Slip Ring Motor Issues

So - have these problems shown since the Thryistor drives were changed to VFD ?

Your text suggests that the motors ran ( well ) for 3 years and then 6 months ago you began having failures. Was that the point after which the drives were changed ?

Sorry for the confusion. The drives/motors ran well for three years after the conversion. Periodic Checks on the rotor brush compartment, after time, showed the brushes to be worn, to the point were the spring tension on some of the brushes was weakened. The vfds would occasionally trip on overcurrent @ start of a run command. Inspection of the rotor brush compartment would sometimes show a bit of build up on the rotor surfaces(s) but, after applying a cleaning stone, usu. trip issue would cease. Production has gone up, so periodic maint. has seen its (unfair) share of cuts due to time constraints.

With VFD operating a SR motor, ie with the SRs shorted you can have problems if the SR brushes are arcing (per Tony's link) so the neglected maintenance could well be the cause of your woes.

Using a contactor to do the shorting though adds another variable.....what condition were the contactor's contacts in?

Not depleted, but could be a chronic issue with contact points on shorting contactor. If the motor brushes are shorted in the motor, it would negate the use of resistors to control speed in the event of vfd failure. The crane would have to have a means to lower a ladle of molten steel in this scenario.

You'd have the same concern if a motor shat itself with a bucket of molten steel hanging from the crane not just if its VFD failed.

Did any of the motors fail with a suspended load in transit, or were you lucky and they failed when starting to lift?

Steeler, was the original system a Kramer thyristor drive?

Following Wal's comment, could you not short the rings on the rotor? That way you take out a possible weak link and reduce the maintenance required.
No bush gear = maintenance free.

Was thinking the same but I think that Steeler has it set up so he can changeover from VFD to resistor control "at the flick of a switch", ergo the shorting contactor on the slip rings.

With a hard wired short, the short would have to be removed manually if the VFD shat and reverting to resistor control is needed to lower the bucket of molten steel.

Then again if it is hardwired then a problem is less likely to occur in the first place and all that's needed is a spare VFD. Steeler needs to weigh the risks and benefits.

http://www.lmphotonics.com/slipring.htm

The slip ring set up is o.e.m. design. The motors need to be re-furbed with emphasis on making them more "robust" for the inverter duty application. Sealing is also important to keep conductive contaminants, dusts outside of the stator, and rotor compartments. Changing o.e.m. equipment is not an option, as the original manufacturer performed the upgrade and this could void any warranties, or guarantees. I will double-check all "weak links" in the system, including the brushes for bounce and arcing. I think that is one of the main contributors.. Thanks to all who had input, as this aids in the thought process on corrective, and preventative maintainence. These motors have proven themselves to be true work horses, considering the severe environment. Wifhout regular service, they can fail, hard.

Thanks to Tony S. On the link. Very, very informative.