More Stepper Motor Woes

Hi Guys

I have a project to control 3 small stepper motors and I am having difficulty getting them to behave. I have used larger NEMA34 motors before and had teething problems with them but ended up with a working solution. The small step motors I need to control are CKD brand, 3.75 degree, 24V, 60 ohm, model number J262-859. I have checked their website but cant find ANY info. These motors are about 40mm dia and 12mm thick. I think they are called 'can' type.

I am trying to control them with Compumotor OEM-750 drives and am using the following settings.

motors set at bipolar half winding ie I'm using one end and the centre tap of each phase

supply voltage 75V (Compumotor OEM-300)

10uStep (set as 2000 steps per rev equiv for a 200 step motor, these are 90 step ie 3.75 degrees)
current gain of 32 (default is 8 for these drives, 32 makes it turn)
working in the 0.2A to 0.7A range seems to work OKish at about 0.5A
standby set to 25%

I can get them to turn but they get hot within a few minutes (I cant hold them for more than 2 seconds) so I turn the drive off and cause my brain more pain by trying to work out why.

Any ideas on troubleshooting would be most appreciated.

derekj308

Do you have the coils hooked up to the proper terminals? If you criss-crossed the coils the current limit would have no effect.

Hi H500
I believe I do have them hooked up correctly as I can get them to turn. The motors are 6 wire unipolar and I am running them as series bipolar. When I swap the leads of one phase the motor doesn't turn. I have identified each coil set and the centre tap. Would you explain "criss-crossed the coils" in case I have missed your point. I suspect that the motors have an inductance that is incompatible with the OEM750's but I have no way of testing the inductance of the motors. The manual for the OEM750 says it will do between 0.2mH and 80mH but suggest it works best with inductance between 1 and 10mH.

The way in which I get them to turn is to pump up the current gain but all that does is cause excessive overheating under no load conditions.

I'm not hell bent on using these drives or running the motors in bipolar configuration. I would use a unipolar driver if I was certain that the motors would operate correctly. I dont want to just go and buy a unipolar driver to find out that they don't work with these motors either.

Thanks for the reply.

Cheers
derekj308

You say current is set to .5 amp, this is about 25% high, should be .4 amp (24v/60ohm). Since power dissipation(ie heat) is proportional to square of current, that is about 50% more power. Also, have you measured the temperature. There is no telling what the actual temperature is from your touch test, but probably about 50 or 60 C. Many motors can run hotter than that without problem. Also, many small steppers like those used in disk drives or printers, are designed to be mounted to a relatively large metal frame which also serves as a heat sink. As a test, you might try connecting the coils to a 24v dc supply and just to see how hot it gets.

Derek, by criss-cross, I mean connect the motor phase1- to the controller phase2- terminal.

If it was proper, reversing the +/- on one phase would just make the motor spin backwards. Since yours don't turn.....

Hi Guys

Thanks for all the replies. You'll be happy to know I did get it going eventually. I don't have any specs for the motor other than shown above so I incorrectly assumed it could do 1000 rpm just like my NEMA34 200 step steppers. Not so. It seems to get grumpy over 400rpm. At low rpm it has excellent torque for its size, even run as bipolar using just two of the 4 coils. Even at 400rpm it doesn't have anywhere near the torque as at low rpm. I haven't experimented with running bipolar across all 4 coils and getting it to work so when I do I'll add to this post the results. H500, I did do some experimenting with swapping the leads around and found that there was no noticable difference in performance as long as I used 2 specific coils of the 4 coils together. As you said, swapping on one phase did reverse the direction but there was no performance difference that I could notice by making the shaft stop by hand (it has a gear so I can get enough friction to make it stall). I did notice a drop in torque when I changed the coil to use the terminal at the opposite side of the centre tap ie if I was using coils 2 and 4, I was now using coils 2 and 3. I'll be checking my wiring on my NEMA34's to see if there is more torque to be gained. I think that when I mentioned that I couldn't get them to turn when I swapped the lead on one phase I was probably only just getting them to turn and was trying to run them much faster than they would allow and that swapping the leads when I had two adjacent coils wired made enough of a difference to stop them turning. Thanks jeffs555 I did reduce the current to 0.4A and they are hot but they are an acceptable hot. In the end I was able to run them at 50uSteps (about 30khz out of Mach2) and they are smooooth as. Not expecting any extra resolution since the mechanism I am driving has a backlash of about 0.05mm. Not for long.

BTW we are hot rodding a Roland Modela Player MDX-20. The software is very average and the contoller it comes with doesn't run by pulses to the drives. Once we're finished with this baby we'll be sending constant load toolpaths to it courtesy of Surfcam Velocity. Oh yeah!

Cheers
derekj308

24v is very high for a stepper, so the poor performance is fully expected.

"24v is very high for a stepper, so the poor performance is fully expected."

An old thread but for any newbies or new readers, the above is complete rubbish, I consider 24V to be a low voltage to run a stepper, those on my large CNC run at 76.
Higher voltages are exactly what is required to overcome mediocre performance. Motor power is directly proportional to the supply voltage divided by the square of the inductance of the coils. Power output increases with higher voltage throughout the rev range.
What is more the 24V quoted is the badge voltage on these drives, as per the OPs original post.