Types of DC Generator and its equation

DC generators convert mechanical energy into electrical energy. For this conversion to take place, dc generators require a magnetic field. The magnetic field is produced by the field coil or winding.

DC generators are classified based on the way in which the field windings are excited. The different types of DC generator are shown below

Before getting into the topic, learn the construction of DC Generator and its working principle.

Separately excited DC Generator

It is a type of DC generator, in which the field windings are excited from a separate source of supply. The output voltage generated by the generator depends on the speed of rotation of armature and field current.

The separately excited dc generators are rarely used in practice. The following figure shows the circuit diagram of a separately excited dc generator.

In the above circuit diagram,

I_f – Field current, I_a – Armature current, I_L – Load current,

R_a – Armature winding resistance, V – terminal voltage

Let V_br be the voltage drop at the brush contacts.

Armature current is given by, I_a = I_L

Applying Kirchoff’s Voltage Law to the armature circuit, E_g – I_a R_a – V – V_br = 0

Thus, the generated Emf equation E_g = I_a R_a + V + V_br

Power developed in the DC generator = E_g I_a

Power delivered to the load = V I_a

Self Excited DC Generator

The self-excited dc generator produces DC output, whose field windings are excited by the current produced by the generator itself. No separate source is used for filed excitation.

In self-excited dc generators, some flux is already present in the poles due to residual magnetism. When the armature is rotated with the residual flux, a small amount of output voltage and hence some current is induced in the armature conductors. This current will produce more flux, which in turn produces more current to flow through the field winding. It will continue until the field current reaches its rated value.

There are three types of self-excited DC generators based on the way, in which the field coils and armature are connected. They are

1. Shunt-wound Generator
2. Series wound Generator
3. Compound wound Generator

Shunt-wound Generator

In shunt wound dc generators, the field windings are connected across or in parallel with the armature winding. The field winding has more number of turns and thin wire, having high resistance.

The diagram of dc shunt generator is shown below, in which the load is connected across the armature. A small amount of current will flow through the field winding and more current flows through the armature.

In the above circuit diagram,

I_sh – Shunt field current, I_a – Armature current, I_L – Load current,

R_a – Armature resistance, V – terminal voltage, V_br – Brush contact drop

Armature current is given by, I_a = I_L + I_sh

Shunt field current I_sh = V/R_sh, Where R_sh – shunt field resistance

Terminal voltage equation is given by, V = E_g – I_a R_a – V_br

Power developed in the DC generator = E_g I_a

Power delivered to the load = V I_L

Series wound Generator

In series wound dc generators, the field winding is connected in series with the armature winding. Such generators are called DC series Generator. They have less number of turns with a thick wire having low resistance.

Here, the load is connected in series with the field winding and armature winding. So the load and series field winding carries the armature current.

In the above circuit diagram,

I_se – Shunt field current, I_a – Armature current, I_L – Load current,

R_a – Armature resistance, V – terminal voltage, V_br – Brush contact drop

Armature current is given by, I_a = I_se = I_L

Terminal voltage equation is given by, V = E_g – I_a R_a – I_a R_se – V_br

Power developed in the DC generator = E_g I_a

Power delivered to the load = V I_L

Compound wound DC Generator

The compound wound dc generators have both shunt and series field winding. It is because one field winding is connected in series with the armature and the other is connected in parallel with the armature.

The compound wound dc generators are classified into two different types based on the way of connection

1. Long shunt Compound wound generator
2. Short shunt Compound wound generator.

Long shunt Compound wound generator

The below figure shows the circuit diagram of a long shunt compound wound generator. In this, the shunt field winding is connected in parallel with a combination of series winding and armature conductors.

In the above circuit diagram,

I_sh – Shunt field current, I_se – Shunt field current, I_a – Armature current, I_L – Load current,

R_a – Armature resistance, V – terminal voltage, V_br – Brush contact drop

Armature current is given by, I_a = I_se = I_L + I_sh

Shunt field current I_sh = V/R_sh, Where R_sh – shunt field resistance

Terminal voltage equation is given by, V = E_g – I_a R_a – I_a R_se – V_br

Power developed in the DC generator = E_g I_a

Power delivered to the load = V I_L

Short shunt Compound wound generator

In short shunt DC compound generator, the shunt field winding is connected across the armature conductors and this combination is connected in series with a series field winding. The following figure shows the circuit diagram of short shunt compound generator.

In the above circuit diagram,

I_sh – Shunt field current, I_se – Shunt field current, I_a – Armature current, I_L – Load current,

R_a – Armature resistance, V – terminal voltage, V_br – Brush contact drop

Armature current is given by, I_a = I_L + I_sh where I_L = I_se

Terminal voltage equation is given by, V = E_g – I_a R_a – I_se R_se – V_br

Generated Emf equation, E_g = V + I_a R_a + I_se R_se + V_br

Voltage drop across shunt field winding = I_sh R_sh

Shunt field current I_sh = ( E_g – I_a R_a – V_br )/R_sh, Where R_sh – shunt field resistance

By substituting the value of E_g in the above equation, we get shunt field current I_sh = ( V + I_se R_se )/R_sh

Power developed in the DC generator = E_g I_a

Power delivered to the load = V I_L

Differentially and Cumulatively compound Generator

The compound wound dc generator is constructed in such a way that, the shunt field winding is stronger than the series field winding. Based on this, the Compound wound generators can further be classified into two types:

1. Differential compound generator
2. Cumulatively compound Generator.

If the flux produced by the series field winding opposes the shunt field flux, thereby weakening the total flux then it is called a differential compound generator.

If the series field flux aids the shunt field flux, thereby strengthening the field flux, then it is called a Cumulatively compound wound Generators.

Permanent Magnet DC generators

As the name indicates, the permanent magnet dc generator has a permanent magnet on the inner periphery of the stator. The necessary magnetic field required to produce the rotating torque is developed by the set of permanent magnets.

The main disadvantage of such a generator is that the flux produced by the magnets deteriorates with time which changes the characteristics of the machine.

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