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Steady state PWM of 2 motor phases and current measurments

tprohas

Jul 15, 2011
1
Joined
Jul 15, 2011
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Hi All,

I am driving a 3 phase PMSM as follows:

I set my voltage to 12 volts. I PWM a single High Side (phase A) FET, at low 10% duty cycle (20KHz freq). I Turn ON a single LOW side FET PHase B. This is done for a steady state test. I measure the current via a Phase B low side shunt. This is effectively 2 inductors in series, with series resistance.

If I hand calculate the expected currents, taking into account inductor resistance, shunt resistance, etc. If I Assume 12 volts, and resistance of 0.140 ohms, max possible current is 86Amps. At 10% I would expect about 8.5Amps. At low duty I'm close to my expected value.

The problem is if I increase the duty cycle, to 20 or 30 %, I have a wider margin of error, over 2-4 Amps. I'm trying to understand where the error is. I don't know if reactance of the inductor is causing the issue, and if it is, is there any info on determining this for a PWM duty cycle value (not a Sinusoidal Reactance!).

Any ideas or suggestions?

Tom
 
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