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single phase AC motors

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blazer4

New Member
I don't understand why single phase ac motors need start windings? I know they do, but there is something I'm not getting.
It seems to me the stator poles produce a constantly changing magnetic field which induce a VOLTAGE across the windings, or squirl cage of the rotor. Since the impedance of the rotor is almost purely inductive, the current, and therefore the resulting magnetic field of the rotor should lag that voltage by ~ 90 degrees. Therefore the two magnetic fields of the stator, and rotor should be displaced by ~ 90 degrees, and therefore no need for start windings.
I know I have something wrong, but I don't know what it is. Can someone please straighten this out for me?
 

dknguyen

Well-Known Member
Most Helpful Member
I forgot this stuff as soon as I learned it lol, but here is my professor's animated online notes. I hope they help.

Single phase induction machines (double field theory):
http://www.ece.ualberta.ca/~knight/ee332/other/spim.html

Starting 2-winding machines:
http://www.ece.ualberta.ca/~knight/ee332/other/split_phase.html

If I remember right (I'm just glossing over the links I posted) though it has something to do with fact that the magnetic field is pulsating rather than rotating and two equal and opposite magnetic fields opposing each other at zero RPM - which means you have to start it to cause the magnetic field imbalance which will then continue to perpetuate itself.
 
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