DC GENERATOR INTRODUCTION TO GENERATORS: Electrical generators are standalone machines that provide electricity when power from the local grid is unavailable. These generators supply backup power to businesses and homes during power outages. Generators do not create electrical energy, but they convert mechanical or chemical energy into electrical energy. Based on the output, generators are classified into two types as AC generators and DC generators. DC GENERATOR: A DC generator is an electrical machine that converts mechanical energy into electricity. When a conductor cuts magnetic flux, an electromotive force (EMF) is produced in them based on the principle of electromagnetic induction The EMF so produced is called dynamically induced EMF as it is produced to rotation of conductors. The electromotive force can cause a flow of current when the conductor circuit is closed. The direction of the EMF can be obtained by Flemming’s Right hand rule. CONSTRUCTION:
Cut-section of a DC Machine
Front View of DC Machine
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A DC machine has mainly four components. 1. Field magnet system 2. Armature 3. Commutator 4. Brush & Brush Gear
Field Magnet System: The Field Magnet System is the stationary or fixed part of the machine. It produces the main magnetic flux. The magnetic field system consists of Mainframe or Yoke, Pole core and Pole shoes and Field or Exciting coils.
Field magnet System of DC machine Magnetic Frame and Yoke:
The outer hollow cylindrical frame to which main poles and inter-poles are fixed and by means of which the machine is fixed to the foundation is known as Yoke. It is made of cast steel or rolled steel for the large machines and for the smaller size machine the yoke is generally made of cast iron.
The two main purposes of the yoke are as follows:-
It supports the pole cores and provides mechanical protection to the inner parts of the machines. It provides a low reluctance path for the magnetic flux.
Pole Core and Pole Shoes:
The Pole Core and Pole Shoes are fixed to the magnetic frame or yoke by bolts. Since the poles, project inwards they are called salient poles. Each pole core has a curved surface. Usually, the pole core and shoes are made of thin cast steel or wrought iron laminations. The poles are laminated to reduce the Eddy Current loss. The shape of Pole shoe is referred to as cruciform shape. The poles core serves the following purposes given below:
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It supports the field or exciting coils. They spread out the magnetic flux over the armature periphery more uniformly. It increases the cross-sectional area of the magnetic circuit, as a result, the reluctance of the magnetic path is reduced.
Field pole of a DC machine Field or Exciting Coils:
Each pole core has one or more field coils (windings) placed over it to produce a magnetic field. The coils are wound on the former and then placed around the pole core. When direct current passes through the field winding, it magnetizes the poles, which in turns produces the flux. The field coils of all the poles are connected in series in such a way that when current flows through them, the adjacent poles attain opposite polarity.
Armature:
Armature of DC machine The rotating part of the DC machine or a DC Generator is called the Armature. The armature consists of a shaft upon which a laminated cylinder, called Armature Core is placed.
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The armature core of DC Generator is cylindrical in shape and keyed to the rotating shaft. At the outer periphery of the armature has grooves or slots which accommodate the armature winding. The armature core of a DC generator or machine serves the following purposes.
It houses the conductors in the slots. It provides an easy path for the magnetic flux.
As the armature is a rotating part of the DC Generator or machine, the reversal of flux takes place in the core, hence hysteresis losses are produced. The silicon steel material is used for the construction of the core to reduce the hysteresis losses.
The rotating armature cuts the magnetic field, due to which an e.m.f is induced in it. This e.m.f circulates the eddy current which results in Eddy Current loss. Thus to reduce the loss the armature core is laminated with a stamping of about 0.35 to 0.55 mm thickness. Each lamination is insulated from the other by a coating of varnish.
Armature Winding:
The insulated conductors are placed in the slots of the armature core. This arrangement of conductors is called Armature Winding. The armature winding is the heart of the DC Machine. Armature winding is a place where the conversion of power takes place. In the case of a DC Generator here, mechanical power is converted into electrical power.
