I read that all you need to start an induction motor is to apply the AC voltage, and with time the slippage decreases. How does this happen magnetically?
If a rotor, connected to a moderate sized mass, is stationary at t=0, and you apply a quickly rotating magnetic field, then the stator will spend half of its AC cycle with its magnetic vector ahead of the rotor, and the other half it will be behind. To me that means it rotates the rotor in the +ve direction for 1/2 of the 1/60 cycle (assuming 60Hz), but it rotates it backwards for the other half.
What am I missing?
Best Answer
Most of what is written about induction motors is about 3-phase motors. In a 3-phase motor, the magnetic fields rotate smoothly in one direction. Single-phase motors accomplish the same thing with two windings that, in effect, are energized as a two-phase system. There are several schemes to accomplish that.