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.