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The Impact of Rewinding on Motor Efficiency

Written by Jim Custodio, GE Motors

Pumps & Systems, June 2007

When a motor fails, users can (1) rewind, possibly for high efficiency; (2) replace the failed motor with a newmotor; or (3) invest in a premium efficiency product. Here are the advantages and disadvantages of eachapproach and the precautions that must be taken to assure the best investment.

When a motor fails, the user must decide whether to repair or replace it. To make a proper decision, one mustconsider the cost of the repair, the availability of a replacement, the age of the motor, the electrical designrequired for the application, any special mechanical features, and the urgency of returning the failed motor toservice. Placing the driven equipment back into service is frequently the highest priority, and users often maketheir decision based on this criterion alone. Plant managers tend to be less concerned if the rewound motor is

less efficient when their operation's downtime is costing thousands of dollars a minute.

Increased Profits by Energy Saving

U.S. industry will continue to feel the squeeze on profits, and manufacturers who fail to implement energyreduction programs will find themselves at a competitive disadvantage. In some industries, motor operationcosts may even exceed those of labor cost.

It is no longer practical to view the power bill as a fixed base cost, not worth the effort it would take to reduce.The need to minimize power usage should be as important to the CFO as it is to the plant manager. Due to theimprovements that have occurred in motor technology, even companies that already had energy programs ten

years ago should now reevaluate their criteria.

Future Power Costs

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In 1970, electricity cost the average industrial user about one cent per kilowatt-hour. By 1980 that had jumpedto four cents: a 300 percent increase. Power costs in some areas are already over seven cents. The cost forelectricity may fluctuate with economic cycles, but long-term it will continue to increase.

The nature of today's power bill has also changed. The contract rate of the past now only covers 60 percent to75 percent of the actual amount paid by users. In addition to taxes, today's bill can include such additions as"fuel adjustment" and "demand charges." The bottom line is that electric power can be a major cost element in

your product.

First Cost vs. Operating Cost

Manufacturers who understand their business look at the total cost of a motor, rather than simply making adecision based on initial purchase price.

Let's consider the example of a pre-EPAct 75-hp, 1800-rpm TEFC motor that originally sold for approximately$2700, with an efficiency of 91.7 percent. This motor, operating continuously and using power costing$.07/kW-hr, will in just one year cost $37,414 to operate - or 1,386 percent of the original purchase price! Infact, operating costs will overtake the purchase price after the first 26 days of operation. Even if the motor is

only used for two shifts (assuming 4,160 hours per year), this will still occur after 55 days.

For this reason, any expenditure related to repair or replacement of a failed motor should be evaluated based ona total cost of operation calculation.

An Incorrect Decision Costs Money

Building on the example above, consider the impact on the operating cost of the same 75-hp motor using a moreefficient motor. Let's say the motor efficiency improves to that of an EPAct design with a nominal efficiency of94.1 percent. The new EPAct motor would do the same amount of useful work, but use $979 less energy.

In mid-2001 the motor industry, in cooperation with conservation groups and the DOE, introduced productswith even higher efficiencies: NEMA Premium. With this new specification, the same motor rating would havea nominal efficiency of 95.4 percent and save $1,510 in the first year. These savings continue to accrue as thecost of power goes up. Without increasing the cost of a kilowatt-hour, a NEMA Premium motor would generate$10,563 in savings over seven years, compared to $6,854 for an EPAct level efficiency motor.

Factors in the Rewind Decision

Hundreds of thousands of older T-frame motors were manufactured well before there were any governmentstandards. Many of these still operate in U.S. industry. Each time one fails, an opportunity is created to improvethe user's bottom line.

Power costs will certainly continue to rise and further escalate motor operating expense. So the question of howrepair affects motor efficiency is an important one. Some claim a rewound motor is never as efficient as theoriginal; others say a well-executed rewind can be better than the original design. These differences inperception suggest there may be several factors involved.

Armed with the right information, understanding the factors that affect rewind performance does not need to becomplicated. Let's examine the various types of motor losses and how they are influenced by engineeringdecisions.

Keep in mind that actual motor losses may differ between two motors of the same design, depending on how themotor is used. Figure 1 shows how motor losses vary with load.

Figure 1, left. How motor losses vary with load.


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As a motor approaches 100 percent of rated load, losses increase dramatically, with most of the increase foundin the form of rotor and stator losses. The age of the motor is also a factor. Figure 2 shows the progression ofmotor efficiencies through the years, driven by improvements in engineering design and material technologies.(Note that these ratings are for typical GE motors from 1944 up to EPAct; actual efficiencies will vary frommanufacturer to manufacturer.)

Figure 2, left. The history of motor efficiency, TEFC, 1800-rpm.

The distribution of losses will also be different forvarious motor designs. Variations in speed, design andenclosure will all affect loss distribution, as shown inFigure 3.

Figure 3, left. Representative losse s as a percentage of total losses .

The ability of the repair shop to analyze and replace thoseparts which most influence losses, such as the stator core, thewindings and the rotor, will affect the outcome of a rewind.

With all that in mind, let's take a look at losses in a typical50-hp, 1800-rpm, TEFC standard efficiency design. Thedistribution of losses is shown in Figure 4.

Figure 4, le ft. Losse s in a 50-hp, 1800-rpm, TEFC Pre-EPAct motor.

{mospagebreak}

The table in Figure 5 shows how these losses can be reduced. Following thatare detailed explanations of the techniques.

