Energy Savings by means of Energy Efficient Electric Motors and the Factors influencing Energy Management
Electric motors convert about 70% of the world's energy so it is not a surprise that they have been a key part of the world energy conservation plan. Any improvement of the effi-ciency levels of electric motors can have a measurable impact in the efforts of conserving energy and re¬ducing operating costs...
- Dr L Ashok Kumar
Energy management embodies engineering, design, applications, operation, and maintenance of electrical power system to provide for the optimal use of the electrical energy. Optimal in this case refers to the design or the modification of a system to use minimum overall energy where the potential or real energy savings are justified on an economic or cost benefit basis. Optimization also involves factors such as comfort, healthful working conditions, the practical aspects of productivity, aesthetic acceptability of the space, and public relations.
Any process requires a certain minimum consumption of energy. Energy additions beyond this minimum require an evaluation of the incremental cost of more efficient or techniques versus the resulting energy savings or costs. Some of the energy-intensive industries have long found it competitively advantageous to design for energy management and conservation. Savings realized by reducing energy usage and preventing economic losses by minimizing the probability of fuel supply curtailment are two economic incentives to develop an energy management program on a facility-by-facility basis. Regarding all the devices, in general, energy conservation methodologies can be categorized into the following four areas.
Electric motors convert about 70% of the world's energy so it is not a surprise that they have been a key part of the world energy conservation plan. Any improvement of the efficiency levels of electric motors can have a measurable impact in the efforts of conserving energy and reducing operating costs [13. As the world deals with energy consumption and climate changes, many governments and organizations have made tremendous strides in introducing energy-efficient legislatures and standards.
Electric motors convert electrical energy to mechanical energy. As in any other energy conversion devices, some energy is lost in the process. When electric motors convert energy, the electric energy is first converted to magnetic energy and then mechanical motion, when magnet fields interact with each between motor stator and rotor. To establish these magnet fields, stator and rotor windings are deployed. Further, the cooling system is set up to dissipate the heat inside the motor windings, iron core, and motor bearings. As the energy flows through stator winding to rotor winding, energy losses occur in the following areas: stator winding, stator core, rotor winding, rotor core, airflow system, and bearings.
Efficiency of a motor is the ratio of the power output from the motor shaft to the power input to the motor terminals. To understand the efficiency value, it is appropriate to examine the losses during this power conversion. The five major losses in the motor are stator resistive, rotor resistive, core, wind age and friction, and stray load. All of these losses are influenced by motor designing, manufacturing, and testing accuracy.
Stator and rotor windings are designed to create the necessary magnetic fields for torque generation. The stator resistive loss is /2R, proportional to the square of stator current and linearly proportional to the stator resistance. Stator resistance is influenced by motor voltage class and inrush requirement. The higher the terminal voltage, the thicker the insulation of the winding and the smaller the winding conductors will be, in turn, the higher the stator resistance. Similarly, rotor resistive loss is determined by the rotor slot size and rotor bar materials.
The contributing factors to the core loss are the type of electrical steel, frequency of the power supply, and air-gap flux density. One critical choice in design stage is the ratio of losses between copper and iron. Copper losses are electrical losses incurred in the motor windings, while the iron losses happen in the iron cores. Magnetic wedges could be used in the open stator slots to reduce the ripple of the air gap presence caused by the slot openings and the effective flux densities in stator teeth, thus increasing the efficiency of induction motors by decreasing the core loss and stray load loss.
Windage and Friction Loss
The speed of the motor and the type of the cooling fans used influence the windage and friction loss. Because the power of the fan is proportional to the third power of its speed, two-pole motors incur much larger portion of windage and friction loss. This loss adversely impacts 60-Hz machines more than 50-Hz machines.
Stray Load Loss
These losses account for the additional losses that exist only at load and increase with the square of the rotor current. The stray load loss is related to stator and rotor slot geometries, air-gap flux density, and electrical steel and manufacturing processes. Stray load loss occurs at higher frequencies because of the harmonics caused by the stator and rotor slotting and the rotor current. These losses are not easy to predict because they do not really belong to any one of the loss categories. For instance, the load rotor current will create distortions of magnetic field in the laminations between the rotor bars causing additional iron losses, which are not accounted for in the no-load test and which only measures the core losses in the absence of the rotor current. One effective approach to determine the stray load loss is to subtract the stator winding loss, iron loss, and windage and friction loss from the total losses that could actually be measured.
