Hi all! I am trying to control a PMSM in speed mode and analyze the power consumption over different operating points using an oscilloscope. My Vdc is 48V, and the motor is delta configured. In fact, I can control the motor and follows the reference speed perfectly, but the voltage signals generated by the inverter are kinda confusing to me.
Here, for instance, I show a picture of my oscilloscope. In yellow, the voltage between 2 phases of the inverter (hence terminals A & B of my motor). At the begining we can clearly see a sinusoidal PWM, with the correct electrical frequency (motor spinning at 250rpm and 5 pole pairs, so fe=250/60*5=20.8Hz, T=50ms). But then the PWM turns off, I can’t understand why. Furthermore, the current (in cyan) is unbothered. This behaviour repeats periodically over time.
Can anyone elaborate on this behaviour? Is it normal?
Thanks in advance!
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Hi! So the oscilloscope is connected with the GND lead on one phase, and the probe tip connected to another phase? That’s going to make the scope ground swing all over the place with reference to the ODrive’s DC-. If the ODrive is on a power supply, or there’s a ground loop of some kind (even just coupling through the power supply’s y-capacitors), it could be causing a lot of current draw through the scope / GND lead – I almost wonder if you’re blowing a fuse in the scope or something. Usually what I’d recommend is to use an actual differential probe (I use the Micsig DP1500), or to use two scope channels with probes grounded to DC- and connected to e.g. phase A/B, using the scope’s math channel to subtract them to get the relative voltage.
Hi! I’m actually using a differential probe (ELDITEST GE 8100). It seems to work correctly for every other test I’ve done so far. I connect one lead to Output A of the Odrive and another one to Output B. So I should be correctly measuring floating Line-Line voltages.
To describe a little bit better my setup, I have a test motor connected to a load motor. The load motor sets the load torque (via another controler). The test motor is speed controlled via an Odrive S1.