Change synchronous motor to induction motor

Converting a synchronous motor to an induction motor is easily done, to most motors, by shorting the rings, however, you would most likely need to use a properly sized VFD to get the starting torque you need. If you are interested to do such an upgrade you should consider the age of the motor and any other peripherals (gearbox, etc.) in the drive train. It makes little sense to make such an upgrade on a 20 year old motor or one that has been rewound many times. You may be using the best motor shop on earth but burning out the windings takes a toll on laminations. A cost/benefit analysis would be necessary here.

Sizing a replacement is no big deal if you have good documentation of the existing equipment. You need someone to show you the math to make sure they know what they are doing. One more thing, crusher applications (and uphill conveyors) have a unique problem. If you are not coordinating the feed with the crusher speed you may find yourself in an overload situation. Typically VFDs are tuned to protect the motor in overload conditions. Some will shed speed to prevent overcurrent in the motor. You want to make sure your supplier is not doing that and has sized the variable frequency drive to compensate. You are obviously beating rocks to death in a cement plant so I would recommend a 200% overload rating as this is what I have typically seen for crusher applications.

One more point to be taken care about the insulation quality. If it is quite old slip ring motor, then it needs to be checked, whether it can be run with the variable speed drive.

We have a drive which will use the original slip ring resistors and automatically switch in the resistors for the driven speed.
We have replaced a number of old drives with this unit. If you need any additional info please let me know. It has proven successful as it is a cheaper option to replacing both the motor and drive.