Figure 5. Possible methods to

reduce losse s through rewinding.

Stator Losses

Stator losses are primarily I2R losses, released in the form of heat as current passes through the stator windings.


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When rewinding a motor, smaller diameter wire will increase the resistance and therefore I2R losses; largerdiameter will have the opposite effect. If the original wire was aluminum, changing to the same size copper wirewill also reduce resistance and loss. Obviously, using a larger diameter copper wire will affect the bestreduction.

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 startingcurrent by as much as 23 percent.

Core Loss

Core loss is the sum of the eddy current and hysteresis losses that occur while energizing the motor's magneticfield.

Motors are insulated between the core laminations to minimize eddy currents, but the process of stripping candestroy this insulation. When stripping a motor for rewinding, insulation burnout must be done at carefullycontrolled 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 themthoroughly 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 excessiveloading, extended stall conditions, single phasing, or bearing failure leading to rotor striking can all causeincreases 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. Repairshops may have equipment to evaluate the core in its failed condition, but are unable to relate the results tooriginal 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 twoto 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 thatdesign efficiencies are maintained throughout production. The DOE requires that testing laboratories bethird-party certified to assure compliance with the testing procedure defined in IEEE 112 B. This process waswritten into the Federal Energy Act to assure reduced optimistic efficiency claims. Repair shops have no suchrequirement.

Rotor Loss

Rotor losses are I2R losses, released as heat through the rotor slots and endrings. It is unlikely that a repair shopwill be able to improve rotor losses.

Efficiency vs. Time

You have undoubtedly heard or read more than once that motor efficiency naturally decreases through motorlife as a result of "heat aging." This argument says that as the motor starts and stops the core temperatureincreases and decreases, causing deterioration in the core steel's electrical properties and a resulting increase ininternal 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 Motor


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Now 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:

Rewind the motor to the original efficiency.1.Rewind the motor to a higher efficiency.2.Replace with a new motor of same efficiency.3.Replace with a premium-efficiency motor.4.

A fifth option that no one should knowingly choose is to rewind the motor to a lower efficiency - but manyusers unwittingly make this decision. As explained above, it is very easy to damage the stator core insulationwhile 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 Motor

The high-fill rewind produces some efficiency gains whena larger wire size is used. As would be expected, thegreatest efficiencies are realized by retrofitting with newpremium-efficiency motors.

Figure 7 shows the annual operating cost difference for each of the options listed above. Note that operatingcost can be increased by a poor rewind just as much as it can be decreased by a new, premium-efficiencymotor.

Figure 7. Annual Operating Cost Difference: Rewind vs. Premium-Efficiency

Motor Continuous Operation @ $.07/kW-h

The conclusion is obvious: either replace failed motorswith new, premium-efficiency motors, or else exerciseextraordinary care in the rewinding process.

Protecting the Stator Core in a Rewind

No 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 thencause the motor to fail prematurely. You've probably had at least one motor that operated well for years beforeinitial failure, but failed again shortly after being rewound. The failure is more often the result of temperaturerise 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 maybe supplied with either organic or inorganic insulation coatings, or with no coating at all. If uncoated steel isused, 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 toname 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 interlaminationinsulation. Damage can occur even in a low temperature oven when several cores are stacked, and fire from the


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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 andmay actually change its chemistry at higher temperatures. Organic insulation can be damaged in any oven hotenough to burn out the winding insulation.

Inorganic insulation can withstand temperatures up to 700-deg F, allowing the old winding to be burned outsafely if the oven temperature is carefully controlled.

Uncoated semi-processed steel laminations will stick together if they get too hot, increasing eddy current lossesdramatically. Motors built with this type of steel can be stripped at oven temperatures of 700-deg F or below.

Stripping Methods

In the past few years, awareness has grown among users that poor quality motor rewinds can cause an increasein losses. Users have demanded an end to the practice of burning out the old windings at uncontrolledtemperatures.

Motor repair shops that have kept pace with technology have switched to temperature-controlled ovens andhave discontinued the practice of softening varnish with a handheld torch. Ask your rewind shop if they canperform any of the following non-injurious stripping techniques:

Mechanical strippingChemical strippingHigh-pressure water jetsFreezing processUltrasonic stripping

Regardless of the process used, make sure your rewinder can follow the motor manufacturer's recommendedsafe burnout temperature limits. Some do and some don't - which is probably the main reason for one shop'sreputation 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 toperform a qualitative core loss test before and after rewinding (assuming the core is okay to begin with). Thiscan also help you screen the motor population to determine if any given motor is even repairable. More repairshops now offer this service, so ask yours.

Establish a Repair/Replace Policy

In response to the rising cost of electrical power, every company should establish a repair/replace policy to helpmake intelligent decisions.

Give every motor-driven machine in your plant a repair/replace priority, and consider investing in spare motorsfor any continuous process machines that are critical to plant operation. These "critical" machines are excellentcandidates for retrofitting with premium-efficiency motors; the existing standard-efficiency motor should not bekept as a spare.

There is a good deal more to comparing relative in-use costs between motors than simple energy usage. Thelocal power company may offer rebates on new premium-efficiency motors. Extended warranties may beavailable to reduce MRO costs. A new motor could be installed with a variable speed drive to maximize theprocess productivity (the drive may also be covered under a rebate). Inventory costs can be reduced by notstoring 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,


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260-439-3245, 260-439-3338, http://www.gemotors.com/.

Tags: Energy Efficiency , June 2007 Issue , Motors

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