3 reasons for increased efficiency loss in rewound motors

The loss in efficiency of rewound motors

Approximately 300,000 motors are rewound in the UK every year, with an average rating of about 12 kW, so the efficiency of rewound motors is extremely important. The loss in efficiency on rewinding depends on the techniques, processes and skill used to perform the rewind, and is usually between 1 and 2%.

3 Reasons For Increased Efficiency Loss In Rewound Motors
3 Reasons For Increased Efficiency Loss In Rewound Motors (photo credit:

The reasons for increased loss are discussed below. If the choice is between rewinding a standard efficiency motor or purchasing a new high efficiency motor, the difference in efficiency will be 4 to 5% at full load in favour of the high efficiency motor, which will also have a much longer service life.

Generally speaking, there are three factors affecting the efficiency of rewound motors, and let’s talk about each of them //

  1. Increase in Iron Losses
  2. Copper Loss
  3. Mechanical Considerations

1. Increase in Iron Losses

An increase in the iron losses can be caused by //

  • Mechanical damage to the core
  • Thermal damage to the core
  • Electromagnetic changes

Mechanical stress in the core will increase the hysteresis loss, as might happen if the core is fitted into a new frame with an undersized bore. The practice of hammering stator teeth back into place after stripping will result in increased hysteresis locally as a result of the residual stress.

Eddy current loss will increase if the insulation between adjacent laminations is damaged, for example by burring together by filing or by accidental impact.

Steel laminations being destroyed
Steel laminations being destroyed. The vertical slots were originally in straight lines, not broken up as shown. Since their edges are sharp, they can easily cut through the stator winding insulation and short them together, creating an electric heating circuit instead of a motor. (photo credit:

The stator tooth tips and rotor surface are the most vulnerable, since both carry high frequency harmonic fluxes. Skimming to remove damage is rarely acceptable because the air gap is increased and efficiency reduced.

Thermal damage to the oxide or varnish insulation between the laminations is normally regarded as the usual cause of increased iron loss following a rewind. New work in which the increased loss after rewind under carefully controlled conditions for a number of motors was measured has shown that for conventional steels the temperature should not exceed 380°C. Losses increase very rapidly at higher temperatures.

The very highest efficiency motors use thin laminations of high quality steel, coated with a microfilm of varnish and these were found to exhibit no increased loss over the test range of 350 – 400°C.

Most motors are designed to run with flux densities in the stator and rotor core just over the knee of the magnetisation curve. If the winding characteristics are changed after rewind, for example if the number of turns are reduced, the flux density and hence the loss will increase.

Saturation curve of a DC generator
Saturation curve of a DC generator

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2. Copper Loss

Stator copper loss is the largest loss (at full load) in most induction motors. The winding pattern may be changed during rewinding to simplify the process, and in doing so the repairer must consider the effect on flux density and resistance.

Of load losses, stator copper losses (also referred to as I2R losses) are caused by heating from the current flow through the resistance of the stator winding. Techniques for reducing these losses include optimizing the stator slot design. Rotor losses are caused by rotor currents and iron losses.

These losses are reduced for example by increasing the size of the conductive bars and end rings to produce lower resistance. Stray load losses are the result of leakage fluxes induced by load currents. These can be decreased by improving slot geometry of rewound motors.

Full considerations of these losses are very complex, and are beyond the scope of this article. Full details can be found in this reference.

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3. Mechanical Considerations

The concentricity of rotor and stator is very important. It is common practice to metal spray shafts or bearing housings which have been damaged in service. This is acceptable only if special care is taken to preserve concentricity – errors which result in a minimum to maximum gap ratio greater than 1:1.25 will adversely affect efficiency.

Replacement bearings (and lubricants) should be to the original specification, and repairers should be aware that high efficiency motors are beginning to use newer, more sophisticated bearings.

Go back to Rewound motor losses ↑

Rewinding a Large Motor (VIDEO)

References //

  • Electrical Energy Efficiency by Copper Development Association
  • Low voltage motors – Motor guide by ABB

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About Author //


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on


  1. Walter Kramer
    May 02, 2017

    This article is consistent with our experiences.
    We recommend rewinding only if replacement is too expensive or not possible.
    The outcomes depend very much on the skills and experience of the rewinder.

  2. Les Parsons
    May 01, 2017

    Yes. The EASA study performed in the UK, headed by Professor David Walters, showed that motors can be rewound without degradation, using strict procedures. Also one can increase efficiency,by changing design. Example: smaller coil overhangs. Changing from concentric to lap wound.

  3. Claes Hugoson
    Mar 13, 2016

    Please study the report from EASA and the test performed by the university of Manchester were the rewinded motors very often had better performance than the new one.
    ELR and REPA motor have performed the same test with same result!
    Claes Hugoson

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