Speed of rotating magnetic field of a slip ring induction motor

The speed of the rotating magnetic field in a slip ring induction motor is determined by the frequency of the power supply and the number of poles in the motor. The rotation speed of the magnetic field is known as the synchronous speed.

Formula for synchronous speed:
Synchronous speed (Ns) = (120 * Frequency) / Number of Poles

In this case, the frequency of the power supply is given as 50 Hz and the motor has not been specified to have a certain number of poles.

Calculating the synchronous speed:
Synchronous speed (Ns) = (120 * 50) / Number of Poles

Since the number of poles in the motor is not specified, we cannot determine the exact synchronous speed.

However, we are given that the motor is running at a speed of 960 rpm (revolutions per minute).

Calculating the actual speed:
Actual speed = 960 rpm

The actual speed of the motor is less than the synchronous speed because of slip. Slip is the difference between the synchronous speed and the actual speed of the motor. It is expressed as a percentage of the synchronous speed.

Calculating slip:
Slip = (Synchronous speed - Actual speed) / Synchronous speed

Since the actual speed is less than the synchronous speed, the slip will be a positive value.

Now, if we assume the slip to be small, we can approximate the synchronous speed as follows:

Synchronous speed ≈ Actual speed

Therefore, the speed of the rotating magnetic field of the slip ring induction motor running at a speed of 960 rpm would be approximately 960 rpm.

Hence, the correct answer is option 'd) 960 rpm'.