AC Stepper motor problem


I found this a bit strange stepper motor from mixing valve controller. Google wasn't really helpful with this but i assume it's AC-driven 24V stepper.

Lately it's started to behave weirdly, it's stepping way faster than it was originally decade ago and seems to be down on torque and missing steps too.

I wonder if the problem is in driver circuit or in motor itself? What kind of effect that capacitor have if it's dried and down on capacitance?

So the question is do you think that motor itself can cause that sort of problem or is it definitely the driver? And yes i know it's not really arduino related question, but there's arduino piggyback controlling the motor controller so it's kind of related :slight_smile:

Its a synchronous motor, not a stepper. The run capacitor probably needs replacing, perhaps you can check its capacitance value with a multimeter?

Thanks for correcting, i knew its not exactly stepper but didn’t know how i should call motor that runs with steps :slight_smile: Ill try to check it with dmm soon, but i afraid the controller is somehow busted. Yesterday it went sometimes in state it kind of tries to drive the motor in both direction simultaneously judged by buzzing sound from motor and the indicator leds…

Can you trace out the circuit?

Yes, i guess i have to print these and sketch on paper before i fully understand what's happening.

More testing revealed everything is working until the motor is asked to open the valve, closing is not a problem. And it doesn't matter if the motor is connected or not. Times it doesn't work it lights both of the leds (the left is indicator for closing and right one is opening) and there is ~15V ac in opening and closing pins of the motor connectors (the square ones on the side) instead of 22V just in two pins as it should.

There's the board straight from early nineties in case of someone wants to torture his head with this :slight_smile:

Just replace the capacitor. Looks like someone has already been mucking about with a soldering iron. The motor changes direction because there is a cam rotating which makes the two microswitches reverse the motor connection.


Well, i think the whole shit might be actually soldered by hand, but im fairly sure no-one have touched it with iron since. It's from the era when it was profitable to actually produce something in Finland too :slight_smile:

I don't say the cap is necessarily good, but replacing it doesn't fix the problem in controller as it doesn't work even without the motor now.. And those micros are just for end stops. Motor itself seems to work ok and turns also to open direction if i plug it in connector for reversed operation, but ofcourse the closing direction doesn't work in that case.

That board is probably wave soldered, not hand soldered. Wave soldering is a common way of doing through hole.

If something breaks, the first suspect would be those electrolytic capacitors. Those tend to leak after some years. I do see a suspect brown stain on the PCB on the left bottom corner in the image, near the blue "era" block. One of the caps also has a brown stain. I guess that blue thing is part of the power supply (transformer?), making those caps likely related to the power supply. Unstable power can result in all kinds of weird behaviour in an electric circuit.

Other than that I don't see anything suspicious on the board.

Fixing a PCB when you don't know in detail how it's supposed to work, or what could possibly be wrong, is often a dead end. If those capacitors are not the problem I'm afraid it's simply due for replacement.

Yes, only the top board is indeed soldered by hand. Good spot with that cap, i didn't notice that stain before i looked my own pictures..

We replaced all the electrolytic caps on the board and now it's going strong again. Also the logic how the motor is driven seems to be back in normal now. Capacitances were all over the place. Kind of a miracle it even worked this far :slight_smile:

So thanks for the tips everyone. Changing the controller for smarter one would have been wise, but in other hand then my esp8266 project converting dumb controller to smart would have left unemployed :smiley:

Those caps are probably smoothing caps for the power supply, values are normally not critical, and you only got problems when one (or more) totally broke down.

Anyway, good to hear it works again.