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TMS320F28069M: Instaspin current scaling errors

Part Number: TMS320F28069M

Hi,

I am having big issues with current sensing in our custom made board. When running the motor under load there is a huge imbalance in current. First I thought that we had a bad motor, but since then we have tried switching around the phases and a different motor. The same inverter phase is in all cases the one with most current, as per verification with external current probes.

When doing the initial identifications of the motor parameters, all is well, and we store the i_bias and v_bias values in the user.h.

The current measurment is done with three low side shunts 150uOhm each. (The design of the amplification is similar to Ti EVM, but our maximum current is 1100A in stead of 82.5A)

The problem appeared when we looked at the phases of the motor where one of them actually got warmer than the other two, running abt 120A p-p. The imbalance is so big that two different phases on the pcb under load have a different temperature of abt 30 degC. Switching waveforms are all looking very good on the scope.

The I_bias values are looking very good as well, all at 1.001 to 0.999 or similar. No big variance. But something else I saw was that the torque estimation was off as well. When we load the system in our dynamo we have a shaft torque of about 10Nm and in this case the torque estimation in instaspin sw is about 35Nm. So something is very off or odd and no matter how I check and double check everything I cant seem to figure where these errors can originate from.

Can this problem be due to a very low inductance motor? I tried this idea, but there is no big difference between 12kHz or 60kHz in switching frequency, but I fear we must increase the frequency much more? The motor is at 10uH.

Or is there any way to manually scale each sensor individually according to our measurments? The offsets seems ok, as stated above.

Any help or input is very appreciated!

Mikael