Winding to rotor capacitance in 8-pole induction motor?

[Skogsgurra]

I have noticed that there seems to be more capacitive coupling from stator winding to rotor in a 4 kW 8-pole induction motor than there is in a 4-pole motor.
I do not know enough about stator windings to understand why that would be so. There is, perhaps, a self-evident explanation - or it may be a little more complicated. But for now, I have not the faintest idea. Anyone?

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScottyUK]

Could it be a simple answer - that an 8-pole machine is usually a frame size larger than a 4-pole machine?

 

[Skogsgurra]

Didn't think of that. Yes, it is possible. But I have measured BVR (Bearing Voltage Ratio) that is almost twice that of a four-pole machine. That implies that the capacitive voltage divider consisiting of winding-Rotor and rotor-frame capacitance is offset more than the larger diameter itself could be responsible for. Of course, the air gap is probably wider also. That would increase BVR.
Is there anything else one could think of? More slots? Wider slots? Other winding arrangement that makes more of the "hot" part of the winding closer to the slot?

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.