I'm trying to measure the inductance and equivalent series resistance of an universal motor (from a sewing machine, 120V, 0.8A) with a B&K 879B LCR meter.
The measurements vary widely with the selected test frequency:
- 100Hz: 164mH, 60 Ohm
- 120Hz: 160mH, 65 Ohm
- 1kHz: 84mH, 285 Ohm
Measured in series-mode.
DC resistance (measured with multi-meter) is 40 Ohm
I would have expected that the motor, when not rotating, can be reasonably approximated by an ideal inductor with a resistor in series.
I wonder what the main culprit is that makes the measurements so frequency dependent. Stray capacitance between the windings? Skin-effect?