Question

Two alternators are mechanically coupled and driven with a common prime mover. One alternator is delivering power at 50 Hz frequency and other at 60 Hz frequency. The maximum possible speed of set will be

• A. 1000 rpm
• B. 300 rpm
• C. 600 rpm
• D. 500 rpm

Answer 1

The correct option is C 600 rpm

$\text{Synchronous speed,}N=\frac{120f}{P}$

$\text{For 50 Hz generator,}N=\frac{120\times 50}{P_{50}}$

$\text{For 60 Hz generator.}N=\frac{120\times 60}{P_{60}}$

$\Rightarrow \frac{P_{60}}{P_{50}}=\frac{6}{5}$

Since poles must always be even, we will choose the smallest possible even number so that the speed of set is maximum.

$\Rightarrow P_{60}=12and{P}_{50}=10$

$\therefore \text{Speed of set, N}=\frac{120\times 50}{10}=600rpm$