Look at the stator windings for damaged insulation caused by careless replacement of the rotor into the stator. Discoloured insulation is an indication that the winding has been overheated. The cause of overheating must be found and corrected before allowing the motor back into service.
Carefully examine the stator core for signs of rubbing with the rotor, usually caused by u worn bearing. Even slight rubbing of the rotor against the stator will generate enough heat to destroy the stator insulation. Replace the bearings before putting the motor back into service.
Laminated steel core plates which have been badly scored may cause a local hot spot to be generated when the motor is running. This is because the Fe (iron) losses will increase in the damaged area. After the motor has been put back into service with new bearings, check the motor running temperature.
After a short period of service dismantle the motor and check for discolouration at the core damage which will indicate local heating. If you suspect core hot spots then the motor core will need to be dismantled for the laminations to be cleaned and re-insulated definitely a shore job.
The insulation resistance reading is the best indication as to the presence of moisture in the motor windings. Break-downs due to insulation failure usually result in an earth fault, short-circuit turns in a phase or phase-to-phase faults.
Larger motors are usually six-terminal, which means that all six ends of the stator windings are brought out to the terminal block. Links between the terminals are used to stator delta connect the motor. Disconnect the supply leads and remove the links. Test between phases with an insulation resistance tester.
A problem can arise on small, three-terminal motors where the star or delta connection is made inside the motor. Only one end of each winding is available at the terminal block. Phase-to-phase insulation resistance cannot be checked. If a three terminal motor is to be rewound, ask the repairer to convert it to a six terminal arrangement.
