Different Characteristics of DC Generators

There are three fundamental characteristics of a DC generator.

No Load Characteristic of DC Generator

We refer to the no-load characteristic, also like the open circuit characteristic or magnetic characteristic. When we do not connect a load to the DC generator, there is no armature current in it. Because of that, there is no armature reaction as well as there is no armature resistance drop in the generator. Thus, the no-load voltage, i.e. open circuit voltage, appears across the terminals of the generator. The characteristics curve shows the relation between the field current and no-load voltage. It is the open-circuit characteristic. Practically, open circuit characteristics of all types of DC generators are the same, whether it is a separately excited or self-excited dc generator. So, we obtain no-load characteristics of all types of DC generators in the same manner.


Internal Characteristics of DC Generator

On loaded condition, the relationship between the induced EMF in the armature and armature current is the internal characteristic of a DC generator. We call this characteristic as the internal characteristic because the voltage, we deal, here is the generated voltage by the armature inside the generator on loaded condition. It is not the output voltage of the generator. The output voltage is the difference between the armature voltage and armature resistance drop. We measure the output voltage of the generator across the load terminals.

External Characteristics of DC Generator

When we measure the terminal voltage of a loaded DC generator, we get the load terminal voltage of the generator. This voltage is always less than the armature voltage. Because there is an armature reaction in the machine. Also, there is a drop in armature voltage due to armature resistance itself. We refer to the relationship between the armature current and the load terminal voltage as an external characteristic. We call this as external characteristic because the voltage we use for drawing the characteristic curve appears across the external terminals (load terminals) of the generator, not across the armature terminals.

Characteristics of Compound DC Generator

But in compound DC generator, there will be both series field and shunt field. For the shunt field, the external characteristic tries to be downwards at the same time for the series field; it tries to be upwards.

Due to these two fields in the compound generator, the external characteristics may become flat, which may have the same voltage at no load as well as full load condition. We call the compound generator which has the same no load and full load voltage, as the flat compounded generator. In the characteristics of flat compounded generator, the intermediate value of the generator voltage is more between that of zero and full load armature current.

external characteristics of compound dc generator

If the series field ampere-turns of the generator cannot significantly neutralize the effect of armature reaction and resistance, the external characteristic of the generator inclines downwards. We call this generator as under compounded generator. When the strength of the ampere-turns is such that it induces an additional voltage in armature than the reduction of voltage due to armature reaction and armature resistance drop, then the terminal voltage at the rated load is more than that of no load. We refer to this dc generator as an over compounded generator.

Applications of Compounded DC Generators

We use the over compounded generator for long DC transmission to overcome the transmission line voltage drop. And we use a flat compounded DC generator for the load not far away from the location of the generator. But we generally do not use under compounded DC generator because its voltage regulation is always poor.

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