Question

A three phase alternator has negligible stator resistance. A short circuit test is conducted on this alternator. At a particular speed a field current of $I_f$ is required to drive the rated armature current. If the speed of the alternator is reduced to one fourth, the field current required to maintain armature current is

• A. equal to 4$I_f$
• B. equal to $I_f$
• C. cannot be predicted due to insufficient data
• D. equal to 2 $I_f$

Answer 1

The correct option is B equal to $I_f$

For short circuit current, $I_{sc}=\frac{E}{X_s}$
$E\propto f\varphi$
$X_p\propto f$
So, $I_{sc}\propto \varphi$
Hence short circuit current is only a function of excitation.