Selecting energy-efficient motors
DUE TO RAPIDLY rising power costs, there has been a growing demand for energy-efficient motors. Initially, the major emphasis has been the efficiency improvement at full load. But savings are also available at other operating conditions, since the motors have high er power factors, fewer PF-correction capacitors are required, and the branch-circuit losses are lower. These motors offer greater reliability, which will help to reduce operating costs and improve productivity. In addition, en ergy-efficient motors offer far greater application flexibility-stand ard ener gy-efficient motors can be used on many applications where special elec trical design motors (normal efficien cy) are currently being used.
Following is a summary of some of the many reasons why energy-efficie nt motors offer much more than a better way of converting electrical energy to mechanical energy or rotating motion. Highest efficiencies. Energy-effi cient motors have the highest efficien cies of any motor line designed or built. Some people ask, “Hasn’t the industry really gone back to the good old U frame design practices?” Actually, there is no comparison between the normal-efficiency U-frame, or T-f rame motors and the new energy-saving motor efficiencies. A look at the histo ry of 71Jz-hp TEFC 1800-rpm motor efficiencies, for example, shows that a 1944-design motor had an efficiency of 84.5%, a 1955 U-frame 87.0%, a 1965 normal-efficiency T-frame 84.0%, anda 1981 energy-saving T-frame 91.0%.
Lower operating cost. Price varies with horsepower, speed, enclosure and voltage. Operating cost will vary with electric power rates, operating hours, motor load and efficiency.
For a 75-hp 1800-rpm dripproof nor mal-efficiency motur operating contin uously on only $0.05/kWh power, the operating cost will be over 13 times the first cost. The leverage is considerable. The price premium for an energy efficient motor of this rating is approx imately 22% and the power savings are approximately 4%. The payback in this case is 0.42 year or 5 months.
Lower demand charg e. Frequently, utilities apply a demand charge for the maximum kilowatts used during the preceding 12-month period. Each mo tor operating during this peak 15- or 30-min period would contribute toward the charge. Only one 100-hp high efficiency motor can reduce the de mand by 2.9 kW. At a demand charge of $5.70/kW, this represents a monthly charge of $16.53 or $198.36 annually.
Fewer power-factor-correction ca pacitors. Energy-efficient motors have higher power factors as well as higher efficiencies. This means fewer kVARs of capacitors are required for power factor correction. Savings are greater for smaller-horsepower motors.
Lower branch-ci,rcuit losses. High er-efficiency motors have lower full load currents. The energy saved in the motor branch circuits due to lower line currents is in addition to that realized within the motor. In a system with a maximum allowed voltage drop of 3%, the branch-circuit losses are 120 W less for a 50-hp energy-saving motor than a normal-efficiency motor. With power factor-correction capacitors, the branc h-circuit losses can be reduced another 180 W.
Lower no-load losses. Some motors are applied to varying loads or duty cycle applications where they run for a period of time under a no-load condi tion. Even in these cases an energy efficient motor provides savings. The percent reduction in no-load losses is significant with more-efficient motor designs.
Reduced air-conditioning load. As motors convert electrical energy to mechanical energy, they waste a certain amou nt of energy in motor losses or heat. If the motor operates in an area that must be air conditioned, such as a textile plant, these losses contrib ute an added load to the air-condition ing unit. Some users evaluate the sav ings of the more-efficient motor by adding an additional 25% for the sav ings in air-conditioning load.
Savings increase with time. The savings available by using more-effi cient motors are directly proportional to the power costs. Since power costs are forecasted to rise faster than most other costs, the value of energy-effi cient motors will grow with each increase in the electric power rates.
Nameplate efficiency. There are many products being offered to cus tomers as the ideal solution to their rapidly rising power costs. Although large potential savings are promised, the facts are vague. This is not the case with energy-efficient motors.
NEMA has adopted standard MG1- 12.53b, which is an efficiency-labeling standard based on the probability (bell-shaped)-curve concept that once the normal value of efficiency is estab lished for a design, half the motors will be above and half below. The standard, which applies to NEMA designs A and B single-speed, polyphase squirrel cage, integral-horsepower motors in the range of 1-125 hp, calls for the full-load nominal efficiency to be iden tified on the nameplate. This standard recognizes the variations in materials, manufacturing processes, test results, and motor-to-motor efficiency varia tions for a given design. The full-load efficiency for a large population of motors of a single design is not a unique efficiency, but rather a band of efficiencies. The new standard indi cates the minimum and nominal effi ciency to be expected from a motor design and a population of motors.
Any energy-efficient motor that complies with NEMA standards will have the nominal efficiency value on the nameplate. Some manufacturers also put their guaranteed minimum efficiency value on the nameplate.
Int erchangeability. High-efficiency motors have the same hp-frame lineup as established by NEMA for normal efficiency T-frame motors. Therefore, they have the same shaft height and mounting dimensions and are easily interchangeable. If they are used to replace a U-f rame motor, it is neces s.ary to use a transition base.
