Effect of Rotor on Stator Winding Inductance in an Offline Generator

Ah OK. Thanks!

There is something odd in your measurements though. When you say you rotated 45°, how accurate is that?

Roughly measured from the coupling hub at drive-end. First we rotated it to 90 degrees, then measured - same results as at original position. Then we rotated further to 45 degrees. We did it roughly like half of 90 degrees. Then we took another reading. Now things changed.

It was like this:

0 degrees: U-U1 = 4mH, V-V1 = 4mH, W-W1 = 12mH

90 degrees: U-U1 = 4mH, V-V1 = 4mH, W-W1 = 12mH

45 degrees: U-U1 = 12mH, V-V1 = 4mH, W-W1 = 4mH

What else do you think it could be

"... We did it roughly like half of 90 degrees. .."

It is no longer odd with the realization the amount of rotation stated is a rough estimate.

Presence & position of rotor does affect inductance of stator windings.

If rotor is out, stator flux, which did go through high permeability iron rotor now goes through air μ = ≈1.

Similarly, salient pole directly on a stator winding centre reduces total air gap to minimum. Poles are 90 degrees apart, so 45 degree rotation gives maximum.

This is connected with the direct axis and quadrature axis machine parameters & the d-q axis machine theory.

There is still a lesser effect with cylindrical machine rotor. The rotor winding slots are not everywhere around circumference and are a large "air" gap compared to rotor-stator clearance, which may be a few mm.

You may be able to some information by energising each stator at reduced AC mains voltage. A Medium Wave "AM" portable radio may pick up noise from "tracking" in faulty phase. With rotor out, this may localise the slot, as might a search coil & voltmeter.

With a brushless machine the rotor exciter diodes will conduct on one polarity of stator AC, near short-circuit on rotor main field. This means you have to disconnect the main rotor lead or half the diodes - however, they are usually accessible for replacement without removing rotor - there might be diode fuses, easier to remove without loose leads.