I have recently seen a 2-capacitor single phase motor with a forward / reverse cam switch that has only three wires to the motor. That doesn't compute with my knowledge of drum and cam switch control of that load. I understand that with four leads, but not three. How does that work?
It's rare, but it is done in very small sizes.
Motor is built with two identical windings but displaced by 90 degrees or so. The two windings have a common end connected giving three terminals, one to each winding and one to the junction of the two.
Because the windings are identical, you energize one from the line and one through a cap and the motor turns in one direction. Reversing the direct and capacitor connections reverses the motor. This can be accomplished by wiring the cap across the two winding leaads, and moving the hot supply leg from one to the other. Neutral supply leg goes to the junction of the two windings.
This is typically used in small synchronous gearmotors, typically less than
1/10 hp or so. Starting torque is not great, due to the need to make the windings identical. Bodine used to make lots of motors like this.
This is the Furnas page on drum switches but down around page 5 they have lots of different motor wiring info that has saved my bacon a few times. It is worth printing and saving somewhere.
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