a stable trigger on any waveform?

[DazA1]

Is it possible to get an oscilloscope to trigger on any repeating waveform?

I have had no problem setting my trigger until i started looking at the voltage output of a VFD driving a 3 phase induction motor.

I could see the pwm switching when i stopped the scope but could not seem to get a stable trigger at all when running. I tried HF reject on the trigger setup and adjusted the hold off but the thing would not stay still. I had hoped to view one complete cycle so i could see how the pwm changes on start up.

Any ideas?

[Torrentula]

If you're just interested in the startup behaviour you could just single capture it (then the scope will trigger off the first rising edge).

[w2aew]

Is it possible to get an oscilloscope to trigger on any repeating waveform?

I have had no problem setting my trigger until i started looking at the voltage output of a VFD driving a 3 phase induction motor.

I could see the pwm switching when i stopped the scope but could not seem to get a stable trigger at all when running. I tried HF reject on the trigger setup and adjusted the hold off but the thing would not stay still. I had hoped to view one complete cycle so i could see how the pwm changes on start up.

Any ideas?

Maybe use DC coupling on the channel to remove the "wobble" of the baseline as a start (if that's possible given your operating voltages).
You should be able to use DC coupling on the trigger, with the level you have shown, and the holdoff set to something between 12 to 20ms.

Of course, if you're only interested in the startup behavior, then a single shot capture is the way to go (holdoff doesn't apply).

[DazA1]

Ok next time i will try DC coupling on the channel and trigger.

w2aew Like your youtube videos btw. Especially the transmission line termination video i learned a lot.

I think i am right in saying that the phenomenon of vfd reflected voltage is similar. In the previous image i posted the pwm signals (at the vfd) that look perfectly normal but the fast rise times of the switching IGBT's cause problems when the motor cable length is above a certain length (300M in this case). I have read up on this and they say that an impedance mismatch between the long cable and the motor, this is rectified by installing reactors. See the images below and notice the high voltage peaks at the motor, higher than the voltage rating of the motor windings (400v rms).

[DazA1]

And here is the plot of the csv file i saved.

The black trace is at the vfd and the yellow is 300 Meters away at the motor.