topic Re: Double Motor Voltage from Expected in Kinetis Motor Suite

Double Motor Voltage from Expected

derekcook — Thu, 02 Aug 2018 13:14:54 GMT

Hello,

My Motor is rated for 200V at 2000 RPM. I am seeing 200V at 1000 RPM, and 360V at 2000 RPM. Any idea what could cause this? I've been scratching my head for a couple of days on this one and have tried several things. This is the case with both my sensorless and sensored projects.

We can have a meeting with a few members of my team if needed, or you have ideas.

Thanks,

Derek

Re: Double Motor Voltage from Expected

linestream-adam — Fri, 03 Aug 2018 00:31:34 GMT

Derek,

Where is the 200V at 2000 rpm number coming from? Is that a back emf of your motor?

If it is the back emf, than any voltage above the back emf will be used to generate current in the motor phases.

The command voltage is used to both overcome the motor back emf as well as to generate current.

How does this voltage vary with respect to torque on the motor?

Re: Double Motor Voltage from Expected

derekcook — Fri, 03 Aug 2018 00:51:53 GMT

Hey Adam,

200V is the nominal voltage of the motor at 2000 RPM. However, I am seeing 200V on both the A, B, C phases, and the Q reference voltage at 1000 RPM.

I can increase the torque to 70Nm at 1000 RPM, and 2000 RPM, and the voltage does not vary. It stays constant at the speed. The current however, increases with torque as expected. The current I am getting on the A, B and C phases are what I wold expect. It is just the high motor voltage that is unexpected.

Re: Double Motor Voltage from Expected

linestream-adam — Fri, 03 Aug 2018 00:53:44 GMT

Can you provide the datasheet for your motor?

How are you measuring the voltage on A,B,C?

Re: Double Motor Voltage from Expected

derekcook — Fri, 03 Aug 2018 01:15:55 GMT

I have an oscilloscope on phase A, B and C measuring RMS voltage. I have also measured it with a multimeter and current clamp. The Q voltage reference seems to match this voltage.

Here is the datasheet for my motor:

Re: Double Motor Voltage from Expected

derekcook — Fri, 03 Aug 2018 14:54:26 GMT

Hey Adam,

We figured out the voltage problem. Our meter and scope was calculating the peaks from the switching frequency into the RMS voltage. If you run the motor at 2000 RPM, we see 200V on the mean voltage of our scope. If we apply a filter on the scope we see 200V AC RMS.

We also found that to relate Q voltage to motor voltage you have to divide by the square root of 3. If you divide by the Q voltage by the square root of 3 at 2000 RPM, you then get 200V.

I think we have this problem figured out. Just a measurement issue.