Reducing motor losses
Energy can be conserved and electric power bills significantly reduced if motor operating losses are minimized. This article will examine losses and meth ods of reducing them.
Part 2 -Methods, application, and payback
POWER losses in moto rs can be reduced by several diff erent m eth ods. High-effici ency motors, the N A SA developed power-factor controller, and variable-frequency, variable-speed drives are all effective when properly applied.
It is important to understand not only how each of these systems reduces losses and increases the efficiency of a motor, but also which method provides the most effective results and the shortest payback on the investment.
the standard motor efficiency was about equal to that of many of today’s high-efficiency motors. Then better insulating materials were developed, making it possible to push more cur rent through a given wire size and run it at a higher temperature. This, in turn, made it possible to reduce motor core sizes and make the motors smaller in overall dimensions. The net result was smaller, lighter, lower-cost mo tors, which became standard as a result of thr. competitive nature of the industry.
Until a few years ago, initial cost was the major factor in purchasing a motor of a given hp, design, and enclo sure from among suppliers known to provide adequate reliability and availa bility. Operating costs, as determined by motor efficiency, were generally ignored. In the clays of less than 2- cents-per-kWh electrical power, this was permissible, especially since in
industry the cost of electrical power averaged less than 1% of the total value of products shipped. In addition, almost half of all motors were pur chased by original equipment manufac turer (OEMs) to be incorporated into other equipment. The OEM is concerned with the reliability of his final product and its initial cost. He will purchase a motor of adequate reliability at the lowest cost and in most cases ignore the operating losses, which he does not pay and which the buyer of his equipment rarely consider!’.
This situation is changing. With power costs averaging 4 to 5 cents per kWh and still rising, the cost of electri cal power can no longer be ignored. In addition, the availability of additional power is no longer assured as demand begins to exceed utility capacity, and it is essential to the national economy that waste be reduced. All these factors have made the high-efficiency motor desirable in spite of its higher initial cost. Today, the astute motor purchas er analyzes the overall cost of owning a motor, including operating costs, in selecting the motor, and even in purchasing equipment in which an OEM has installed motors. In many cases, the payback time for the increased cost of energy-efficient motors can be short and, where applied correctly, very attractive economically.
Reducing winding losses
Winding losses are the J2R heat losses resulting from current flowing through the windings. They represent 55% to 60% of the total losses. As we have seen, they vary with the square of the current in the stator windings and rotor conductors, and with the resist ance of these windings. The current drawn by the motor is primarily a function of the load on the motor and cannot be reduced substantially, al though improving the motor’s power factor can reduce the current some what. Therefore, th’e best way to reduce J2R winding losses is to reduce the resistance of the windings. This is what is done in energy-efficient motors, sub ject to limitations on physical size and cost.
In the stator, the size and number of conductors can be increased to lower the resistance. In recent years many standard motors in smaller sizes (up to 20 or 30 hp), which make up the numerical majority of motors purchased, have used aluminum wire in their stator windings. However, in creasing the cross section and number of the aluminum conductors would increase the physical size of the wind ing.
NEMA standard motor frame sizes have, for each frame, a standard dimension from the base to the center line of the shaft. Increasing the size of the stator could require a larger dimension, making a larger NEMA frame size necessary. This is undesira ble, since it increases both motor cost and space requirements. For these rea sons, high-efficiency motors almost universally use copper conductors in the stator windings for minimum resistance in minimum dimensions.
Losses in the roto s of squirrel-cage induction motors can also be reduced by increasing conductor sizes. Most rotors of moderate-size integral-horse power motors today have the sec ondary-conductor bars which make up the “squirrel cage” cast in one piece, along with the end rings and fan blades. In this application for ease of casting, aluminum is used. Rotors with copper bars, swaged and brazed into place, would be too costly to manufac ture. By enlarging the slots, aluminum bars of larger cross-section can be formed. Both slot size and geometric shape are very important in motor design. For wound-rotor motors, cop per conductors can be used. Larger total rotor conductor cross-section de creases the J2R rotor losses in either case.
A reduction in the air gap between stator ‘and rotor and a reduction of magnetic flux density (by using more and better magnetic steel cores) will both reduce the magnetic field re quired, which will therefore reduce the current necessary to produce this mag netic field. It is the reactive component of the total motor current, the magne tizing current, that is reduced. This reactive component does not add to the power consumed; but it does increase the J2R losses, which are read on the kWh meter. Reducing the reactive cur rent thus cuts down the PR losses and also improves motor power factor.
In motors designed for minimum J2R losses, each of these factors is used and balanced against the increased cost. A typical high-efficiency motor uses about 20% more copper and 15% more aluminum than the equivalent-hp stan dard-efficiency motor.
