DC compound generator and its load characteristics

        The dc compound generator is the combination of dc series generator and shunt generator. If you understand the working concept of these two generators, the dc compound generator is ease to understand.

    The load characteristics of series generator show the rising or booster characteristics and shunt generator shows the almost constant voltage with respect to load.

    The combination of these two generators is known as compound generator. It has shunt wound and series wound.

    Compound generators are classified into two dc machines.

• Long shunt compound generator 
      
• short shunt compound generator

    If the series field aids the shunt field then, it is called cumulative compound generator. If the series field opposes the shunt field then, it is called as differential compound generator. Accordingly, the internal wound connections.

Short shunt compound generator

By kvl , kcl

The equation of generated emf from circuit diagram,

Eg- Generated voltage

Vt- Terminal voltage

Ia-  Armature current

Il-  Load current

Ish - Shunt field current   

ra, rse, rsh, armature ,series , shunt resistance. In this case, the armature reaction drop and brush drop is neglected.

Long shunt compound generator

by,  kvl and kcl

The equation of generated emf from circuit diagram,

Load characteristics of dc compound generator

    When the load increasing the load current in increases. The series field current is increases and flux is increasing and this series flux aids the shunt field flux there by generated emf is also increased. however, the terminal voltage is decreased in case of dc shunt generator.

    The series field mmf are made such that the terminal voltage produces as same as rated voltage then, it is called as flat compound. The terminal voltage produces greater than rated voltage then, it is called over compound. The terminal voltage produces less than rated voltage then it is called as under 

compound.

Applications of dc compound generator

    This type of generator is used to maintain the constant voltage across the electrical appliances such as elevators, conveyers, air compressors and high torque load etc.

    

DC Series Generator and its characteristics with circuit diagrams

    

    The dc series generator has a lot of applications when the appliances needs the voltage boosters. In other words, this type of generator gives output as rising voltage characteristics instead of constant voltage as like as dc shunt generator.

DC series generator

    

    The generator which field winding is connected in series with the armature conductor in known as dc series generators.

    The circuit diagram for dc series generator is given below

The equation of dc series generator,

Eg = Vt + Ia*(Ra+Rse) + Armatue Reaction drop + Brush Voltage drop

    

    Just neglect the armature reaction drop and brush drop. So, easier analysis of machine.

    Assume the generator is operating constant speed N by the external prime mover or diesel engine. The armature current is equal to the field current and load current because of series connection.

    First and fore most step is to conduct the no load test or occ test in dc series generator.

    The load on the generator is removed and run the machine under constant speed N the emf generation in the generator is given by the equation,

The emf equation of dc generator Eg = (NՓP/60)*Z/A

    The generating emf in directly proportional to the field flux and this field flux is proportional to the armature current Ia.

    Magnetization characteristics of dc series generator

If we notice that the magnetizing characteristics

   Even though the field current is zero there is some voltage across the terminal because of residual magnetism in the poles. 

    The curve start rising linearly and saturated at one point because of core saturation.

Load characteristics of dc series generator

    Then load is fed into generator and operating at constant speed such that armature current is constant. By varying the field current the load characteristics are plotted between terminal voltage Vt and field current If.

    Here, external and internal characteristics are plotted in the same graph.

Nature of voltage rise in DC series generator

    The addition of load causes armature current flow through the circuit the effect of armature flux on main field causes armature reaction leads to demagnetization. which reduces the voltage at terminal from no load to full load. The voltage drop at armature conductors is Ia Ra.

    As you can see the load increases and Vt is increases and that is why the dc series generator is operated in boosters and when its is need by rising voltage characteristics.

    DC series generator is also used in regenerative breaking in locomotives when it is running freely.

Try to solve this problem,

    

    A DC series generator delivers a load current of 50 amps at 500 volts. The resistance of armature is 0.05 ohms and series resistance are 0.03 ohms. Find the induced emf, power developed in armature and power delivered to load, if contact drop is 1 volt per brush. Neglect armature reaction. so, it is ease to work.

