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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).

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?

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  • \$\begingroup\$ 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. \$\endgroup\$ – tomnexus Jan 3 '15 at 19:37
  • \$\begingroup\$ Isnt the LCR frequency meant as a sine wave? \$\endgroup\$ – Golaž Jan 3 '15 at 20:19
  • \$\begingroup\$ 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. \$\endgroup\$ – tomnexus Jan 3 '15 at 20:42
  • \$\begingroup\$ 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. \$\endgroup\$ – tomnexus Jan 3 '15 at 20:54
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Your RLC meter assumes that the parameter you select is the only significant attribute of the item being measured, so when you tell it to measure R it assumes that the stepper motor coil is a pure resistor with no inductance or capacitance. In reality the coil has significant inductance which will increase the reading. At 10kHz the coil's impedance is mostly inductive, so the L reading should be fairly accurate but the R reading won't be.

For PWM current control the inductance value isn't critical - it just has to be high enough to keep current ripple down to an acceptable amount. However the resistance value is critical, as it determines the average current through the coil. At high frequencies the resistance will increase due to skin effect, but in the frequency range you intend to use and with the small diameter of the coil wire, its AC resistance should not be much higher than your DC measurement.

Ignore the RLC meter's AC resistance measurement. The DC value should be close enough. Use the lowest available PWM frequency that still maintains continuous current flow in the coil at the lowest PWM ratio you intend to use.

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