Question

Whenever a synchronous machine operates at normal conditions, what is the relation between the rotor speed N in rpm, the frequency f in Hz, and the number of poles P?
1. \(f = \frac{{PN}}{{120}}\)
2. \(N = \frac{{120\;f}}{{s\;P}}\)
3. \(f = \frac{{120\;N}}{P}\)
4. \(N = s \times \frac{{120\;f}}{P}\)

Answer 1

Correct Answer - Option 1 : \(f = \frac{{PN}}{{120}}\)

Concept:

The synchronous speed of a synchronous machine is given by

\({N_s} = \frac{{120f}}{P}\)

Where f is frequency and P is the number of poles.

A synchronous machine always rotates with a synchronous speed.

Explanation:

From the above relation, the frequency can be written as,

\(f = \frac{{PN}}{{120}}\)