The question:

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The answer:

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).



PE Electrical and Computer: Power Practice Exam, 2020

ISBN 978-1-947801-16-5


1 Answer 1


When the stator connection is reversed, the synchronous speed becomes negative 1800 RPM. Slip is the difference between synchronous speed and rotating speed. The slip changes from something like 50 RPM to something like 3550 RPM. The induced voltage in the rotor is proportional to slip. That would cause a huge increase in rotor current it it were not for the effect of the rotor reactance. It is still a pretty big increase because the current increases from something near rated current to something above locker rotor current.

Regarding Plugging Torque

Although I agree that (C) is the correct answer, I don't agree that the torque will be larger than at start up. The image below from Fitzgerald, Kingsley, Umans, Electric Machinery supports my assertion. Considering the induced voltage is an acceptable line of reasoning, but it is better to consider the Steinmetz equivalent circuit. However the solution of the equivalent circuit is probably not sufficient because it does not consider the effects of the size, shape and position of the rotor bars. In many motor designs, the minimum torque during acceleration has a minimum between standstill and operating speed. That may also mean that the torque rises as the speed increases in the reverse direction. The torque in the plug reverse region may be higher or lower than locked-rotor torque depending on rotor design.

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