

1.6.7: A motor required to deliver a load which follows the following duty cycle

50 kw for 10 minutes

No load for 4 minutes

25 kw for 10 minutes

No load for 6 minutes

The cycle is repeated indefinitely. Find the suitable capacity of a continuously rated motor for the purpose.[NOV/DEC 2011]

2 2 2 P( 1 t1  P2 t 2  .....  Pn t n Peq = t1  t 2  ....  t n

2 2 2 2 (50 10)  0(  )4 (25 10)  0( 6) Peq = 10  4 10  6 31,250 Peq   1041.66 30

Peq  32.27kw

12. Based on rms torque, estimate the kw rating of a 750 r.p.m motor used for driving equipment having the following load torque curve.

1. For the first 10 seconds, the torque is constant at 40 kg-m

2. For the next 30 seconds, the torque varies linearly with time from 35 kg-m to 15 kg-m.

3. For the last 50 seconds the torque is constant and equal to 10 kg-m.[MAY/JUNE 2011]

2 2 2 T(1 t1  T2 t 2 .....  Tn n )t Teq = t1  t 2 ....  t n

2 2 2 (40 10)  (20 30) (10 50) Teq  10 30 50 33000 Teq   366.66 90

Teq 19.15 N  m

2NTeq O / P power, Peq  60 2 75019 5. Peq  60

Peq 1504.03watts

Peq  5.1 kw

1.6.1The temperature rise of motor after operating for 30 minutes on full load is 20˚C and after another 30 minutes it becomes 30 ˚C on the same load. Find the final temperature rise and time constant. [APRIL/MAY 2010]

Given data:

t1=30 min, t2= 30+30=60 min, 1  20C, 2  30C

t1  τ 1  F (1 e )

t 2  τ 2  F (1 e )

t1 t1 30    τ τ τ 1 F (1 e ) 1 e 20 1 e  t 2  t 2   60    2 τ τ 30 τ F (1 e ) 1 e 1 e 30 60   2 e τ  e,u τ  u 20 1 u 1 u  2  30 1 u (1 u) (1 u) 20 1 1   0.66  30 (1 u) 1 u 1 1 u  0.66 1.5151 u u 1.5151 u  0.5 1  e τ  u 30  e τ  0.5 30   Ln0.5 τ 30   0.693 τ 30 τ   43.29Min        Heating time constant 0.693

t 1  (1 e τ ) 1  F  30  20 (1 e 43.29 )  F  20 (1 0.6)  F  20 0.5  F 40 C Final temperatur e rise F     

UNIT- 2: DRIVE MOTOR CHARACTERISTICS

2.1. Mechanical characteristics One of the essential requirements in the section of a particular type of motor for driving a machine is the matching of speed-torque characteristics of the given drive unit and that of the motor. Therefore the knowledge of how the load torque varies with speed of the driven machine is necessary. Different types of loads exhibit different speed torque characteristics. However, most of the industrial loads can be classified into the following four categories. 2.1.1. Speed-Torque characteristics of various types of load.

Constant Torque characteristics:

Most of the working machines that have mechanical nature of work like shaping, cutting, grinding or shearing, require constant torque irrespective of speed. Similarly cranes during the hoisting and conveyors handling constant weight of material per unit time also exhibit this type of characteristics

Torque Proportional to speed: Separately excited dc generators connected to a constant resistance load, eddy current brakes have speed torque characteristics given by T=k

Torque proportional to square of the speed: Another type of load met in practice is the one in which load torque is proportional to the Square of the speed. Eg: Fans rotary pumps, compressors and ship propellers.

Torque Inversely proportional to speed: Certain types of lathes, boring machines, milling machines, steel mill coiler and electric Traction load exhibit hyperbolic speed-torque characteristics.

2.1.2 Four Quadrant Operation

For consideration of multi quadrant operation of drives, it is useful to establish suitable conventions about the signs of torque and speed. A motor operates in two modes – Motoring and braking. In motoring, it converts electrical energy into mechanical energy, which supports its motion. In braking it works as a generator converting mechanical energy into electrical energy and thus opposes the motion. Motor can provide motoring and braking operations for both forward and reverse directions. Figure shows the torque and speed co-ordinates for both forward and reverse motions. Power developed by a motor is given by the product of speed and torque. For motoring operations power developed is positive and for braking operations power developed is negative. In quadrant I, developed power is positive, hence machine works as a motor supplying Mechanical energy. Operation in quadrant I is therefore called Forward Motoring. In quadrant II, power developed is negative. Hence, machine works under braking opposing the motion.

Therefore operation in quadrant II is known as forward braking. Similarly operation in quadrant III and IV can be identified as reverse motoring and reverse braking since speed in these quadrants is negative. For better understanding of the above notations, let us consider operation of hoist in four quadrants as shown in the figure. Direction of motor and load torques and direction of speed are marked by arrows.

Hoist consists of a rope wound on a drum coupled to the motor shaft one end of the rope is tied to a cage which is used to transport man or material from one level to another level . Other end of the rope has a counter weight. Weight of the counter weight is chosen to be higher than the weight of empty cage but lower than of a fully loaded cage. Forward direction of motor speed will be one which gives upward motion of the cage.

Load torque line in quadrants I and IV represents speed-torque characteristics of the loaded hoist. This torque is the difference of torques due to loaded hoist and counter weight. The load torque in quadrants II and III is the speed torque characteristics for an empty hoist. This torque is the difference of torques due to counter weight and the empty hoist. Its sigh is negative because the counter weight is always higher than that of an empty cage.

The quadrant I operation of a hoist requires movement of cage upward, which corresponds to the positive motor speed which is in counter clockwise direction here. This motion 30

will be obtained if the motor products positive torque in CCW direction equal to the magnitude of load torque TL1. Since developed power is positive, this is forward motoring operation.

Quadrant IV is obtained when a loaded cage is lowered. Since the weight of the loaded cage is higher than that of the counter weight .It is able to overcome due to gravity itself. In order to limit the cage within a safe value, motor must produce a positive torque T equal to TL2 in anticlockwise direction. As both power and speed are negative, drive is operating in reverse braking operation. Operation in quadrant II is obtained when an empty cage is moved up. Since a counter weigh is heavier than an empty cage, it’s able to pull it up.

In order to limit the speed within a safe value, motor must produce a braking torque equal to TL2 in clockwise direction. Since speed is positive and developed power is negative, it’s forward braking operation. Operation in quadrant III is obtained when an empty cage is lowered. Since an empty cage has a lesser weight than a counter weight, the motor should produce a torque in CW direction. Since speed is negative and developed power is positive, this is reverse motoring operation.

2.1.3 Mechanical characteristics D.C. Shunt motor:

A constant applied voltage V is assumed across the armature. As the armature current Ia, varies the armature drop varies proportionally and one can plot the variation of the induced emf E. The mmf of the field is assumed to be constant. The flux inside the machine however slightly falls due to the effect of saturation and due to armature reaction. The variation of these parameters is shown in Fig. Knowing the value of E and flux one can determine the value of the speed. Also knowing the armature current and the flux, the value of the torque is found out.

This procedure is repeated for different values of the assumed armature currents and the values are plotted as in Fig. (a). From these graphs, a graph indicating speed as a function of torque or the torque-speed characteristics is plotted Fig. (b)(i).

As seen from the figure the fall in the flux due to load increases the speed due to the fact that the induced emf depends on the product of speed and flux. Thus the speed of the machine remains more or less constant with load. With highly saturated machines the on-load speed may even slightly increase at over load conditions. This effect gets more pronounced if the machine is designed to have its normal field ampere turns much less than the armature ampere turns. This type of external characteristics introduces instability during operation Fig. (b)(ii) and hence must be avoided. This may be simply achieved by providing a series stability winding which aids the shunt field MMF.

Mechanical characteristics D.C. Series motor:

Following the procedure described earlier under shunt motor, the torque speed Characteristics of a series motor can also be determined. The armature current also happens to be the excitation current of the series field and hence the flux variation resembles the magnetization curve of the machine. At large value of the armature currents the useful flux would be less than the no-load magnetization curve for the machine. Similarly for small values of the load currents the torque varies as a square of the armature currents as the flux is proportional to armature current in this region. As the magnetic circuit becomes more and more saturated the torque becomes proportional to Ia as flux variation becomes small.

Fig. (a) shows the variation of E1, flux, torque and speed following the above procedure from which the torque-speed characteristics of the series motor for a given applied voltage V can be plotted as shown in Fig.(b) The initial portion of this torque-speed curve is seen to be a rectangular hyperbola and the final portion is nearly a straight line. The speed under light load 32

conditions is many times more than the rated speed of the motor. Such high speeds are unsafe, as the centrifugal forces acting on the armature and commutator can destroy them giving rise to a catastrophic break down. Hence series motors are not recommended for use where there is a possibility of the load becoming zero. In order to safeguard the motor and personnel, in the modern machines, a 'weak' shunt field is provided on series motors to ensure a definite, though small, value of flux even when the armature current is nearly zero. This way the no-load speed is limited to a safe maximum speed. It is needless to say, this field should be connected so as to aid the series field.