Eg = 500 + 50*(0.05+0.03) +(2*1)

induced emf=Eg= 506 volts

Power developed in armature

P armature = Eg*Ia = 506*50 = 25.3KW

P load = Vt*Ia = 500*50 = 25KW

Parallel Operation of DC Generator and Load sharing with Problem

    The need for parallel operation is to share the load when demand for higher power rating is very high in the bus bar lines

    The bus bar is made up of thick copper wire. rather than one single higher power rating dc generation, the small dc machines are operated is parallel. Because of maintain and repairs, continuous power supply.

Conditions for parallel operation

• The terminal voltage of incoming dc generator should be equal to the bus bar voltage.
• The polarity of bus bar should be matched with the polarity of dc generator.
• The prime mover speed should be same for all the machine connecting in parallel.   

Paralleling  a dc generator to bus bars

    Consider the dc shunt generators G1 and G2 are connected to bus bars. The switches S and S1 are closed. Initially, the shunt generator G2 switches S' and S2 are open.

    The G2 is to be connected to the bus bars to share the part load of G1. The generator G2 is run at rated speed by prime movers connected to G2. The switch S' is closed between negative terminal of bus bar and generator G2.

    The voltage of G2 is varied by shunt field rheostat there by varying the field current.

    The shunt field rheostat is adjusted such that, the voltmeter reads zero voltage for parallel operation.

    The switch S2 is closed. In this condition, the induced emf in generator G2 is exactly equal to the bar voltage and there is no current flow through G2. This condition is called as floating generator.

 

    The field current of generator G2 is increased, there by current I2 is flowing through bus bars. Likewise, the field current of generator is decreased, current I1 is flow through bus bars. The total current in the load is IL.

IL = I1 + I2

Load Sharing

    The values of equal voltage intervals up to rated voltage and armature current are obtained. So, We can plot the drooped characteristics of generator G1 and G2.

    The combined characteristics of two dc generators.

    The two generators divide the load depends on dropping characteristics of each  generator.

E1,E2 - Generated voltage of G1 and G2.

R1,R2 - Armature resistance

V- terminal voltage

problems on parallel operation of dc generators

 for example, two shunt generator operating is parallel output current of 600 A

Generator G1, Armature resistance Ra1 is 0.02 ohm and Generated voltage E1 = 455v

Generator G2, Armature resistance Ra2 is 0.025 ohm and Generated voltage E2 = 460v

Find out terminal voltage and output power of each dc machine.

I1 + I2 = 600 

(E1 - v)/Ra1 + (E1-v)/Ra2 = 600

E1= v + I1 Ra1

455 = v + I1  00.2

E1 = v + I2 Ra2

460 = v + (600-I1)0.025

By solving these two equation,

I1 = 222.2.2A

I2 = 377.77A

v = 450.56

output power of generator G1 

P1=V*I1=100.12KW

output power of generator G2

P2=V*I2=170.2KW

What is Commutator and Commutation in DC Machine ?

    We all know that dc machine is an electromechanical device. It acts as two types of electric machines either dc generator or dc motor. In case of generator, the output electric power is taken out from the machine.

    The dc generator converts mechanical motion into electrical power by electromagnetic induction. The induced emf produced alternative current in nature. but, the need is direct current connect to Electrical Appliances! how to convert alternating current into unidirectional pulsating current.

The answer to the complex questions is 'commutator'.

What is commutator?

    A commutator is a rotary electrical switch divided by segments and fixed with armature conductors. The function of commutator in dc generator is to collect current from the armature conductors and fed into load.

Commutation in dc machine

    The commutation is nothing but the process of reversal of electric current in a coil with the help of carbon brushes and commutator segments.

    The update of slip ring commutator is split ring commutator. The slip ring commutator is a device used to take output current as bidirectional. Where as split ring commutator is fixed with armature conductors. so, every half cycle the direction of electric current reversed. consider the dc machine with 2 poles double layer lap wound armature conductors and rotating in anti clock wise direction.

    The armature coil is connected with split ring commutator segments the number of armature coil is equal to the number of commutator segments. The mica insulation provided in between the commutator segments.

