MRO Today - A rewind that's right

A rewind that's right
Know the process of rewinding an energy-efficient motor and make sure your outside service center is doing the job correctly

by Chuck Yung

Your company spent a lot of money replacing standard-efficiency electric motors with energy-efficient models, and now one has failed. Do you rewind it or replace it? Sure, the rewind will save money, but can an energy-efficient motor be rewound without reducing its efficiency? The answer is probably yes, if your outside service center follows some basic guidelines.

By dealing with a qualified service center, and verifying a few simple things, you can be reasonably sure your rewound energy-efficient motor will maintain its original efficiency. A savvy maintenance team member should be prepared to discuss specific procedures with the service center before sending an energy-efficient motor for rewind.

What makes it energy efficient?
There is really no magic in an energy-efficient motor. When power input is converted to power output, some energy is always lost. By addressing the areas where power is lost — to heat, friction and windage — motor manufacturers have increased the efficiency of their products. They use longer stator cores and correspondingly longer rotors, for example, to reduce losses in the core. They also use more copper than usual in the windings to reduce copper losses.

The external fan on totally enclosed, fan-cooled (TEFC) models is also as small as possible, minimizing the horsepower diverted to cooling the motor. Manufacturers of energy-efficient motors use open or shielded bearings and are careful to install only a specified quantity of grease during assembly. Over-packed bearings increase friction, which lowers motor efficiency.

Breaking it down
The stator core is composed of laminations, thin pieces of steel coated with an insulation to reduce eddy currents in the core. Assuming the failure didn’t blow a hole in the core (thereby reducing the mass), the next concern is whether the windings burned out at an appropriate temperature.

To facilitate removal of the old windings, most service centers process stators in burnout ovens to incinerate insulating varnishes and epoxies. Care must be taken to maintain the appropriate oven temperature in order to preserve the inter-laminar insulation of the core.

The insulating coating (organic, chemical or oxide) on each thin piece of laminated steel in the core reduces efficiency-sapping eddy currents. Newer motors tend to have lamination insulation capable of withstanding higher temperatures than older motors. Because winding insulation materials (varnishes and epoxies) burn at lower temperatures than the inter-laminar insulation, the burnout process (properly done) will not harm the inter-laminar insulation.

Some manufacturers of burnout ovens state temperatures below 750 degrees F are safe. A recent study conducted in the United Kingdom also settled on 750 F for standard laminations, and found oxide-coated newer steel laminations could withstand temperatures in excess of 900 F with no loss of efficiency.

An oven with a chart recorder will provide documentation that your motor was not burned out at an excessive temperature.

Core testing
A core-loss test should be performed before and after the old windings are removed. The watts/pound readings help determine if the core is still good and safeguard against burnout-related problems. Commercially available core testers simplify the process for the service center and usually provide print-outs documenting the core’s condition.

Core loss also can be measured with a watt meter if an adequate voltage supply is available.

The new winding
When current passes through a conductor, heat is generated. For a given current, a larger conductor will heat up less than a smaller conductor. To maintain the motor’s original efficiency, it makes sense that the wire size in the new winding be no smaller than in the original winding.

Looking at it another way, the cross-sectional area of the conductors determines the amount of copper in a motor. A larger cross-sectional area reduces copper losses (heating due to resistance) and increases efficiency.

Based on current density, this is usually reported as circular mils per amp (CM/A). The CM/A for the new winding must not be lower than the original winding or efficiency will drop. For reference, motors manufactured before 1964 had at least 550 CM/A. From 1964 to the advent of the energy-efficient motor, T-frame motors had lower current densities (350 to 450 CM/A) to meet demands for lower-cost, lighter-weight motors. Today’s high-efficiency models have densities of 600 to 1000 CM/A.

Make sure the service center you deal with understands that changing current density affects efficiency and doesn’t reduce wire size just to make a particular motor easier to rewind. Assuming no damage to the stator core and no reduction in the circular mils/amp, the potential efficiency of a rewound energy-efficient motor should remain the same.

Which winding is best?
Random-wound motors have either lap or concentric windings. Manufacturers use concentric windings in smaller motors, because they are suited for automated production of thousands of identical motors. This keeps new motors economical, which benefits the buyer. The down side is not every turn in a concentric coil is equally effective.

