IJSAR Journal of and Computing (IJSAR-JEC) ISSN: 2408-7653 Volume 2, Issue 1 (March 2015), 9-16. www.mdcjournals.org

Research Article

SOFT , AN ALTERNATIVE TO MOTOR CONTROL. Obeta, C.N.1, Mgbachi, C.A.2 Udeh, I.J.3 Department of Electrical/Electronic Engineering, Enugu State University of Science and (ESUT) Enugu1,2,3. [email protected], [email protected], [email protected]

ABSTRACT: Today the world is witnessing energy crises and this necessitates the efficient utilization of and electronic helps in accomplishing this task of efficient energy usage. Accurate control of applied voltage to electric motor is very vital for its starting and normal speed control. This paper examines the development of a soft starter through the use of silicon control (SCR) to manipulate the voltage delivered to electric motor. A variable resistance is inserted in series with the gate circuit to trigger the SCR at any point –in-time along the wave form, to allow for time proportional power control to the load. The result is that the rated input voltage is reduced according to the magnitude of the resistance.

Keywords: Applied voltage, Soft starter, Silicon control rectifier, Variable resistance.

1.0 INTRODUCTION

The basic control parameters on electric motors are speed, , flux, voltage and current, though they are all interrelated. Soft starter in this case is any device that controls the power to a load by means of controlling the applied voltage or current [1]. Soft starter can be applied in both starting and normal speed control of motors [2]. Starting of small electric motor can be direct-on-line, here starting current is not excessive but limited to few amperes [3]. The starting current of an (AC) motor can vary from 3-7 times the nominal current [4]. This is because a large amount of energy is required to magnetize the motor enough to overcome the inertia the system has at stand still. The high current drawn from the network can cause problems such as voltage drop, high transient and in some cases uncontrolled shut down [5]. High starting current also causes great mechanical stress on the motor’s bars and windings, and can affect the driven equipment and the foundation consequent upon the afore –mentioned. Larger motors requiring high starting current must be provided with means of limiting the starting current to the value that will not damage the coil [6]. This can be done by connecting the motor first in star where the applied voltage is motor 1/3 of the rated voltage and later in delta when the full motor voltage is applied [7]. Another voltage reduction method is the use of autotransformer where the voltage is injected gradually [8]. Silicon controlled rectifier (SCR)is a solid state switching device that can provide fast infinitely variable proportional control of [9]. It has been in use since 1950s but today, they are smaller, safer, versatile and very reliable [2]. In most power electronic applications, SCR are commonly used due to their ruggedness, simplicity, cheapness and market availability even for very high rating applications [10]

Many methods have been adopted to control the applied voltage to influence the speed. In this paper, a soft starter is developed with a silicon controlled rectifier (SCR) to control the applied voltage. The voltage can be reduced by delaying the triggering gate pulses of the SCR. If these pulses are delayed, the voltage is reduced from the rated input voltage and the motor starts safely and if running, the speed reduces. More losses are produced in the voltage and power due to the change in . To recover these losses in power, a special SCR called Gate Turn off (GTO) which forces the current to flow almost in phase with voltage is used. The gate terminals are shorted and a common control pulse is used to control the speed of the motor.

1.1 SCR firing modes.

There are two main categories by which SCR can control power to the load – zero and non-zero voltage crossover firing schemes.  zero voltage cross over firing: In zero voltage crossover firing, the control circuit allows scr to turn on and off only when the voltage sine wave across the SCR is zero. Consequently, the load power is turned on or off only when | 9 IJSAR Journal of Engineering and Computing (IJSAR-JEC) ISSN: 2408-7653 www.mdcjournals.org

IJSAR Journal of Engineering and Computing (IJSAR-JEC) ISSN: 2408-7653 Volume 2, Issue 1 (March 2015), 9-16. www.mdcjournals.org

the instantaneous value of the sinusoidal wave form is zero. This can occur in time proportional and demand oriented transfer (DOT) firing modes.

 Time Proportional: This firing mode has a fixed time base with the power output proportioned over the time base. Load power is controlled by switching the SCR “on” for a number of complete electrical cycles, and then “off” for a number of complete electrical cycles. The output power to load depends on number of complete electrical sine waves. In this mode, it behaves like a magnetic contractor which means that the load is switched on with a regular time T for a variable time t. So power delivered is given as P = V2 x t R T Where V = Rms line voltage and R = load resistance

It is commonly used as a replacement for mechanical contractors to eliminate radio frequency interference (RFI) and allow faster switching times. Time proportional controllers in 50Hz AC. power supply with 2seconds time base are shown below:

a.

a. 75%

b. 50% c. 25%

Fig. 1 a-c Controllers with percentage power output in 50Hz.

| 10 IJSAR Journal of Engineering and Computing (IJSAR-JEC) ISSN: 2408-7653 www.mdcjournals.org

IJSAR Journal of Engineering and Computing (IJSAR-JEC) ISSN: 2408-7653 Volume 2, Issue 1 (March 2015), 9-16. www.mdcjournals.org

a. 75%

b. 50% c. 25%

Fig. 2 a-c Controllers with percentage power output in 60Hz.

