# Servo Motor



## drsparky (Nov 13, 2008)

DC servo needs a feedback signal and normal use a square wave for control of position. You may need to view the output on a scope meter or an oscilloscope. A normal millimeter will not give you a true picture of the output.


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## 5volts (Jan 11, 2008)

lectricboy said:


> I went on a service call at our local museum. They have a turntable display wih a 90 VDC servo motor. It is controlled by a Dayton DC power supply/speed control. I checked the output voltage from the controller and got 106-107 VDC. Is this too much voltage for this motor? Does the 10% rule apply to DC motors? The motor is making a terrble noise and won't run. Is it the motor or the controller? Please forgive me, I just don't work on DC motors very often.


Just wondering can you turn the shaft by hand freely or does it have a brake? If it has a brake is it releasing? Not sure what type of meter you used to measure the output but as mentioned a regular meter will not be accurate. 

If the shaft does turn freely you can try shorting out the servo motor leads with a jumper at the pothead/connector and then try to turn the shaft to see if the internal permanent magnets are OK. It should feel locked when shorted.

Its possible the drive could be missing pulses because of a faulty cable/encoder/feedback or drive output circuit. But in my experience with servo motors I have found either the brake is not releasing when it should be or the feedback cable is bad or there is a mechanical bind somewhere. Haven't seen many bad servo motors or servo drives. But most of our stuff is PLC controlled and the software and drives have a lot of diagnostics for troubleshooting. Get back to us and let us know what you found, good luck:thumbsup:


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## John Valdes (May 17, 2007)

drsparky said:


> DC servo needs a feedback signal and normal use a square wave for control of position. You may need to view the output on a scope meter or an oscilloscope. A normal millimeter will not give you a true picture of the output.


Doc, I think he has a 90 volt permanent magnet DC motor, not a servo motor. He did not mention any feedback.

OP. The small power supply you speak of is most likely a mini DC drive. Does it have a pot on it to adjust the speed? Heres how to set it up correctly.

Put your meter leads on the connections at the motor or control. (you have only 2 wires, right?) A1 & A2?

Turn speed pot fully clockwise (full speed) and check voltage. It should be at 90 VDC. If not adjust pot (Max) until you achieve 90 VDC. 
Now turn the speed pot all the way CCW until the motor stops. Now turn the (Min) pot CW just until it tries to turn. Back off just a tiny bit and your set.
10% over speed is fine. The true way to test speed is with voltage. If the motor does a good job at 90 volts leave it at 90. If you need a little more speed just adjust the (max) pot, not to exceed 10% of full voltage.


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