Question

A 20 pole, 25 Hz, alternator is directly driven by a 60 Hz synchronous motor. Then the number of poles in the synchronous motor are

• A. 24
• B. 10
• C. 12
• D. 48

Answer 1

The correct option is D 48

Let synchronous speed of motor $=N_{sm}$
Also, $N_{sm}=\frac{120\times f_m}{P_m}$

$\therefore N_{sm}=\frac{120\times 60}{P_m}$

synchronous speed of alternator,

$N_{sg}=\frac{120\times f_g}{P_g}=\frac{120\times 25}{20}=150rpm$

Since alternator and motor are directly coupled

$N_{sg}=N_{sm}$
(or) $150=\frac{120\times 60}{P_m}$

$\Rightarrow P_m=48$