Mechanical characteristics D.C. compound motor:

Two situations arise in the case of compound motors. The mmf of the shunt field and series field may oppose each other or they may aid each other. The first configuration is called differential compounding and is rarely used. They lead to unstable operation of the machine unless the armature mmf is small and there is no magnetic saturation. This mode may sometimes result due to the motoring operation of a level-compounded generator; say by the failure of the prime mover. Also, differential compounding may result in large negative mmf under overload/starting condition and the machine may start in the reverse direction. In motors intended for constant speed operation the level of compounding is very low as not to cause any problem. Cumulatively compounded motors are very widely used for industrial drives.

High degree of compounding will make the machine approach a series machine like characteristics but with a safe no-load speed. The major benefit of the compounding is that the field is strengthened on load. Thus the torque per ampere of the armature current is made high. This feature makes a cumulatively compounded machine well suited for Einstein College of Engineering intermittent peak loads. Due to the large speed variation between light load and peak load conditions, a y wheel can be used with such motors with advantage. Due to the reasons provided under shunt and series motors for the provision of an additional series/shunt winding, it can be seen that all modern machines are compound machines. The difference between them is only in the level of compounding.

2.1.4 Mechanical characteristics Single Phase Induction Motor

Types of Single Phase Induction Motor

The single phase induction motors can be classified according to the phase difference produced between the currents in the main and auxiliary winding. The classifications are:

1. Split – Phase motors

2. Capacitor – Start motors

3. Capacitor – run motors

4. Capacitor – Start and run motors

5. Shaded – Pole motors

Split – Phase motor

It consists of two stator windings. One is the main winding or running winding and another is auxiliary winding or stator winding. These two winding axes are displaced by 90 electrical degrees. The auxiliary winding has high resistance and low reactance and main winding has low resistance and high reactance. Ir is the current flowing through the running winding and Is is the current flowing through the starting. These two currents are out of phase. The auxiliary winding is used only for starting period. The auxiliary winding is used only for starting period. When the motor speed is about 75% of synchronous speed, the auxiliary winding is disconnected from the circuit. This is done by connecting a centrifugal switch in the auxillary circuit. After this, motor runs because of main winding only.

In the Speed – torque characteristics of Split – phase induction motor it shows that upto 75% of speed, main and auxiliary windings are present in the circuit and after 75% of the speed is attained, only the main winding is present in the circuit. The starting torque of the motor can be increased by connecting a resistance in series with the auxiliary winding. Split phase induction motor is also called resistance start induction motor.

It is mainly used for loads that require low or medium starting torque. The applications are: 1. Fans

2. Blowers

3. Centrifugal pumps

4. Washing machines

The characteristics of this motors are:

1. The starting torque is 100% to 250% of the rated value.

2. The breakdown torque is upto 300%

3. The power factor of this motor is 0.5 to 0.65

4. The efficiency of the motor is 55% to 65%

5. The power rating of this motor is in the range of ½ to 1 HP

Capacitor Start Single Phase Induction Motor

It is one type of single phase induction motor. Here, a capacitor is connected in series with the auxiliary winding. It is also used to get higher starting torque. Single – Phase supply is applied to the two windings. The starting current Is leads the line voltage, because of the capacitor present in the auxiliary winding. The running current Ir lags the line voltage. The phase displacement between the two currents is approximately equal to 900 during Starting Auxiliary C Centrifugal winding switch Ia

I Ia I Im a  V V Main m I winding

Cage Im Rotor

Again the auxiliary winding is disconnected from the circuit by centrifugal switch at 75% of the synchronous speed, ie. the capacitor is used during starting period only. The direction of rotation of the motor can be changed by changing the connections of one of the windings.

Characteristics of these motors are:

1. The starting torque is 250% to 400% of the rated value.

2. The breakdown torque is upto 350%

3. Power factor of the motor is 0.5 to 0.65

4. The power rating of the motor is 1/8 to 1 HP.

5. The efficiency of the motor is 55% to 65%.

Capacitor – Run Motor

In this motor, a capacitor is permanently connected in series with the auxiliary winding. Here, the centrifugal switch is not needed and therefore the cost of the motor is less.

Auxiliary C winding

Ir Ia

V Main winding

Cage Rotor 200

150

100

Percent torque Percent of 0 20 40 60 80 100 Percent of synchronous Speed

The capacitor value is in the range of 20.50 µF. The capacitor is AC paper oil type. The starting torque has to be sacrificed because the capacitor chosen is a comprimise between the best starting and running conditions.

Characteristics

1. The starting torque is 100% to 200% of rated value.

2. The breakdown torque is upto 250%

3. The power factor of the motor is in the range of 0.75 to 0.9

4. The efficiency of the motor is 60 to 70%

5. The power rating of the motor is 1/8 to 1 HP.

Capacitor – Start Capacitor – run Motor

Here two capacitors are used. One capacitor Cs is used for starting purpose and those another capacitor Cr is used for running purpose. In this motor, we can get high starting torque, because of two capacitors.

Ir Cs

Cr V Main winding Centrifugal switch Rotor

Main and 300 auxiliary winding

200

Main Percent torque Percent of 100 winding

0 20 40 60 80 100 Percent synchronous Speed

The value of starting capacitor Cs is large and the value of running capacitor Cr is small. The running capacitor Cr is permanently connected in series with auxiliary winding. When the motor speed picks upto 75% of synchronous speed, the centrifugal switch is opened and the starting capacitor Cs is disconnected from the circuit.The capacitor Cs is used for developing high starting torque and capacitor Cr is used to improve the power factor.

Characteristics

1. The starting torque is 200% to 300% of rated value.

2. The rated breakdown torque is upto 250%

3. The power factor of the motor is in the range of 0.75 to 0.9

4. The efficiency of the motor is 60 to 70%

5. The power rating of the motor is 1/8 to 1 HP.

Shaded Pole Motor

Construction

Shaded pole motor is a split phase type single phase induction motor. It has salient poles on the stator excited by single phase supply and a squirrel cage rotor. A position of each pole is surrounded by a short circuited turns of copper strip called shading coil. It has no

commutator, brushes, collector rings, contactors, capacitors or moving switch parts and so it is relatively cheaper, simpler and extremely rugged in construction and reliable.

The characteristics of these motors are

1. The starting torque is 40% to 60%

2. The breakdown torque is upto 140%

3. The power factor of the motor is in the range of 0.25 to 0.4

4. The efficiency of the motor is 25% to 40%

5. The power rating of the motor ranges upto 40W.

2.1.5 Mechanical characteristics Three Phase Induction Motor

Consider a speed torque characteristic shown in fig. for an induction machine, having the load characteristic also superimposed on it. The load is a constant torque load i.e., the torque required for operation is fixed irrespective of speed. The system consisting of the motor and load will operate at a point where the two characteristics meet. From the above plot, we note that there are two such points. We therefore need to find out which of these is the actual operating point. To answer this we must note that, in practice, the characteristics are never fixed; they change slightly with time. It would be appropriate to consider a small band around the curve drawn where the actual points of the characteristic will lie. This being the case let us considers that the system is operating at point 1, and the load torque demand increases slightly. This is shown in fig, where the change is exaggerated for clarity. This would shift the point of operation to a point 10 at which the slip would be less and the developed torque higher. The difference in torque-developed 4Te, being positive will accelerate the machine. Any overshoot in speed as it approaches the point 10 will cause it to further accelerate since the developed torque is increasing. Similar arguments may be used to show that if for some reason the developed torque becomes smaller the speed would drop and the effect is cumulative. Therefore we may conclude that 1 is not a stable operating point.

Let us consider the point 2. If this point shifts to 20, the slip is now higher (speed is lower) and the positive difference in torque will accelerate the machine. This behavior will tend to bring the operating point towards 2 once again. In other words, disturbances at point 2 will not cause a runaway effect. Similar arguments may be given for the case where the load characteristic shifts down. Therefore we conclude that point 2 is a stable operating point torque, Nm From the foregoing discussions, we can say that the entire region of the speed-torque characteristic from s = 0 to s = ^s is an unstable region, while the region from s = ^s to s = 0 is a stable region. Therefore the machine will always operate between s = 0 and s = ^s. Modes of Operation

The reader is referred to fig which shows the complete speed-torque characteristic of the induction machine along with the various regions of operation.

Let us consider a situation where the machine has just been excited with three phase supply and the rotor has not yet started moving. A little reaction on the definition of the slip indicates that we are at the point s = 1. When the rotating magnetic field is set up due to stator currents, it is the induced emf that causes current in the rotor, and the interaction between the two causes torque. It has already been pointed out that it is the presence of the non-zero slip that causes a torque to be developed. Thus the region of the s = 0 and s = 1 is the region where the machine produces torque to rotate a passive load and hence is called the motoring region. Note further that the direction of rotation of the rotor is the same as that of the air gap flux.

Suppose when the rotor is rotating, we change the phase sequence of excitation to the machine. This would cause the rotating stator field to reverse its direction | the rotating stator mmf and the rotor are now moving in opposite directions. If we adopt the convention that positive direction is the direction of the air gap flux, the rotor speed would then be a negative quantity. The slip would be a number greater than unity. Further, the rotor as we know should be "dragged along" by the stator field. Since the rotor is rotating in the opposite direction to that of the field, it would now tend to slow down, and reach zero speed.

