Voltage Regulation

When the alternator is subjected to a varying load, the voltage at the armature

terminals varies, the magnitude of this change in terminal voltage depends upon

the load current I and power factor cos Ø.

The voltage regulation of an alternator is defined as the increase in terminal

voltage when full load is thrown off, assuming field current and speed remaining

the same.

The percentage regulation is defined as the ratio of change in terminal voltage form

full load to no load and rated terminal voltage.

i.e. Percentage regulation = Eo - V /V X 100

where E0 is no load terminal voltage and V is full load rated terminal voltage.

Note : In case of leading power factor E0 is less than V, so regulation will be -ve.

Determination Of Voltage Regulation

It is not usually possible or desirable to determine the voltage regulation by direct

testing, indirect methods being adopted which do not need the alternator

loading. For this purpose OC and SC tests are performed to give OC and SC

characteristics. The various methods used for determination of voltage regulation

are given below:

Synchronous EMF or Impedance Method.

After determining Re and Xs from OC and SC tests the open circuit voltage can be

determined form the following relation

E0 = √(V cosØ + IRe)2 + (V sin Ø + IXs)

2

And %age regulation = Eo - V /v X 100

For leading pf Ø is taken negative and for unity pf Ø is taken zero.

The synchronous impedance obtained by this method is always higher than actual

value owing to the fact that the synchronous impedance Zs at short-circuit

condition is too large due to very low degree of saturation. Hence this method is

called the pessimistic method.

Amp-Turn or MMF Method.

From the OC and SC characteristics determined from OC and SC tests the field

currents If1 and If2 are determined to give rated voltage V on no-load (neglecting

armature resistance drop) and cause short-circuit current, equal to full load current,

on short-circuit. If1 gives full load rated voltage and If2 gives demagnetizing

ampere-turns at full load.

This method of determination of synchronous impedance is known as optimistic

method since it gives values lower than actual values. The reason of it is that the

excitation to overcome armature reaction is determined on unsaturated part of the

saturation curve.