Reducing core losses
Core losses, as we have seen, consist of two components-hysteresis losses and eddy-current losses. They repre sent about 20 % to 25% of the total losses. Hysteresis losses can be reduced by using silicon steel rather than car bon steel in the stator and rotor lami n ations. For a given magnetic flux den sity and steel thickness, the hysteresis losses are a function of the material used. Typical good-quality carbon steel will have losses of 4.5 to 5 watts per pound (W /lb) at a flux density of 15 k il ogauss (kG). At the same flux densi ty, even a low grade of silicon steel has losses of only 3.6 W/lb, and higher grades can have losses as low as 2.5 W /lb . However, silicon steels cost at least twice as much per pound as car bon steels-from 200 % for the lowest grades to over 230 % for higher gr ades-so the improvement in losses must be weighed against the increase in cost.
Eddy-current losses can be reduced by making the core laminations of thin n er steel. For a med i um-grade sili con steel, goi ng f rom 24-gauge to 26- gauge laminations can reduce eddy current losses by about 15 , and going to 29-gauge laminations can reduce these losses by almost 20%. Material costs will increase by only about 5%, but there will be increased manufac turing costs as a result of the larger number of laminations required to make up cores of a given size.
Both hysteresis and eddy-current losses can be reduced by reducing the flux density. This can be accomplished by lowering the current in the wind ings, but only a small improvement is possible here , mainly by reducing the air gap between the stator and rotor. A greater improvement can be obtained by increasing the core size, since for a given total magnetic flux a larger core will result in a lower flux density as a result of the increased cross-sectional area. In addition, larger windings to reduce PR losses generally require a physically larger core. Increasing core diameter will increase the distance from the base of the motor to the center line of the shaft, requiring a larger NEMA frame size, so the increased core area is usually obtained by increasing core length, resulting in a longer motor. Greater motor length does not increase frame siz.e. The decreased flux density reduces the CORE DESIGN AND ASSEMBL Y has a major effect on core losses. Assembled core is slammed into machine (photo near right),
to loosen laminations so insulation can pene trate. and then dipped in special insulating liquid (photo far right) so that f1nal core will have lowest eddy-current losses. Hysteresis losses are determined by quality of core steel. Slot design is also important in increas ing efficiency.
required magnetizing current in some measure, resulting in improved power factor and a small reduction in PR losses.
The reduction obtainable in core losses must be balanced against the increased cost of materials. Typically, a high-efficiency motor uses thin lami nations of silicon steel, with about 35% more steel in the core than a standard motor of the same horsepower.
Reduc ing mechanical and stray –load losses
Mechanical losses are those resulting from friction and windage within the motor, representing only about 5% to 8% of the total losses. Friction is essentially bearing friction, and a slight improvement can be obtained by using high-qualit y, low-friction bear ings. High-quality bearings are essen tial to maintain the closer clearances required by reduced air gaps between the stator and rotor in any case. Wind age losses are caused by friction of the air against the rotating parts of the motor and of the cooling air against the internal cooling fans and circulat ing through the motor. Small improve ments are obtained by optimizing fan blade design and the paths of circulat ing cooling air. The possible improYe ment in efficiency is small, but the increased cost of better bearings and well-designed fan blades is also small. Energy-efficient motors use quality bearings internal fans, and air-cooling systems as a matter of course.
The term “stray-load” losses is a catch-all for some actual losses caused by leakage flux induced by motor cur rents, for variations in losses with load which, for convenience, are assumed to be constant, for losses resulting from nonuniform current distribution in stator and rotor conductors, and the like. They amount to about 11% to 14% of the total losses. Attention to the involved design of stator and rotor slot geometry an<l insulation is impor tant. Little else can be done to reduce these losses per se, but motor improve ments made to reduce winding and core losses and total motor current will also
have the effect of reducing these losses. Careful manufacturing and hand ing of core laminations also will help. A well designed high-efficiency motor will inherently have low stray-load losses.
High-efficiency motors are selected tion offset the higher initial cost. How ever, there are some other benefits that accrue to users. The lower-loss design s produce less heat and therefore ru n at lower operating temperatures. Some are manufactured with alumin u m housings, which reduce operating temThese lower operating temperatures will result in longer motor life, sin ce the life of insulating materia ls de creases as temperature increases. It is a rule of thumb that the life of insu la tion is doubled for every lOoC reduct ion in operating temperature. While insu lation failure is not the only cause of motor failure, reduced insulation tem perature will certainly increase average motor life, assuming that the insuthe same or higher operating temperatures as those in standard motors Reduced operating temperatures will also improve the overload capacity of these motors. Since insulation is at a lower temperature to begin with, the motor can be overloaded for longer periods or greater amounts before the insulation reaches the maximum allowable total temperature. For the same reasons, these motors can be operated in higher ambient tempera tures or at higher altitudes (where the thinner air provides less cooling) with no derating or less derating than stan dard motors. They will also tolerate wider variations in appl ied voltage without overheating. The lower operat ing temperatures should improve the effectiveness of lubrication and in crease the life of motor bearings. Finally, the reduction in losses means less waste heat is produced. In an air conditioned plant with large numbers of motors, this might reduce the over all cooking load. (Of course, in a plant that requires heating, the reductio n in heat might increase the heat requ ired from the heating system.) Generally speaking, high-efficiency motors can not only pay for their higher initial cost in power savings; they also should have longer life and a wider range of application than standard motors.