Viewed from the end of a stator core, the concentric winding has coils with two, three or more different slot spans. Each span has a different angle (expressed as “chord factor”), which determines the effectiveness of the turns within that coil. Depending on the chord factor, 10 turns in a coil spanned at 1 to 9 won’t have the same strength as 10 turns of a coil spanned at 1 to 10 or 1 to 8. If the turns in one span are half as effective as the turns in another, twice as many turns would be required. That would double the coil’s resistance.

The distance around the coil also changes with the span, so a larger span for the same 10 turns would require a longer conductor. Because a longer conductor has greater resistance, the total winding resistance depends on the span(s) selected.

The length of conductors is also affected by the distance that the coil ends extend beyond the core. Keeping these “coil extensions” to a minimum by careful fitting helps control the “mean turn length,” the average distance around each coil. The shorter this length, the lower the total winding resistance, which in turn reduces the copper losses and improves efficiency.

Winding resistance
Because all coils in a lap winding have the same mean turn length, it’s often possible, by careful fitting, to produce a winding with a lower resistance than the original winding. Lower resistance means less efficiency is lost in the windings. All else being equal, a carefully fitted rewound motor can be more efficient than the original. As a general practice, the service center should replace the stator winding with exactly the type of winding removed from it. This means the same wire size, winding type, turns, span and coil extension.

For energy-efficient motors, conversion from concentric to lap should be done only if mean-turn-length calculations for both windings prove total winding resistance can be reduced. It’s often possible to improve the efficiency of a two-layer concentric winding, while a three-layer concentric winding is always more efficient than its lap-wound counterpart.

No matter which winding type is used, winders must carefully fit the coils and avoid lengthening coil extensions to ease insertion. As noted above, lengthening extensions increases total winding resistance, reducing efficiency.

Bearings
Quality bearings of C-3 internal clearance are the standard for electric motors. Sealed bearings prevent contamination and require no periodic lubrication; unfortunately, they also create more friction than shielded or open bearings, resulting in a slight drop in efficiency. When efficiency is a concern, remain with the open or shielded bearing style installed by the manufacturer.

For greater reliability in some applications or environments, it may be worthwhile to install sealed bearings, despite the expected efficiency drop. A better alternative is to consider installing non-contact seals, which exclude contaminants without causing friction.

Grease intervals, quantity and viscosity will also impact the efficiency of an energy-efficient motor. Follow manufacturer’s guidelines to maintain optimum efficiency.

Windage
External fans are often overlooked as a cause of efficiency loss. Windage varies among fan designs, depending on fan diameter (the most significant variable), the number and size of blades, the material, and surface finish. Replacing a damaged fan with one that’s not identical will impact efficiency. Even painting a fan could alter efficiency.

Most fans today are plastic or aluminum. Replacements should be identical to the originals whenever possible. If your chemical processes make aluminum impractical, be sure you (or your service center) discuss alternatives with the motor manufacturer to avoid harming efficiency.

What to expect
Core-loss tests are extremely important for verifying the condition of the stator core. Before- and after-burnout watts/pound readings allow the service center to document core condition. Tests can be made with a commercially available core tester or with a watt meter and a power supply.

The service center should have an adequate test panel, instruments and power supply to perform no-load tests on motors it repairs. This should include an accurate bridge to measure extremely low winding resistance. Digital equipment is generally better for measuring the extremely low resistance encountered with AC windings. Winding temperature at the time of measurement should be recorded and corrected to a standard temperature (usually 25 C.) The temperature control on the burnout oven should be accurately calibrated. The burnoff process should also be monitored closely to ensure temperatures do not exceed safe levels.

Dynamometer testing of large electric motors is expensive because of the setup time, test time and power consumption. With smaller motors, the expense is difficult to justify. Make the load-test decision based on how critical the application is, as well as energy and operating costs.

One final point
It is seldom practical to increase the efficiency of motors not originally built as energy efficient. The stator core and rotor losses depend upon the core length and type of steel used in the laminations. The frame is usually sized to fit the existing core. Adding laminations to the stator and rotor, then rewinding the stator and rebuilding the rotor are rarely feasible, much less cost-effective. The slot configuration was designed to hold a specific amount of copper, so increasing the cross-sectional area of the conductors in order to reduce copper losses is seldom possible.

Chuck Yung is a technical support specialist at the Electrical Apparatus Service Association, an international trade association of firms that sell and service electrical and mechanical apparatus.

This article appeared in the December 1998/January 1999 issue of MRO Today magazine. Copyright, 1998.