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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?

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    \$\begingroup\$ I expect that the increase in resistance is due to the emerging eddy current losses as frequency rises and the decrease in inductance is due to eddy currents forming parallel inductances hence they reduce the net inductance. \$\endgroup\$ – Andy aka Jun 15 at 9:25
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    \$\begingroup\$ @Andyaka That makes a lot of sense to me. The motor is designed for 60Hz with phase angle control. At those higher frequencies, the laminations are probably not very effective at keeping the eddy currents at bay. Can you convert your comment into an answer? \$\endgroup\$ – Stefan Jun 15 at 18:50
  • \$\begingroup\$ done. See below. \$\endgroup\$ – Andy aka Jun 15 at 20:06
  • \$\begingroup\$ "... with a B&K 879B LCR meter" - did you make the measurements in series mode, or parallel mode? \$\endgroup\$ – Bruce Abbott Jun 15 at 20:46
  • \$\begingroup\$ @BruceAbbott series mode. Added it to the question \$\endgroup\$ – Stefan Jun 15 at 20:57
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Can you convert your comment into an answer? – Stefan

I expect that the increase in resistance is due to the emerging eddy current losses as frequency rises and, the decrease in inductance is due to eddy currents forming parallel inductances hence they reduce the net inductance.

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Universal motor have brushes. You should take it a part before measuring. Anyway measuring value can depend on frequency due to parasitic element. you must use measure frequency to match working frequency for accuracy.

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  • \$\begingroup\$ What parasitic element are you referring to? I would expect that the brushes contribute to the ESR, and maybe make the measured inductance dependent on the position of the rotor, but all those measurements were made in the same position. \$\endgroup\$ – Stefan Jun 15 at 18:46
  • \$\begingroup\$ I just refer to element then unexpected to affect LC value such as capacitance between wire or magnetic interference between coils. \$\endgroup\$ – M lab Jun 17 at 4:35

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