Confused about brushed DC motor controller - Page 1
Forum section: electronics / general

author: eTimesV
author_rank: Contributor
timestamp: July 10, 2024, 10:55:40 pm
content: hi,While I don't really need to know how this motor and its controller work since, it works, it's bugging me not understanding how it works. So...I got it used. It's probably 20-25yrs old.I want to use it for a fixed-post style machine to grind telescope mirrors, in case anyone's wondering (similar to watch?v=PSGVzPTocWE (leaving off the youtube.com part since I don't want the large youtube thumbnail here and don't see any other way to turn it off)Infos:Groschopp Motor (probably fromhttps://www.groschopp.de/en/motors)Mot PM1 85-40160V0.95A110W3000/minIsol. Kl. BNr. 7832309WK 1054502IP 54Controller:PMGN942K MotorreglerThe controller had some some more stuff attached when I got it: a PID (that had cut-off thermocouple wires coming out of it) and a relay, presumably the PID turned this controller/motor off and on based on a temperature reading. I removed those since they weren't relevant for my use case.Some pics:(I 3D printed the red case based onhttps://www.thingiverse.com/thing:2938921)I believe, as you can see in the pictures, it is a simple brushed DC motor.There are three wires coming out of it: blue, brown and green-yellow. The green-yellow is ground and is electrically connected to the housing of the motor (I tested for continuity).I did hook up a cheap Aliexpress PWM board I had and was able to spin the motor. However my PWM was only rated for maybe 36V max and I figured if I used that instead of the original controller that I'd be missing out on a lot of torque since the motor says it's rated at 160V. Oh, I think I also just hooked up a 12V battery and got it spinning too.I thought the old controller was doing some kind of PWM, or I suppose it could just be changing the DC voltage (although I think that's not a great torque-preserving strategy), but when I hooked it up to an oscilloscope I didn't see anything like I expected.A potentiometer connected to the controller adjusts the motor speed.Here is a video showing the set up and the oscilloscope screen while changing the speed.In the first part I have the leads connected between the yellow-green ground wire and the brown wire going to the motor and in the second part I have the ground connected to the brown motor wire and the probe connected to the blue motor wire.In the first case, the signal does not appear to change one bit as I change the speed.In the second case, the rather odd-shaped signal grows in voltage as I increase the speed.Some questions...1. What kind of control signal is that?2. Is there a name for it? And/or is it some strategy pre PWM of giving pulses of high voltage to get the most out of a brushed motor?3. Why does the pulse look oddly shaped (I think)...or did I connect the oscilloscope wrong or incorrectly set the settings?4. It's giving the 160V motor just over 200V. I guess that's similar to driving steppers, that you give them way more voltage than they're rated at?5. Why does the measurement of GND-to-brown wire show about 350V peak to peak? And did I just damage my oscilloscope which is rated to 300V per channel? (I did use my battery-powered scope rather than my mains-powered one to avoid blowing up my scope by making a grounding mistake since I'm not too sure about oscopes and high AC voltages).6. Are there any PWM DC motor controllers for this 160V motor? I didn't find any, at least not any hobbyist ones. I did't look into the much more expensive industrial products. I would really like to control the motor speed via a microcontroller. From what I read about spoofing a hardware potentiometer with electronics it wasn't completely straightforwards for this case, but I didn't research it in depth yet.

author: eTimesV
author_rank: Contributor
timestamp: July 23, 2024, 08:57:26 pm
content: ok, this is clearly the wrong forum, or, there is something wrong with my question (if so let me know maybe I can address it).Does anyone have a suggestion for a better forum to ask this question? Maybe practicalmachinist or cnczone or ...?

author: Benta
author_rank: Super Contributor
timestamp: July 23, 2024, 09:24:58 pm
content: You're on the right forum, but in the wrong section.Try moving to "Beginners".I may be reading your scope wrong, but you seem to be AC coupling the measurement. That makes very little sense for a DC motor.And a safety hint: green/yellow connected directly to the metal of something is called "protective earth" or PE. "GND", "ground" etc. can be anything. Measuring between PE and one of the terminals will show nothing sensible.Showing videos is OK, but pics taken at salient points are better for analyzing the signal and scope settings.Cheers.

author: eTimesV
author_rank: Contributor
timestamp: July 24, 2024, 09:53:19 am
content: thanks a lot for the reply and tips.I'm a little confused then about the PE comment. Don't I have to clamp the probe ground to something? I was thinking the green/yellow wire was the circuit ground since it was the ground wire coming from the wall outlet.I did connect the probe across the two "power" lines to the DC motor in the 2nd part of the video, as noted.And shouldn't AC coupling still show the changeable part of the signal, removing any DC offset?I'll have to see if I can find time to do the test again with DC coupling.

author: Benta
author_rank: Super Contributor
timestamp: July 24, 2024, 09:12:09 pm
content: Stop diddling around more before you kill yourself. Seriously.There's an excellent thread on how to use scopes as sticky in the Beginners section.