Similar to the gas mileage of a car, motor efficiency may not be as straightforward as it appears. Per NEMA recommendations, the efficiency shown on motor nameplates represents its nominal value. Because of various factors affecting motor efficiency, there is a tolerance band associated with this nominal value. This tolerance is set at 20% of the nominal losses [33. For example, the nominal efficiency of a 500-hp random wound motor is 95.8%, and the difference between the nominal and the minimum is 0.8%, which is a substantial change from the nameplate or a nominal efficiency value.
The major contributor to the loss variance of motors is the stray load loss. Stray load loss is sensitive to manufacturing processes such as punching laminations, stacking stator and rotor cores, die-casting aluminum rotors, and machining air gaps. Variations in the measured losses due to manufacturing could reach 10% [33.
If suitable, the percentage of stray load loss of the rated power could be determined based on Figure 1, and it decreases with the power output. These values typically are maximum values and used with IEEE test method El and IEC 60034-2-1 assigned stray load loss method. Therefore, the tested stray load loss could be smaller than the assumed values. This means that any stray load loss variation will only help narrow the tolerance. In recent years, successful efforts were made to reduce this loss. Without this reduction of the stray load loss, it would be difficult to achieve NEMA premium or international efficiency (IE) 3 levels.
Since congress passed Energy Policy Act (EPACT) legislature in 1992, the efficiency of electric motors has improved substantially. NEMA MG1 Tables 12-11, 12-12, 12-13, and 12-14 detail the requirements for motors ranging from 1 to 500 hp, 60 Hz and 50 Hz, and low and medium voltages. Table 12-14 covers 50-Hz machines as a part of harmonization with IEC and will not be discussed here. Tables 12-11 and 12-12 govern low-voltage, 60-Hz machines, while Table 12-13 governs medium-voltage, 60-Hz machines. Both open and totally enclosed fan-cooled (TEFC) machines are covered in these tables.
The difference between nominal values and minimum values is 20% variance of rated losses. The step change of efficiency values in above tables is determined with a 10% loss change, and the efficiency values are rounded up to one decimal point. This tolerance reflects the total variation from three key areas: manufacturing, testing equipment, and testing laboratories with a range from 4.5% to 18.9% [33. Within a single manufacturing facility and testing department, one band of tolerance of 10% loss variation is achievable as suggested by the data in the "Manufacturing" section.
IEC 60034-30 defines the efficiency requirement outside of United States and Canada and encompasses both 50- and 60-Hz machines. IE1 to 1E4 are four established efficiency levels ranging from the lowest to the highest. 1E2 and 1E3 of this standard are now in harmony with NEMA MG1 Tables 12-11 and 12-12 through the adoption of 60-Hz efficiency values from NEMA. For comparison, Table 1 illustrates the relationship between IEC and NEMA on efficiency levels. 1E4 marking is currently for information only in IEC and may require permanent magnet (PM) motors to achieve these efficiencies. PM motors will not only increase efficiency but also reduce motor sizes.
Table 1: Typical Range Of Efficiencies For Dry-Type Transformers: 25%-100% Loading (Informative)
Tolerance: NEMA Versus IEC
NEMA addresses the efficiency by listing both nominal and minimum efficiency values, while IEC addresses the efficiency tolerances in IEC 60034-1 by listing the tolerance percentages with the nominal values listed in IEC 60034-30. The nominal values represent the average values of a large population of motors of the same design. The distribution of the motor efficiency shall remain the same although some motor efficiency could be higher and lower than the nominal as long as the efficiency values do not fall outside of the nominal minus the tolerance.
