Hello THooper
From the images, it is clear that the rotor has dissipated too much power within a period of time, and probably repeated periods of time.
When the motor is operating at high slip (during start or stall) there is high rotor dissipation. This also reflects as additional stator current and stator heating, however the bulk of the slip loss is in the rotor.
If the rotor has a thermal mass that is less than the load requires, you will slowly overheat the rotor with repeated starts. If the rotor time constant is much too low, then the rotor may fail relatively early. The rotor time constant and the stator time constant are not necessarily coordinated, so, with a short time constant rotor, yoou will get rotor failure without stator failure or damage.
A crusher is generally a high inertia machine and so it needs a long time constant rotor. The motors are typically rated with a maximum locked rotor time or a maximum load inertia. These can vary tremendously between motors and it is important to use a motor that matches the application.
If the rotor thermal mass is adequate for the driven load, but there are repeated starts, then the total rotor dissipation will cause a melt down.
Another cause of excessive rotor heating is negative sequence currents due to voltage imbalance and slip losses due to supply harmonics.
I have experienced local harmonic voltage on the supply above 12% due to the number of VFDs on the supply. This will certainly increase the continuous rotor power dissipation and when this occurs in conjunction with high inertia loads and frequent restarts or stalls, then rotor failure can occur.
I would check the motor thermal rating relative to the load, I would look for overload conditions that cause frequent stalls, I would look for phase imbalance conditions and also check the voltage harmonics.
This could be a) a rotor not suitable for the application, or b) a power quality issue or c) both.
Best regards,
Mark
Mark Empson
Advanced Motor Control Ltd