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In a recent question of mine about microwave plates, jpa correctly identified the type of motor used for microwave plates as a specific variant of AC synchronous motor, equipped with gears that reduce the speed to some 5RPM.

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A single-phase (or two-phase derived from single phase) stator winding is possible, but in this case the direction of rotation is not defined and the machine may start in either direction unless prevented from doing so by the starting arrangements.

What I am unclear on - what is the principle of operation of such motors?
What construction property of the motor is responsible for the start direction being unpredictable but the motor never being stuck and unable to start?

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The wikipedia link in the answer you dismissed to ask this question gives you the answer: "A single-phase (or two-phase derived from single phase) stator winding is possible, but in this case the direction of rotation is not defined and the machine may start in either direction unless prevented from doing so by the starting arrangements "

When AC is applied, the motor is unstable until it starts rotating, so it starts rotating (maybe a butterfly in China flaps its wings). It could (in theory) stay stationary, but only in the manner that if you stand a broomstick on end, it could stay balanced.

Maybe a better example is a piece of paper - stand a piece of paper on edge, and it falls one way or the other. Once it starts falling one way, it keeps falling that way unless you do something like blow or push it back the other way. The motor is unstable, it starts rotating one way or another, and once it starts rotating, it keeps going. You can push it back the other way - if you take one of these and wedge something in the mechanism so it can't continue, it will reverse.

If you want it to only rotate one way you simply build a ratchet or other clever mechanical in the mechanism so if it starts backwards it immediately reverses.

For example, this sort of fishtank circulation pump: http://www.tunze.com/149.html?&user_tunzeprod_pi1%5Bpredid%5D=-infoxunter052

Obviously, a propeller that runs backwards doesn't do what you want, but if the motor starts backwards, what happens is that rather than pushing water out the front, the rotor pulls itself forwards out of its housing. When it does that it then hits a carefully positioned part of the housing in front of the propeller, and bounces off. It's now turning in the opposite direction, and carries on doing so. When it's turning in that (correct) direction, it's pushed back in the housing, clear of the thing that it bounced off, so continues to turn in that direction.

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  • $\begingroup$ I'm not asking for how unidirectionality is achieved and I understand the idea of unstable balance, but in case a motor has a considerable load, the torque in the unstable balance position tends to be too low to overcome that initial load/friction; heavier synchronous motors do require startup devices/properties; I wonder why, no matter how unlucky you are, you never get the "coin standing on edge" situation with these simple synchronous motors that don't have these extras. (btw, I quoted the same phrase from the Wikipedia in my question, it's very scarce in details.) $\endgroup$
    – SF.
    May 7 '15 at 12:44
  • $\begingroup$ I'll hazard a guess that the backlash in the gears allows the motor to start with very little torque. $\endgroup$
    – regdoug
    May 7 '15 at 14:13
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Most low-power, single-phase synchronous motors are shaded-pole motors, in which a turn of copper wire is placed around some fraction of each of the poles for the main coil. This creates a phase shift in the magnetic field that causes the motor to turn in a particular direction at start-up.

Note that the rotor can be a simple squirrel cage rotor that is powered by induction, or it can be a permanent magnet. The former type is used in applications in which some "slip" is acceptable. The latter type is used in electric clocks for strictly synchronous operation.

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  • $\begingroup$ But this one seems not to be a shaded-pole motor, as its start direction is random. I understand how the shaded pole enforces both startup from arbitrary position and direction. In this case direction is not enforced - but still we get a sure-fire start. $\endgroup$
    – SF.
    May 7 '15 at 15:40
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I've got a few of the circular motors in my junk box and took one apart. With what I've found on the net, and what I see in the unit I have, here is what I found. The circular units are for very small wattage requirements and usually are applied in equipment, like the fish tank pump and microwave turntable discussed here. The unit I have came out of a commercial printer as a feed roller driver. The round units are low cost and do not need a winding armature; a simple round winding into which is set a ceramic rotor on a bearing. Without a magnetic conductor, or armature, there is no room to put a shaded pole copper ring. That means the motor needs some method to insure it rotates in the proper direction. And that is where a simple mechanical cog comes into play. The rotor has a feature that looks like a bow-tie and above that a small gear, which drives #1 reduction gear. This #1 gear has a friction arm below it. As the #1 gear turns, the arm turns with the gear until it hits a stop, and the gear keeps turning. If the motor starts the "right" way, the arm moves away from the bow-tie feature. If the motor started the "wrong" way, the arm moves toward the bow-tie and the design is such that at the prescribed angle the rotor/bow-tie slams into the arm. The angle seems away from a magnetic cogging pole, about in the middle between 2 magnetic "bumps." When the impact occurs, you can see the rotor bounce off the arm and instantly starts in the "right" direction, which moves the arm out of the way.

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