Method of Excitation of DC Generator, Separately Excited Generator, Self-Excited Generator, Shunt Generator, Series Generator, Compound Generator

Method of Excitation of DC Generator

An electromagnet generates the necessary magnetic field for a d.c. generator to function. When current is passed through the field winding in this electromagnet, the necessary magnetic flux is generated.

The field winding is also called exiting winding and current carried by the field winding is called an exciting current.

Excitation is the process of supplying current to the field winding, and method of excitation is the process of supplying the exciting current.

There are two Method of Excitation of DC Generator.

1.     Separate excitation

2.     Self excitation

So Depending on the Method of Excitation of DC Generator used, the d.c. generators are classified as,

1.     Separately excited generator

2.     Self excited generator

In separately excited generator, a separate external d.c. supply is used to provide exciting current through the field winding.

The d.c. generator produces d.c. voltage. If this generated voltage itself is used to excite the field winding of the same d.c. generator, it is called self excited generator.

1. Separately Excited Generator

The generator is referred to as a separately excited generator when the field winding is supplied by an external, separate d.c. supply, or when the excitation of the field winding is separate. Fig. 7.39 depicts a schematic representation of this type.

This type of generator’s field winding contains a sizable number of turns of thin wire. This results in a winding that is longer but has a smaller cross section. In order to reduce the field current, this field winding has a high resistance.

Voltage and current Relations

The field winding is excited separately, so the field current depends on supply voltage and resistance of the field winding.

So For armature side, we can see that it is supplying a load, demanding a load current of IL at a voltage of Vt which is called terminal voltage.

Now Ia = IL

The internally generated e.m.f The terminal voltage, Vt, which is a component of E and supplies the load’s voltage, is not equal to Vt when supplying a load. This is because a voltage drop equal to IaRa volts occurs across the armature winding when armature current Ia flows through it due to armature winding resistance Raohms. Along with the terminal voltage Vt, the induced e.m.f. must provide this drop. The resistance Ra is made to be very small in order to maintain a minimal Ia Ra drop. The voltage drop at the brush contacts, also known as brush contact drop, also exists in addition to the drop. However, this drop is small and is therefore typically disregarded. So in all, induced e.m.f. E has three components namely,

i) Terminal voltage Vt

ii) Armature resistance drop IaRa

iii) Brush contact drop Vbrush

So voltage equation for separately excited generator can be written as,

E = Vt + IaRa + Vbrush

Where E = ϕPNZ / 60A

Generally Vbrush is neglected as is negligible compared to other voltages.

2. Self Excited Generator

It is referred to as a self-excited generator when the field winding is supplied by the generator’s own armature. Field cannot be excited in such a generator without generated e.m.f., and there cannot be generated e.m.f. without excitation. Therefore, it is only natural to wonder how this kind of generator operates. The field poles’ lingering magnetism under normal circumstances is the answer to this.

The field poles have some magnetic flux even though the generator is theoretically not operating because there is no current flowing through the field winding. Remaining magnetism and residual flux are terms used to describe this phenomenon. Due to this residual flux, the generator develops a small em.f. when it is started, which causes a small current to flow through the field winding. The flux generated tends to rise as a result. The induced e.m.f. is then raised as a result. The field current and flux will increase in the future. Up until the generator develops rated voltage across its armature, the process is cumulative and continuous. This is how self-excited generators build voltage.

Based on how field winding is connected to the armature to derive its excitation, this type is further divided into following three types.

i)  Shunt generator

ii) Series generator

iii) Compound generator

So Let us see the connection diagram and voltage, current relations or these types of generators.

i. Shunt Generator

When the field winding is connected in parallel with the armature and the combination across the load then the generator is called shunt generator.

The field winding has large number of turns of thin wire so it has high resistance. Let Rsh be the resistance o the field winding.

Voltage and Current Relations

From the Fig. 10. 45 , we can write

Ia = IL + Ish

Now voltage across load is Vt which is same across field winding as both are in parallel with each other.

Ish = Vt / Rsh

While induced e.m.f. E, still required to supply voltage drop IaRa and brush contact drop.

E = Vt + IaRa + Vbrush

E = ϕPNZ  / 60A

So In practice, brush contact drop can be neglected.

ii. Series Generator

When the field winding is connected in series with the armature winding while supplying the load then the generator is called series generator.

It is shown in the Fig. 7.41.

Field winding, in this case is denoted as S1 and S2. The resistance of series field winding is very small and hence naturally it has less number of turns of thick cross-section wire as shown in the fig,7.41.

So Let Rse be the resistance of the series field winding

Voltage and Current Relations

As all armature, field and load are in serious they carry the same current.

Ia = Ise = IL

Where Ise = Current through series field winding

Now in addition to drop IaRa, induced e.m.f. has to supply voltage drop across series field winding too. This is IseRse i.e. IaRse as Ia = Ise. So voltage equation can be written as,

E= Vt + IaRa + IaRse + Vbrush

E= Vt + Ia (Ra + Rse ) + Vbrush

Where E = ϕPNZ / 60A

iii. Compound Generator

In this type, the field winding is connected to the armature in series and parallel, respectively. The same poles are used to support the series and shunt field windings. Compound generators are further divided into the following categories based on how the shunt and series field windings are connected:

a) Long shunt compound generator,

b) Short Shunt compound generator

a) Long shunt compound generator

In this type, shunt field winding is connected across the series combination of armature and series combination of armature and series field winding as shown in the Fig. &.42.

Voltage and current relations are as follows.

From the ig. 7.42

Ia = Ise

and

Ia = Ish + IL

Voltage across shunt field winding is Vt.

Ish = Vt / Rsh

Where Rsh = Resistance of shunt field winding.

So voltage equation is,

E = Vt + IaRa + IaRse + Vbrush

Where Rse = Resistance of series field winding.

b) Short Shunt compound generator

In this type, shunt field winding is connected, only across the armature excluding series field winding as shown in the Fig.7.43.

Voltage and current relations are as follows.

For the Fig. 7.43, Ia = Ise + Ish

Ise = IL

Ia = IL + Ish

The drop across shunt field winding is drop across the armature only and not the total Vt, int his case. So drop across shunt field winding is E – IaRa.

Ish = E-IaRa / Rsh

So Now the voltage equation is E = Vt + Ia Ra + IseRse + Vbrush

Ise  = IL

E = Vt + IaRa + IL Rse + Vbrush

Neglecting Vbrush, we can write,

E = Vt + IaRa + IL Rse

E – IaRa  = Vt + IL Rse

Ish = [Vt + ILRse] / Rsh

Any of the two above expressions of Ish can be used, depending on the quantities known while solving the problems.