50Hz a.c .supply is 50 cycles per second and the two seconds time base is 100 cycles. In 75% power output, the on cycle should be 75 in number and the off cycle 25. Same applies to 50% and 25%. For 60Hz supply, the two seconds time base is 120cycles, hence 75% is 90cycles on and 30cycles off.

♦ Demand Oriented Transfer (DOT):

The circuit is designed in such a way that the power output from SCR depends on the load. If 50% power output is required in a 50Hz a.c. power supply, then time base should be two cycles (40msec) with one cycle (20m sec) on and one cycle (20msec) off. If 75% power out put is required, then time base should be 4 cycles. (80msec) with 3 cycles (60msec) on and off for a cycle (20msec). Standard control signals for DOT pack 0-5mA 4-20mA 0-20mA 10-50mA 0-50mA 0-5VDC 1-5mA 0-10VDC

• Non-zero voltage cross over firing:

This is referred to as phase Angle firing, where each a.c voltage cycle is dissected into small parts. Phase control is the most common form of thyristor power control. The thyristor is held in off condition-which means all current flow in the circuit is blocked by the thyristor except a minute leakage current. Then the thyristor is triggered into an “on” condition by the control circuitry. The unit controls down to the sub-cycle level resulting in extremely tight and accurate control. It supplies a saw tooth wave form and may be used on all types of loads. Power to load is controlled by governing the point at which SCR is turned on in each half cycle of the full a.c. electrical sine wave. After triggering, the remainder of the a.c. cycle is applied to the load. Phase Angle controller with percentage power output is shown below.

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IJSAR Journal of Engineering and Computing (IJSAR-JEC) ISSN: 2408-7653 Volume 2, Issue 1 (March 2015), 9-16. www.mdcjournals.org

Being a rectifier, two devices in inverse parallel are needed to control a.c. power. Standard signals for phase angle pack are: 0-5mA 4-20mA 0-20mA 10-50mA 0-50mA 0-5VDC 1-5mA 0-10VDC

II. IMPLEMENTATION OF SOFT STARTER

The new approach involves judicious placement of substantial resistance in series with the gate circuit. Increasing the resistance raises the threshold Level and with the load current half sine wave chopped up to a greater degree, triggering of SCR is delayed and the load receives less average power because power is delivered for less time throughout the cycle. Reducing the resistance causes more power to be delivered to the load because triggering will start earlier and the delay angle is reduced. | 12 IJSAR Journal of Engineering and Computing (IJSAR-JEC) ISSN: 2408-7653 www.mdcjournals.org

IJSAR Journal of Engineering and Computing (IJSAR-JEC) ISSN: 2408-7653 Volume 2, Issue 1 (March 2015), 9-16. www.mdcjournals.org

Fig.3. Circuit and wave form when resistance is inserted.

In phase-firing, timed gate pulses are fed to the SCRs and in this control mode, each of the two SCRs in an a.c. is turned on only for a portion of the cycle. Phase angle fired mode of SCR provides a feature called soft start. Soft start limits the amperage output on start – up and ramps the amperage slowly from zero output to full operating power. Soft start slowly increase the amount of voltage applied to the load over a user specified time base. The output voltage is ramped from zero to the desired output at known rate. This feature prevents inrush current when controlling variable resistance or inductive coupled loads.

Fig. 4. Increase of voltage by soft start.

The average d.c. output is given by

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IJSAR Journal of Engineering and Computing (IJSAR-JEC) ISSN: 2408-7653 Volume 2, Issue 1 (March 2015), 9-16. www.mdcjournals.org

E (cos  - cos ) 2

Where  is trigger angle and  is cut – off angle

Fig. 5. Trigger and cut off angle.

The rms value of the output voltage is

E  (x-+ 1 sin (2) 2 2 Where  is the conduction angle.

Control using soft starter can be open or closed loop. Components of basic soft starter include power , (SCR) rated three times the line voltage and PID controller or to control the firing angle of SCR to make it conduct at the required part of the supply voltage cycle.

Table 1. SCR Firing Angle VS Power Output to the Motor.

Firing angel () Output power (degree) (%) 30 83.3 45 75 60 66.7

90 50 120 33.4

135 25

150 16.68

180 100

The graph of the variation with the code was drawn in MATLAB environment as shown below.

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IJSAR Journal of Engineering and Computing (IJSAR-JEC) ISSN: 2408-7653 Volume 2, Issue 1 (March 2015), 9-16. www.mdcjournals.org

% INFLUENCE OF FIRING ANGLE ON PERFORMANCE OF SCR.