Therefore this region (s > 1) is called the braking region. (What would happen if the supply is not cut-off when the speed reaches zero?) . There is yet another situation. Consider a situation where the induction machine is operating from mains and is driving an active load (a load capable of producing rotation by itself). A typical example is that of a windmill, where the fan like blades of the windmill are connected to the shaft of the induction machine. Rotation of 39

the blades may be caused by the motoring action of the machine, or by wind blowing. Further suppose that both acting independently cause rotation in the same direction. Now when both grid and wind act, a strong wind may cause the rotor to rotate faster than the mmf produced by the stator excitation. A little reaction shows that slip is then negative.

Further, the wind is rotating the rotor to a speed higher than what the electrical supply alone would cause. In order to do this it has to contend with an opposing torque generated by the machine preventing the speed build up. The torque generated is therefore negative. It is this action of the wind against the torque of the machine that enables wind-energy generation. The region of slip s > 1 is the generating mode of operation. Indeed this is at present the most commonly used approach in wind-energy generation. It may be noted from the torque expression of equation that torque is negative for negative values of slip.

2.2Braking of DC motors

Sometimes it is desirable to stop a d.c. motor quickly. This may be necessary in case of emergency or to save time if the motor is being used for frequently repeated operations. The motor and its load may be brought to rest by using either (i) mechanical (friction) braking or (ii) electric braking. In mechanical braking, the motor is stopped due to the friction between the moving parts of the motor and the brake shoe i.e. kinetic energy of the motor is dissipated as heat. Mechanical braking has several disadvantages including non-smooth stop and greater stopping time.

In electric braking, the kinetic energy of the moving parts (i.e., motor) is converted into electrical energy which is dissipated in a resistance as heat or alternatively, it is returned to the supply source (Regenerative braking). For d.c. shunt as well as series motors, the following three methods of electric braking are used:

(i) Rheostatic or Dynamic braking

(ii) Plugging

(iii) Regenerative braking

It may be noted that electric braking cannot hold the motor stationary and mechanical braking is necessary. However, the main advantage of using electric braking is that it reduces the wear and tear of mechanical brakes and cuts down the stopping time considerably due to high braking retardation.

2.2.1 Dynamic braking of dc motor

In this method, the armature of the running motor is disconnected from the supply and is connected across a variable resistance R. However, the field winding is left connected to the supply. The armature, while slowing down, rotates in a strong magnetic field and, therefore, operates as a generator, sending a large current through resistance R. This causes the energy possessed by the rotating armature to be dissipated quickly as heat in the resistance. As a result, the motor is brought to standstill quickly. Fig. (i) shows dynamic braking of a shunt motor. The braking torque can be controlled by varying the resistance R. If the value of R is decreased as the motor speed decreases, the braking torque may be maintained at a high value. Ata low value of speed, the braking torque becomes small and the final stopping of the motor is due to friction. This type of braking is used extensively in connection with the control of elevators and hoists and in other applications in which motors must be started, stopped and reversed frequently.

We now investigate how braking torque depends upon the speed of the motor.

Referring to Fig.(ii),

where k2 and k3 are constants

For a shunt motor, f is constant.

Braking torque, TB α N

Therefore, braking torque decreases as the motor speed decreases.

2.2.3 Plugging of dc motor

in this method, connections to the armature are reversed so that motor tends to rotate in the opposite direction, thus providing the necessary braking effect. When the motor comes to rest, the supply must be cut off otherwise the motor will start rotating in the opposite direction.

Fig.(ii)

Figure shows plugging of a d.c. shunt motor. Note that armature connections are reversed while the connections of the field winding are kept the same. As a result the current in the armature reverses. During the normal running of the motor [See Fig. (i)], the back e.m.f. Eb opposes the applied voltage V. However, when armature connections are reversed, back e.m.f. Eb and V act in the same direction around the circuit. Therefore, a voltage equal to V + Eb is impressed across the armature circuit. Since Eb~ V, the impressed voltage is approximately 2V. In order 10 limit the current to safe value, a variable resistance R is inserted in the circuit at the time of changing armature connections. We now investigate how braking torque depends upon the speed of the motor.

Referring to Fig. (ii),

For a shunt motor, f is constant.

Braking torque, TB = k5 + k6N

Thus braking torque decreases as the motor slows down. Note that there is some braking torque (TB = k5) even when the motor speed is zero.

2.2.5 Regenerative braking of DC Motor

In the regenerative braking, the motor is run as a generator. As a result, the kinetic energy of the motor is converted into electrical energy and returned to the supply. Fig. shows two methods of regenerative braking for a shunt motor.

(a) In one method, field winding is disconnected from the supply and field current is increased by exciting it from another source [See Fig. (i)].As a result, induced e.m.f. E exceeds the supply voltage V and the machine feeds energy into the supply. Thus braking torque is provided upto the speed at which induced e.m.f. and supply voltage are equal. As the machines lows down, it is not possible to maintain induced e.m.f. at a higher value than the supply voltage. Therefore, this method is possible only for a limited range of speed.

(b) In a second method, the field excitation does not change but the load causes the motor to run above the normal speed (e.g., descending load on a crane).As a result, the induced e.m.f. E becomes greater than the supply voltage V [See Fig. (ii)]. The direction of armature current I, therefore, reverses but the direction of shunt field current If remains unaltered. Hence the torque is reversed and the speed falls until E becomes less than V.

2.3. Braking of AC motors

When a motor is switched off it `coasts' to rest under the action of frictional forces. Braking is employed when rapid stopping is required. In many cases mechanical braking is adopted. The electric braking may be done for various reasons such as those mentioned below:

1. To augment the brake power of the mechanical brakes.

2. To save the life of the mechanical brakes.

3. To regenerate the electrical power and improve the energy efficiency. 43

4. In the case of emergencies to step the machine instantly.

5. To improve the throughput in many production processes by reducing the stopping time.

In many cases electric braking makes more brake power available to the braking process where mechanical brakes are applied. This reduces the wear and tear of the mechanical brakes and reduces the frequency of the replacement of these parts. By recovering the mechanical energy stored in the rotating parts and pumping it into the supply lines the overall energy efficiency is improved. This is called regeneration. Where the safety of the personnel or the equipment is at stake the machine may be required to stop instantly.

Extremely large brake power is needed under those conditions. Electric braking can help in these situations also. In processes where frequent starting and stopping is involved the process time requirement can be reduced if braking time is reduced. The reduction of the

1. Dynamic

2. Regenerative

3. Reverse voltage braking or plugging

These are now explained briefly with reference to induction motors.

DYNAMIC BRAKING

In dynamic braking the motor is disconnected from the supply and connected to a dynamic braking resistance RDB. In and Fig. this is done by changing the switch from position 1 to 2. The supply to the field should not be removed. Due to the rotation of the armature during motoring mode and due to the inertia, the armature continues to rotate. An emf is induced due to the presence of the field and the rotation. This voltage drives a current through the braking resistance. The direction of this current is opposite to the one which was owing before change in the connection. Therefore, torque developed also gets reversed. The machine acts like a brake. The torque speed characteristics separate by induction machine under dynamic braking mode is as shown in Fig. (b) for a particular value of RDB. The positive torque corresponds to the motoring operation. Here the machine behaves as a self-excited generator. Below a certain speed the self- excitation collapses and the braking action becomes Zero. Process time improves the throughput. Basically the electric braking involved is fairly simple. The electric motor can be made to work as a generator by suitable terminal conditions and absorb mechanical energy. This converted mechanical power is dissipated/used on the electrical network suitably.

REGENERATIVE BRAKING:

In regenerative braking as the name suggests the energy recovered from the rotating masses is fed back into the a.c. power source. Thus this type of braking improves the energy efficiency of the machine. The armature current can be made to reverse for a constant voltage operation by increase in speed/excitation only. Increase in speed does not result in braking and the increase in excitation is feasible only over a small range, which may be of the order of 10 to 15%. Hence the best method for obtaining the regenerative braking is to operate, the machine on a variable voltage supply. As the voltage is continuously pulled below the value of the induced emf the speed steadily comes down. The field current is held constant by means of separate excitation.

This has many advantages over its rotating machine counterpart. Static set is compact, has higher efficiency, requires lesser space, and silent in operation; however it suffers from drawbacks like large ripple at low voltage levels, unidirectional power flow and low over load capacity. Bidirectional power flow capacity is a must if regenerative braking is required. Series motors cannot be regeneratively braked as the characteristics do not extend to the second quadrant.

PLUGGING:

The third method for braking is by plugging. Fig. shows the method of connection for the plugging of a shunt motor. Initially the machine is connected to the supply with the switch S in position number 1. If now the switch is moved to position 2, then a reverse voltage is applied across the armature. The induced armature voltage E and supply voltage V aid each other and a large reverse current flows through the armature. This produces a large negative torque or braking torque. Hence plugging is also termed as reverse voltage braking.

