Voltage Regulation of Alternator

The voltage regulation of an alternator is defined as “the rise in voltage at the terminals, when the load is reduced from full load rated value to zero, speed and field current remaining constant”.

With the change in load, there is a change in terminal voltage of an alternator or synchronous generator. The magnitude of this change not only depends on the load but also on the load power factor.

It is also defined as “the rise in voltage when full load is removed divided by the rated terminal voltage, when speed and field excitation remains the same.” It is given by the formula,

Methods to determine voltage regulation

Direct method of determining the voltage regulation is employed for smaller machines.

In this direct method, a three phase load is connected to star connected alternator with the help of Triple Pole Single Throw switch. The field winding of alternator is excited using an external DC supply. A rheostat is connected in series with the field winding, to control the flux produced in the field winding.

Adjust the rheostat of the field winding so that, to produce the rated terminal voltage. Close the load switch, apply the full load and measure the voltage at full load V.

Then the entire load is thrown off while the speed and field excitation are kept constant. The open circuit or no-load voltage E_o is measured and now the regulation can be determined from the below equation.

But in the case of large machines, it becomes very difficult to determine the voltage regulation by direct loading method. So it is very important to switch over to the indirect methods of determination.

Now’ let’s discuss about the different Indirect methods of determining the voltage regulation.

1. Synchronous Impedance method or EMF method
2. Ampere turns method or MMF method
3. Potier method or Zero Power Factor(ZPF) method
4. American Standard Association(ASA) of Modified MMF method.