PowerPro Anomaly

[JPG]

If I were monitoring for over current conditions with a stepping motor, I would be observing current to the motor coils. Not something we can easily do here.

P.S. your current measurements taken using what device, and at what input voltage?

I was monitoring both the AC in and one of the motor drives. AC Amps in on 120 VAC with a DMM with clamp-on ammeter. Motor drive on an oscilloscope with a clamp on amp probe. The drive current is pulsed, pulse waveform varies with speed, I was using pulse amplitude and measured ~3.6 Amps at the trip point, independent of speed. Current on both AC In and motor drive increase uniformly in response to mechanical load until they trip. There are six current drive wires to the motor, for now I'm assuming that maps to three phases of current drive with independent returns. I have just one current probe for the scope. There appear to be eight current drive pulses per revolution per line.

Note - I was quoting Jim (McCann, likely) in that statement regarding AC Current Limiting protection, from the last part of this: https://www.shopsmith.com/mediawiki/ind ... em_Solving The other sources mentioned overload protection, which may or may not be the same thing. There could also be multiple overload protections.

- David

Thank You.

I would think pulse frequency would vary with rpm. rather then fixed 8 p/rpm.???

[DLB]

Thank You.

I would think pulse frequency would vary with rpm. rather then fixed 8 p/rpm.???

I had to do some recalculating to answer. Commanded and measured RPM on the PP display are upper spindle RPM, not motor RPM. IIRC I calculated the ratio 5:8 Motor:Spindle based on spindle diameter. Then in reality 'measured' 3:5 based on synchronizing the drive pulses with AC line, which occurred very near 6000 RPM commanded. And I presume to = 3600 motor RPM.(?) In that specific case there were 8 drive pulses per cycle AC. And therefore, 3:5 ratio between actual motor RPM and commanded RPM. The small difference in ratios may (or not) relate to what Dennis referred to as 'pitch diameter' in another thread. I don't have the right kind of tach to actually measure.

At commanded RPM = 6000, there were 8 pulses in 16.7 mSec. At commanded RPM = 600, there were 8 pulses in 167 mSec. I believe these are 3600 and 360 motor RPM, respectively. Conclusion: Fixed at 8 pulses per revolution. Pulse frequency varies with respect to time, but maintains 8 / rev.

- David

[JPG]

Makes sense, thank you.

[edma194]

Conclusion: Fixed at 8 pulses per revolution. Pulse frequency varies with respect to time, but maintains 8 / rev.

Did you mean "pulse width varies with RPM, but maintains 8/rev." ? Or perhaps varying pulse current?

[JPG]

Now you got me wondering(again).

[edma194]

I understand what he's saying, just trying to find a way to make sense to me. The frequency does vary by RPM or time, but there's clearly more to it than that. Or is there not?

[JPG]

I thought I understood as well. It being a stepping motor requires us to exit the analog world and think regarding pulses of equal magnitude and possibly equal duration.Pulse width modulation may not be being used here.

Not intuitively obvious is as the frequency of those 8 pulses per revolution is varied, the rpm WILL also vary. A stepping motor MUST maintain geometric synchronization with the input pulses or it will not rotate at a fixed rate(stall/chatter...) I assume pulse width/amplitude modulation is to deliver more torque when the rpms are lagging due to load and vice versa.

Tis a balancing act fer sure.

[DLB]

At any 'fixed' motor speed, the primary response of the controller to increased load is increased current pulse amplitude. The waveforms are imperfect, I showed that in another thread: viewtopic.php?p=286136#p286136 Without better synchronization between scope and motor it would be difficult (impossible?) to determine if there are any other changes to the waveform. I don't think PWM is being used. Amplitude modulation is definitely being used. Frequency or phase modulation is what I'm least sure of.

I did note in the post I referred back to (last part) that there was one pulse in the string of eight that was different than the rest. I don't have an explanation for that. It could be inferred that some unusual form of PWM is being used but not as a primary response to load.

The system always exhibits some RPM jitter, even with no load external to the motor. So when I say 'fixed' speed I mean within that jitter range. IMO the controller is constantly measuring and correcting RPM via a feedback loop that includes software. And as I said, primarily via current amplitude modulation.

- David

[JPG]

I am guessing that the controller actually controls pulse voltage which results in the current differences you are observing.

That voltage control includes waveform.