The machine instantly comes to rest. If the motor is not switched off at this instant the direction of rotation reverses and the motor starts rotating the reverse direction. This type of braking therefore has two modes viz. 1) plug to reverse and 2) plug to stop. If we need the plugging only for bringing the speed to zero, then we have to open the switch S at zero speed. If nothing is done it is plug to reverse mode. Plugging is a convenient mode for quick reversal of direction of rotation in reversible rives. Just as in starting, during Plugging also it is necessary to limit the current and thus the torque, to reduce the stress on the mechanical system. This is done by adding additional resistance in series with the armature during plugging.

UNIT III

3.1. DC MOTOR STARTERS

At the time of starting of induction motor speed is zero, so back emf is also zero, as back emf is zero & rotor coil is sorted(in case of squirrel cage rotor),starting current is about 5 to 6 times higher than the rated current. so because of this high voltage drop gets created in the supply line & equipment connected to this line will experience this. So to reduce the high current generally we use star delta starter. at the time of starting we use star connection so the voltage got reduce then at the time of run we use delta

Connection.. Starters also increase the starting torque & improves the power factor.

3.1.1. Need and types of Starters

Starter is used to reduce starting current. starting current is usually large because of inertia, characteristics of motor winding(at starting slip=1,rotor resistance low implies less back emf).So at starting we use starter .for example. STAR DELTA starter. by using a starter at least the switching is improved which is much important for safe and longer life time use. simply a DOL can be used for lower rated motors (up to 07 HP)in this case. for higher rated motors (up to 15 HP)we can use a star delta connection. a delta connected motor takes 1.7 times line current than a star connected motor. think ! at starting the current is almost halved .Now a days we use drives (power electronics products which not only takes care of starting of a motor (improving power factor) also controls it as per requirement.

3.1.2. DC motor starters

1. Two point starter 2. Three point starter 3. Four point starter

3.1.3TWO POINT STARTER

At no load, T=0 therefore the motor achieves dangerously high speed. So motor should not be started without load. Shows connection diagram of a two point starter.

The starting resistance is connected in series with the armature of a series motor. The no load release coil is connected in series with the armature. After closing the supply, the handle is moved from OFF position to stud no.1. Then full starting resistance is included. Therefore the starting current is reduced. Then the starting resistance is gradually cut down and the motor gathers speed, which will then develop back emf.

No load release coil (NLR)

The no load release coil is in series with the armature. Therefore load current flows through this coil also. Since the speed of DC series motor is inversely proportional to the load current, the motor should not be allowed to operate at no load. If the motor happens to be operated at no load then it achieves dangerously high speed. So the no load release coil gives necessary protection to the motor.

When the load current becomes zero, the NLR coil is de-energized and releases the handle, which goes back to OFF position

3.1.4.THREE POINT STARTER

The connection diagram of a three point starter is shown in figure

When the handle is moved to ON position the soft iron, which is attached to the handle, is attracted by the electromagnet. When the handle is in ON position, the motor achieves its rated full speed, which develops back emf. This back emf then regulates the armature current.

 The starting resistance (R1 to R6) is connected in series with the armature of a DC motor  A handle, which can be moved over the starting resistance against the spring.  A no-voltage release (NVR) coil is connected in series with the field winding.  An OLR (Over Load Release) coil is connected in series with the armature  A movable arm is placed near the OLR coil.

Operation

The handle is moved over the starting resistance after switching on the supply. When the handle is at stud no.1, the full starting resistance is included in series with the armature. Therefore the starting current is reduced. When the handle is further moved, the resistances are cut out gradually. At the same time, the motor develops back emf when it gathers speed.

Protective Devices

Two protective devices are incorporated in the starter. They give the necessary protection to the motor from over load and power supply failure.

(i) NVR (No Voltage Release)

The NVR is an electromagnet. The coil is connected in series with field winding. When the handle is in ON position, the no volt coil is magnetized and attracts the soft iron and keeps the handle in ON position against the spring tension. In the case of failure or disconnection of the supply or a break in the field circuit, the NVR coil is de-energized thereby releasing the arm, which is pulled back by the spring to the OFF position.

(ii) OLR (Over Load Release)

The over current release consists of an electromagnet connected in the supply line. If the motor becomes over loaded beyond a certain predetermined value, the line current (or) armature current increase and hence the attracting power of electromagnet increases, then the movable arm is lifted and short-circuits the electromagnet(NVR). Hence the arm is released and returns to OFF position.

Demerit of 3-point starter

The motor speed can e ieased eakeig the flu [N α /Φ]; hile eploig this method, to decrease the flux, the field current is to be decreased to achieve speeds above the rated speed. To achieve higher speeds, the field current is to be reduced to a very low value. This low value of current also passes through NVR, which is unable to create enough electromagnetic pull to overcome the spring tension. Hence the arm is pulled back to OFF position. This is an undesirable feature of 3-point starter which makes it unsuitable for variable speed motors.

3.1.5.FOUR POINT STARTER

Shows connection diagram of a four point starter. In this starter, the HOLD ON coil has been taken out of the shunt field circuit and has been connected directly across the supply line through a protecting resistance HR. It should be noted that with this arrangement, any change of current in the shunt field circuit does not affect the current passing through the HOLD ON coil because the two circuits are independent of each other. It means that the electromagnetic pull exerted by the HOLD ON coil will always be sufficient and will prevent the spring from restoring the handle to OFF position no matter how the field rheostat is adjusted for speed control.

3.2. AC MOTOR STARTERS:

An induction motor is similar to a poly-phase transformer whose secondary is short circuited. Thus, at normal supply voltage, like in transformers, the initial current taken by the primary is very large for a short while. Unlike in DC motors, large current at starting is due to the absence of back emf. If an induction motor is directly switched on from the supply, it takes 5 to 7 times its full load current and develops a torque which is only 1.5 to 2.5 times the full load torque. This large starting current produces a large voltage drop in the line, which may affect the operation of other devices connected to the same line. Hence, it is not advisable to start induction motors of higher ratings (generally above 25kW) directly from the mains supply. Various starting methods of induction motors are described below.

3.2.1. AUTO TRANSFORMERS:

It is also known as autotransformers or compensator. It consists of an autotransformers with necessary switches or three phase transformers reduced voltage is applied to the motor when the motor pick up 80%. So, that the transformer is out- out and full voltage is given to the motor most of the autotransformer are provided with 3 sets of tops so as to reduce the voltage to 85, 60 up to 50% of the line voltage.

3.2.2. STAR – DELTA STARTER:

This method is used in the case of the motor which one built t run normally with a delta connected starter winding. It consists of two way switch connect the motor in star for starting and then delta for normal running. The star connected applied voltage by a factor of 1/3 and hence the torque developed because 1/3 of that of which would have been developed if the motor war directly connected in the delta.

The Three types of star delta starters are

1. Hand Operated

2. Semi Automatic

3. Fully Automatic

3.2.3. DIRECT ON LINE STARTER (or) D.O.L. STARTER:

When fully voltage is supplied across the starters of induction motor, lot of current in drawn by the winding. This is because at the time of starting the induction motor are started using direct on line starter on heavy starting current will flow through the winding such as heavy starting current of short duration may not cause to the motor. Since the construction of induction motor are rugged.

More over it takes time for temperature rise to endanger the utilization of motor windings. But this heavy impression current will cause large voltage drop with the linear during the period of motor

A direct alternate method at starting of induction motor is application up to starting of induction motor. The ON push button is pressed coil A becomes energized and if open contacts are closed when OFF button push button is pressed in a will get energized and main contacts of the conductor open when the motor starts, in case of overload on the motor the contact of over load may be opened and sub sequently the motor will stop.

3.2.4. ROTOR RESISTANCE STARTER

Shows a rotor resistance starter. This starter can be used only for slip ring induction motor. As shown in figure external or starting resistance is connected in the rotor terminals.

In this method, the motor is always started with full line voltage, applied across the stator terminals. The value of starting current is adjusted by introducing a variable resistance in the rotor circuit. At starting, the full resistance is included and hence the starting current is reduced. The resistance is gradually cut out of the rotor circuit as the motor gathers speed.

PRIMARY RESISTANCE STARTER

A variable resistance is connected in series with the supply terminals of the motor. The purpose of this resistance is to reduce the supply voltage. This reduced voltage is given to the motor terminals.

The reduced voltage limits the starting current. If the voltage across the terminal is reduced by 50%, then the starting current is reduced by 50%, by torque is reduced to 25% of the full voltage value.

UNIT IV

4.1. Conventional methods

The speed of a d.c. motor is given by:

N α Eb / Φ

N = K (V- IaR) / Φ r.p.m.

Where R = Ra for shunt motor

= Ra + Rse. for series motor

From exp, it is clear that there are three main methods of controlling the speed of a d.c. motor, namely:

(i) By varying the flux per pole (Φ). This is known as flux control method.

(ii) By varying the resistance in the armature circuit. This is known as armature control method.

(iii) By varying the applied voltage V. This is known as voltage control method.

Speed Control of D.C. Shunt Motors

The speed of a shunt motor can be changed by (i) flux control method

(ii) armature control method (iii) voltage control method. The first method (i.e.flux control method) is frequently used because it is simple and inexpensive.

4.1.2. Flux control method

It is based on the fact that by varying the flux Φ, the motor speed (N α 1/ Φ) can be changed and hence the name flux control method. In this method, a variable resistance (known as shunt field rheostat) is placed in series with shunt field winding as shown in Fig.

