A rewind should retain the number of turns in the winding to maintain the starting and speed-torque performance of the original motor. Depending on how the original winding was inserted, by machine or by hand, it may be possible to increase the cross-sectional area of the copper when rewinding.
Any gain in efficiency will then be dependent on how much additional copper area can be included in the rewind. If three perfectly round wires are touching at the edges, the lines through the centers form an equilateral triangle with sides equal to the diameter D of the wires. The area of the sector of each wire inside the triangle is equal to one-sixth of the area of the wire because the angles of the triangle corners are 60°.
The area of the sectors of the three wires is then The highest level of slot fill possible using wires of the same size, without deforming the wires, is then equal to If someone can take a “standard efficiency” (whatever that is) motor with a possible slot fill of 60% and increase that slot fill to 90%, then the effect on stator resistance and stator losses cannot be greater than a reduction of 33%.
If the stator loss is 33% of the total loss, then the reduction in total loss is 11%. This is only approximately one NEMA nominal efficiency band. So, what is the real likelihood that someone can achieve significantly higher efficiency for a standard-efficiency motor just by packing in more wire?
The typical energy-efficient motor is unlikely to have a slot fill of 60%, so it is unlikely that the total losses of such a motor could be reduced by 10% or one NEMA band by packing in larger wire. As for achieving a premium efficiency level, consider that a 10-hp, four-pole, energy-efficient motor might have an efficiency of 89.5%.
To meet a premium efficiency level, efficiency must be raised to at least 91.7%. That increase corresponds to three NEMA bands of efficiency or a reduction in total losses equal to 23%. Achieving premium efficiency by a rewind is unlikely. Read More