    Each coil having two ends is connected to commutator. Let's see what happens when its running as dc generator. connected with load from the dc generator diagram. consider coil a connected with commutator segment 1 and 2 in simple ways.  

case1

    The brush is in exact position of commutator segment 1. The width of the brush is equal to the commutator segment.

    The coil between 4 and 1 is Ic and other coils carries Ic. By kcl, the current 2I enter into the commutator segment 1.

case 2

 

    The commutator is rotated such a way that 3/4th and 1/4th of commutator segment 1 and 2. 3/4th of current is entering into segment 1 and 1/4th of current is entering into segment 2. By KCL, the resultant current is 2I.

case 3

    In this case, brush is equally distributed in the commutator segment 1 and 2. At this instant, no emf is induced in the coil a. Because of armature conductors is out off main field flux. so, the current is 2I.

case 4

    The commutator is rotated such a way that 3/4th and 1/4th of commutator segment of 2 and 1. 3/4th of current is entering into segment 2 and 1/4th of current is enter into segment 1. By KCL, the resultant current is 2I.

case 5 

    The brush is exactly is in commutator segment 2 and current from the coil between 2 and 3 is Ic. another coils current from 4 to 1 is Ic. The resultant current is 2Ic

Effects of Commutation 

    The commutation discussed above process is called as Linear commutation. The speed of the dc machine is high such that RPM is high. The time required to shift the commutator segment is very less in terms of milliseconds. so, the contact of brushes produces motor sparks in brushes when the current reversal is occurs. It might damage the commutator segments and brushes. There are some methods to improve commutation in dc machine.

    

Armature Reaction in DC machine and it Effects

     The Armature Reaction happens in both dc generator and motors. Consider the dc machine is act as a generator for better understanding. 

    What is Armature Reaction ?

    It is the Effect of armature flux on the main field flux. when the armature carries current Ia produces armature flux. 

    Consider the dc generator is rotating in clockwise direction and it has 2 poles. The main field flux Փm from north to south. According to the flemming right hand rule, the armature current is flowing in the conductor.

    We can define the direction of armature flux using right hand thumb rule. The physical mean point between the north and south poles called as Geometrically Neutral Axis GNA. The axis which passes the zero crossing of resultant magnetic field on the air gap is called as Magnetic Neutral Axis MNA.

    The Armature flux Փa is perpendicular to the main field flux Փm and the Resultant flux is Փr. The Armature flux Փa crosses the main field flux causes Cross Magnetization effect.

    The Variation of armature current Ia results in variation of armature flux Փa. The resultant flux shifts the magnetic neutral axis. There will be poor commutation in that machine. The angle between the Geometric Neutral axis and magnetic neutral axis is Ɵ.

    The main field flux is opposed by demagnetizing flux  Փd causes demagnetizing effect. The conductors in the armature which are responsible for demagnetization effect is 4Ɵ in terms of angle.Where Ɵ is the electrical angle.

Analysis of Armature reaction in terms of Ampere Turns

Let zbe the total number of armature conductors. I be the current in the armature.

demagnetization per pole = 2Ɵe

pole pitch = 180 electrical degree

cross magnetization per pole = 180-2Ɵe

Number of conductors = 2*Number of turns

Total ampere turns = ZI/2A

A- number of parallel paths

Total ampere turns per pole = ZI/2AP

P-number of poles

    In order to reduce the effect of armature reaction, Compensating winding is provided in between the poles called inerpoles.

For example, The 8 pole generator has a output current of 200A and 500v having lap winding. The armature with 1280 conductors and 160 commutator segments. If brushes are advanced by 4 segments find out demagnetizing ampere turns and cross magnetizing ampere turns.

In lap winding, Number of parallel paths = Number of poles

A=P=8

Z=1280 conductors and 160 commutator segments.

