Characteristics of DC Generators: Series, Shunt and Compound

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Characteristics of DC Generators

The speed of a DC generator is made constant by the prime mover. Under such conditions, the performance of the generator is given by the relation among the excitation, terminal voltage and load. These relations are given graphically in the form of curves, which are called as characteristics of DC generators. These characteristics show the behaviour of the DC generator under different load conditions.

The following are the main characteristics of a DC generator

Open Circuit Characteristics or Magnetisation Curve

This is the graph plotted between the generated EMF at no-load (E₀) and the field current (I_f) at a given constant speed. It is also known as no-load saturation curve. Its shape being practically the same for all types of DC generators whether separately-excited or self-excited.

Internal Characteristics

It is the graph plotted between generated EMF (E) on-load and the armature current. Because of the effect of armature reaction, the magnetic flux on-load will be less than the flux at noload. Therefore, the generated EMF (E) under loaded condition will be less than the EMFgenerated (E₀) at no-load. As a result of this, the internal characteristics curve lies just below the open circuit characteristics.

External Characteristics or Load Characteristics

The external characteristics or load characteristics is the plot between the terminal voltage (V) and load current (I_L). Since, the terminal voltage is less than the generated voltage due to armature and series field copper losses. Hence, the external characteristics curve will lie below the internal characteristics curve by an equal amount to voltage drop due to copper losses in the machine.

Characteristics of DC Series Generator

In a DC series generator only one current flows through the whole machine. Therefore, the armature current, load current and excitation all are same.

Open Circuit Characteristics (O.C.C)

The curve (1) in the plot shows the O.C.C of a series DC generator. It is the graph plotted between the generated EMF at no-load and field current. The O.C.C can be obtained by disconnecting the field winding from the machine and is excited separately.

Internal Characteristics

The internal characteristics of a DC series generator is the graph plotted between generated EMF (E) on-load and the armature current. Because of the effect of armature reaction, the magnetic flux on-load will be less than the flux at no-load. Therefore, the generated EMF (E) under loaded condition will be less than the EMF generated (E₀) at no-load. As a result of this, the internal characteristics curve lies just below the open circuit characteristics [See the curve (2)].

External Characteristics or Load Characteristics

The external characteristics or load characteristics is the plot between the terminal voltage (V) and load current (I_L}). Since, the terminal voltage is less than the generated voltage due to armature and series field copper losses, which is given by,

$$\mathrm{V \: = \: E \: - \: I_{a}(R_{a} \: + \: R_{se})}$$

Therefore, the external characteristics curve will lie below the internal characteristics curve by an equal amount to voltage drop due to copper losses in the machine [see the curve (3)].

Characteristics of DC Shunt Generator

In a shunt generator, the armature current splits up into two parts: one is I_sh flowing through the field winding and the other is I_L which goes to the external load.

Open Circuit Characteristics

The curve (1) in the figure shows the open circuit characteristics of a DC shunt generator which is same as that of the series generator.

Internal Characteristics

When the load is connected to the generator, the generated EMF (E) is reduced due the reduced flux per pole because of the effect of armature reaction. Therefore, the generated EMF under loaded condition is less than the generated EMF at no-load. As a result of this, the internal characteristics drops down slightly [see the curve (2)].

External Characteristics

It gives the relation between the terminal voltage (V) and the load current (I_L). It is also known as load characteristics or terminal characteristics.

$$\mathrm{V \: = \: E \: - \: I_{a}R_{a}}$$

Therefore, the external characteristics lies below the internal characteristics by an amount equal to voltage drop due to armature resistance.

External or Load Characteristics of DC Compound Generator

In compound DC generators, both the series and shunt fields are combined. Depending upon the connection of field winding with the armature, the compound generators are of either shortshunt or long-shunt type. The cumulative compound generators are mostly used; hence we shall discuss the characteristics of cumulatively-compound DC generator.

In a cumulatively-compound DC generator, the series field aids the shunt field. The degree of compounding depends upon the series field excitation with the increase in the load current.

Over-Compounded Generator

If the series field turns are so adjusted that with the increase in the load current, the full-load terminal voltage is greater than the no-load terminal voltage, then the generator is known as over-compounded generator [see the curve (1)].

Flat-Compounded Generator

If the series field turns are so adjusted that, the full load terminal voltage is equal to the noload terminal voltage, the generator is known as flat-compounded generator or level compounded generator [see the curve (2)].

Under-Compounded Generator

If the series field turns are so adjusted that, the terminal voltage at full load is less than the terminal voltage at no-load, the generator is known as under-compounded generator [see the curve (3)].