We have a strange issue with a 1FK7061-4CH71-1BA0 servo motor locking up.  The strange part about it is that it is during commissioning when there is no power on the machine.

This particular motor is one of many on a machine, all of the same part number.  It is connected to an inline gearbox (nothing connected to gearbox output shaft) which rotates freely but when you connect the power cable to the motor, a 6FX8012-5DN06-1BA0, the motor shaft locks up and cannot be rotated.  Disconnect the power cable again and the shaft can be moved.
If you connect another cable from a different motor (but still a 6FX8012-5DN06-1BA0) it may or may not lock the shaft; some do and some don't!!

I might have expected this with a braked motor where the power cable is connected at both ends however this is NOT a braked motor and the drive end of the cable is disconnected laying on the floor...  There isn't even any power on the machine!!

Even more strangely, the problem has disappeared this morning; power cable connected or not the shaft rotates freely.

This is not the first time we've seen this.  About six months ago we had a motor like this but everything worked fine when the machine was powered up, commissioned and put in to service.  We've had no issues since...

Not really a problem (yet) but more interested as to why this is occuring and whether anyone has encountered this?

Marko Bursic

It's not locked, it just acts as a generator. When you turn the shaft, the energy fills the bank capacitors. 

 I assume you are referring to the capacitors in the drive?  If so, this cannot be the case as the leads are not connected to the servo - they are unconnected laying on the floor.  The only connection to the power cable is at the motor end.

Also, if this was the case why is only one motor doing this and not all of them.

In order for the motor to act as a generator you have to be able to turn the shaft which I cannot...therefore this is unlikely.