Correct Answer - Option 2 : Inversely proportional to number of poles
In a three-phase induction motor, synchronous speed is
\({N_s} = \frac{{120f}}{P}\)
Where,
Ns = Synchronous Speed in rpm
f = supply frequency in Hz
P = number of poles
⇒ Synchronous speed (Ns) ∝ (1 / P)
Therefore, the synchronous speed of the induction motor is inversely proportional to number of poles
Rotor full load speed, Nr = Ns (1 – s)
Where s = full load slip
- Synchronous speed is the speed of rotation of the magnetic field in a rotary machine.
- An induction motor always runs at a speed less than synchronous speed.
- The rotating magnetic field which is produced in the stator will generate flux in the rotor which will make the rotor rotate.
- But due to the lagging of rotor flux current with respect to stator flux current, the rotor will never reach its rotating magnetic field speed i.e. the synchronous speed.
- If the motor is running at synchronous speed, then the torque generated by the motor equal to zero.
- If the rotor runs at synchronous speed as the field of the stator, this will cause the slip to become zero, then there will be no torque generated in the rotor.
