| The Impact of Rewinding on Motor Efficiency |
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| Written by Jim Custodio, GE Motors | |
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The table in Figure 5 shows how these losses can be reduced. Following that are detailed explanations of the techniques.
Figure 5. Possible methods to reduce losses through rewinding.Stator LossesStator losses are primarily I2R losses, released in the form of heat as current passes through the stator windings. When rewinding a motor, smaller diameter wire will increase the resistance and therefore I2R losses; larger diameter will have the opposite effect. If the original wire was aluminum, changing to the same size copper wire will also reduce resistance and loss. Obviously, using a larger diameter copper wire will affect the best reduction. Another option for reducing stator losses is to reduce the number of wire turns. Use this method with caution. While full load efficiency may be increased, starting current will go up and power factor will be reduced. Both starting and maximum torques will be increased. A change from ten turns to nine turns would increase starting current by as much as 23 percent. Core LossCore loss is the sum of the eddy current and hysteresis losses that occur while energizing the motor's magnetic field. Motors are insulated between the core laminations to minimize eddy currents, but the process of stripping can destroy this insulation. When stripping a motor for rewinding, insulation burnout must be done at carefully controlled temperatures. Otherwise it's easy to overheat the laminations, breaking down the core insulation and actually increasing core loss. Not all repair shops use the same insulation burnout techniques; investigate them thoroughly before deciding where to have the motor rewound. Another item that is often ignored is the condition of the core after motor failure. Failures caused by excessive loading, extended stall conditions, single phasing, or bearing failure leading to rotor striking can all cause increases in the core loss. It is very unlikely that the original core loss data would be available from a ten year old T-frame motor. Repair shops may have equipment to evaluate the core in its failed condition, but are unable to relate the results to original factory core loss specifications. Applying even the best techniques to improve the efficiency may be inappropriate without the original core loss information. The repair shop would have to conduct a full efficiency test using a dynamometer, which takes two to three hours depending on frame size, in order to validate the finished motor. Manufacturers do this on every motor they design and have programs registered with the DOE to assure that design efficiencies are maintained throughout production. The DOE requires that testing laboratories be third-party certified to assure compliance with the testing procedure defined in IEEE 112 B. This process was written into the Federal Energy Act to assure reduced optimistic efficiency claims. Repair shops have no such requirement. Rotor LossRotor losses are I2R losses, released as heat through the rotor slots and endrings. It is unlikely that a repair shop will be able to improve rotor losses.
Efficiency vs. TimeYou have undoubtedly heard or read more than once that motor efficiency naturally decreases through motor life as a result of "heat aging." This argument says that as the motor starts and stops the core temperature increases and decreases, causing deterioration in the core steel's electrical properties and a resulting increase in internal losses. In fact, this is only a problem if an aging type of steel is used in the core. Most manufacturers use non-aging steel that does not lose its electrical properties over time. Rewind vs. a New MotorNow that you know some of the pitfalls of rewinding, let's reexamine our options in the face of a motor failure. Provided that downtime isn't the critical factor, a user now has these choices:
A fifth option that no one should knowingly choose is to rewind the motor to a lower efficiency - but many users unwittingly make this decision. As explained above, it is very easy to damage the stator core insulation while stripping out the old winding and so increase core loss by three times or more. Figure 6 compares the typical results of each of the choices, with assumptions for the "poor quality rewind" figured at three times the original core loss.
Figure 6. Efficiency Gain (Losses): Rewind vs. Premium-Efficiency MotorThe high-fill rewind produces some efficiency gains when a larger wire size is used. As would be expected, the greatest efficiencies are realized by retrofitting with new premium-efficiency motors. Figure 7 shows the annual operating cost difference for each of the options listed above. Note that operating cost can be increased by a poor rewind just as much as it can be decreased by a new, premium-efficiency motor.
Figure 7. Annual Operating Cost Difference: Rewind vs. Premium-Efficiency Motor Continuous Operation @ $.07/kW-hThe conclusion is obvious: either replace failed motors with new, premium-efficiency motors, or else exercise extraordinary care in the rewinding process. Protecting the Stator Core in a RewindNo single aspect of the rewind process is as important as preserving the electrical integrity of the stator core. Not only can insulation damage increase core loss, but the resulting rise in motor temperature could also then cause the motor to fail prematurely. You've probably had at least one motor that operated well for years before initial failure, but failed again shortly after being rewound. The failure is more often the result of temperature rise than of defective materials or faulty workmanship in the new windings. The obvious question is "what is a safe insulation burnout temperature?" Unfortunately, there is no simple answer. Manufacturers use a wide variety of materials for the core. Steel may be supplied with either organic or inorganic insulation coatings, or with no coating at all. If uncoated steel is used, the motor manufacturer will add an oxide insulation coating while annealing. Each of these lamination insulations has a different limit in temperature that it can withstand before deteriorating, so it's impossible to name a temperature that is safe for all motors. But there are some guidelines. In all cases, the stripping operation must control the core temperature to prevent damage to the interlamination insulation. Damage can occur even in a low temperature oven when several cores are stacked, and fire from the burning organic materials results in increased temperature beyond the oven setting. If the motor has an organic lamination insulation, it will begin to deteriorate rapidly at around 500-deg F and may actually change its chemistry at higher temperatures. Organic insulation can be damaged in any oven hot enough to burn out the winding insulation. Inorganic insulation can withstand temperatures up to 700-deg F, allowing the old winding to be burned out safely if the oven temperature is carefully controlled. Uncoated semi-processed steel laminations will stick together if they get too hot, increasing eddy current losses dramatically. Motors built with this type of steel can be stripped at oven temperatures of 700-deg F or below. Stripping MethodsIn the past few years, awareness has grown among users that poor quality motor rewinds can cause an increase in losses. Users have demanded an end to the practice of burning out the old windings at uncontrolled temperatures. Motor repair shops that have kept pace with technology have switched to temperature-controlled ovens and have discontinued the practice of softening varnish with a handheld torch. Ask your rewind shop if they can perform any of the following non-injurious stripping techniques:
Regardless of the process used, make sure your rewinder can follow the motor manufacturer's recommended safe burnout temperature limits. Some do and some don't - which is probably the main reason for one shop's reputation over another for better quality rewinds. There is really only one way to make sure that losses have not been increased in the process, and that's to perform a qualitative core loss test before and after rewinding (assuming the core is okay to begin with). This can also help you screen the motor population to determine if any given motor is even repairable. More repair shops now offer this service, so ask yours. Establish a Repair/Replace PolicyIn response to the rising cost of electrical power, every company should establish a repair/replace policy to help make intelligent decisions. Give every motor-driven machine in your plant a repair/replace priority, and consider investing in spare motors for any continuous process machines that are critical to plant operation. These "critical" machines are excellent candidates for retrofitting with premium-efficiency motors; the existing standard-efficiency motor should not be kept as a spare. There is a good deal more to comparing relative in-use costs between motors than simple energy usage. The local power company may offer rebates on new premium-efficiency motors. Extended warranties may be available to reduce MRO costs. A new motor could be installed with a variable speed drive to maximize the process productivity (the drive may also be covered under a rebate). Inventory costs can be reduced by not storing repaired motors that could become obsolete before they are needed again. These are only a few of the possibilities. Many motor distributors are willing to assist you in evaluating a program. Check the Internet for the one nearest you. John Sajovic is the marketing manager for GE Motors, 1635 Broadway 19-5, Fort Wayne, IN 46802, 260-439-3245, 260-439-3338, http://www.gemotors.com/. Comments (0)
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