Overcurrent occurs after unkown incident when pushing motor to max.

[roar]

Custom board with STM32G474QE and MCSDK 5.Y.4

Hi,

we have a 5 pole pair PMSM which is running at 800VDC consuming ~36Amps at 1800rpm during testing in water. We run it in speed mode, which results in a Torque Ref of >21400 read in MotorPilot(~72Amps) where the limit is 100Amps. The motor can run at this speed for quite some time, but is not intended to do so for very long.

The problem occurs when we increment the speed only by 50-100rpm more, the motor speeds up and reaches the desired speed. Shortly after, we hit overcurrent and the motor shuts down properly.

If we inspect the HighFrequencyPlot looking at Ia and Ib, we can see that something is happening right before the currents reach IQMAX . (see the second to last waveform, inside the circle)

 

If we lower the VBUS to e.g. 600V, 550V or 500V, the same thing happens but at a lower rpm and Torque Ref.

Can anyone help me identify what could be the issue here?

[DZann.1]

In case of sensorless, I think that the problem is the circle limitation. When the motor generate a high BEMF that is quite equal to the VBUS available (decreased by the death time, ...), the circle limitation change the angle. For better understand this situation, the library should calculate the modulation index (the ratio between the desidered voltage and the available voltage). When the ratio is greater than 95% this could happens.

[roar]

Thanks for the reply. I failed to mention that we use Hall sensor for speed measurement, not sensorless operation.

I will try to look at Circle Limitation if there is something that we can do there.

Although I was kind of expecting one of the PID regulators to prevent this from happening, and just settle at a speed depending on available voltage.

[roar]

Here is a screenshot of Ia and Ib when we have VBUS 600V, 1800rpm.  

Just to be clear; the Overcurrent fault is not what we worry about, in fact, it is probably what is protecting our system. The small dips and inconsistencies in right before the peak+shutdown is the problem that we can't figure out.

[THA.1]

Did you enable flux-weakening? If not, check your motor's back-emf constant. If it reaches VBUS around 1800 rpm, it becomes uncontrollable above 1800 rpm without flux-weakening. ​

[roar]

Thanks for the reply.

No, we have not enabled flux-weakening. The B-EMF Constant of our motor is 179.5Vrms/krpm, and at 1800rpm this results to 456V, so we should be in the clear here. Please let me know if there is something wrong with my assumptions.