Stray Load Loss: NEMA versus IEC
IEEE 112 Section 5.7.4 Table 2 shows the stray load loss as follows: 1.8% of rated power for motors between 1 and 90 kW, 1.5% of rated power for motors between 91 and 375 kW, 1.2% of rated power for motors with ratings between 376 and 1,850 kW, and 0.9% for 1,851 kW and higher. A comparison between the stray loss values calculated according to NEMA/IEEE and IEC standards is illustrated in Figure 1. The difference in stray load loss shows that the motor could be rated with different levels of efficiency depending on which standard is considered.
In the old IEC 60034-2 standard, the stray load loss was assumed to be 0.5% of the motor rated power. The IEC 60034-2 2007 edition has eliminated this assumption and published a curve (Figure 12 in Section 184.108.40.206.3) showing the variation of additional losses in percentage of the input power instead of the rated output power. The standard also describes equations to calculate the stray load loss for several power ranges.This change is significant since it eliminates the major difference between NEMA and IEC values, which could cause a full percentage point change in efficiency, thus making the efficiency comparison between IEC and NEMA motors more meaningful.
A. Housekeeping Measures
Energy conservation can be obtained by proper maintenance and operation. These activities include the following: shutting off unused equipment; improving electricity demand management; reducing winter temperature settings; turning off the lights; and eliminating steam, compressed air, and heat leaks. Proper lubrication of equipment, proper cleaning and replacement of filters in equipment, and periodic cleaning and lamp replacement in lighting systems will result in optimal energy use in existing facilities.
B. Equipment and Process Modifications
These modifications can be either applied to existing equipment or taken into consideration during designing and planning. Such modifications may include the use of more durable or more efficient components; the implementation of novel, more efficient design concepts; or the replacement of an existing process with one using less energy.
C. Better Utilization of Equipment
This can be achieved by properly examining the production processes, schedules, and operating practices. By these measures, one can obtain considerable energy saving. Generally speaking, many industrial plants are multiunit, multiproduct installations that evolved as a series of independent operations with minimum consideration of overall plant energy efficiency. Hence, in general, through the following procedures the plant efficiency can be improved:
• proper sequencing of process operations;
• rearranging schedules to utilize process equipment for continuous periods of operation to minimize losses associated with startup;
• scheduling process operations during off-peak periods to level electrical energy demand;
• Conserving the use of energy during peak demand periods.
By relamping, installing adjustable-speed drives (ASDs) in ventilation systems, and considering solar effects in commercial facilities, considerable energy saving can be obtained.
D. Reduction of Losses in Building Shell
Adding insulation, closing doors, reducing exhaust, utilizing heat, etc., can reduce heat losses. The key element of the energy management process is the identification and analysis of energy conservation opportunities (ECOs). The energy survey and energy balances identify energy wasting situations and differentiate between those that can be corrected by maintenance and operation actions from those that require capital expenditures. The former can be corrected in a short time and the results are almost immediate. The latter will require some investment and delivery time for materials and equipment. This paper is a summary of Chapter 5 of IEEE Std 739-1995 . Please refer to the original documentation for a detailed discussion.
Power losses in the electrical distribution system within factories, plants, and buildings occur through the operation of equipment inefficiently and through a distribution system design with losses in conductors and transformation equipment. The evidence of these losses may be seen in the plant distribution system by measuring the voltage difference between the service entrance and the terminal of the load. The National Electrical Code (NEC) (NFPA 70-1996) gives requirements for voltage drops in [2, Sections 210-19(a) and 215-2]. ANSI C84.1-1989 also contains voltage drop requirements . One has to realize that the NEC and ANSI requirements are considered as minimum. Power flow and voltage drop calculations should be performed routinely in the design stage, during construction, system operation, and major renovation so that proper material, equipment, and tap setting may be specified for the distribution.
A. Loss Reduction Opportunities in the Industrial Plant
1) Current-Carrying Conductors
The I2 R losses in electrical conductors can be reduced by selecting an increased wire size in cabling and by using a heavier cross section in busbars. The economic incentive can be determined by analyzing the duty factor, the load factor, the electricity price, and any changes in conduit size due to increasing the conductor cross section area or wire gauge.