The shunt field rheostat reduces the shunt field current Ish and hence the flux .Therefore, we can only raise the speed of the motor above the normal speed (See Fig). Generally, this method permits to increase the speed in the ratio 3:1.Wider speed ranges tend to produce instability and poor commutation.

Advantages

(i) This is an easy and convenient method.

(ii) It is an inexpensive method since very little power is wasted in the shunt

field rheostat due to relatively small value of Ish.

(iii) The speed control exercised by this method is independent of load on

the machine.

Disadvantages

(i) Only speeds higher than the normal speed can be obtained since the total

field circuit resistance cannot be reduced below Rsh—the shunt field

winding resistance.

(ii) There is a limit to the maximum speed obtainable by this method. It is

because if the flux is too much weakened, commutation becomes poorer.

Note. The field of a shunt motor in operation should never be opened because its speed will increase to an extremely high value.

4.1.3. Armature control method

This method is based on the fact that by varying the voltage available across then armature, the back e.m.f and hence the speed of the motor can be changed. This is done by inserting a variable resistance RC (known as controller resistance) in series with the armature as shown in Fig.

N V IaRa RC where RC = controller resistance

Due to voltage drop in the controller resistance, the back e.m.f. (Eb) is decreased. Since N Eb, the speed of the motor is reduced. The highest speed obtainable is l hat corresponding to RC = 0 i.e., normal speed. Hence, this method can only provide speeds below the normal speed (See Fig.)

Disadvantages

(i) A large amount of power is wasted in the controller resistance since it

carries full armature current Ia.

(ii) The speed varies widely with load since the speed depends upon the

voltage drop in the controller resistance and hence on the armature

current demanded by the load.

(iii) The output and efficiency of the motor are reduced.

(iv) This method results in poor speed regulation.

Due to above disadvantages, this method is seldom used to control tie

speed of shunt motors.

Note. The armature control method is a very common method for the speed control of d.c. series motors. The disadvantage of poor speed regulation is not important in a series motor which is used only where varying speed service is required.

Speed Control of D.C. Series Motors

The speed control of d.c. series motors can be obtained by (i) flux control method (ii) armature-resistance control method. The latter method is mostly used.

4.1.4. Flux control method

In this method, the flux produced by the series motor is varied and hence the speed. The variation of flux can be achieved in the following ways:

(i) Field diverters.

In this method, a variable resistance (called field diverter) is connected in parallel with series field winding as shown in Fig. Its effect is to shunt some portion of the line current from the series field winding, thus weakening the field and increasing the speed (N α1/Φ). The lowest speed obtainable is that corresponding to zero current in the diverter (i.e., diverter is open). Obviously, the lowest speed obtainable is the normal speed of the motor. Consequently, this method can only provide speeds above the normal speed. The series field diverter method is often employed in traction work.

4.1.5. Armature diverter

In order to obtain speeds below the normal speed, a variable resistance (called armature diverter) is connected in parallel with the armature as shown in Fig. The diverter shunts some of the line current, thus reducing the armature current. Now for a given load, if Ia is decreased, the flux f must increase (T α IaΦ). Since N α 1/Φ, the motor speed is decreased. By adjusting the armature diverter, any speed lower than the normal speed can be obtained.

Tapped field control.

In this method, the flux is reduced (and hence speed is increased) by decreasing the number of turns of the series field winding as shown in Fig. The switch S can short circuit any part of the field winding, thus decreasing the flux and raising the speed. With full turns of the field winding, the motor runs at normal speed and as the field turns are cut out, speeds higher than normal speed are achieved.

Armature-resistance control

In this method, a variable resistance is directly connected in series with the supply to the complete motor as shown in Fig. This reduces the voltage available across the armature and hence the speed falls. By changing the value of variable resistance, any speed below the normal speed can be obtained. This is the most common method employed to control the speed of d.c. series motors. Although this method has poor speed regulation, this has no significance for series motors because they are used in varying speed applications. The loss of power in the series resistance for many applications of series motors is not too serious since in these applications, the control is utilized for a large portion of the time for reducing the speed under light-load conditions and is only used intermittently when the motor is carrying full-load.

4.1.6. WARD-LEONARD SYSTEM

In this method, the adjustable voltage for the armature is obtained from an adjustable- voltage generator while the field circuit is supplied from a separate source. This is illustrated in Fig.The armature of the shunt motor M (whose speed is to be controlled) is connected directly 60

to a d.c. generator G driven by a constant-speed a.c. motor A. The field of the shunt motor is supplied from a constant-voltage exciter E. The field of the generator G is also supplied from the exciter E.

The voltage of the generator G can be varied by means of its field regulator. By reversing the field current of generator G by controller FC, the voltage applied to the motor may be reversed. Sometimes, a field regulator is included in the field circuit of shunt motor M for additional speed adjustment. With this method, the motor may be operated at any speed upto its maximum speed.

4.1.7. Advantages

(a) The speed of the motor can be adjusted through a wide range without

resistance losses which results in high efficiency.

(b) The motor can be brought to a standstill quickly, simply by rapidly

reducing the voltage of generator G. When the generator voltage is

reduced below the back e.m.f. of the motor, this back e.m.f. sends this

current through the generator armature, establishing dynamic braking.

While this takes place, the generator G operates as a motor driving

motor A which returns power to the line.

supply is not available .

The disadvantage of the method is that a special motor-generator set is required for

each motor and the losses in this set are high if the motor is operating under light

loads for long periods.

4.2. PHASE CONTROLLED RECTIFIER FED DC DRIVES

Here AC supply is fed to the phase controlled rectifier circuit. AC supply may be single phase or three phase. Phase controlled rectifier converts fixed AC voltage into variable DC voltage. Here, the circuit consists of SCRs. By varying the SCR firing angle (delay angle) the output voltage can be controlled. This variable output voltage is fed to the DC motor. By varying the motor input voltage, the motor speed can be controlled.

4.2.1. Block diagram

Advantages of Thyristorised drives.

1. Basic operation is simple and reliable. 2. Time response is faster. 3. Operating efficiency is high, about 95%. 4. Small size. 5. Less weight 6. Low initial cost.

Disadvantages of Thyristorised drives.

1. The overload capacity is lower 2. Under certain operating conditions, the power factor in the AC supply is low.

Types of phase controlled rectifier drives

1. Single phase Half Wave Converter drives 2. Single phase Semiconverter drives 3. Single phase Fully Controlled Rectifier drives

4.2.2. SINGLE PHASE HALF WAVE CONVERTER DRIVES

Assume armature current Ia is constant. Here, the motor is separately excited DC motor. Motor is operated from single phase half wave controlled rectifier. Motor field winding is fed through separate DC source.

During the positive half cycle SCR t is foad iased. At ωt = α, “C‘ t is tiggeed ad comes to the ON state. Then the positive voltage is fed to the motor.

At ωt = π, feeheelig diode oes to the foad iased ad “C‘ oes to the OFF state, because of reverse voltage.

During the negative half cycle, SCR T is OFF state, and free wheeling diode conducts upto 2 π + α.

α to π –T ON

π to π + α –FD ON

Duig the peiod, π to π + α, uet is positie ut output oltage is zeo eause of closed path (FD-motor-FD).

Hee aig the fiig o dela agle α, the output oltage a e aied. This aiale voltage fed to the motor, then the motor speed can be changed. It is the one quadrant drive, because the output voltage and current is always positive.

4.2.3. SINGLE PHASE SEMICONVERTER DRIVES

Shows single phase Semiconverter drive. Assume armature current Ia is constant. DC motor is operated from single phase semiconverter or half controlled rectifier. Semiconverter consists of tow SCRs, two diodes and one free wheeling diode (FD). Freewheeling diode is connected across the load. Here, the load is DC motor.

Duig the positie half le to π, “C‘ T1 and diode D1 is forward biased. At

ωt = α, “C‘ T1 is triggered. Then SCR T1 and diode D1 comes to the ON state. Duig the peiod α to π, “C‘ T1 and diode D1 is ON state. In this period, we can get positive output voltage and positie uet. At ωt = π, “C‘ T1 and diode D1 is turned OFF.

Duig the egatie half le π to π + α, the feeheelig diode conducts. In this period, current flow through FD and motor i.e., closed circuit. Here we can get positive output current and zero output voltage.

At ωt = π + α, “C‘ T2 is triggered. Then SCR T2 and diode D2 comes to the ON state. During the peiod π + α to π, “C‘ T2 and diode D2 is ON state. Now we can get, positive output voltage and positive output current. This voltage is fed to the DC motor. This converter also offers one-quadrant converter drive, because the output voltage and current is always positive. It is shown figure.

This converter is used upto about 15KW DC drives.

4.2.4 SINGLE PHASE FULLY CONTROLLED RECTIFIER DRIVES

Shows single phase full converter drive. Assume armature current Ia is constant. Here, the load is DC motor. Full converter consists of 4 SCRs and load.

Duig the positie half le to π “C‘ T1 and T2 are forward biased.

At ωt = α, “C‘ T1 and T2 are triggered and comes to the ON state. These two SCRs conducts upto π + α. Duig the peiod α to π + α, “C‘ T1 and T2 ON state.

