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VFD Question

3 phase voltages and setup for VFD

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Carl Wilson 417/01/2020 12:00:02
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Old mart should have added "on this site". Lot of clever clogs here.

SillyOldDuffer17/01/2020 13:09:21
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Posted by Steviegtr on 16/01/2020 22:49:14:

...

The windings in most multi speed motors have multiple windings around a pole. So 4 windings on single speed & 8 windings on 2 speed & so on. This does not mean the motor goes from a 4 pole motor to a 8 pole motor as some have suggested. Its just a different winding on the same pole.

Not wrong exactly, but I think this depends on exactly what's meant by a 'pole'. To an electrical engineer a physical pole isn't necessarily the same as a magnetic pole.

If it walks like a duck and quacks like a duck it is a duck. When someone says a 'motor goes from a 4 pole motor to a 8 pole motor', they mean electrically, not literally. I prefer the electrical engineering view because Stevie's definition of 'pole' gives the wrong answer from:

RPM = Frequency * number of poles

I find it easier to think of motor poles in terms of effect rather than their physical layout and like to believe only motor designers have to do both! I could be wrong again...

Dave

Andrew Johnston17/01/2020 13:14:34
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Posted by Steviegtr on 16/01/2020 22:49:14:

The windings in most multi speed motors have multiple windings around a pole. So 4 windings on single speed & 8 windings on 2 speed & so on. This does not mean the motor goes from a 4 pole motor to a 8 pole motor as some have suggested. Its just a different winding on the same pole.

I don't follow that? The equation for the synchronous speed (in rpm) of an induction motor is 2 times frequency (Hz) times 60, all divided by the number of poles. Assuming that the frequency is fixed then the only way to change the speed is to change the number of poles. Of course windings aren't added or removed, simply connected in a different sequence. But the upshot is to change the number of poles.

Andrew

Edit: Thought better of it

Edited By Andrew Johnston on 17/01/2020 13:18:13

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