160 commutator segments = 360 degree

1 commutator segments = 2.25 degree

4 commutator segments = 4*2.25 = 9

1 degree electrical = p/2 mech degree

so, 36 degree electrical 

Demagnetizing ATd/Pole = 800 Ampere Turns

Cross magnetizing ATc/Pole = 1200 Ampere Turns

Different Types of DC Generator and their Circuit diagrams

 

The Classification of dc machine is divided by their field excitation. The field of excitation gives different characteristics. Because of different types of connections between field winding and Armature.

    You may ask a question, why this field excitation configurations gives types of dc machine?

    From the emf equation of the dc generator we can say,

E=NՓPZ/60A 

     The generated voltage E is directly proportional to the speed of the dc machine. The term RPM is related with the field current. Because of electromagnetic induction, rate of change of flux is proportional to the induced emf.

    The basic two divisions of generator are

• Self excited dc generator
• separately excited dc generator

    Again, self excitation is classified into

• Series generator
• shunt generator
• compound generator

The compound generator is again classified into

• Long shunt generator
• Short shunt generator

 Each generator has its own open circuit or magnetization characteristics and load or terminal characteristics.

Separately excited dc machine

    As you guess, the name separately excited says that the field winding is excited with separate external power supply. There is no electrical connection between the armature and field winding the circuit diagram is shown below.

from the circuit diagram, we can write the kvl and kcl equation

Vt = Ea - IaRa

Ia=IL

The Load current is equal to the armature current. The brush drop is neglected.

    Vt- Terminal voltage

    Ea- Generated voltage

    Ia-  Armature current

 

    Ra- Armature resistance

The separately excited machine is used for supply source to the speed control of dc motor.

Series generator

    In this machine, the field winding is connected in series with armature. so, the load current is equal to the armature current.

By kvl, the equation,

Vt = Ea - Ia(Ra+Rse)

Ia=IL=Ise

Ise - Series current

Rse-series field resistant

    In this type of machine is used in demand of load current supply.

Shunt generator

    The word shunt means that field winding is connected in parallel with armature. As you can see in the circuit diagram the armature current is divided in to load current and shunt field current.

Vt = Ea - IaRa

Ia=IL + Ish

    The voltage regulation of shunt generator is high. because the no load to full load voltage is almost constant. Therefore, it is used in electro-plating and battery charging applications.

Compound generator

    There are two windings present inside the compound generator one is series field another one is shunt field.

    If the series magnetic field aids the shunt magnetic field then its is called as cumulative mode.

    If the series magnetic field opposes the shunt magnetic field then it is called as differential mode.

    Compound generator is classified into long shunt and short shunt.

Short shunt

Vt = Ea - IaRa - IseRse

Ia=IL + Ish

Ish = Vt + RseIse/Rsh

Long shunt

Vt = Ea - IaRa - IseRse

Ia = Ise = IL + Ish

Ish = Vt/Rsh

The compound machines are used in places like Hotels, Offices, Shopping malls to generate constant voltage Supply.

Self Excited or DC Shunt Generator with circuit Diagrams

     

    In this type of dc machine, the field winding is connected parallel to the alternator. The simple parallel connection plays a major role in voltage built up in the shunt generator. we will see about what are the interesting things happen in magnetization and load characteristics of self excitation machine.

 Applying kirchoff voltage law to the circuit,

 Vt = Ea - IaRa - Vb

Vt- Terminal voltage

Ea- generated or induced voltage

Ia- Armature current

Ra- Armature resistance

Applying kirchoff current law,

Ia = IL + Ish

    Practically, there will be voltage drop due to brush but it may be around one or two volt per brush. So, we neglect the Vb.

Vt = Ea - IaRa

Observing the shunt generator, we can classify into open circuit or magnetization characteristics and terminal characteristics

Open circuit or magnetization characteristics of dc shunt generator

    In this case, the load of the dc machine is removed. At this point the load current flowing through the circuit is zero.(IL= 0)

    We can say,

Ia=Ish

Ea = Vt - IshRa

    The terminal voltage is equal to the field winding of the machine. because it is connected across the armature. When the rotor is rotated, the residual magnetic flux present in the poles induces emf in that armature.

    Because of the induction of emf, there will be minimum amount of current flowing through the field winding and this current aid the magnetic flux in the air gap. So, the voltage develops like a loop up to the saturation of point.

