In an induction motor if the rotor is locked, then the rotor frequency of the induction motor will be:

Rotor Frequency in an Induction Motor:

The frequency of the rotor current in an induction motor, also known as the rotor frequency, is dependent on the slip of the motor. Slip (s) is defined as the difference between the synchronous speed (N_s) and the actual rotor speed (N_r), expressed as a fraction of the synchronous speed:

s = (N_s - N_r) / N_s

The rotor frequency (f_r) is given by:

f_r = s × f_s

Where,

• f_r = Rotor frequency
• s = Slip
• f_s = Supply frequency
   

Condition of Locked Rotor:

When the rotor is locked, it means that the rotor is not rotating (N_r = 0). In this condition, the slip (s) is equal to 1 because the rotor speed is zero:

s = (N_s - 0) / N_s

s = 1

Therefore, the rotor frequency (f_r) is:

f_r = 1 × f_s

f_r = f_s

Conclusion:

The correct answer is:

2) equal to supply frequency