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Economist
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Servo Getting Hot

10/03/2012 8:30 PM

Servo motor getting hot. Tuned all the control parameters manually but still keeps getting hot. Any suggestions?

Gets hot even on idle. Idle voltage floating around 0 +/-110mV and current around 0.2A.

Servo not oscillating. You can barely hear it.

Low inductance motor.

Your input is appreciated

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Epke
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#1

Re: Servo Getting Hot

10/03/2012 8:40 PM

is it a new or old motor? did you check the signals(pulses) to the motor?

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bigg
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#2

Re: Servo Getting Hot

10/03/2012 10:20 PM

How hot? In my limited exposure to servo motors, they seemed to run hot but didn't burn out. I think they run hotter than induction motors we are used to. The ones I've seen don't have external fans either. On the other hand, 0.1 V x 0.2 A = 0.02 W. That's not enough power to heat much of anything. Are you sure you're measuring this right? Maybe you're not allowing time for the built-up heat to dissipate?
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Just an Engineer
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#3

Re: Servo Getting Hot

10/03/2012 11:09 PM

Check your measurements of voltage and current again, but check both AC and DC. You might see almost no current in the AC range while there could be a steady state DC in the background, or vice versa.

The heating potential in the coil (while not operating) as previously pointed out is observed current x observed voltage across the coil.

I've seen coils get "warm" when on poorly regulated DC supplies and the AC ripple was the source.

the other possibility that I'm certain you have already ruled out is potential heat transfer from another item (Like transmitted through the shaft).

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#4
In reply to #3

Re: Servo Getting Hot

10/03/2012 11:41 PM

I am now thinking that there is a ripple AC current that is causing the excessive heating. The pwm frequency is fixed at 20 khz and there is no way to increase it. The voltage at idle is around around 0, but the ac current may be too high for that short on time. How to measure this? I can't see it on a multimeter.

A way to reduce the ripple current is to use an inductor in series. How to properly size it? I am researching this now.

Btw, i like your handle

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Epke
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#5
In reply to #4

Re: Servo Getting Hot

10/04/2012 1:35 AM

use an oscilloscope? what kind of voltage does the servo use?

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#6
In reply to #4

Re: Servo Getting Hot

10/04/2012 11:00 PM

Your question has the beginning of an answer in it. A multimeter is insufficient to handle the situation. You need an oscilloscope, 1 MHz bandwith sufficient to see it all. Dirt cheap on eBay.

To illustrate, I assume bipolar drive (not bipolar transistors, but chopped drive from both + and - power sources). If you drive sequence is 0,+,-,0,+,-,0,0,0, as an example, the multimeter measures the average of sorts, that is nominally zero. And miss all the action taking place.

Whatever your situation exactly is, get your instrumentation fit at least the minimum requirements. THEN come back, if needed at all.
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Economist
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#8
In reply to #6

Re: Servo Getting Hot

10/09/2012 12:34 PM

the problem was resolved using an inductor in series.. Turns out the ripple current was too high (~5.5A of 11ms and was enough to heat the motor).. I burnt the 3A fuse on a multimeter trying to measure the current

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bigg
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#9
In reply to #8

Re: Servo Getting Hot

10/10/2012 9:59 AM

Thanks for the feedback. Taught me something.
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mvzing
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#7

Re: Servo Getting Hot

10/05/2012 6:40 PM

How hot is the servo?? i send you this link:

"The internal temperature of a motor producing rated torque, operating at or below an ambient temperature of 40 degrees Celsius, and mounted to the recommended heat sink should not normally exceed this 155 degrees Celsius limit." Yaskawa inc. Sigma V

http://yaskawa.com/site/webfaqmotion.nsf/99395dc89505d18b86256eaf00620582/86256ed000746b6a8625777c006639d2!OpenDocument
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