Can a Rewind Up Motor Efficiency?

Can You Rewind a Standard-Efficiency Motor to Bring It to a Higher Efficiency?

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Can you rewind (plus new bearings, etc.) a standard-efficiency motor to bring it to a higher efficiency than it was rated for? Can you approach premium levels? How about rewinding premium to approach 'super' premium? Our customer's rewinder says it can come close. If true, what does that do to the ROI advertised by the motor people? We advise them to go with new motors, but we need facts.

—From March '11, Control Design

Answers

Enough Gains Unlikely

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 that triangle is

Equation 1

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

Equation 2

The highest level of slot fill possible using wires of the same size, without deforming the wires, is then equal to

Equation 3

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.

Standard wire sizes differ in area by about 12.3%. Starting at 60% fill and using a single wire size, the closest one can come to 90% fill is 85%. This further reduces the likelihood of a significant increase in efficiency by changing the wire copper area.

Now, it might be possible to make some other "improvements" by changing the number of turns and trying to reduce some of the other types of losses, such as core loss, but that requires more than just a normal rewind. It also could have adverse effects on the starting performance of the motor.

And note that the above scenarios are being done by hand winding and not by high-volume production-winding equipment in a motor plant that relies on 80-82% slot fill. For hand winding, figure a 5 hp motor being wound in 2-3 minutes in production or 2-3 hours by hand. Will the additional cost offset the energy savings from the slightly higher efficiency?

None of this has addressed that when a motor manufacturer raises the efficiency, the stack length of active laminations in the motor is usually increased and the steel material is also changed to one with lower losses. Neither of these can be done in a rewind situation.

William Hoyt, Industry Director,
National Electrical Manufacturers Assn. (NEMA),
www.nema.org

Small Efficiency Gains

The efficiency of any motor can be increased if the area of the copper wire conductors in the stator winding is increased. In the case of a pre-EPAct motor, there is a high probability that this can be done. In most cases, though, the increase in efficiency will be less than half of a percentage point of efficiency, which would not be enough to change the efficiency to EPAct or premium levels.

For example, a 50 hp, 1800 rpm TEFC motor at the NEMA premium level would have a nominal efficiency of 94.5%. Historical data for pre-EPAct motors used by the U.S. Department of Energy indicates a typical nominal efficiency of 91.3% for this rating. The difference in efficiency would be 3.2%, thus far greater than the 0.5% maximum approximation.

The primary reason for the relatively small increase in rewound motor efficiency is that stator copper (winding) losses, although usually the largest component in motor losses, are typically only 35-40% of the total losses (table).

Table of Motor Losses

The windings tend to be tighter in the slots in EPAct motors than they are with pre-EPAct motors, thus reducing the potential for efficiency improvement by increasing wire area. To illustrate this point, the EASA/AEMT Rewind Study (www.easa.com/energy) found that the most common efficiency improvement for rewound EPAct motors using the good practices detailed in the study was 0.2 percentage points. The windings of premium efficient motors typically either do not have slot space available to allow an increase in wire area, or their wire area per amp of rated current is so great that an increase in wire area would have a negligible effect on efficiency.

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