Commutator:
Commutator of DC machine The commutator, which rotates with the armature, is cylindrical in shape and is made from a number of wedge-shaped hard drawn copper bars or segments insulated from each other and from the shaft. The segments form a ring around the shaft of the armature. Each commutator segment is connected to the ends of the armature coils. It connects the rotating armature conductors to the stationary external circuit through brushes. It converts the induced alternating current in the armature conductor into the unidirectional current in the external load circuit in DC Generator action, whereas it converts the alternating torque into unidirectional (continuous) torque produced in the armature in motor action.
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Brushes & Brush Gear:
Brush of DC Machines Carbon brushes are placed or mounted on the commutator and with the help of two or more carbon brushes, current is collected from the armature winding. Each brush is supported in a metal box called a brush box or brush holder. The brushes are pressed upon the commutator and form the connecting link between the armature winding and the external circuit. They are usually made of high-grade carbon because carbon is conducting material and at the same time in powdered form provides a lubricating effect on the commutator surface. Bearings:
The ball or roller bearings are fitted in the end housings. The function of the bearings is to reduce friction between the rotating and stationary parts of the machine. Mostly high carbon steel is used for the construction of bearings as it is a very hard material.
Shaft:
The shaft is made of mild steel with a maximum breaking strength. The shaft is used to transfer mechanical power from or to the machine. The rotating parts like armature core, commutator, cooling fans, etc. are keyed to the shaft.
Armature Winding:
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Lap Winding: finish end of one coil is connected to a commutator segment and start end of the adjacent coil is situated under the same pole. Wave Winding: coil progressive passes every North and South pole till it returns to the coil side where it started. Equilizer ring: these are low resistance copper wires that connect points in armature winding which under ideal conditions should remain at the same potential. They relieve the brushes from circulating currents Dummy coils: these are not electrically connected to the rest of the winding. They only preserve mechanical balance.
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Terms Associated with Conductors:
Conductor (Z): The length of a wire lying in a magnetic field in which EMF is induced is called conductor. Turn (T): When two conductors are connected in series, so that the EMF induced in them help each other is known as a turn. Coil: Two coils along with their end connections constitute one coil. A coil may be single turn or multi-turn. Single turn coils have two conductors but multi-turn conductors have many conductors per coil side. Winding: Number of coils arranged in coil group is called winding.
Pole pitch: No. of conductors per pole Coil Pitch: it is the distance measured in terms of armature slots between two sides of a coil.
Front pitch (Yf): Distance in terms of no. of armature conductors between the second conductor of one coil and the first conductor of the next coil which are connected to the same commutator segment.
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Back pitch (Yb): Distance in terms of no. of armature conductors between the first and last conductor of the coil
Resultant pitch (YR): Distance in terms of no. of armature conductors between the start of one coil and start of the next coil to which it is connected.
Commutator pitch (YC): Distance in terms of no. of commutator segments between the segments to which two ends of a coil are connected.
WORKING PRINCIPLE: Flemming’s Right Hand Rule:
“Hold the right hand fore-finger, middle finger and the thumb at right angles to each other. If the forefinger represents the direction of the magnetic field, the thumb points in the direction of motion or applied force, then the middle finger points in the direction of the induced current.” Principle of Operation:
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The working principle of a DC generator is based on Faraday’s laws of electromagnetic induction. When a conductor is placed in a varying magnetic field, an electromotive force gets induced within the conductor. For production of dynamically induced EMF, three things are necessary. Magnetic field Conductor Motion of the conductor with respect to the magnetic field In DC generators, the magnetic field is provided by the field magnet system. Conductors are placed on the armature and armature is being rotated by prime-mover. As the armature rotates, it cuts the air gap flux. Rate of change of flux produces EMF in the armature coils.
In the above figure, a single turn coil ABCD is rotated in the magnetic field. The coil rotates along its own axis xx’. While rotating, flux is cut by the coil and therefore EMF is induced in it. The magnitude of induced EMF is proportional to the rate of change of flux linkage and its direction is given by Flemming’s right hand rule. When coil is in position shown in the figure, the flux linkage with its coil is maximum but no flux is cut by the coil sides AB & CD. So, no EMF is induced in it.