Transformers in the U.S. are manufactured according to IEEE Std C57.12.00-1993, IEEE Std C57.12.01-1989, NEMA ST 20-1992, UL 1561-1994, and UL 1562-1994 -. However, the transformer standards do not require efficiency in the transformer design. The goals of the standards are safety, convenience, compatibility, security, reliability, noise control, and other engineering and environmental parameters. Energy savings can be achieved by specifying and purchasing efficient transformers and operating the transformer efficiently. The relationship between transformer materials and efficiency at various loads is shown in Table I.
Reactors are wound devices similar in many respects to transformers. They are used in reduced-voltage motor starting, current limiting, reactive power compensation, harmonic filtering, and reactance grounding. It is not economical to custom design smaller reactors solely for energy Conservation.
The opportunity does exist for energy conservation in the larger reactors. One can use a no-load formula for shunt reactors and a load loss formula for series reactors to perform the total loss cost evaluation.
An improvement of power factor is achieved by using capacitors that can provide both economic and system advantages. Direct economic advantages are attained when monetary incentives, such as a power-factor penalty, are enforced. Operational benefits, such as improved system efficiency, release of system capacity, reduction of power losses, and voltage profile improvement, may also be obtained.
5) Power Quality Improvement
Energy savings can be achieved by improving the quality of the power supply to the utilization equipment. The induction motor derating factor curve from NEMA is shown in Fig. 1.
Heating And Cooling Equipment
A. Material and Space Heating
Industrial plants normally require certain types of heating systems for personnel comfort and/or processes. Ultraviolet, infrared, and resistance heating are common for the material heating system. Electric space heating systems generally consist of either electric boilers or electric heating coils installed in air-handling units. Heat is distributed to the occupied spaces.
1) Energy Conservation Opportunities for a Heating System
a. Ultraviolet heating
• Combination of UV wavelength, particularly those from UV-A and UV-B bands, can improve producing quick curing of various surface coatings and inks.
• Contrasted with the conventional drying process for curing inks on products, UV heating can achieve as much as 80% energy savings.
b. Infrared heating
• Products with a suitable configuration, i.e., reasonable rates of surface area to mass, should be evaluated for electric infrared heating.
• According to the process requirement, the heat source(s) should be carefully selected.
c. Resistance heating
• Apply electrical energy only when the product is present.
Figure 1: Derating factor curve provided by NEMA.
• Install the appropriate thermal insulation to minimize heat losses from the process.
d. Space heating:
• It should be equipped with an on/off switch or temperature setting mechanism for the entire space served for demand control.
• Use high-voltage electric or electrode boilers and eliminate transformers for large space heating systems.
• Provide zoning capability to shut down unused areas.
B) Cool Storage System
Cool storage is a load management technique that shifts the electrical requirements for air conditioning or process cooling from daytime to nighttime hours. This displacement of load reduces the peak electricity demand and can result in significant electrical bill savings. Cool storage extracts heat from a storage tank or tanks at night to produce chilled water or ice and uses the storage devices to absorb heat from the load the following day. Since the refrigeration work is done off peak, less work is required during the day so the peak demand in the electrical system is reduced.
1) Energy Considerations
The energy performance of a thermal energy storage system can result in significant reduction in overall energy consumption. A report in the May 1993 ASHRAE Journal documents the retrofit of a Texas Instruments Incorporated electronics manufacturing plant in Dallas to a full-shift thermal storage system. The ten-year-old 1.1 million square feet factory has achieved a reduction of 30.2% peak electrical demand, and a reduction of 28.3% annual cooling electricity usage.
When ice storage systems are used, there may be a reduction in kilowatt-hour consumption of 20% due to air-cooled night condenser temperature and cold air. Retrofitting rooftop units with central storage systems, sized at 60% of the air-conditioning load, can also give around 30% demand savings and 15%-28% energy savings.
C) Refrigeration Equipment
Refrigeration is a process of transferring energy from a low-temperature source to a higher temperature sink, by circulating a refrigerant through an expansion or metering device from a high-pressure side containing a condenser to a low-pressure side containing an evaporator.
Table II: Fuel Requirements And Electricity Consumption Of Industry
1) Energy-Saving Features for the Refrigeration Equipment
a) Centrifugal chiller capacity control
• Capacity control for part load performance and shut down are important features for energy efficiency.
