Episode #47: How To Troubleshoot Servo Motors

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The Motion Control Show

Now that we understand the basics of servo motors, their thermal properties, and their speed/torque curves after the last three episodes and discussing it, now we get to get into some of the fun part of really figuring out where the rubber meets the road and how the problems arise.  I'm Corey Foster at Valin Corporation.  Reach out to us with the email address and website here.  I hope this helps.

The performance questions we're going to be talking about today are:
  • Why is my motor overheating?
  • Why doesn’t my motor get as much speed as it should?
  • Why doesn’t my motor get as much torque as it should?
  • Why did my motor burn up when it's only supposed to be operating in the continuous region of the curve?

But at first there are definitely some steps you need to take in order to make sure some of the more easy-to-resolve issues are taken care of first:
  • Check your program. Are you actually commanding the speed that you think it should be?
  • Check the drive configuration because there are some drives that will actually cap the torque output to the continuous torque to keep you from accidentally burning up the motor during the tuning, initial programming and configuration.
  • Double, triple and quadruple-check your sizing. What assumptions did you make that perhaps are not accurate?  Double-check those.  And triple.  And quadruple-check them.

So why is your motor overheating?  Remember the assumptions here that I talked about in previous episodes.  Your motor’s overheating because you're putting too much power in and it's not able to dump all that power out.  Typically, that is going to be when you're over the continuous region that's above this line here.  But now you may think that you're below this line, that continuous line.  However, how is the motor getting rid of the heat?  What is it mounted to?  How much airflow does it get?  What is the ambient temperature?  Maybe the ambient temperature that you're starting at is higher than what you assume.  If you were starting a motor at 20C, that's room temperature, and the motor is going to burn up at 110, that's 90 degrees differential that you can heat the motor up before it burns up.  But if it's at the ambient temperature of 40C, then you only have a temperature rise of 70 degrees before it burns up.  So, that's less power you can put into it.  Gearboxes are not the greatest heatsinks.  The motor mounted to a big metal plate is a much better heat sink than a gearbox.  So those are some different things that have to be looked at in order to understand why it's overheating.  Is the power requirement going in more than you are expecting it to be?  Again, it goes back to the sizing.

Why doesn’t my motor get as much speed as it should?  One of the more typical answers here is the bus voltage.  You may size a motor by looking at this 240 VAC curve out here, but then you're only using one phase of that three-phase power and you're only using 208.  Well, that's this curve here.  So, you're not going to get beyond this curve to get to the higher voltage.  So, are you using the correct voltage?  The other thing to look at here is this slant.  If you are taking more torque than you think and you're hitting into this line here, this is below 4000 RPM if it takes that much torque to get there.  But, maybe you were hoping to get to 4500 RPM.  Don't forget that you need some torque in order to get there and the torque reduces at the higher speeds, so that is something to be aware of.  But it's often tied to the voltage.  People will size for 240 VAC, but then maybe they only use 110 VAC and that cuts your speed in half right there.

Why doesn’t my motor get as much torque as it should?  I mentioned this before, but sometimes drives are actually undersized for the motor.  Let's say a motor gets maximum torque at 2 amps, but the drive itself only puts out 1 amp.  That means you're going to get a reduced amount of torque because it doesn't have as much current available to power the motor as it could. 

One of my favorites is: Why did my motor burn up when it's only operating in the continuous region of the curve?  There's so many things going on here.  Why do you think it's only supposed to be operating in the continuous part of curve?  Did you check your sizing and all that?  Have you looked at any mechanical jams to make sure that the motor isn't working harder than it supposed to be?  Or mechanical stiction or spots where the motor might be running over or against a rough spot in the mechanics? 

I had one time where a customer was so sure that they didn't have any mechanical problems.  I went out there and watched it and it turned out that sure enough there was a mechanical jam.  But it only really showed up as a problem when it hit that mechanical jam and then the operators went off to their break.  By the time the operators came back, the motor had had a chance to heat up over time because it was sitting there working hard as it heated up over their break.  When they came back and then it would fault out.  It would either fault out just before the break ended or just after they got back when they tried to make the next move.  I had to prove to the customer that they were having a mechanical problem.  There wasn't a motor performance issue. 

Now what's particularly interesting about this is that this happens every once in a while when there's operators who have the ability to reset the drive and will reset the system in order to recover from a mechanical jam.  If you go back to how I explained the thermal protections in the last episode, there is a time delay from when the motor gets hot to when a thermistor or thermal switch can capture that temperature increase.  Usually the primary protection for the motor is the drive calculating that temperature increase.  The problem is when the operator resets the drive to recover from the fault, that drive then starts back over at its original assumption of what the ambient temperature is, 20C, room temperature, for example, maybe 30C.  But maybe the motor has gotten hot up over 50 or 60 degrees.  If they reset it quickly enough over and over again, the motor can go from 50 degrees to 60 degrees to 80 degrees to 100 degrees over time.  But in that same time frame, the heat transfer hasn't gotten through to the thermal switch or thermistor yet for that protection to kick in.  It has to happen pretty quickly.  But I have definitely seen that happen several different times where the motor gets hot and burns up before the thermal switch catches it.

I'm Corey Foster at Valin Corporation.  If you need any help or assistance, reach out to us at this email address and website here.  Follow our #motioncontrolshow.  I hope this helps.

If you have any questions or are just looking for some help, we're happy to discuss your application with you.  Reach out to us at (855) 737-4716 or fill out our online form.