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When the coil is rotated in clockwise direction, the coil sides begin to cut the field first slowly then at gradually increasing rate. So, the EMF gradually increases and becomes maximum when the loop rotates through 900. Direction of induced EMF is B-A & C-D.
In the next quarter cycle, between (900-1800), the rate at which conductors cut the flux gradually decreases. So, the EMF reduces gradually and becomes zero when the loop again becomes parallel to the magnetic field.
In the third quarter of revolution, i.e. between 1800-2700, the rate at which conductors cut across the magnetic field increases and the EMF induced also becomes maximum gradually. But the direction of induced EMF is now A-B & D-C. In the fourth quarter of revolution, i.e. between 2700-3600, the induced EMF decreases as the coil moves and becomes zero when it reaches 3600. This cycle is repeated in each revolution of armature. The EMF generated is of pulsating nature and hence called as Alternating EMF.
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The current induced in the coil is collected and conveyed to the external load circuit through slip rings. To obtain unidirectional current, split rings or commutators are used. In the first half cycle, current flows along ‘B-A-M-L-D-C-B’. ‘a’ acts as negative pole and ‘b’ acts as positive pole. In the next half cycle, position of segments ‘a’ and ‘b’ are also reversed. So, M and L are again in contact with negative and positive segment respectively.
So, the current collected or the voltage appearing across the brushes is unidirectional. This EMF has ripples. To have a constant waveform, large number of commutator segments are used. The voltage generated by one single coil is small. Hence, several turns in series are used.
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EMF Equation of DC generators:
푑ɸ The Expression of Dynamically induced EMF is = i.e. rate of change of flux linkage. 푑푡 Let’s assume, P – Number of poles of the machine ɸ – Flux per pole in Weber Z – Total number of armature conductors N – Speed of armature in revolution per minute (r.p.m) A – Number of parallel paths in the armature winding In one revolution of the armature, the flux cut by one conductor is = Pɸ Wb 60 Time taken to complete one revolution is: t= sec 푁 Pɸ Pɸ푁 The average induced e.m.f in one conductor: = 60/푁 60 The number of conductors connected in series in each parallel path: Z/A
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Therefore, the average induced EMF across each parallel path or the armature terminals is given by:
Pɸ푁 푍 Pɸ푁푍 Eg = x = Volts 60 퐴 60퐴 If we take, n= 푁 i.e., Number of rotations per minute, then the EMF equation becomes: 60 Pɸ푛 Eg = Volts 퐴 In case of lap winding, A=P, So;
ɸ푁푍 Eg = Volts 60 In case of wave winding, A=2, So,
Pɸ푁푍 Eg = Volts 120 In the above equation, we can see that, P, Z, A are constants, Hence,
Eg α Nɸ Classification of DC Generators: Depending on the manner in which the field winding gets supply, DC generators are of the following types.
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Separately Excited DC Generator A DC generator whose field winding or coil is energized by a separate or external DC source is called a separately excited DC Generator. Here field current is independent of armature current.
Here,
If=Field current
Ia=Armature current
IL=Load current V= Terminal voltage
Eg= EMF generated
Ra= Armature resistance
Rf=Field Resistance
Ia= IL
V= Eg- Ia Ra Or
V= Eg- Ia Ra-Brush Drop
Electrical power developed in the armature= EgIa Watt
Electrical power delivered to the load= VIL Watt Self Excited DC Generator
Self-excited DC Generator is a device, in which the current to the field winding is supplied by the generator itself. In self-excited DC generator, the field coils may be connected in parallel with the armature in the series, or it may be connected partly in series and partly in parallel with the armature windings.
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Series Generator
A series-wound generator the field coils are connected in series with the armature winding. The series field winding carries the armature current.The series field winding consists of a few turns of wire of thick wire of larger cross-sectional area and having low resistance usually of the order of less than 1 ohm because the armature current has a very large value
Here,
Rse= Series winding resistance
Ia= IL= Ise
V= Eg- Ia Ra- Ia Rse
= Eg- Ia (Ra+ Rse)
= Eg- Ia (Ra+ Rse)- Brush Drop
Shunt Wound Generator
In a shunt-wound generator, the field winding is connected across the armature winding forming a parallel or shunt circuit. Therefore, the full terminal voltage is applied across it. A very small field current Ish, flows through it because this winding has many turns of fine wire having very high resistance Rsh of the order of 100 ohms. The current field Ish is practically constant at all loads. Therefore, the DC shunt machine is considered to be a constant flux machine.
