# Measuring inductance and ESR of a motor using LCR meter

I am building a circuit, where a MCU will drive an unipolar stepper motor through MOSFETs and constant current source (not included in the schematic).

simulate this circuit – Schematic created using CircuitLab

The MOSFETs will switch at a frequency of somewhere in 10's or 100's kHz square wave. To design a constant current source, I need to measure the inductances of the coils and also their ESR. I have a LCR meter and multimeter at hand, but am unsure of how exactly to measure it.

If I measure the resistance of the coils (L1, L2, L3 or L4) with a multimeter I get around 5.5Ohms, if I do the same with the LCR meter set at 10kHz or 100kHz, I get like 500Ohm and 450uH.

How should I measure inductance and ESR of the coils?

• If you have an LCR meter with variable frequency, you're all set. The L should be fairly invariant over frequency. The Z (which I think is what you're seeing as 500 ohm) will increase. Commented Jan 3, 2015 at 19:37
• Isnt the LCR frequency meant as a sine wave? Commented Jan 3, 2015 at 20:19
• Yes frequency is a sine wave by default. If you have a simple model of the motor, resistor in series with inductor, then you need to find both components. The LCR does this. I know you're switching a square wave, in use, then you will use a different formula, L dI/dT etc, but you still need to know L. Commented Jan 3, 2015 at 20:42
• Also, perhaps I don't understand, but I've done a bit of reading... If you have PWM control of the MOSFETs, you shouldn't need a constant current source, just a constant voltage. OR you could have a constant current source, to keep the current reasonable without measuring it, but bypass the CC source to ground with a reasonable size capacitor, so the source never sees the 100 kHz. Commented Jan 3, 2015 at 20:54