Servo Motor Troubleshooting

dronai · Oct 11, 2014

CNC machine: Servo went out, and was replaced. Still having problems, encoder cable, and drive replaced. Motor runs and faults. Amps go from 8 to 26 on the fault. We thought mechanical, and binding, but removed and tested, still showed erratic current at some point, and faulting. Swapped drives, and motors with the Y axes. Problem stays at X. Swapped cable connections at the board, and both X and Y work. But the problem didn't swap ? WTH?

JRB · Oct 11, 2014

Had the same problem once. After replacing nearly everything, we found corrosion in multi-pin power cable connector?

Mike_RH · Oct 11, 2014

What is the make / model of the drive?
Do you get a fault / error code ?
What is it?
Does the fault occur at the same physical position on the axis?

Steve Bailey · Oct 11, 2014

The spike in current is caused by a change in load. The obvious thing to look for is mechanical binding. Increased load would also be due to sudden acceleration at that point. If there is a problem with the encoder signal at that point, the drive may think it is out of position and be trying to correct. A problem with the encoder signal could be in the encoder itself or the cable. If the motor moves the flex in the encoder cable at that point could be causing the problem.

dronai · Oct 11, 2014

JRB said:
Had the same problem once. After replacing nearly everything, we found corrosion in multi-pin power cable connector?

Thanks, will check

dronai · Oct 11, 2014

Mike_RH said:
What is the make / model of the drive?
Do you get a fault / error code ?
What is it?
Does the fault occur at the same physical position on the axis?

Everything is very old Allen Bradley
No fault displays back then
No on the position.

Steve Bailey said:
The spike in current is caused by a change in load. The obvious thing to look for is mechanical binding. Increased load would also be due to sudden acceleration at that point. If there is a problem with the encoder signal at that point, the drive may think it is out of position and be trying to correct. A problem with the encoder signal could be in the encoder itself or the cable. If the motor moves the flex in the encoder cable at that point could be causing the problem.

Mechanical binding has been ruled out and proven
Thanks for your input
I need my employer to chime in here.

rpoet · Oct 11, 2014

How fast does the drive fault; is it immediately? Are you certain that the 3 power wires to the motor are phased properly? Are the A and B signals on the encoder swapped? Both of these things will cause the drive to fault; it will be fighting itself and give up.

-rpoet

dronai · Oct 12, 2014

rpoet said:
How fast does the drive fault; is it immediately? Are you certain that the 3 power wires to the motor are phased properly? Are the A and B signals on the encoder swapped? Both of these things will cause the drive to fault; it will be fighting itself and give up.

-rpoet

Fault occurs a different times, sometimes right away.
Phasing is correct.
A & B signals, the guys I'm working with know their stuff.
When the cables were swapped, everything ran as it should. That proves mechanical and drives are good. The only issue is why the problem didn't follow the bad cable ? That leaves the multi-pin power cable connection as a good possibility.

rpoet · Oct 12, 2014

dronai said:
Fault occurs a different times, sometimes right away.
Phasing is correct.
A & B signals, the guys I'm working with know their stuff.
When the cables were swapped, everything ran as it should. That proves mechanical and drives are good. The only issue is why the problem didn't follow the bad cable ? That leaves the multi-pin power cable connection as a good possibility.

It does sound like the cable/connector is the next thing to check. Intermittent faults can be a pain to track down and fix; you have my best wishes.

-rpoet

Servo Motor Troubleshooting

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