Question

A 440 V, 50 Hz, 4 pole, 3-phase star-connected alternator has a rated full load current of 50 A. Its resistance per phase can be neglected. To induce rated voltage on no-load, a field current of 2 A is needed. When short circuited, the field excitation needed to circulate rated current in the phase is 1.5 A. When the machine is delivering rated load and rated voltage and a leading phase angle of ${30}^o$, the field current is ____ A.

• A. 1.8

Answer 1

The correct option is A 1.8
The field current required to circulate rated current under short circuit conditions is the field current required to overcome armature reaction.



To maintain rated voltage condition we have to supply additional field current to overcome armature reaction,

$I_f=\sqrt{2^2+{1.5}^2-(2\times 2\times 1.5\times cos{60}^o)}$

$=1.80A$