DC field slowly shorting?

[lrob]

I have a 250 HP dc motor that seems to slowly short the shunt field. The motor is a shunt, series motor. The shunt field had similar problem two years ago. I am looking at application speed. I was also considering if there is something in the Drive that could be out of adjustment and slowly cooking the field. I am considering having a tech. in to check out the drive. I was looking for feedback as to what is most common problem that might cause my situation?

 

[Skogsgurra]

You should absolutely have a drive technician check your drive and motor. Sounds like your field rectifier is outputting too high a voltage or that you are stopping the motor without switching off the field supply and without having a separate cooling fan for those cases.

Does the motor get hot at standstill? Does it smell burnt or hot? Turn-to-turn shorts can develop over time if the field winding gets hotter than normal. You need a "drive doctor". Soon!

 

[lrob]

We have a separate cooling blower. I do not believe the motor was getting hot at standstill. It did smell burnt. We currently have the motor removed from the drive for repair when it is returned. I will check the field voltage in the stopped mode. I think I will also bring in a drive tech. to give the drive a checkout and see if I am missing something. Thanks for the advice.

 

[mc5w]

I had an instance where copper oxide buildup on the outside of the field supply fuses caused the field current to go down to 80% of normal. I went and pulled the fuses and cleaned them up with #220 or #240 silicon carbide abrasive paper per the recommendations of the Consumer Product Safety Commission. Go to

http://www.inspect-ny.com

and look up aluminum wiring issues.

I reinstalled the fuses and now the field current was 125% or 130% of nameplate. Had to readjust the field resistor. It was really amzing what a little bit of copper oxide was doing to the circuit! Somebody else just kept adjusting the field resistor to compensate for the copper oxide instead of cleaning it out. A bit like running with a pebble in your shoe!

Mike Cole, mc5w@earthlink.net