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I just disassembled the shifter assembly in a washing machine. It uses a synchronous 120VAC gear motor. The motor says "CW/CCW Z.P." It has two leads for the AC power and no other inputs.

I discovered that the direction sometimes reverses if I allow the motor to stop, then quickly cycle the input power, then re-apply the power. However, this is not reliable, as sometimes it reverses and sometimes it doesn't.

How is this motor being reversed by the washing machine control board? There must be a reliable method, otherwise the shifter arm assembly would be damaged.

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A basic 2-wire synchronous motor will start randomly in either direction, or may fail to start at all.

Usually, cheap uni-directional motors, as used in clocks for instance, have a mechanical spring/cam arrangement that engages if the motor starts in the wrong direction, and flicks the armature into the other direction.

A synchronous motor may be built with a directional bias, perhaps shading rings over part of the pole face, but this increases power consumption when running.

Another possibility that might allow the controller to start the motor in the direction of choice with just two wires could be a magnetic bias to hold the rotor in a quadrature position with power off, then the controller starts power with the appropriate phase. This is pure speculation on my part, I've never seen it described.

If the motor will start in either direction, I would expect the washing machine mechanics to be able to tolerate this. If it's not meant to start in either direction, then perhaps any starting mechanism has broken.

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  • \$\begingroup\$ I just tested the motor while installed in the mechanism. When it hits the mechanical limit, after exerting a fairly large strain on the bracket, it automatically reverses. You can actually see the bracket warp a bit as it strains against the limit, but then it reverses every time. Is this common to synchronous motors? \$\endgroup\$ – Ryan Griggs Apr 4 '17 at 18:53
  • \$\begingroup\$ That sounds like a spring arrangement, that flicks the motor into running in the opposite direction. \$\endgroup\$ – Neil_UK Apr 4 '17 at 19:55
  • \$\begingroup\$ Is that what "T.Z." means on the motor label? \$\endgroup\$ – Ryan Griggs Apr 4 '17 at 23:37

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