Capacitances of various coil sizes for AC hipoting

[VLFit]

I realize that with an infinite number of coil geometries, wire sizes, designs, voltages, etc that it is impossible to know the capacitance of a coil to estimate the AC voltage charging current necessary to hipot it, but, does anyone have any rough rules of thumb that might define even a dozen typical sizes of coils, probably stators mostly? Possibly someone has worked out a few examples of motor or generator coils based on voltage rating, MWs, HP, diameter, # of turns, etc for a few voltage sizes: 2400V, 4160V, 13.8 kV, etc. Also, do many motor shops perform power factor testing on rewinds? Thanks.

 

[waross]

"I realize that with an infinite number of coil geometries, wire sizes, designs, voltages, etc that it is impossible to know the capacitance of a coil to estimate the AC voltage charging current necessary to hipot it"
Energize the coil through a high value resistor with a fairly low DC voltage. Measure the time constant and from that calculate the capacitance. Don't forget the discharge diode.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[VLFit]

Let me think about that and how it would correlate to the 50/60 Hz charging current. Thanks.

 

[electricpete]

For formwound stators, the insulation absorption current (vs simple capacitive current) is going to complicate things compared to an R-C circuit model. Of course we already knew the model isn't perfect, maybe good enough..I'm not sure.

Another thought would be to test with low voltage power frequency ac if that is readily available.

Rough estimate can be made using parallel plate capacitor model C = epsilon * A / d
Area is the total area of all core slots.
d is ground insulation thickness
epsilon between 2 and 4 times epsilon0. I think epoxy mica vpi insulation system would be towards 4.


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(2B)+(2B)' ?

 

[electricpete]

Related thread here focusing on motor winding capacitance to ground
thread237-278636

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(2B)+(2B)' ?

 

[ScottyUK]

Doble have, or at least had, a fairly large database of machine data. You might be able to extract the data you want from there. They will almost certainly charge for access to it.