DC motors provide quick and efficient through dynamic or regenerative braking. The speed of a dc motor can be smoothly controlled down to 0 r/min and then accelerated in the opposite direction immediately.
NEMA has classified power supplies for dc motors by an alphabetic order of increasing magnitude of ripple current. A dc motor may be operated on a power supply having a letter designation occurring prior in the alphabet to the letter stamped on the nameplate. Operating a dc motor on a power supply having a letter designation occurring later in the alphabet than the letter stamped on the nameplate requires derating the motor due to the increased losses. The power supply codes, specified in 12.65 of NEMA MG 1-1993, are shown in Table III.
1) High-Efficiency DC Motor Application:
• Select the motor size to closely match the load. Since the brush current density is low and it does not commutate and film well, an oversized motor runs inefficiently.
• Select an efficient power supply, preferably NEMA Class C.
B. AC Motors
AC motors are manufactured as single-phase and polyphase. Single-phase motors are generally used in applications where the power supply is single phase and the power requirement is less than 1 hp. Three-phase motors are used in those applications that require more than 1 hp and where a three-phase power supply is available. Induction and synchronous motors are two types of ac motors that are available in the market. Squirrel-cage inductions motors, the workhorses of the industry, are the standard induction motor. Wound-rotor induction motors are constructed using a wound rotor accessible through slip ring. Wound-rotor motors are generally started with resistance in the rotor circuit to limit the starting current. Synchronous motors are manufactured in two major types: nonexcited and dc excited. Nonexcited synchronous motors, ranging to 30 hp, employ a self-starting circuit and require no external excitation supply. DC excited synchronous motors require dc supplied through slip rings for excitation. Synchronous motors can operate at lag lagging, leading, or unity power factor and, thereby, can provide power-factor correction.
1) ASDs: Induction motors, either general or special purpose, may be suitable for operation on inverter drives to achieve variation in the output delivered from the driven device by speed control rather than by means of mechanical control of the flow. When the application requires variable or constant torque from low to base speed, or beyond, an ASD may be appropriate.
2) Motor Sizing and Energy Management: There are three duty cycles for sizing motors. "Duty cycle" refers to the energization/de energization and load variations with respect to time for any application. These are listed in Table I
3) Example—Motor Sizing Determination Varying Duty Cycles: A motor-driven process with varying duty cycle is given in Table V. Columns 2 and 3 show the time duration for each part of the cycle and its corresponding horsepower requirements.
The rms horsepower for the motor is equal to 46.3 hp ((1 115 200/520)1/2). The rms horsepower determines the motor thermal capacity at constant speed to allow for ±10% voltage variation and the resulting additional motor heating. To ensure proper operation of the motor during the worst case scenario, a 10% allowance is added to the rms horsepower in sizing the motor. Therefore, a minimum of 50.9 hp (46.3*1.1) motor is required for the process. The motor usable horsepower is determined by nameplate horsepower, service factor (SF), and must be equal to or greater than the required horsepower. In this example, one can either choose a 50-hp motor with 1.15 SF or a 60-hp motor with 1.0 SF.
The other constraint is that the motor must be capable of carrying the peak horsepower (torque) value from the duty cycle at 90% rated voltage. Since motor breakdown torque (BDT) is reduced by the voltage square, the required BDT can be determined from
%BDT = (Peak Load/Nameplate * 0.92) * 100 + 20. (1)
20% margin is added in the equation to prevent the operation from being too close to the actual breakdown torque. For those two possible choices, the % BDTs for 50- and 60-hp motors are 202% and 172%, respectively. Since the BDT for either the 50-or 60-hp motor is 200% (NEMA MG 1-1993), the 60-hp motor is the better choice since only 172% of the torque is required at the peak-load horsepower.
C. Elevator and Escalator
The power demand depends upon the speed, weight, and acceleration of the elevator car and load. Older elevators will run better and more efficiently and provide better service simply by replacing the outdated switch and relay control system with a microprocessor control.