    The minimum value of resistance to excite the shunt field is known as critical resistance. If the resistance of field winding is beyond the critical resistance there will be no further increase in induced emf.

Rsh < Rcritical

The shunt resistance should less than the critical resistance.

Terminal or load characteristics of dc shunt generator 

    In practical if we increase the load, the demand of load current is high results in decrease in the terminal voltage Vt. because of armature voltage drop IaRa across the generator. Under no load conditions the terminal voltage is nearly equal to the induced emf because of IaRa drop on no load is small.

    The drop due to demagnetization effect of armature reaction. Because of increase in load current causes the flux weakening there will be reducing magnetic flux.

    To neglect the armature reaction the compensating winding is provided in between the field winding of self excited machine.

    The load current increases with decrease in terminal voltage due to decrease in field current Ish. In separate excited dc generator the field current is kept as constant in case of self excitation field current is variable.

Problem

    For example,

    A 10kw 250v dc shunt machine has armature field resistance of 0.1 ohm and 125 ohm.

    If we analyse the total armature power,

Ia = IL + Ish

Ish = 250/125 = 2A

IL = 40A

Ia = 42A

Ea = Vt + IaRa

             =250 + (42*0.1)

Ea = 254.20 volt

Conclusion

Notice that only field current is 2A and load current is 40A

The total Armature power developed Pa = 10.67 KW

comments

What is the advantage of shunt machine while working as motor?

Separately Excited DC generator with Diagrams

  

    The name separately excited tells you, The field winding is excited by an external independent DC power supply.i.e, There is no electrical connection between armature winding and field winding.

    The DC generator has no load and load characteristics. The load characteristics is again classified by internal and external characteristics.

Open Circuit or magnetization or No load characteristics of separately excited dc generator

    Consider the load is removed from the dc generator. In this situation the load current (IL=0).

By Kirchoff voltage law, We can write the equation,

Vt = Ea - IaRa

Vt- Terminal voltage

Ea- generated voltage

Ia- Armature current

Ra- Armature resistance

    Practically, there will be voltage drop across the brush in the range of one or two volt per brush. So, it is neglected to get a round off  voltage. 

    Even though the field current is zero, There will be some emf present in armature. Because of residual magnetism in the poles.

From the emf equation of DC generator,

 we can say generator voltage is directly proportional to the speed of the Electrical Machine.

Ea α N

    At No load condition, generation of voltage across the dc generator varies with field current at fixed RPM N1,N2,N3.The graph shows that for the generation of voltage is increased with increase in fixed rpm for different values.

Load characteristics:

External characteristic (I vs Vt)

The plot between output parameters terminal voltage vs load current gives terminal characteristic. 

When the terminal voltage Vt decrease with increase in demand of current from the more load. Because of drop across the armature ohmic drop. There is No load at terminal the Load current is Zero (Ia=0).

Vt = Ea - IaRa

Vt = Ea

Internal characteristics (Ia vs Ea)

    We all know that compensating winding is provided in between the poles for neglecting Armature Reaction. 

    Consider dc generator is not present with compensating winding.If the load current increases IL= Ia for separately excited dc generator. The armature reaction causes flux weakening, there will be reducing flux so the terminal voltage will be decreases.

Conclusion

    The Major advantage of the separately excited machine is separate DC power supply to field winding.This is more stable than other machine when we operate.  

   For example, The separately excited dc generator has a terminal voltage of 240v and induced emf of 250v. If Ra = 0.1 ohm, What will be full load current and output power?

EMF equation of DC Generator

The generator of emf in dc machine is contributed by the faraday's law of electromagnetic induction.

    Whenever the change in magnetic flux in the coil, the emf is induced in the coil.

Practically the rate at which the conductor is cutting the magnetic field induces emf in that coil.

