HOT Stepper on my Pick and Place machine

[rx8pilot]

The Theta (part rotation) stepper motor on My Quad IV-C get very hot and I am trying to understand what is normal.

This is an old machine that has been proven over decades so I doubt there is a fundamental flaw. I have never done any designs with steppers but what little I know is that they do get hot while sitting still. My question is - how do I know how hot?

This motor spends most of its time sitting still with no load. Clearly, the driver circuit does not ease up on the current too much during that time. It seems to work ok but there is a periodic failure that could be related to the motor not turning or losing position. That failure could also be in the control system, but for now I am just trying to eliminate the stepper motor. I have already replaced the motor itself to get a 'second opinion' it gets just as hot as the first one.

In a commercial stepper driver from the 90's, would you expect current or other adjustments to be available? Maybe the driver simply needs to be tuned. The replacement PCB is $1,800 so I am highly motivated to either determine all is well or fix it myself if it is indeed not right.

Hoping someone knows something about old industrial stepper drives. Thanks in advance.

[amyk]

What is "very hot"? Stepper motors can certainly get hot enough to burn skin, and still be OK. I have a few old printers that use them, and they have a cooling fan and warning labels about the heat -- you can see an example of one here:

https://www.orientalmotor.de/media/files/17112005105540.pdf

More info:
http://www.parkermotion.com/dmxreadyv2/faqsmanager/faqsmanager.asp?question=965

[rich]

Some drives of that vintage have 2 current settings (Irun and Ihold).
For example, the micro stepping drivers in my CNC use an external resistor to set Irun.  Then a 2nd resistor sets the current reduction - if its not fitted then Irun == Ihold.
The NEMA 23 motors claim max ambient 40°C, and a max rise of 90°C, so a worse case skin burning peak of 130°C. My setup runs approx. 70-80°C peak (in 20°C ambient).

[rich]

I meant to also ask if the periodic failure still occurs with your second motor fitted, and are the bearings of the first motor still smooth with minimal free play.

[rx8pilot]

What is "very hot"? Stepper motors can certainly get hot enough to burn skin, and still be OK.

I think it is less than skin burning heat. I don't have a thermocouple to measure so I have to go with the 'How long can I put my finger on it' test. About 2-3 seconds without major pain.

I meant to also ask if the periodic failure still occurs with your second motor fitted, and are the bearings of the first motor still smooth with minimal free play.

I have not yet run a batch of boards so I am not sure if there is any difference yet. The original motor seemed to be in good mechanical health. No shaft play, no noises, free movement, etc.

It makes sense that they would run hot, but as I read/learn more I am surprised just how hot (dangerous to touch). It sounds like there is probably no issue at all, just alarming. I will take a look at the drive PCB as close as I can looking for anything related to holding current. I am a little nervous to take the card out of the cage since it is (apparently) working.

[rich]

Assuming a sensible ambient, that sounds unnecessarily warm for a stepper motor which is mostly stationary and even when moving does little work. However it could just be designed without a reduced holding current.

If it doesn't work with motor #2 then you'll need to start measuring things. My vintage CNC, had extra and missing steps caused by noisy power supplies, drooping power supplies, vibration sensitive connectors and interference from the spindle inverter.

Other random thoughts:
If X,Y are driven by steppers what is their operating envelope.
If theta should have current reduction and doesn't anymore, perhaps the driver is also over temp.

[rx8pilot]

So I replaced the Theta motor and placed a couple thousand parts without errors. The motor still gets rather hot. Next step is to pull the driver board and do an inspection.



Sent from my SAMSUNG-SM-G890A using Tapatalk

[cncjerry]

Most of the time it is software configuration not telling the driver to disable the motor when not busy.  You usually don't need the motor windings holding in a low torque requirement application. 

[cobra18t]

I think it is less than skin burning heat. I don't have a thermocouple to measure so I have to go with the 'How long can I put my finger on it' test. About 2-3 seconds without major pain.

Well, it is not the best measurement method in the world, but that sounds like 50-60degC. It is warm, to be sure, but does not seem outside the operating range of a stepper. Most stepper drivers have a potentiometer or DIP switch set somewhere to adjust the current. You could turn it down if it is still a concern.