Typically, motors have electromagnet structures that consist of thin iron leaves bonded together with insulating resin to build up to the needed magnetic flux for the power the motor needs. The insulated leaves prevent self-induced currents from the magnetic field in the iron core. If this insulation begins to burn away, 'shorting' the leaves together, it causes high induced currents that cause both high current consumption in the motor and a lot of internal heating - making the condition worse. The motor may appear to work normally, but actually has reduced shaft torque and is a fire risk. If you put your hand near it while it is running, it will probably be very hot.
I would just replace the motor at this point.