>> F=[ 30 45 60 90 120 135 150 180]; P=[ 83.3 75 66.7 50 33.4 25 16.68 100]; plot(P,F,'r') ylabel('Firing Angle ');xlabel(' Load Power( Red)'); grid on

180

160

140

120

100

80 Firing Angle Firing 60

40

20

0 0 10 20 30 40 50 60 70 80 90 100 Load Power( Red)

Fig. 6. Variation of firing angle () with percentage power output of SCR.

From the table and graph above, the firing angle is inversely proportional to the power output, so the smaller the angle, the higher the output power to the load except at full half cycle (1800) when the output power is 100%. Increasing the resistance raises the threshold level and with the load current half sine wave chopped up to a greater degree, triggering of SCR is delayed and the load receives less average power because power is delivered for less time throughout the cycle. Reducing the resistance cause more power to be delivered to the load because triggering will start earlier and the delay angle is reduced.

III. DESCRIPTION

Basically an SCR power controller consists of the following; semiconductor power device (SCR and diodes), a control circuit normally referred to as the firing circuit, a means to dissipate the heat generated from the semiconductor device and protective circuits (fuses and transient suppressors). Judicious placement of substantial resistance in series with the gate circuit is vital and it is necessary that the SCRs used on power control are rated at high enough voltage to withstand industrial voltage peaks. The higher the peak voltage rating of SCR, the safer it will be. Reduced voltage starting can be accomplished in several ways. A common method is the use of autotransformer. This is called reduced voltage autotransformer (RVAT). Starting similar to this is reactor and primary resistor starter. These days they are called solid state reduced voltage (SSRV) starter. They use high speed switching device called SCRs to switch on for only a portion of each half of the sine-wave line power. By doing so, the rough average of voltage getting to the motor is reduced proportionately by the amount of time the switch is delayed. By moving the “gate” point further back in the sine-wave, the rms voltage is increased until the SCR is being gated at the zero-cross point and the motor is getting full line voltage. | 15 IJSAR Journal of Engineering and Computing (IJSAR-JEC) ISSN: 2408-7653 www.mdcjournals.org

IJSAR Journal of Engineering and Computing (IJSAR-JEC) ISSN: 2408-7653 Volume 2, Issue 1 (March 2015), 9-16. www.mdcjournals.org

3.1 Advantages of soft start

Improved Efficiency: The efficiency of soft starter system using solid state is more owing to voltage consumption. Controlled startup: The starting current can be controlled smoothly by easily altering the starting voltage and this ensures smooth starting of the motor without any jerks. Controlled acceleration: Motor acceleration is controlled smoothly. Low cost and size: This is ensured with the use of solid state switches.

IV. CONCLUSION Soft start is the best approach for motor starting and speed control as the output voltage is ramped from zero to the desired output at known rate. This feature prevents inrush current when controlling the inductive load. This could be achieved using a variable three pot resistance in gate circuit.

REFERENCES

[1] Omega Technology Company “User’s Guide power controllers”. http://www.omega.com [2] Ramamoorty, M. and Arunachalam, M., “ A solid state controller for induction motors”, 1977 IEEE Annual meeting on industrial applications,12 october, 2013. [3] Kamakhya Basumatary “speed control of ” http://www.academia.edu/11064/speedcontrol. [4] Ram amoorty, M and Wani, S.N, “Choper controlled slip-ring induction motor with closed loop control” 1EEE Trans. On industrial electronic and control instrumentation, vol. IECI -24 No.2, PP.153-161, May 1977 [5] Basu,P.R. “Variable speed induction motor using thyristor in the secondary circuit 1EEE Transactions on power Apparatus and systems, vol. PAS-20 pp.509-514. 2x, March/April 1971. [6] Canlos. A. martins, Adriano. S. carvolho. “Technological Trend on induction motor electric drives” IEEE Porto power Tech conference, vol.2, set 2001, ieeexplore.ieee.or/xpcs/abs – all.jsp arn umbar = 964725 & Is number = 20828. retrieved Dec., 5,2014. [7] Induction motor - Wikipedia, the free encyclopedia. http://www.wikipedea.org/wiki/inductionmotor. retrieved Nov. 5,2014 [8] Andrzej M. Trzyradlowski “Introduction to modern power electronics” John Wiley and sons pp. 190-220, 2010.

[9] Arthur Holland, “Holland Technical skills document”, 26 April, 2001 www.dcnz.co//scr.pdt. Retrieved Jan. 4, 2015.

[10] Allen E Boldt, ”Using SCR switching circuit in Application to motor requirement speed regulation, torque capabilities and stopping methods”, www.thyristorscr.co/news/z. Retrieved Jan. 10, 2015.

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