Motor Rotation

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Brad Friedlin

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My question concerns inductive motors with a start and a run winding. I understand that the start winding has more resistance and therefore more magnetism during startup of the motor. The start winding is offset slightly from the run winding. The induced magnetic pole in the rotor is attracted to its opposite pole in the stator and rotation begins in that direction. What happens if when the motor is at rest it is in a position where the initial attraction is in the wrong direction of desired motor rotation?

I am a machinery technician (not an electrician) and am tasked with teaching other machinery technicians about electrical theory, potential, current and PTC relays, switches, breakers, run and start caps, inductive motors as they relate to air conditioning and refrigeration, and testing and troubleshooting these components as well. Every now and then I still have questions like this and I am hoping to use this forum as a tool to learn more and resolve questions from my students that I cannot answer.

I would appreciate any help available from this resource.

Thanks!
 
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hillbilly1

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North Georgia mountains
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Owner/electrical contractor
If memory serves me correctly, the start winding is 90 off from the run winding. It always runs the same direction regardless of where it stopped. To change rotation the start winding "polarity" ( I know its the wrong term, but it's the best I can come up with right now) is reversed by swapping only the start winding connections. due to the centrifugal switch or current relay, the start winding is taken out of the circuit at a predetermined speed or amp draw, so reversing the winding while the motor is running will do nothing, but the next time it is started it will run the opposite direction.
 
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gar

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Location
Ann Arbor, Michigan
Occupation
EE
080825-2012 EST

Brad Friedlin:

A single phase motor with one winding is not self starting, and will just sit in one position and hum. But if you can mechanically start it rotating in a direction then it will continue to rotate in that direction. Energy from the oscillating magnetic field pulses the rotor at just the correct time to keep it spinning. This is like someone pushing a swing at the correct time.

A single coil will produce a magnetic field at a fixed angle in space. If an AC current is applied, then this magnetic field will oscillate from N to S and back to N every cycle, but the major alignment of the field will be at the same angle in space except for the polarity change.

To make an AC induction or synchronous motor self starting you need some approximation to a magnetic field whose direction rotates in space. That rotating field can then drag the rotor with it.

Two coils at different angles to one another in space can produce that rotating field if they are excited at different phase angles. The relative phasing of the excitation will determine the direction of rotation of the field and therefore the motor rotor direction.

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