The total magnetic flux cut by armature conductor = flux per pole × no of  poles

                                                                                 = P × Փ

p =  Number of poles

Փ = Magnetic flux per pole

N is the speed of the armature conductor in revolution per minute (RPM)

We can take n revolution in one minute or 60 seconds

                                N number of Revolution = 1 minute = 60 seconds

The time required to take one revolution     

1 revolution = 60/N seconds

The emf  induced per coil,

Z = Total no of conductors

A =  Number of parallel paths

Consider A number of parallel paths in armature and Z number of conductors. So, Z/A gives the number of conductors in each path.

In Lap winding,

            Number of parallel paths = Number of poles in dc generator

                                                A=P

Replace A=P

 In Wave winding,

    Number of parallel paths is always two 

For example, 6 pole dc machine having 480 conductors driven at a speed of 1200 rpm and  flux per pole is 0.012 wb

case 1

    Consider a lap winding

     A=P

case 2

    Consider Wave winding

    

    Number of parallel paths is always two

    A=2

Conclusion

    In dc generator emf is generator by converting mechanical energy into electrical energy is the other hand the have to give supply to dc motor in this case emf generator in the armature opposes the supply voltage called as back emf Eb

Comments 

    Is Back emf Eb equal to generated emf in dc generator?

Principle of Operation of DC Generator in Easy Ways

 

    The first electromagnetic generator was invented in 1831 by British scientist Michael Faraday. The reverse operation of  electric DC generator is DC motor.

Laws contributing in dc generator

    We have to understand the concept of laws contributing in dc generator before understand the operation of dc generator.

    Wherever the change in magnetic flux near the coil, the emf is induced in that coil. This is called as faradays law of electromagnetic induction.

    Just take a piece of bar magnet and the LED circuit with coil. If we move the bar magnet front and back near the coil, the LED will be glowing.

 

Fleming right hand rule

    In general, the Flemings right hand rule in defines the operation of dc generator. The thumb represents the direction of force in conductor.

    The fore finger represents the direction of magnetic field and the middle finger represents the direction of current in the coil.

 

Operation or dc generator

    In dc generator, we should rotate the shaft of armature through Non-conventional sources. The Armature shaft having conductors converts mechanical energy in to electrical energy. 

The armature conductors are going to rotate and magnetic field and at stationary so is known as dynamically induced emf.

    The emf induced in the conductor is minimum when the conductor is parallel to the magnetic field.

    

 e=0

    

The conductor in inclination to the magnetic field.

 

e=-nblvsinƟ

 

    The negative sign is derived from Lenz law. The induced emf produces current in the circuit always opposes the rate of change of flux. Ɵ is an angel between flux and conductor and v is the velocity of the conductor.

    The maximum emf is induced in the conductor when the conductor is perpendicular to the magnetic field.

e=-nblv

 

Consider a simple dc generator with permanent magnet. The coil is placed in between the permanent magnet. The magnetic field lines are from north to south pole. The coil is having two conductor AB and CD. The conductor AB is always connected with S2 and B2 likely the conductor CD is always connected with load.

    consider the generator armature is always rotated in the clockwise direction.

 

Positive cycle

    When the conductor is parallel to the magnetic field there is no magnetic field cut by conductor so, emf is zero in the conductors.

    The conductors are inclined to the magnetic field partially the magnetic field is cut by conductors so, there is induced emf in that conductors according to Fleming right hand rule. The current is flowing from the conductor ABCD to load through B1 B2.

    When the conductor is perpendicular to the magnetic field the maximum emf is induced in the conductors. The current is flowing from ABCD the conductor to load through B1 B2.

 

Negative cycle

    Remember the conductors are always connected to the slip rings S1 and S2. In this case the emf is zero because of no magnetic fields are cut by conductors.

    When the conductors are in inclined angle partially the emf is induced in the conductors but, the current is flowing from the conductors DCBA to load through B2 B1. 

    The emf is maximum when conductor is perpendicular to magnetic field likely the current is flowing from the conductor DCBA to load through B2 B1.

    The number of cycles per second called hertz.

 

Conclusion 

    There is no major difference between the operation of dc generator and dc motor. The mechanical energy is converted in to electrical energy vice versa. 

 

Comments

     Which material is used in brush? Why?