When an IE3 motor fails, plants always face the same dilemma: should the motor be sent to a rewind workshop and rewound, or should a new IE3 motor be purchased from stock? At first glance the answer looks simple; the rewind fee is usually lower than the price of a new motor, and "the cheaper option wins" seems obvious. Yet the right decision rests not on the immediate invoice but on the electricity the motor will consume over its lifetime — that is, on efficiency loss and total cost of ownership (TCO). In this article we examine in detail the real cost of rewinding an IE3 motor, what happens to the efficiency class after a rewind, and in which cases buying a new IE3 is more economical.

This is not merely a technical preference; it is an investment decision that lands directly on the electricity bill. On a motor running 16-24 hours a day, an efficiency loss of a few percentage points exceeds the price gap between a rewind and a new motor within just a few years. The decision must therefore be made by weighing the motor's operating profile, age, and how many times it has already been rewound. As HEM Motor, with our identity as both manufacturer and supplier, we offer a consumption-based perspective so that plants can make this comparison correctly.

Stator and windings of a rewound IE3 electric motor in a rewind workshop

Why Is the Rewind Decision Not Made on Price Alone?

A rewinder quotes you only the rewind fee, and that figure looks small next to a new motor. But this comparison is incomplete, because a motor's real cost is not its purchase price. About 95-97% of an industrial electric motor's lifetime cost comes from the electricity it consumes, and only 2-3% from its purchase price. The price of the motor is the tip of the iceberg; the real cost is the energy it draws for years.

This is where the concept of total cost of ownership (TCO) comes in. TCO is the sum of purchase (or rewind) cost, installation, lifetime energy consumption, and maintenance expenses. Evaluating a rewind decision by the workshop invoice alone means ignoring the largest item in this equation. If efficiency on a high-running motor drops by a few points, the lost energy exceeds the rewind fee many times over within a few years.

For example, if the efficiency of a continuously running 30 kW motor falls by just two points after a rewind, thousands of extra kWh are consumed per year. This loss repeats every year and accumulates as long as the motor stays in the field. That is why the right question is not "is a rewind cheaper than a new motor?" but "what will this motor cost me in total over the coming years?" Our article on how to calculate total cost of ownership (TCO) builds the framework for this calculation step by step.

Is the Efficiency Class Preserved After a Rewind?

A common misconception is the assumption that "the motor was IE3, so once rewound it will be IE3 again." Yet the efficiency class is tightly bound to the motor's physical design and winding quality. Even though a good rewind workshop can preserve the original winding data, in practice many factors can lower efficiency:

  • Winding wire cross-section and fill factor: The original factory winding is machine-made to keep the slot fill factor at the maximum. In a manual rewind, the same copper cross-section and tightness cannot always be achieved, which increases copper losses (I²R).
  • Thermal damage to the stator core: To remove the old winding, the stator is usually placed in an oven. Excessive heat degrades the insulation layer of the silicon laminations and increases iron (core) losses. This loss does not return, no matter how well the winding is done.
  • Insulation and impregnation quality: The penetration depth and thermal conductivity of the varnish affect the motor's cooling and therefore its efficiency.

For this reason it is wrong to say "every rewind preserves the efficiency class." A first rewind in a qualified workshop may lower efficiency only slightly; but a poor-quality rewind can drop the motor noticeably below the IE3 class. The decisive factor here is who performs the rewind, with what measurements, and with what material quality. Our article on winding and insulation class (F/H) in IE3 motors details the effect of winding quality on life and durability.

The Cumulative Effect of Repeated Rewinds

A motor's first rewind and its third rewind are not the same. In each rewind cycle the stator is heat-treated again, the slot insulation is renewed, and the core pack is stressed a little further. These effects are cumulative:

  • First rewind: In a good workshop the efficiency loss may remain limited; the motor is still a reasonable choice.
  • Second rewind: Thermal aging in the core pack increases, and efficiency falls more noticeably.
  • Third and subsequent rewinds: Iron losses rise permanently; the motor can no longer reliably deliver the IE3 efficiency printed on its nameplate.