At ωt = π + α, “C‘ T3, T4 are triggered and SCR T1 and T2 comes to the OFF state. Now SCR T3 and T4 conducts upto 2 π + α.

From the above discussion, the full converter is also called two quadrant converter. It means, the average output voltage is either positive or negative byt output current is always positive. It is shown

THREE PHASE DC DRIVES

For large power dc motor drives, three-phase controlled rectifiers are used. Three phase controlled rectifier circuits give more number of voltage pulses per cycle of supply frequency. This makes the motor current continuous and filter requirement also less. The number of voltage pulses per cycle depend on the number if thyristors and their connections for three phase controlled rectifiers. Semi converters and full converters are most commonly used in practice.

4.2.5. THREE PHASE SEMI CONVERTERS CONTROLLED RECTIFIER

The circuit diagram for a three phase semi converter feeding a separately excited dc motor is shown in figure. It is a one quadrant converter because the average output voltage and current is always positive. The field winding of the motor is also connected to three phase semi con vert er. It is use d up to abo ut 120 KW ratings.

The fiig agle α a e aied fo to π. Duig the peiod π/ ωt π/, “C‘ T1 is forward biased. If T1 is tiggeed at ωt = π/ + α, “C‘ T1 and diode D1 conduct and line to line voltage vac appears across the motor terminals.

At ωt = π/, ac starts to be negative and free wheeling diode Df conducts. The motor current continues to flow through Df and T1 and D1 are turned off.

In the absence of freewheeling diode SCR T1 would continue to conduct until SCR T2 tiggeed at ωt = π/ + α ad the feeheelig atio ould e aoplished through T1 and D2 .

4.2.6. THREE PHASE FULLY CONTROLLED RECTIFIER

Three phase full converters are used industrial applications upto 1500 KW drives. It is a two quadrant converter i.e., the average output voltage is either positive or negative but average output current is always positive. Shows the power circuit diagram of 3-phase full converter feeding a separately excited dc motor.

The “C‘s ae tiggeed at a iteal of π/. The feue of output ipple oltage is fs. The filtering requirement is less than that of three phase semi converter. At ωt = π/ + α, “C‘ T6 is already conducting and SCR T1 and T6 conduct and the line-to-line voltage vab appears across the motor terminals.

At ωt = π/+ α, “C‘ T2 is fired and SCR T6 is reversed biased immediately. SCR T6 is commutation. During inteal π/ + α ωt π/ + α, “C‘s T1 and T2 conduct and the line to line voltage vac appears across the load. If the SCRs are numbered as shown the firing sequence is 12, 23, 34, 45, 56& 61.

4.3. DC CHOPPER DRIVES

4.3.1. Concept

DC motor speed can be controlled through DC chopper. Fixed DC voltage is fed to the DC chopper circuit. DC chopper converts fixed DC into variable DC voltage. This variable DC voltage is fed to the motor. By varying the DC voltage, the motor speed can be controlled.

4.3.2. Advantages of DC Chopper Control

1. High efficiency 2. Flexibility in controls 3. Light weight 4. Small size

Application of DC Chopper Drives

1. Battery operated vehicles 2. Traction motors control in electric traction 3. Trolly cars 4. Hoists

Types of DC Chopper Drives

1. First quadrant chopper or type A chopper 2. Second quadrant or type B chopper 3. Two quadrant type A chopper or type C chopper 4. Two quadrant type B chopper or type D chopper 5. Four quadrant chopper or type E chopper.

4.3.3. FIRST QUADRANT OR TYPE-A CHOPPER OR MOTORING CHOPPER

In this chopper circuit diagram consists of chopping device (SCR, Power MOSFET, BJT, etc), free wheeling diode and motor.

Input DC supply is fed to the chopper circuit. Then the chopper CH1 is turned on by applying trigger pulse. Now the input voltage is fed to the motor. During the ON time(Ton) of the chopper, output voltage is equal to input voltage i.e., Vo=Vs .

After Ton period, chopper CH1 is turned off. Now the load is disconnected from the supply but the motor current flows through the free wheeling diode. In the turn-off period(Toff), output voltage is zero but load current flow through the fee wheeling diode.(FD-motor-FD). i.e, Vo=0

Again the chopper CH1 is turned on and this cycle is repeated. Here assume armature current is constant.

Average output voltage is given by

Va = Vsα

Whee α = dut atio o dut le of the chopper = TON/T

TON = Turn on time of chopper

TOFF = Turn off time of chopper

T = Total time or chopping period (TON + TOFF) = 1/f

Vs = Supply voltage f = Chopping frequency

B aig the o tie o off tie of the hoppe, the dut le α, the output voltage can be changed. This variable output voltage is fed to the DC motor. Then the DC motor speed can be controlled.

In the type A chopper offers one-quadrant drive. It means, the average output voltage and output current is always positive. i.e., power flows from source to load.

4.3.4. SECOND- QUADRANT, OR TYPE-B CHOPPER

Second quadrant or type-B chopper is shown in figure. In this chopper, the load must contain the dc source E, like a dc motor (or a battery).

When CH2 is on, output voltage is equal to zero, i.e, Va = 0 but load voltage E drives current through L and CH2. During on time of the chopper (TON), inductor L stores energy.

When CH2 is off, output voltage Va = [E+L di/dt] exceeds source voltage Vs. As a result, diode D2 is forward biased and conducts, thus allowing power to flow to the source. Chopper CH2 may be on or off, load current I0 flows out of the load. Here load current I0 is treated as negative.

The power flows from load to source because output voltage Va is always positive and load current Ia is negative. As load voltage Va is greater then the source voltage Vs, type B chopper is also called as step-up chopper or boost converter. It is also known as regenerative chopper.

4.3.5. TWO QUADRANT TYPE-A CHOPPER OR TYPE-C CHOPPER

Type C chopper is obtained by connecting type-A and type-B chopper in parallel as shown. Here, the output voltage Va is always positive but the load current Ia is positive as well as negative.

When chopper CH1 or FD conduct, the output voltage and load current is always positive. In other words, CH1 and FD operate together as type-A chopper in first quadrant. When chopper CH2 or diode D2 conduct, the output voltage is positive but the load current is negative. In other words CH2 and D2 operate together as type-B chopper in second quadrant.

Average load voltage is always positive but average load current may be positive or negative as explained above. Therefore, power flow may be from source to load (first quadrant operation) or from load to source (second- quadrant operation).

Choppers CH1 and CH2 should not be on simultaneously otherwise direct short circuit will occur. This type of chopper configuration is used for motoring and regenerative braking of dc motors. The operating region of this type chopper is shown

4.3.6. TWO QUADRANT TYPE-B CHOPPER OR TYPE-D CHOPPER

The power circuit diagram for two quadrant type B chopper or type D chopper is shown. The output voltage is equal to supply voltage i.e, V0=Vs when both CH1 and CH2 are on and output voltage is equal to negative value of supply voltage i.e. Va = - Vs when both choppers are off but both diodes D1 and D2 conduct.

Average output voltage Va is positive when on time of the choppers (TON) is more than their turn off time (TOFF) shown in figure. Average output voltage Va is negative when choppers turned off time (TOFF) is more than their turn on time (TON) as shown

The direction of load current is always positive because choppers and diodes can conduct current only in the direction of arrows shown in figure. Here output voltage Va is negative, the power flows from load to source (power flow is reversible). The operation of this type-D chopper is shown by hatched area in first and fourth quadrants in figure

4.3.7. FOUR QUADRANT CHOPPER OR TYPE-E CHOPPER

Shows the power circuit diagram for a four quadrant chopper or type-E chopper.

It consists of four power semiconductor switches CH1 to CH4 and four power diodes D1 to D4 I anti parallel. Working of this chopper in the four quadrants is explained:

Forward motoring mode: For first quadrant operation of figure CH4 is kept on, CH3 is kept off and CH1 is operated. With CH1, CH4 on, load voltage is equal to supply voltage i.e, Va = Vs and load current Ia begins to flow. Here both output voltage Va and load current Ia are positive giving first quadrant operation. When CH1 is turned off, positive current freewheels through CH4, D2 in this way, both output voltage Va, load current Ia can be controlled in the first quadrant. First quadrant operation gives forward motoring mode.

Forward braking mode: Here CH2 is operated and CH1, CH3 and CH4 are kept off. With CH2 on, reverse (or negative) current flows through L, CH2, D4 and E. During the on time of CH2 the inductor L stores energy. When CH2 is turned off, current is fed back to source through diodes

D1, D4 note that there [E+L di/dt] is greater than the source voltage Vs. As the load voltage Va is positive and load current Ia is negative, it is second quadrant operation of chopper. Also power is flows from load to source. Second quadrant operation gives forward braking mode.

Reverse motoring mode: For third quadrant operation of figure. CH1 is kept off, CH2 is kept on and CH3 is operated. Polarity of load emf E must be reversed for this quadrant operation. With

CH3 on, load gets connected to source Vs so that both output voltage Va, load current Ia are negative. It gives third quadrant operation. It is also known as reverse motoring mode. When

CH3 is turned off, negative current freewheels through CH2,D4. in this way, output voltage Va and load current Ia can be controlled in the third quadrant.