Conformation with NEMA stan dards. If a motor meets NEMA stan dards, the nominal efficiency will appear on the nameplate as indicated above. This efficiency will be deter mined in accordance with NEMA test ing Standard MG1-12.53a, which is based on IEEE 112 Test Method B. In addition, these motors meet other NEMA standards with no compromise on inrush currents, starting or break down torques. Therefore, there are no specialized application rules or knowl edge required in their application.
Protection and control. Since ener gy-saving motors use a conventional design approach and meet NEMA standards, they can be applied with conventional, NEMA -standard over load, short-circuit, single-phasing, re verse-phase, and stall protection. In addi.tion, these motors employ the same control as a normal-efficiency motor when applied for reversing, plugging, overhauling loads, adjustable frequency, and other diverse applica tions.
Cooler nnd quieter opemtion. An added benefit of a motor with a higher efficiency is that there are fewer losses-less heat to dissipate. This results in a cooler-operating motor requiring less supplemental cooling normally supplied by the external fan on a TEFC design. Using a smaller or more-efficient fan can also reduce sound level.
Longer insulation life. Energy-sav ing motors operate below class B tem perature rise, and most are below class A rise at rated load. These motors are built with either class B or class F insulation. The result is a reduction of operating temperatures of at least 20°C. Since the theoretical life of insu lation doubles with every lOOC reduc tion in temperature, the insulation life increases up to four times the standard motor insulation life.
Improved bearing life. A common cause of bearing failure is too little lubricant. The longer the grease life, the less chance there is for bearing failure. Since the life of grease is directly affected by the motor operat ing temperature, the energy-efficient design offers greater reliability of the bearing system.
Lubricant life can be estimated from the bearing temperature. The tempera ture rise of bearings in a TEFC motor is approximately 112 to 2fs of the winding
temperature rise. The energy-efficient design increases the lubricant life 200% compared to a normal-efficiency motor. Increased lubricant life will most certainly contribute toward greater bearing and motor reliability.
Less starting thermal stress. These motors also have less thermal stress during starting and short-term tem perature peaks caused by abnormal operating conditions.
The rate of rise for an energy-saving motor is much lower than the rate for a standard motor. This results in lower total temperatures. The more-efficient design has a 30°C lower peak tempera ture after there are two overload trips
from a cold start and a 51oc lower temperature after one trip following a hot restart. Since rapid changes in temperature coupled with long-term thermal aging are a frequent cause of motor failure, the more-efficient de sign is clearly a premium quality motor that will give greater reliability.
Greuter stall capacity. The lower operating temperature and slow rate of rise of energy-efficient motors produce a greater stall capacity, providing bet ter protection against failure due to single-phasing.
Less susceptible to impaired venti lation. The energy-efficient motor is less susceptible to thermal damage from loss of ventilation. Although the AC induction motor of normal-efficien cy design is a very reliable device and requires little maintenance, it is essen tial that there be no obstructions of the cooling system. Since higher-efficiency designs operate at lower temperatures, they are less susceptible to dar.wge from impaired ventilation.
Better buy than the old U-.frame motors. When the motor industry shifted to the T frame with class B insulation, some motor users continued to purchase U-frame motors with their lower-temperature class A insulation. Although it is true that U-frame motors operated cooler, the T-frame normal-efficiency designs have equal or even greater reliability because the improved insulation system was de signed to withstand higher tempera tures.
Some motor users accepted class B insulation but insisted on class A rises for additional insurance against failure available with this added thermal margin.
The energy-efficient T-frame motors offer cooler operation and extra ther mal margin for significantly improved reliability. In addition, they have high er efficiencies, higher power factors, lower prices, lower noise, lighter weight, and longer life than the old U-frame motors. There simply is no comparison.
Higher service factors. To improve motor efficiency, the designer fre quently adds additional electromagne tics. This increases the motor’s over load capability in addition to its effi ciency. The industry motor standard for service factor is 1.0 (zero overload) for totally enclosed fan-cooled motors and 1.15 {15% overload) for dripproof designs.
All dripproof and TEFC energy-sav ing motors have service factors of 1.15, and some even have thermal margins that would allow 30 to 40% overload. However, it is not recommended that energy-efficiency motors be operated continuously at these high service fac tors, because the efficiencies drop off at loads above nameplate rating and this could shorten bearing life and cause shaft breakage.
Motor users or specifiers who delib erately oversize motors to compensate for unknowns or cyclical loads can now more closely match the motor size to the load.
Better suited for energy-manage ment systems. High-efficiency motors experience less stress from repeated starting associated with energy-man agement systems. Certain applications, particularly large fans and some com pressors, have hard starting duty due to the considerable inertia that must be accelerated to full-load speed. Energy efficient motors hold up much better than standard designs under these con ditions.
Thermal margin for sprcd con trol. The added thermal margin is also helpful where motors are being used on inverter drives where the nonsinusoi dal power supply produces extra losses and hPat, and the motor is operating at low speed with less than normal cool ing. The result can be a lower initial motor cost for the OEM.