So rewinding the same motor again and again means working with a slightly less efficient motor each time. When a motor arrives at the rewind shop for the second or third time, it is usually time to buy a new one. This is a sensible threshold both in terms of efficiency and the fatigue accumulated in the frame. Repeated rewinds also mean a fresh risk of failure and downtime each time, which adds to the cost as lost production.

Cost comparison of a new IE3 efficient electric motor versus a rewound old motor

When to Rewind and When to Buy New?

The decision cannot be reduced to a single rule; but the operating profile and power range give a clear direction. The general framework is as follows:

When Rewinding Makes Sense

  • Low-running motors: For motors on a standby line, switched on occasionally, or running only a few hours a day, the energy cost is low, so a rewind can be economical.
  • High-power, special-frame motors: For very high power or non-standard sizes with long lead times, a rewind can be a practical bridge to get production back up quickly.
  • First rewind and a qualified workshop: If the motor has never been rewound and is given to a measured, high-quality workshop, the efficiency loss may stay at an acceptable level.

When Buying a New IE3 Makes Sense

  • Continuous (S1), many-hour motors: On pump, fan, compressor and conveyor motors running 16-24 hours a day, the efficiency gap creates fast payback; a new IE3 is almost always profitable.
  • Motors arriving for a second/third rewind: Because of accumulated efficiency loss, these motors no longer deliver IE3 performance.
  • Standard, stock-available powers: In common powers between 0.75 and 90 kW, a new IE3 motor is supplied quickly, so waiting for a rewind is unnecessary.

Our article on rewind or buy new handles this decision holistically. The return on replacing an old motor with an efficient one is shown with real consumption figures in our article on the payback of replacing an old motor with IE4.

A Simple Payback Logic

You do not need complex formulas to decide; the logic is this. First determine the motor's annual running hours and its power. Then estimate the expected efficiency loss after a rewind (for example, two points). Multiplying the efficiency loss by the power and the annual running hours gives you the extra energy the rewound motor will consume each year. Multiply that extra consumption by the unit electricity cost and the "hidden annual cost" of the rewind appears.

On a high-running motor this hidden cost usually closes the price gap between a new motor and a rewind within one or two years. So even though rewinding looks cheap on day one, in continuous operation it becomes more expensive within a few years. On a low-running motor, this equation turns in favor of the rewind. That is why the decision depends not on power but on running hours and the efficiency gap. The same logic also reveals when it is wiser to replace a motor with an IE4 super premium model.

Verifying Rewind Quality: Questions to Ask

Even if you decide to rewind, you should request clear information from the workshop to secure the outcome:

  • Is the stator oven temperature controlled, and is the core pack protected against thermal damage?
  • Are the original winding data (number of turns, wire cross-section, connection diagram) preserved?
  • After rewinding, are insulation resistance, current imbalance and no-load running tests performed?
  • Are the varnish and insulation materials used suitable for the original insulation class (F/H)?

If you cannot get clear answers to these questions, it is hard to trust that efficiency will be preserved after the rewind. If the uncertainty is high and the motor runs many hours, the safest route that eliminates the risk is a new IE3 motor. In our efficient electric motors category, we supply the most-requested power and speed combinations quickly from stock, so you can restart production without waiting in the rewind queue.

Frequently Asked Questions

Does a rewound IE3 motor stay in the IE3 efficiency class?

It cannot be guaranteed. The efficiency class depends on the winding wire cross-section, slot fill, and especially the thermal condition of the stator core pack. In a first rewind at a qualified workshop, efficiency may drop only slightly; but a core pack stressed during oven baking, or a low-quality rewind, can drop the motor noticeably below the IE3 class. Preserving the efficiency class is a result of rewind quality, not of the nameplate.

Why does rewinding a high-running motor often cost more?

Because most of a motor's lifetime cost is energy consumption. On a motor running 16-24 hours a day, a few points of efficiency loss means thousands of extra kWh per year. Since this loss repeats every year, the "cheap"-looking rewind invoice exceeds the cost of a new motor within a few years.

When is a rewind still sensible?

For low-running standby motors, very high-power or long-lead-time special motors, and motors rewound for the first time at a qualified workshop, a rewind can be economical. In these cases the low running hours keep the annual cost of efficiency loss limited, and a rewind becomes a practical way to get production back up quickly.