Slip ring induction motor fault

What would the current curve of the stator of a slip ring induction motor look like if the resistive load of the rotor suddenly rose
towards infinity?
I imagine it would behave like the primary current of a transformer with no load on the secondary and drop to a low level.
I have a weird problem with a slip ring induction motor turning a 100 ton converter. The thermal overload is set to 100A and sometimes the motor behaves normally and rotates the converter drawing around 60A on each phase. Increasingly often, in fact about 99% of the time now, the converter turns very slowly and sometimes even stops completely. The stator current shoots up to 160A per phase and the thermal overload trips. A mechanical overload in the converter or gearbox seems unlikely as the gearbox is also driven by a separate low speed induction motor on a separate axle and this behaves normally, drawing 25A on each phase when rotating the converter.
I think a bearing fault on the main motor is most likely, but I am also interested in the possible effects of a fault in the external resistive load on the rotor, which consists of four resistors shorted sequentionally by contactors as the converter gathers momentum.
Of course the best solution would be to rip out this ancient crap and replace it with an inverter driven induction motor.
Svenne
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If all *three* phases of the resistor load opened, then you're right it would drop the primary current pretty low.
But if only one phase of them opens then the current would rise in the other phases and trip the overload. You don't mention if it happens during the starting sequence or after its up to speed, but a faulty contactor on one of the steps of the starting resistor is possible. So as the controller steps through the sequence, at some point the rotor gets 'single-phased' and the stator current will rise. I'd check all the start-sequence contactors (including the full-run contactor) on the resistor bank, and the connections between the contactors and the resistor itself.
For a bearing fault, you should be able to tell pretty easy when 'jogging' the equipment with the low speed motor (or at least hear some grinding if the area is quiet enough to listen for it). A momentary 'catch' in the bearing caused by something like a broken ball or roller (assuming it isn't a film-lubricated bearing) might momentarily increase the load, but a large inertia such as your 'converter' would just carry through the rotation and further mangle that particular ball/roller. By now, the bearing would be in shambles and screeching like h___. If it is a film-lubricated bearing, then a lack of oil would make it run very hot (seen them actually glowing red on large machinery).
Of course one of the obvious questions to ask is, "Has any maintenance been done or changes?" A change in shaft alignment is another thing that can make drive current rise, but it would be high all the time, not intermittent.
As far as 'ancient crap' versus inverters, well how long has this equipment been running? Yes, it has a few contactors and such that need to be dressed and PM once in awhile, but it is pretty robust. Modern inverters work well, but you need a whole bank of test equipment to troubleshoot them and they are a bit more fussy about their environment, power supply quality, etc...
daestrom
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The fault has been located and you are right. There was a bad contact on the final rotor resistance shorting contactor so that it was intermittently running with one phase open. This caused the stator current to shoot up and the thermal overload to trip.
I wasn't expecting this, I expected a fault on one of the rotor resistance contactors to give a high resistive load in the rotor circuit causing a lower current to flow and the motor run at lower power drawing less current. What seems to be happening is that the slip speed is increasing as the rotor slows down and the motor effectivly becomes a transformer feeding current into the rotor resistor bank, which then glows hot.

No maintenance has been done for at least a decade or so.

Again, you seem to be right. The motor and its control circuit has been running for over a decade without any maintenance being done. The equipment is so old that new contacts for the resistor shorting contactor are now unobtainable. A new contactor has now been installed and the motor is up and running problem free again. It will be worthwhile to include examination of the contactor contacts in the yearly maintenance schedule.
Thanks for the help. I now know a bit more about fault finding on slip ring induction motors.
Svenne
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