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I noticed that the rotating plate of many microwave ovens usually rotates with a specific direction, even if sometimes this changes. What makes the plate rotating clockwise/counterclockwise?

I know that in many two-phase AC synchronous motors a capacitor is added to create an initial phase shift which (I imagine) determines the rotating direction. However, can it be that this direction reverses e.g. depending on the initial position of the rotor? Or is it that in some case this capacitor is not added?

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Microwave turntables typically use a permanent magnet single phase synchronous motor, no capacitor, as they're cheap.

For a perfectly symmetrically manufactured motor, the startup direction depends on chance. In a real motor, asymmetries might favour one direction over the other, so it may show a preference for going in one direction.

For the purpose of turning severe hot and cold spots into less severe hot and cold rings, it doesn't matter which direction it goes in, so there's no effort or cost spent in getting it to go one way.

When this type of motor is used in clocks, a common mechanism is a small spring that gets wound up if the motor starts in the wrong direction. The motor stalls against the spring, which then flicks the motor the right way.

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  • \$\begingroup\$ The clocks that I've opened up have used shaded poles to determine startup direction, which works just as well on synchronous motors as it does on the more typical induction motor. \$\endgroup\$ – Dave Tweed Dec 6 '19 at 12:23
  • \$\begingroup\$ I own an oscillating fan heater that uses a synchonous motor to drive the oscillation and stalls the motor to affect a change of direction. \$\endgroup\$ – Jasen Dec 7 '19 at 8:44

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