When the motor is stationary,rotor conductors are being cut by rotating flux at synchronous speed.therefore,the frequency of rotor current(or emf)is the same of supply frequency.but when the rotor starts revolving,the rate at which the rotor conductors are being cut by the rotating flux depends upon the relative speed between the rotor and the stator revolving magnetic field , called the slip speed.

Frequency of Rotor Currents at Standstill

The frequency of rotor currents at standstill is an important concept in electrical engineering, especially in the study of induction motors. The answer to this question is option 'C', which means that the frequency of rotor currents at standstill is equal to the supply frequency (f). Let's understand this concept in detail.

What is Standstill?

In an induction motor, standstill refers to the condition when the rotor is not rotating, i.e., it is stationary. At standstill, the relative speed between the stator magnetic field and the rotor is zero. This condition is also known as locked rotor condition.

Why is Frequency of Rotor Currents Important?

In an induction motor, the stator winding is connected to the AC power supply, which produces a rotating magnetic field. This rotating magnetic field induces a voltage in the rotor winding, which in turn produces a current. This current is known as the rotor current.

The frequency of rotor currents is important because it determines the torque produced by the motor. The torque produced is proportional to the product of rotor current and the rotor magnetic field. Therefore, if the frequency of rotor currents is low, the torque produced will be low, and the motor may not start.

What is the Frequency of Rotor Currents at Standstill?

At standstill, the rotor is not moving, and hence, the relative speed between the stator magnetic field and the rotor is zero. Therefore, the frequency of the rotor currents is also zero. However, once the motor starts rotating, the relative speed between the stator magnetic field and the rotor increases, and the frequency of rotor currents also increases.

The frequency of rotor currents is directly proportional to the slip of the motor. Slip is defined as the difference between the synchronous speed of the motor and the actual speed of the rotor. At standstill, the slip is maximum (equal to 1), and hence, the frequency of rotor currents is zero.

Conclusion

In conclusion, the frequency of rotor currents at standstill is zero. Once the motor starts rotating, the frequency of rotor currents increases and is proportional to the slip of the motor. However, during normal operation, the frequency of rotor currents is not usually measured, as it is not a direct parameter of interest.