Correct Answer - Option 2 : 100 Hz
Concept:
The synchronous speed of an alternator is determined by the frequency of the source and the number of poles.
Derivation:
- 3-phase supply with a frequency of ‘fs’ is given to the 3-phase distributed winding the rotating magnetic field is set up by the windings.
- The speed of the rotating magnetic field is in synchronous with the supply frequency and hence called synchronous speed ‘Ns’ (in rpm).
- 3-phase windings are wound for a specific even number of poles ‘P’, 2/4/6/8…
- One cycle of AC current through the windings makes the pole axes move along one pair of poles (P/2).
- Hence fs cycle per second AC current will give rise to the speed of rotating magnetic field as :
\(\frac{{{N_s}}}{{60}} = \frac{{{f_s}}}{{\frac{p}{2}}}\) rps.
- Rearrangement of the above equation will give the equation for synchronous speed in rpm as:
\({N_s} = 120\frac{{{f_s}}}{p}\) rpm.
Calculations:
Given synchronous speed Ns = 600 rpm.
Number of poles p = 20
Frequency, fs = (600 × 20)/120
Frequency, fs = 100 Hertz.