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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?

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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.

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You're absolutely right that a rotor inside a magnet on a single-phase circuit won't start moving on its own. The key to getting it started is having a rotating magnetic field. The field of a magnet in a single-phase circuit doesn't rotate; it just switches polarity. Once the rotor is turning that's all it needs, but when it's stationary it needs a push to get it moving.

There are several ways to create that push. Many motors have an additional winding, called a starter winding, that's offset from the main winding. The power to that winding goes through a capacitor (that's the thing inside that cylinder on the top of the motor), which shifts the phase of the power coming into the starter winding. So as the incoming voltage rises to its peak, the starter winding produces its full magnetic field, and then, a bit later, the main winding hits its peak. The net magnetic field changes its direction as the main winding kicks in. That pulls the rotor a bit. Then the voltage switches direction, the momentum of the rotor keeps it moving, and the pull in the opposite direction pulls it around. Again, the starter winding gives the magnetic field a bit of rotation, and that, again, pulls the rotor more.

Once the rotor is up to speed, a centrifugal switch shuts off the starter winding, and the main winding is left on its own to provide all the power.

As you put more load on the motor, the rotor slows down, and lags further behind the magnetic field. That produces a stronger pull, which produces more torque, which adjusts for the increased load on the motor.

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  • \$\begingroup\$ +1. From the philosophical perspective, you could say that a single phase motor with a start winding, relay and capacitor is self-starting because the start features are incorporated into the motor. \$\endgroup\$ – mkeith May 27 at 18:41
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You were right if you talked about single-phase induction motors with no additional provisions. Such induction motors are rare for that very reason. You can find them as electrical brakes for example, because that starting problem doesn't apply then.

If you insisted on single-phase and single-coil (well, sort of), you had to create an anisotropy in the magnetic field in the rotor. That's done in a shaded pole motor. Such a motor has more standstill torque in one direction than the other, so it starts by itself.

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