# Induction motor with reversing stator magnetic field

The question:

The strength of the torque depends on the difference in speed between the stator's rotating magnetic field and the rotor's rotation. The larger the difference in speed the greater the induced voltage, current and torque.

When the motor is starting up, let’s assume the magnetic field is rotating synchronous speed. The difference in speed between the rotor and stator (motor) is 1,800 RPM, which means there will be a large induced voltage, current and torque. The rotor will start to rotate to catch up with the stator’s rotating magnetic field.

As the stator increases its speed to 900 RPM, the difference in speed between the stator and rotor will only be 900, so the induced voltage, current and torque will be less.

Suddenly, the rotating magnetic field is flipped and rotated counterclockwise. So the speed will be -1800 RPM. The difference between the rotating magnetic field and the stator will be very large. The relative velocity of the stator and the rotor will be large. This will induce an even larger voltage and current in the rotor, which will also generate a larger torque than at start-up.

Immediately following the switch, the rotor will still turn in its same direction, but will be subject to an opposite torque, voltage and current. So (A) is incorrect.

The rotor will be subject to a torque opposite to its direction, so it will begin to slow down. However, the induced voltage will immediately increase. So (B) is incorrect.

When the rotor is slowing down, shouldn't the voltage reduce? I don't understand why the voltage will INCREASE!

Since the induced voltage increases, the induced current will increase. So (C) is correct.

The stator will reverse direction and the rotor will slow down. So (D) is incorrect. The correct answer is most nearly, (C).

Source:

NCEES

PE Electrical and Computer: Power Practice Exam, 2020

ISBN 978-1-947801-16-5