Microprocessor collective control systems are capable of interpreting every aspect of elevator operation: velocity, position, direction, loading level, waiting time, door operation, car assignment, energy usage, and diagnostic—all in "real time." With this fed back, the controller can issue changes in less than a fraction of second. The microprocessor can analyze traffic patterns.
Table IV: Duty Cycle Of Motor Load
Table V: Varying Duty Cycles Of A Motor Load
and compute the best method of moving cars. The microprocessor is capable of equalizing the loading over time. This will help to lengthen the drive and machine life cycles and their associated efficiencies. The microprocessor control should be sensitive to "full load up" and "empty load down" situations where the torque required to operate the elevator is the greatest. Anticipating peak traffic times is critical. One can also link a microprocessor controller to the building security system to manage the after-hours access privilege.
1) Energy Conservation Opportunities for Personnel Transportation System
a) Hydraulic elevators:
• performing passenger traffic studies carefully to select the appropriate speed and car capacity;
• installing microprocessor elevator controller for performance optimization;
• installing high-efficiency motors for the pumping unit;
• Reducing required torque by utilizing a 2 : 4 roping configuration.
b) Traction elevators:
• performing an elevator system and traffic study to assure the comfort and convenience of passengers as well as energy conservation;
• installing high efficiency motors for the hoisting assembly;
• Installing a silicon-controlled rectifier (SCR) to supply the power to the dc hoisting assembly; the efficiency of SCR drives can reach 95% and has the potential of 10%-35% energy savings compared to the motor generator set;
• installing microprocessor elevator controller for performance optimization;
• installing variable-voltage variable-frequency (VVVF) four-quadrant control pulsewidth-modulated (PWM) drive system with full regenerative power to achieve highest operation efficiency.
c) Escalators and passenger conveyors:
• using high-efficiency motors;
• adjusting operation hours to meet the traffic demand;
• using ASDs that would allow the unit to slow down when the unit has no load; the unit is then gently accelerated when passengers are present, which can provide a superb quality ride and the potential of 30% energy savings.
2) Energy Conservation Opportunities for Material Handling System
Material-handling systems are selected using the following rules to obtain the greatest efficiency:
• avoiding unnecessary movement;
• whenever possible, carrying out the process while material is in motion;
• whenever possible, moving the material in a straight line;
• using systems integration to reduce the time and energy in material handling;
• using an ASD for those operations that require wide range of motor speed;
• using linear induction motor (LIM) systems where practical; losses due to the inefficiency of gears, clutches, bearing, or shaft are avoided;
• using power drums for driving belt conveyors for the packing, handling, and sanitary applications in food processing.
Production of compressed air in industry consumes approximately 10% of all electrical energy. Compressed-air production, distribution, and end use can be inefficient when not carefully controlled. A typical compressed-air system wastes approximately 15% of the electrical energy consumed. Further, around 80% of the remaining energy is discarded as heat that could be easily reclaimed for space or process heating.
1) Energy Saving Opportunities of the Compressor:
The major energy-saving recommendations for designing, selection, operation, and maintenance of compressors are listed below.
a) Design and selection of equipment:
• Do not purchase or retain oversized compressors.
• Consider running costs of compressor when purchasing new equipment.
• Ensure adequate receiver capacity for peak demand period. The receiver acts to smooth out the air demand of the factory. The more peaky the demand, the larger the receiver should be. Typically, the receiver should be able to store at least 0.282 52 ft3 (8 L) of volume for every 2.118 87 ft3/min (60 L/min) free air delivery at full load of 101.5 lbf/in2 (700 kPa).
• Select all flexible air hoses and distribution pipework to achieve low air velocity. Do not construct excessive long air delivery lines.
• Arrange all air distribution pipework to slope down to suitably located draining points, preferably fitted with automatic traps.
• Ensure distribution pressure loss does not exceed 7.25 lbf/in2 (50 kPa).
• Provide air dryers that produce the required pressure dew point for the application.
• Equip them with an efficient aftercooler separator to condense and remove as much oil and water as possible.
b) Operation and maintenance:
• Repair all air leaks promptly.
• Install a pressure regulator at each point of use and adjust it down to the minimum pressure required for efficient and reliable operation.
