AC Motor Efficiency - A Guide to Energy Savings
What we have done, and what we are doing
Part 7: A MOTOR AS PART OF A SYSTEM
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Selecting a Motor
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Other Components
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Picking the Best Applications
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Other Determinants
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Motor selection is a complicated process containing numerous tradeoffs. Efficiency is only one of several important considerations. The objective of informed motor selection is to arrive at the best possible installation consistent with minimum cost, horsepower and frame size for the specified life expectancy, load torque, load inertia and duty cycle of the specified application. To satisfy the torque, horsepower, and speed requirements of a large variety of motor applications, polyphase AC motors are designed and manufactured in four groups classified Design A, B, C, and D by NEMA. Each classification of motors has its own distinctive speed–torque relationship (Figure 21) and inherent expectations regarding motor efficiency.
Figure 21 Motors intended for loads that are relatively constant and run for long periods of time are of low slip design (less than 5%) and are inherently more efficient than Design D motors which are used in applications where heavy loads are suddenly applied such as hoists, cranes and heavy metal presses. Design D motors deliver high starting torque and are designed with high slip (more than 5%) so that motor speed can drop when fluctuating loads are encountered. Although Design D motor efficiency can be less than other NEMA designs it is not possible to replace a Design D motor with a more efficient Design B motor because it would not meet the performance demands of the load. The motor with the highest operating efficiency does not always provide the lowest energy choice. Figure 22 compares the watts loss of a NEMA Design D and a Design B motor, in a duty cycle which accelerates a load inertia of 27 lb. ft.2 to full speed and runs at full load for 60 seconds. During acceleration, the lower curve represents the performance of the Design D motor, while the upper curve reflects the NEMA Design B motor. The shaded area between the curves represents the total energy difference during acceleration. In this example, this area is approximately 6.0 watt–hours, the energy saved accelerating this load with a Design D motor instead of a Design B. During the run portion of this duty cycle, the energy loss differential favors the NEMA Design B, because it has a higher operating efficiency. In this example, the energy saved operating this load with a Design B motor instead of a Design D motor is approximately 2.8 watt–hours.
Figure 22 The bar chart shown in Figure 23 summarizes acceleration and running loss/cycle on both the NEMA Design B and Design D. Comparing the total combined acceleration and running portions of this duty cycle, indicates a total energy savings of 3.2 watt–hours favoring the use of the Design D motor, even though the Design B motor has an improved operating efficiency. The key is the improved ability of the Design D motor to accelerate a load inertia at minimum energy cost.
Figure 23 Other Components Affecting Efficiency Because a motor buyer selects the most efficient motor of a given size and type does not mean that energy savings are being optimized. Every motor is connected to some form of driven equipment: a crane, a machine tool, a pump, etc., and motors are often connected to their loads through gears, belts or slip couplings. By examining the total system efficiency, the component which offers the greatest potential improvements can be identified and money allocated to the component offering the greatest payback. In the case of new equipment installations, a careful application analysis including load and duty cycle requirements, might reveal that a 7–1/2 HP pump, for example, could be utilized in place of a 10 HP pump, thereby reducing motor horsepower requirements by one–third. By reducing the mass of the moving parts, the energy required to accelerate the parts is also proportionally reduced. Or in the instance of an air compressor application, the selection as to the size and type of compressor relative to load and duty cycle will affect system efficiency and energy usage. Of course, the most efficient equipment should be selected wherever possible. Reduced system efficiency and increased energy consumption is also possible with existing motordrive systems due to additional friction which can gradually develop within the driven machine. This additional friction could be caused by a build–up of dust on a fan, the wearing of parts causing misalignment of gears or belts, or insufficient lubrication in the driven machine. All of these conditions cause the driven machine to become less efficient which causes the motor to work harder. Rather than replace the existing motor with a higher efficiency model, replacing either critical machine components or the machine itself may result in greater system efficiency and energy savings. Changes in operating methods, such as applying variable frequency drives to centrifugal pumps and fans which currently employ dampers, variable inlet vanes or throttling valves, can save ten times the energy of replacing the motor alone. Energy Efficient Motors may be the most cost effective answer for certain applications. Simple guidelines are listed below:
Using these simple guidelines, followed by an analysis and cost justification based on various techniques can yield results that will influence motor choice beyond just initial cost consideration. OTHER DETERMINANTS OF OPERATING COST Although efficiency is a commonly used indicator of energy usage and operating costs, there are several important factors affecting motor operating costs. Rated performance, as well as selection and application considerations of polyphase motors, requires a balanced power supply at the motor terminals. Unbalanced voltage affects the motor’s current, speed, torque, temperature rise and efficiency. NEMA Standard MG 1–14.35, recommends the derating of the motor where the voltage unbalance exceeds 1% and recommends against motor operation where voltage unbalance exceeds 5%. Voltage unbalance is defined in Figure 24.
Figure 24
Voltage unbalance is not directly proportional to the increase in motor losses, as a relatively small unbalance in percent will increase motor losses significantly and decrease motor efficiency as Figure 25 shows. An effort to reduce losses with the purchase of premium priced, premium efficiency motors that reduce losses by 20% can easily be offset by a voltage unbalance of 3.5% that increases motor losses by 20%. Energy cost can be minimized in many industrial applications by reducing the additional motor watts loss due to voltage unbalance. Uniform application of single-phase loads can help assure proper voltage balance in a plant’s electrical distributor system used to supply polyphase motors. One of the most common sources of motor watts loss is the result of a motor not being properly matched to its load. In general, for standard NEMA frame motors, motor efficiency reaches its maximum at a point below its full load rating, as Figure 26 shows. This efficiency peaking below full load is a result of the interaction of the fixed and variable motor losses resulting in meeting the design limits of the NEMA standard motor performance values, specifically locked rotor torque and current limits. Power factor is load variable and increases as the motor is loaded as Figure 26 shows. At increased loads, normally in the region beyond full load, this process reverses as the motor’s resistance to reactive ratio begins to decrease and power factor begins to decline. In some applications where motors run for an extended period of time at no load, energy could be saved by shutting down the motor and restarting it at the next load period.
Figure 25 Proper care of the motor will prolong its life. A basic motor maintenance program requires periodic inspection and, when encountered, the correction of unsatisfactory conditions. Among the items to be checked during inspection are: lubrication, vibration, ventilation and presence of dirt or other contaminants; alignment of motor and load; possible changing load conditions; belts, sheaves and couplings; and tightness of hold-down bolts.
Figure 26
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