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Here, Rsh= shunt field winding resistance
Ish= Field winding current
푉 Ish= Rsh
Ia= IL+ Ish
V= Eg- Ia Ra Or
V= Eg- Ia Ra-Brush Drop
Electrical power developed in the armature= EgIa Watt
Electrical power delivered to the load= VIL Watt
Compound Wound Generator
In a compound-wound generator, there are two field windings. One is connected in series, and another is connected in parallel with the armature windings. There are two types of compound- wound generator. If the magnetic flux produced by the series winding assists the flux produced by the shunt winding, then the machine is said to be cumulative compounded. If the series field flux opposes the shunt field flux, then the machine is called the differentially compounded.
It is connected in two ways. One is a long shunt compound generator, and another is a short shunt compound generator. If the shunt field is connected in parallel with the armature alone then the machine is called the short compound generator. In long shunt compound generator, the shunt field is connected in series with the armature.
Short Shunt Compound Wound Generator
In a Short Shunt Compound Wound Generator, the shunt field winding is connected in parallel with the armature winding only.
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Here, Ise= IL
푉+퐼퐿Rse Ish= Rsh Eg− Ia Ra = Rsh
Ia= IL+ Ish
V= Eg- Ia Ra-ILRse
Or, V= Eg- Ia Ra-ILRse-Brush Drop
Electrical power developed in the armature= EgIa Watt
Electrical power delivered to the load= VIL Watt Long Shunt Compound Wound Generator
In a long shunt-wound generator, the shunt field winding is parallel with both armature and series field winding.
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Here, 푉 Ish= Rsh
Ise= Ia= IL+ Ish
V= Eg- Ia Ra-IseRse
V= Eg- Ia (Ra+Rse) Or,
V= Eg- Ia (Ra+Rse)- Brush Drop
Electrical power developed in the armature= EgIa Watt
Electrical power delivered to the load= VIL Watt Characteristics of DC Generators: The characteristic of the DC generators explains the relations between the loads, excitation and terminals voltage through the graph. Following are the three important characteristics of a DC Generator.
Magnetization Characteristic (Eg Vs If)
This characteristic gives the variation of generating voltage or no-load voltage with field current at a constant speed. It is also called no-load or open circuit characteristic.
Internal Characteristic (E Vs Ia)
Internal characteristic of DC Generator plots the curve between the generated voltage after taking armature reaction drop into account and load current.
External Characteristics or Load Characteristics (V Vs IL)
External or load characteristics give the relation between the terminal voltage and load current at a constant speed.
Magnetization curve or open circuit characteristics: This curve is common for all types of DC generators whether self or excited. When supply is not given the magnetic dipoles of an electromagnet are randomly oriented. Hence net magnetization is zero. But when current flows these dipoles align themselves in a particular direction due to which magnetic field is produced and flux is established. As the current magnitude increases, more and more dipoles get oriented or aligned. Hence the relationship between current and flux becomes linear. However, after certain point, to increase flux, high number of ampere
Smitarani Sahoo, CVRGU, BBSR turns are required and the increase in flux is negligible. This point is called saturation. Hence relationship becomes non-linear. This curve giving relationship between flux per pole and field ampere turns per pole is called magnetization curve. Since the generated e.m.f in DC machine depends on flux and speed, the generated e.m.f is directly proportional to flux per pole at constant speed. If a curve is drawn between the generated e.m.f on no load and field current when the machine is running at a constant speed, the curve obtained is similar to saturation curve. These curves are called magnetic characteristics or open circuit characteristics. This curve does not start from zero due to residual magnetism.
Characteristics of separately excited DC generators: As there is no connection between the field and armature windings, field current or exciting current is independent of load current. If a curve is drawn between the flux per pole and load current keeping field current constant, it is a straight line. However, due to armature reaction, the actual flux is less than ideal flux, so is the actual EMF generated in the armature. From the generated EMF some voltage is dropped in the armature winding resistance which is directly proportional to the load current.
