Motor Effeciency

If the rewind shop is good at what they do there should be no degrading of the motor.

Some motor winding designs can be improved on as well so it may be possible to increase the power and or efficiency of a motor at times as well but only the rewind shop can tell you if your motor is capable of having that done.

your motor should be rewound exactly the same, the rewind shop if they don't have a coil layout should take the motor coils out one by one, note how they are fitted into the stator and copy the layout making notes and drawings in case they get this motor again. By copying and ensuring they have the same CSA of wire used, the same number of turns in each coil, the coils are installed as they came out, then there will be drop in efficiency.

If there is a problem with the gap between the rotor and stator, then that will have an adverse effect on the motor efficiency

I am agreeing with Brich...so not to derate efficiency, motors should be rewind with the same manufacturer's winding data; such as lenght of winding coil, original insulation or if not much higher insulation than the manufacturer. You should also take good care of the motor laminated slots when rewinding so as to get back the rated efficiency.

Yes, the efficiency will drop, how much it drops will depend on the disassembly, reassembly and curing process and temperatures used. The characteristics of the stator will be affected resulting in an efficiency drop in the order of 2 to 5% depending on care taken and the process used.

This means an original motor of 88% could now be 85% efficient, as a rough guideline.

If the process is harsh and less well controlled, particularly the oven and temperature gradient control, this could be much worse than above.

During more than 30 years, we repaired electrical motors. We never had problems with less efficiency of a repaired motor.

Mostly we also use copper wire and isolation material which can stand a higher temperature. Isolation classe H against isolation classe F or B.

In those cases the motor will survive much longer an overload before burning, in case there is something wrong with the machine driven by this motor.

The motor protection has much more time to react, before the motor burns.

Also when heat exchange is not good because it is a hot sunny day outside, the motor will last longer.

In some cases we actually change and improve the type of winding (one layer versus two layer winding) to obtain less 5th and 7th harmonics.

This gives us a better motor after rewinding.