Evaluate Your Motor Service Center

A good motor service center SHOULD:

Conduct a stator core test before and after winding removal. Repair or replace defective stator core laminations.
Calibrate all test equipment and measuring devices at least annually against standards traceable to the National Institute of Standards (NIST) or an equivalent standard laboratory. This will insure the accuracy of the readings taken during repair.
Measure and record winding resistance and room temperature. The resistance measured in all three phases should be balanced. If it is unbalanced by more than 5% the motor should be tested further.
Have the appropriate power supply for running the motor at rated voltage. Measure and record no-load amps and voltage during the final test. Tests at reduced voltage may not reveal certain motor problems.
Have a quality assurance program. This will ensure that you get the same quality of repair each time.
Have and use, at a minimum, the following test equipment: ammeter, voltmeter, wattmeter, ohmmeter, megohmeter, high potential tester. Other useful test instruments include surge testers, core loss testers, dynameters, tachometers, vibration meters and milliohmmeters.
Balance the rotor. Out of balance rotors can cause vibration, which wears out bearings prematurely and increases friction losses, resulting in loss of motor efficiency and higher operating costs.
Repair or replace all broken or worn parts and fits. Bearing fits should be measured and if worn be restored to manufacturer’s specifications. In the absence of manufacturer’s guidelines, EASA has published bearing fit charts to aid the motor service center.

A good motor service center SHOULD NOT:

Overheat the stator. The safe limit for organic lamination insulation is 680°F and 750°F for inorganic material measured at the core. Most service centers use heat to soften old windings for removal. Overheating the stator during stripping can cause core damage.
Sandblast the core iron. Blasting with sand or other hard materials can cause short circuits between laminations.
Knurl, peen or paint bearing fittings. Knurling, peening or painting these fits could cause them to become loose during service.
Use an open flame for stripping. Using uncontrolled heat causes loss of core plating and can warp the core.
Short the lamination when grinding or filing. Excessive grinding and filing can increase core losses resulting in decreased motor efficiency.
Increase the air gap. Increased air gap produces higher magnetizing current, which increases stator and rotor copper losses and decreases power factor.
Increase the resistance of the stator windings. Increased resistance results in increased stator copper losses and decreased motor efficiency.
Make mechanical modifications without your prior approval. This includes, among other things, changing fans, types of bearings, shaft material and seals. Making changes in the manufacturer’s original design can decrease motor efficiency.
Change the winding design without your prior approval. This change can affect the overall efficiency, torque and other characteristics of the motor.