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I have five small brushless DC motors from KDE, model# KDE2315XF-2050. The specs for this motor state that the winding resistance is 0.034 Ohms, and having conversed with KDE, they state the winding inductance is ~3.6uH as measured with a scope at 150kHz.

I want to verify these values, and according to KDE they use the method outlined in Tektronix's document "Capacitance and Inductance Measurements Using an Oscilloscope and a Function Generator", document ID 75W-28152-1.

KDE modified some of the variables in their example video on YouTube such that the voltage amplitude is 2.5V, and the test frequency is 150kHz.

I performed this test myself using a value of R reference being measured at 995.9 Ohms (using a calibrated Fluke 289 multimeter), freq generator set at 2.5V amplitude and 150kHz. I then measured the windings of 5 motors. The following are the average results for all five motors:

Average Inductance = 3.46uH

Average Resistance = 0.5972 Ohms

When I measure the DC resistance of all five motors using the Lo Ohms setting of the Fluke 289 I get an average resistance reading of:

0.031 Ohms.

Using the scope and the I-V method the inductance jives with the specs, but not the resistance. I trust my multimeter to provide an accurate DC resistance measurement.

If I adjust the voltage amplitude and frequency to 5V and 50kHz I get:

R = 0.08584 Ohms
L = 3.814uH 

So my question is this, why is there such a difference in the winding resistance between the two methods of measurement (scope vs. multimeter)?

As an FYI I do not have a high end scope of function generator, so I do expect a difference but not so much. Also note it has been 20+ years since I have worked with electronics at this level. If I am missing something please forgive me.

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The motor resistance is specified at DC, not at high frequency. At high frequency, you get a skin effect, eddy currents,... which add looses. For this reason it is normal that you get different results DC vs. HF.

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  • \$\begingroup\$ Thanks for your reply. So what's the point then of having the calculation for resistance in the Tektronix document? \$\endgroup\$
    – mike
    Feb 21 at 17:20
  • \$\begingroup\$ @mike So you can account for the resistance effects that do not appear at DC. \$\endgroup\$
    – DKNguyen
    Feb 21 at 17:21
  • \$\begingroup\$ Thank you for the reply. You guys have given me a few things to consider and brush up on. \$\endgroup\$
    – mike
    Feb 21 at 17:26

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