Power Electronics Circuits
Which of the following correctly explains why an induction motor cannot run at exactly the synchronous speed while operating as a motor?
- APermanent magnets are embedded in the rotor, and their magnetic force pulls back against the rotating field, so the rotor always lags the field by a fixed angle
- BAt synchronous speed the stator current becomes zero and the motor is disconnected from the supply
- CAt synchronous speed the rotor conductors do not cut the rotating field, so no current is induced and no torque is produced
Show answer and explanation
Answer: C. At synchronous speed the rotor conductors do not cut the rotating field, so no current is induced and no torque is produced
An EMF is induced in the rotor conductors of an induction motor according to their speed relative to the rotating field, and a current flows. The force between this current and the field becomes the torque. If the rotor turned at the synchronous speed, the relative speed would be zero, so neither induced current nor torque would arise, and the rotor would slow down, unable to overcome the load and friction. An induction motor therefore turns slower than the synchronous speed by just enough (the slip) to produce the torque required.
- A. The rotor of a typical squirrel-cage induction motor has no permanent magnets. Synchronous motors are the ones that use permanent magnets.
- B. Magnetizing current keeps flowing in the stator. Even at synchronous speed, the motor is not disconnected from the supply.
- C. Correct. The slip lets current be induced in the rotor and produces torque.