Runaway Servo Motor

[RobW]

Super Max mill
Model = YCM-40
Date = 2-9-95
Ser# = 508743
Centroid Controls
Model = M-50 INV
Ser# = K5147

I receive the Error: W Axis Runaway Drive after turning on the machine and hitting Home. The W Axis rotates rapidly for a couple degrees. I've verified the cable as good. How can I determine if the encoder is bad, or the drive is at fault?

[cncsnw]

Was anything replaced or disconnected and reconnected, since it was last known to move W without error?

This sounds a little like a polarity mismatch. You could try reversing the W plus and minus motor power leads, on terminals 7 and 8 of the servo drive.

However, more likely it is a failed servo drive (power FET shorted through).