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Characteristics of DC series generators: Internal characteristics: the load current, field current and armature current are same in DC series generator. The internal characteristic lies below the open circuit characteristics due to demagnetizing effect of armature reaction. External characteristics: the terminal voltage of the generator is obtained by subtracting the voltage drop in armature and series field winding. i.e. V=Eg-Ia(Ra+Rse) In the initial portion of the curve, the relationship between the voltage and current is linear or directly proportional due to simultaneous increase in flux. However, when saturation approaches, the increase in flux is less as compared to Ohmic drop. So, in the later stage of the curve, Ohmic drop dominates and hence terminal voltage reduces or shows a drooping characteristics. The maximum value of load resistance for which the generator will be able to excite is called critical load resistance. So, it is clear from the external characteristics that, the terminal voltage first increases and then reaches maximum and reduces finally. If the generator is operated in the drooping portion of the characteristics, it gives approximately constant current irrespective of the load resistance.
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Characteristics of shunt generator: Voltage built up of shunt generator: When the generator is started, due to residual magnetism, small amount of flux is present in air gap which on being cut, produces a small e.m.f OA. This e.m.f produces a current (oa) in the armature which is supplied to field as well. Now, the field flux is strengthened and more e.m.f is produced represented by OB. Again, OB produces a current (ob) which strengthens the flux present before. As e.m.f is directly proportional to flux, e.m.f generated also increases represented by oc. In this way, the voltage in a DC shunt generator gradually builds up.
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Critical field resistance RC: Maximum voltage which a generator can produce is given by the intersection of open circuit characteristics and field resistance line. If OA represents the field resistance line, OA’ represents the maximum e.m.f that can be generated. If the field resistance is increased, it can be represented by line OB which has a greater slope. The e.m.f generated is represented by OB’. If Rsh is further increased represented by line OC, then the e.m.f generated is OC’. With further increase in field resistance, the lines do not intersect at all. So, no e.m.f is generated. Hence it can be concluded that, the field resistance represented by line OC is critical resistance. If the winding resistance is more than this value, voltage will not build up and machine will fail to start.
Load characteristics: Terminal voltage on no load condition depends on the shunt field resistance represented by OA. As the generator is loaded by decreasing resistance of the external load circuit, terminal voltage falls due to three reasons.
Due to voltage drop across armature winding & brush contact resistance Armature reaction The decrease in terminal voltage due to the first two reasons causes reduction field current which in turn decreases the EMF so also the terminal voltage.
It can be observed that, at a certain value of Ia, the effect of armature reaction and terminal voltage drop is such that, Ia decreases even with decrease in load resistance. Under short circuit condition, Vt is 0, but small Ia prevails due to residual magnetism. So DC shunt generators are self-protective against accidental short circuits. The generated EMF under short circuit conditions is very small and is almost neutralized by armature reaction. This is the reason that shunt generators often fail to build up after a severe short circuit.
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Critical speed: It is that speed for which a given shunt field resistance represents critical field resistance.
Characteristics of compound wound generator: Constructionally the categorization of compound generator are different. However functionally, it depends on the flux created by the series and shun filed winding. Whether it is long shunt or short
Smitarani Sahoo, CVRGU, BBSR shunt, the current flowing in the series and shunt field winding is different, so also the flux produced by them. If the series flux supports the shunt flux, it is called cumulatively compound DC generator and if the series flux opposes the shunt flux, it is called differentially compound DC generator. In case of cumulatively compound DC generator the terminal voltage is more than the no load voltage. In case of differentially compound DC generator, terminal voltage is less than the no load voltage. If the series winding flux is so adjusted that, the terminal voltage is equal to the no load voltage, then it is called flat compound DC generator.