Reverse braking mode: Here CH4 is operated and other devices are kept off. Load emf E must have its polarity reversed to that shown in figure. For operation in the fourth quadrant. With CH4 on, positive current flows through CH4, D2, L and E. During the on time of CH4 inductor L stores energy. When CH4 is turned off, current is feedback to source through diodes D2, D3. Here load voltage is negative, but load current is positive leading to the choppers operation in the fourth quadrant. Also power is flows from load to source. The fourth quadrant operation gives reverse braking mode. The devices conduction in the four quadrants are indicated in figure.

UNIT V

CONVENTIONAL AND SOLID STATE SPEED CONTROL OF AC DRIVES

The speed of an induction motor can be controlled by two major methods. They are:

1) Stator side control 2) Rotor side control

The first method is applicable for both squirrel-cage and wound-rotor motors. The second method can be used only for wound-rotor motors.

Stator side control means, we have to vary the stator side parameters i.e., supply voltage, frequency, no. of poles etc.

Conventional Methods Of Speed Control

Types of stator side control

1. Stator voltage control 2. Stator frequency control 3. V/f control 4. Pole changing method Types of rotor side control

1. Adding external resistance in the rotor circuit. 2. Cascade control. 3. Slip power recovery scheme

5.1. Stator Side Control

5.1.1. Stator voltage control

The speed of the induction motor can be controlled by varying the stator voltage. This method of speed control is known as stator voltage control. Here, the supply frequency is constant. The stator voltage can be controlled by two methods.

i) Using auto transformer ii) Primary resistors connected in series with stator winding.

i) Using auto transformer.

The speed of the induction motor can be controlled by using auto transformer. It is shown. The input to the auto transformer is a fixed as voltage. By varying the auto transformer, we can bet variable ac output voltage without change in supply frequency. The variable voltage is fed to the induction motor. Then the induction motor speed also changes. ii) Primary resistors connected in series with stator winding:

By varying the primary resistance, the voltage drop across the motor terminals is reduced. That is, reduced voltage is fed to the motor. Then the motor speed can be reduced. It is one method of conventional speed control of induction motor. The control method is very simple. The main disadvantage is that more power loss occurs in the primary resistors.

5.1.3. Stator frequency control

The stator frequency control is the one of the methods of speed control for a 3-phase induction motor. Here , we can vary the input frequency of the motor. Under steady state condition, the IM operates in the small-slip region, where the speed of the induction motor is always close to the synchronous speed of the rotating flux.

Ns = 120f/P

Where, f = frequency of the supply voltage

P = Number of poles

5.1.4. Voltage/frequency control

The voltage/ frequency control is one method of speed control for three-phase induction motor. Fo the ef euatio, the aigap flu is gie Φ = /πT1Kw) (V/f)

To maintain airgap flux constant, the parameters V, and f must be changed so as to maintain (V/f) ratio constant. This is known as Voltage/frequency control.

Fixed AC voltage is fed to the rectifier circuit. It converts AC to DC. This DC supply is fed to the inverter circuit. It converts DC into variable AC voltage and variable frequency. This output is fed to the stator of the induction motor. By varying V, f and maintaining (V/f) ratio constant, the induction motor speed can be changed. It is one of the most powerful methods of speed control for induction motor.

5.1.5. Pole changing method

For a constant frequency, the synchronous speed of the motor is inversely proportional to the number of poles.

Ns α /P

By changing the poles, the motor synchronous speed can be varied. Provision for changing the number of poles has to be incorporated at the manufacturing stage and such machines are called pole changing motors or multi speed motors.

Rotor side control

5.1.7. Adding external resistance in the rotor circuit

This method is applicable only for slip ring induction motor. The external resistance can be added in the rotor circuit.

A simple and primitive method of speed control of a slip ring induction motor is by mechanical variation of the rotor circuit resistance, as shown

2 2 2 The toue euatio of the idutio oto is T α sE 2 R2 / R 2 + (sX2)

Advantages:

1. Smooth and wide range of speed control. 2. Absence of in-rush stating current. 3. Availability of full-rated torque at starting. Disadvantages:

1. Reduced efficiency because the slip energy is wasted in the rotor circuit resistance. 2. Speed changes vary widely with load variation.

5.1.8.Cascade control

Another method of speed control of slip ring induction motor is cascade control. It is also known as tandem control. It is shown

It consists of two slip ring induction motors. The motor 1 is called main motor M1. This motor is coupled with 2 motor. The 2 motor is called auxiliary motor M2.

A three-phase supply is fed to the stator of the main motor M1. The slip ring voltage of motor

M1 is fed to the stator of the auxiliary motor M2. This method of connection is called cascade connection or tandem connection.

In this cascading method, if both motors produce the torque in the same direction it means, cumulative cascading and opposite direction it means, differential cascading.

5.2. Slip power recovery system:

This system is mainly used for speed control of slip ring induction motor. Controlling the power flow in the rotor circuit can control either by varying the stator voltage or the speed of slip ring induction motor.

The slip power can be recovered to the supply source can be used to supply an additional motor which is mechanically coupled to the main motor. This type of drive is known as slip poe eoe sste ad ipoes the oeall effiie of the system.

5.2.1. Conventional Kramer System

Hence speed control by varying slip is obtained by supplying counter emf to the rotor at slip frequency or injected emf method. Also known as Kramer System control method.

In this method, the speed control obtained just by injecting emf voltage in the rotor circuit. This method explained as follows:

Construction

Here we use totally two drives with one rotory converter, in it one drive is slipping induction motor drive which is directly connected to 3 phase AC source and an other drive is DC shunt drive D which is mechanically coupled with 3 phase slip ring induction drive and electrically oupled ith oto oete hih oets AC slip poe ito DC ad fed to die D. Usig separate exciters with dc source excites both DC and rotary converters fields. 86

Operation

Because of injecting the emf to the motor side given the following results. That is the injected emf is phase opposition to the rotor emf therefore the resistance is increasing and vice versa.

In the Kramer Control Method, from the above circuit diagram the main motor M is connected with 3 phase AC source with frequency of f. Its slip power is taken through slip ring and rotary converter set which converts AC slip power into DC and fed to DC shunt drive motor. And both DC shunt and rotary converters excitation is obtained by using separate dc source.

As soo as slip poe output is oe eas the ai oto M u ith the lo speed. Because the rotary converter get more power from main motor rotor, it converts more amount of DC and fed to DC motor so it try to run in high speed, but by varying the excitation we can maintain the speed. Similarly if slip power output from main motor is less means the motor runs in high speed. Hence, rotary converter converts AC to DC in less. Therefore the speed of DC motor reduced, so by decreasing the excitation we can get increase in speed in main motor.

This method is used only below synchronous speed so this is called as sub-synchronous control method. Since no injection of emf in the rotor side, this is only the drawback in this method. To overcome this we have to go to static scherbius method

Advantages

1. This method is adopted for any speed within the working range 2. If the rotary converter is over excited, then it will take leading current which compensates for the lagging current drawn by slip ring induction motor and therefore improves the power factor of the system.

Scherbius System

Construction

Slip ring induction motor ------connected with source and its slip rings used to

take the slip power

Rotary Converter------which is used to convert rotary slip power into dc

And fed to DC motor which is connected with slip

rings of SRIM

DC Motor ------which is mechanically coupled with induction

generator and this operates source and dc

armature source from exciter.

Operation

In Scherbius system due to slip power availability in slip rings of SRIM we can run the DC motor and induction motor generator set.

If the slip power output from slip ring induction motor (SRIM) is less means the main drive (SRIM) runs with rated speed or in high speed. Hence DC motor gets small amount of DC power input from rotary converter. Therefore the induction generator, generates fed low emf to source.

Suppose the slip power output from slip ring induction motor is more, ie, main slip ring induction motor is more therefore main slip ring drive runs with low speed (more slip). Then DC motor gets more DC power input from rotory converter. Hence, induction generator generates more power and fed into source. Then by varying the field winding flux we can vary the

induction motor generation. Hence, the SRIM speed controlled by varying the field regulator of the dc motor.

Stator Voltage Control by using AC Voltage Regulator

I Φ idutio otos toue Te is proportional to the square of the stator voltage suppose reduction in the supply voltage will reduce the motor torque and hence the speed of the drive.

In addition the motor terminal voltage is reduce to K<1 where KC1 then the motor torque is gives by

2 2 2 Te = /ωs (KV) /(r1 + r2 ) + (X1 + X2 ) r2

Where

r2 = (r2/S)

Te = Torque in NM

ωs = Angular velocity in rad/secs.

V1 = starter terminal total resistance

2 (r1 + r2 ) = stator terminal total resistance

2 (X1 + X2 ) = Stator terminal total inductive reactance

Construction

Six thyristor T1 to T6 connected in Each phase of stator terminals in anti parallel connection of two thyristors to each phases.

All three parallel connected AC regulators connected with supply and another terminal with stator terminal of motor.

Operation

By controlling the firing angle of the thyristors connected in anti-parallel in each phase, the rms value of the stator voltage can be regulated. By varying the firing angle of each thyristors used in AC egulato e a a the oltage applied to the stato teial of Φ iduction motor. Hence the speed and torque explained in following graph.