• After reducing the pressure required at each point of use, adjust the operating pressure of the compressor to the minimum possible above the highest point of use pressure.
• Switch compressor off when not in use.
• Load and unload compressors progressively for multiple compressor installations.
• Eliminate all misuse of compressor air.
• Utilize wasted heat from compressor for heating water or air.
• Ensure that inlet air to the compressor is drawn from a cool source.
• Clean compressor intake filter regularly.
• Investigate whether air cylinders with air-actuated return stroke can be operated with a lower return air pressure.
• Isolate or eliminate redundant distribution pipework.
• Set up a regular maintenance program and monitor the trend.
The accuracy of instrumentation in testing motors is crucial. A 0.5% full-scale error of a wattmeter can cause one-band efficiency variation [43. In addition, NEMA round robin testing confirmed that 0.9% efficiency variation was due to testing .
Table 6 is a comparison between commonly used efficiency tests that are listed in IEEE and IEC standards. These test methods are similar to each other, with the exception of the eh-star test.
NEMAMG1—Part 12 specifies that squirrel cage motors rated between 1 and 500 hp should be tested by using IEEE 112—Method B, otherwise they should be tested by employing IEEE 112 Method E segregation of losses and direct measurement of stray load loss. For large motors, NEMA MG1—Part 20 refers also to IEEE 112 stating that stray load loss should be determined through direct measurement unless otherwise specified.
There are two direct test methods to determine the total losses for induction motors: IEEE 112—Method B [63 and IEC 60034-2-1 residual losses method 220.127.116.11.1 [53. IEEE 112—Method B is recommended for induction motors in the power range of 1-373 kW while IEC method for 1-150 kW. IEEE Method B requires three tests to determine the motor losses and obtain the efficiency: a thermal test at rated load, a no-load test, and a variable-load test at rated conditions.
The iron (core) loss and windage and friction loss are determined through the no-load test, while stator and rotor /2R losses are evaluated through the load test. The stray load loss is calculated as the difference between the total measured losses and the sum of the conventional losses (core, windage and friction, stator, and rotor /2R losses). These values of stray load loss are plotted versus the square of load torque values and then smoothed by using a linear regression.
The key differences between IEEE Method B and IEC residual loss method are the temperature rise determination and core loss [93. Since the temperature rise of the stator winding in IEEE 112—Method B is measured with thermocouples, this approach should be more accurate than one in IEC residual loss that uses interpolated values of the resistance. Core loss in Method B stays constant for all load points, while IEC residual loss method determines a unique core loss at each load point.
The indirect methods for total losses include the reverse rotation test (RRT) [53, [63 and the eh-star circuit method. RRT requires the motor to be coupled to the load and is very time consuming, while the eh-star does not require the coupling and could be performed relatively simple. IEC 60034-2-1 states that the stray load loss from eh-star is suitable for motors between 1 and 150 kW and requires the winding to be connected in star.
A complete comparison between these methods is given by Aoulkadi and Binder [93. A series of 15 motors, powers of 5.5 and 11 kW and two, four, and six pole, were tested by using IEEE 112—Method B, IEC 60034-2 residual loss, RRT method, eh-star, and equivalent no-load method. The test results show good agreement between input—output methods (IEC 60034-2 residual loss and IEEE 112—Method B) and eh-star circuit. Based on the results from this study, the RRT method yields higher stray load loss than the input—output methods.
This paper discusses the energy management and energy conservation for motors, systems, and electrical equipment. The energy management is critical to any society's future economic prosperity, industrial development, and environmental well being. The effective energy conservation must be taken into consideration in all aspects of design, manufacture, operation, and maintenance of any equipment while preserving safety, health, aesthetics, and without any or minimal impact on the environment. Since electric motors constitute about 70% of energy consumption, special attention is given to their types, design, characteristics, applications, sizing, and their utilization. A comprehensive and updated standard on energy management and energy conservation such as IEEE Std 739-1995 can ensure that a continuing high standard of expertise in energy management and energy efficiency is available to industry, commerce, and the public.
Dr L Ashok Kumar
Dept. of EEE
PSG College of Technology,
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