Conditions for self-excitation: There must be residual magnetism in the filed poles. The connection between field coils and armature coils must be in proper direction. For a series generator, resistance of the external circuit must be less than critical resistance. For a shunt generator, field winding resistance must be less than critical field resistance and load resistance must be greater than critical load resistance. Causes of failure of voltage build up: No residual magnetism in the poles. Improper field connections. In case of series generators if the circuit resistance is more than critical resistance. In case of shunt generators, if the critical values are not checked upon thoroughly. Application of DC generators: Separately excited DC generators are used for speed control of DC motors.
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Series generators are used for regenerative braking of induction motors and DC motors. They are also used to compensate the line drop. In this case the generators are called as boosters. Shunt DC generators are used for power supply and battery charging. Cumulatively compound DC generators are used for lighting and power supply. Differentially compound DC generators are used basically for welding purposes. Power flow diagram:
So it can be concluded that, Output=input –losses Or Input= output +losses output output Hence, efficiency= = input output+losses Efficiency of a DC machine: There are three efficiencies of a DC machine. Mechanical efficiency Electrical efficiency Overall or commercial efficiency
electrical power developed by armature Mechanical efficiency= mechanical power input
퐸푔퐼푎 = BHP of prime mover x 735.5
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electrical power delivered Electrical efficiency= electrical power developed 푉퐼 = 퐿 퐸푔퐼푎 electrical power delivered Overall or commercial efficiency= mechanical power input 푉퐼 = 퐿 BHP of prime mover x 735.5
=ηm x ηe Losses of a DC generator:
Copper loss: It occurs in those parts of the machine which contains copper like armature winding, field winding, 2 compensating winding and inter-poles. Armature copper loss is given as Ia Ra which accounts for around 30%-40% of the full load losses.
2 2 Field copper loss is around 20%-30%. Its formula is Ise Rse and Ish Rsh. Shunt field copper loss is practically constant while series field copper losses are variable. Inter-pole and compensating winding drops occur only in those machines which contain those elements. Copper loss is a variable loss. Rotational loss: These losses depend upon speed of the machine and magnetic field strength. Basically, it is of two types. Iron loss and mechanical loss.
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Iron-loss: The losses are named so because it occurs in those parts of the machine which consists of iron. This is also of two types. Hysteresis and Eddy current loss. Hysteresis loss occurs due to repeated cycles of magnetization and demagnetization. This is a constant loss and is given by the formula,
1.6 Wh=ηh (Bm) fV Watt Where,
ηh=Hysteresis Co-efficient
Bm= Maximum flux density f= frequency of EMF or current V= Volume of the core material Silicon steel is used to reduce hysteresis loss. Eddy current losses occur in armature core, teeth and pole faces. These occur due to small circulating currents in the core and is given by the formula
2 2 2 We=ηe Bm f t V Watt
ηh=Constant
Bm= Maximum flux density f= frequency of EMF or current V= Volume of the core material T=thickness To reduce eddy current loss, armature core and pole shoes are laminated. Total iron losses are 20- 30% of full load losses. Mechanical losses: These consists of losses due to friction and windage loss. It is a constant loss whose value is around 10-20% of full load losses. Stray load loss: This losses result form distortion of flux owing to armature reaction, non-uniform distribution of current among the parallel path and short circuit current in the commutator. The magnetic and mechanical losses are collectively called stray power losses. Stray power loss added with shunt copper loss is called constant loss.
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Condition for maximum efficiency: output Generator efficiency=η= input = 푉퐼퐿 Output+losses
푉퐼퐿 = 2 퐼푎푅푎+푊퐶+푉퐼퐿 Where,
Wc= constant loss
Ra=total resistance of armature circuit Efficiency will be maximum when denominator will be minimum.
푉퐼퐿
2 퐼푎푅푎 + 푊퐶 + 푉퐼퐿
Dividing by VIL 1 η= 퐼 푅 푊 퐿 푎+ 퐶+1 V V Now,
d 퐼 푅 푊 ( 퐿 푎 + 퐶 + 1)=0 d퐼퐿 V V 푅 푊 푎- 퐶=0 V 2 V퐼퐿
푅 푊 푎= 퐶 V 2 V퐼퐿
2 푊퐶 IL Ra=푊 or IL=√ 퐶 푅푎
This indicates that maximum efficiency is obtained when variable losses are equal to constant loss.
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