In the above graph, load torque, TL, a is the opeatig poit at ated oltage ad OA is the oto speed. Fo edued stato oltage K=. is the opeatig poit ad OB is the reduced motor speed for load torque TL.

This method is normally adopted for motors having large value of slip (Sm) for Low-slip motors, the range of speed control is very narrow.

Variable voltage Input Inverter Control (VVI)

Construction

Six pulse bridge converter is used to convert the normal three phase AC supply to DC power at variable DC voltage.

LC acts as a filter which is used to filters the harmonics and ripples in six pulse converter output.

The output of converter and LC filter is given to a Φ idge iete ith fee heelig diode. The conductions may be 180o (or) 1200.

Operation

Duig the speed otollig atio usig this Φ idge iete ith its odutio is of 180o (or) 1200 .

In 1800 mode, each thyristor conducts 180o and in 1200 conduction mode, each thyristor conducts 1200 . during operation at a time two thyristors are conducting in 1200 mode. One from upper group and another one from the lower group. During operation of each voltage is displaced by 1200

By varying the ON and OFF period of thyristor the average output voltage and frequency are otolled. The output of the iete is gie to the stato idig of Φ idutio oto, oth voltages and frequency are varied, inverter control method is better than AC voltage controllers practically.

In this case the variable voltage obtained only by changing the firing angle in converter side.

STATOR FREQUENCY CONTROL

According to the synchronous speed formula in Φ induction motor the relationship between speed and frequency is given by

Ns = 120f/P

Where Ns = Synchronous speed in rpm

f = Supply frequency in Hz

P = No of poles

By changing the supply frequency, motor synchronous speed can be altered and thus torque and speed of Φ induction motor can be controlled. For a three-phase induction motor per phase supply voltage is given by

V1 = √ π f1 N1 Φ K

The above equation shows that under rated voltage and frequency operation, flux will be rated.

In case supply frequency is reduced with constant V1, then the air gap flux increases and the induction motor magnetic circuit gets saturated.

With constant supply voltage, if the supply frequency is increased, the synchronous speed and therefore motor speed rises. But with increase in frequency flux and torque also get reduced. This can be obtained be feeding Φ induction motor through three-phase inverters as follow

0 In the above figure each thyristor conducts for 180 of a cycle. Thyristor pair in each arm, ie T1, 0 T4, T3, T6 and T5, T2 are turned ON with a time interval of 180 then the operation explained with the help of following thyristor conduction diagram.

0 0 If means that T1 conducts for 180 and T4 for the next 180 of a cycle. Thyristors in the upper 0 0 group ie. T1, T3 ,T5 conduct at an interval of 120 . it implies that it T1 is fied at ωt = then T3 0 0 ust e fied at ωt = and T5 at ωt = same is true for lower group of SCRs. On the basis of this firing scheme, a table is prepared as shown

VARIABLE VOLTAGE OUTPUT INVERTER CONTROL

In this control instead of changing the applied input voltage to the inverter, the output voltage only varied by using voltage variac (or) Autotransformer in the output side of the inverter. Which is shown.

Construction

 Φ uotolled oete is used fo oetig AC poe ito DC poe.  LC filters adopted for smoothing and removing Harmonics  Φ Bidge iete is used to oete DC to AC aodig to pulse iput ad output is fed to motor through Φ aia.  Φ aia o Auto tasfoe is oeted etee oto stato teial ad Bidge Inverter.  I this otol, AC poe is oeted iot DC usig Φ idge etifie, the DC is oeted ito aiale AC Φ idge iete. The iput oltage to the AC oto Φ oto is otaied though a Φ auto-transformer which is used to the variable output 93

oltage to Φ idutio oto. The egulato otols the epetitio ate of the gate signals to the thyristors also controls the variable autotransformer to give the require volts/hertz.

PWM Inverter Control

 Normally the output from the inverter is squarware with some harmonic contents. So we have to remove (or) reducing the Harmonic contents by using some voltage control techniques.  Hence, we use the pulsing technique to control the alternating voltage output of a static inverters. This explained as follows with some classification.

PWM Techniques

This PWM techniques are classified into

1. Single pulse PWM 2. Multiple pulse PWM 3. Sinusoidal pulse PWM  In all the above cases, the magnitude of the fundamental voltage is controlled by variation of the total ON-time during a half cycle. In single pulse width modulation, one pulse is produced in each half cycle. Then output contains significant harmonics.  In multiple pulse width modulation, each half cycle having number of pulses with equal width.  Similarly in sinusoidal PWM the pulse width is varied through the half cycle in a sinusoidal manner. The pulses should be regularly spaced and the pulse width at a particular position should be proportional to the area under the sinewave at that position.  PWM wave is produced by means of a control circuit in which a high frequency triangular waveform is mixed with a sinusoidal waveform of the desired frequency, voltage control is obtained by varying the widths of all pulses without affecting the sinusoidal relationship. With the pulse width sinusoidal modulation, the output voltage and current have less Harmonic contents, this method is shown. V/F CONTROL ON CYCLO CONVERTER 3Φ AC DRIVE SPEED CONTROL

Cyclo converter is a device which is used for converting direct AC into variable AC output frequency changer.

By using this, we can control both voltage and frequency. So this method is suitable for controlling both voltage and frequency (V/F control ) the following circuit explain this method.

The above circuit consists of totally two sets of thyristors positive and negative groups with Φ poe suppl fo oetig Φ aiale oltage. That is aoe iuit consists of positive and negative group of converters.

When positive group converter is turned ON (ie Thyristor in positive group only triggered) output obtained is positive and vice-versa for negative group also.

For providing this both groups of thyristor must be supplied with appropriate firing signals so that, both groups are producing the same average voltage allowing current to flow in either direction.

Independent control of output frequency and voltage is obtained with only one parameter variation ie the firing points of the controlled rectifiers to be varied. Similary the frequency of the output voltage is controlled by the rate at which the firing points are varied so output voltage is controlled by the maximum rate at which the firing points are varied. So output voltage is controlled by the maximum excursion of the firing points.

A cyclo-converter fed AC motor drive will respond to a change in polarity of the input signals by changing direction of rotation of the motor without the use of contactors to reverse phase sequence. Anyhow, by using this cyclo-converter drive we can drive the induction motor in all four quadrants. This method contain the following limitations.

Limitation

1. Harmonic contents more with low power factor. 95

SLIP POWER RECOVER SCHEMES

Static Kramer Method.

Construction

Φ “‘IM- stato is oeted ith soue of Φ AC suppl though sith “1. Rotor is connected with starting resistances (variable) only at the time of starting only through S2. Slip igs, slip poe is olleted ad fed to solid state oete. ‘etifie Φ idge oete rectifier which is used to convert the slip power AC into DC and fed to DC motors armature through S4.

Operation

In slip power recovery scheme, the energy in the rotor is (slip rings) converted to useful energy and it is used to control the speed.

“lip ig idutio otos oto iuit fed, the slip poe though slip igs though slip igs and rectified by using a diode bridge rectifier and this rectified dc is fed to armature of the separately excited dc motor. Which is mechanically coupled to the induction motor.

The system is started is started by switching ON TPST1(S1) and TPST2 (S2). As soon as the motor attains steady speed, the DC motor is energized by switching TPST2(S2) OFF and TPST3(S3) and

TPST4(S4) are ON (ie starting resistances are cutout). Then speed control is achieved by varying the field current, of the motor. An emf proportional to the back emf of the DC motor may be considered to be injected into the rotor circuit of the induction motor to cause variation in speed of the system.

STATIC SCHERBIUS SCHEME FOR SPEED CONTROL

Construction

Φ “‘IM-stato is oeted ith soue of Φ AC suppl though sith S1. Rotor is connected with starting resistances (variable) only at the time of starting through S2. Slip rings slip power is collected and fed to solid state converter Inverter. Inverter set output is connected ith Φ-AC though Φ tasfoe. L ats as filte to eoe Haois Φ ipples.

Operation

The induction motor is started using three rheostats in the motor circuits by closing T1 and

T2 switches. As soon an the motor attains the steady state speed, the starting resistance is cutout.

Then, AC slip-power collected from slip-rings of SRIM is first converted by the three phase diode bridge, then turned back into AC power at line frequency by the thyristorised inverter and finally returned to the supply network by means of the transformer in figure. Which brings the rotor circuit voltage upto the value corresponding to the voltage of the AC supply network.

The speed of the induction motor is regulated by controlling the firing angle of the inverter.

The gate pulses are provided by the firing circuits, synchronized with the supply voltage. Both the rectifier and inverter are line-communicated by the alternating emfs appearing at the slip- rings and supply network respectively.

The system is started by switching ON first S1 and then S2. As soon as the motor attains a steady slate speed, the rectifier inverter set as well as the transformer is connected to the supply network by switching S2 OFF and S3 , S4 , ON.

Advantages

The problem of commutation near synchronous speed disappears.

The cyclo-converter is to be controlled so that its output frequency tracks precisely with the slip- frequency

Dis- advantages

Control of scherbius drive is same what difficult Cost is more

Application

Used in flywheel energy storage system