Abstract
This entry on electric motors is written from the perspective of a mechanical engineer involved in the design and specification of mechanical systems powered by electric motors. The entry describes several motor types and their intended applications. Specific components and performance criteria for motor selection are illustrated. The entry concludes with a discussion on motor efficiencies and the use of adjustable speed drives (ASDs) to enhance electric motor performance.
INTRODUCTION
This chapter on electric motors is written from the perspective of a mechanical engineer involved in the design and specification of mechanical systems powered by electric motors. The reader may have the impression that electrical engineers have the prime responsibility to specify electric motors, and it may come as a surprise that in most cases, it is the mechanical engineer specifying the electric motor. This is primarily because most of the mechanical equipment specified by mechanical engineers is packaged with electric motors. However, careful coordination with an electrical engineer on the electrical service requirements for the motor is essential in any design.
Electric motors find wide use in the industrial, commercial, and residential sectors of the United States. The following table summarizes the extent of these applications (Table 1).
Electric motors are responsible for a significant fraction of this nation’s electrical consumption (Table 2). As an example, electric motor energy use represents 60% of the total energy consumption in the United States’ industrial sector. A basic knowledge of electric motor design and applications equips the engineer with the skills to provide motor selections that insure reliable service with minimal energy consumption.
As might be expected, there are about as many electric motor classifications as there are applications. The criteria listed in Table 3 should be considered when selecting motors for a given application.
A general classification of common motor types with comments on their applications and special characteristics is given in Table 4. The three basic types of motors are alternating current induction/asynchronous, alternating current synchronous and direct current. Over 90% of the electrical energy input to electric motors serves to power alternating current induction motors.[2]
Because induction motors account for the highest percentage of energy use among motors, it is important to have a basic understanding of how these motors work and what technologies are available to enhance the performance of these motors. Induction motors are constructed to run on single-phase or three-phase power. Most homes in the United States are supplied with single-phase power; therefore, the majority of motors used in these homes are single-phase induction motors. Even though numerically the majority of motors fall in this category (see Table 2), the highest percentage of energy use results from the use of larger three-phase induction motors.
One other important distinction between the smaller single-phase motors and the three-phase motors is the drive method used in the application. Generally, single-phase motors have their shafts directly connected to the device; e.g., a motor driving a small hermetically sealed compressor or a small bathroom exhaust fan. When the motor fails, the whole device generally requires replacement. Larger three-phase motors are coupled to their devices with belts or couplings that allow for relatively easy motor replacement.
Another distinction between single-phase and three-phase motors is the method used to start the motor. In a three-phase motor, there are three stator (the fixed coil) windings spaced at 120° intervals around the rotor (the rotating member of the motor). These motor windings result in a rotating field around the rotor, thus starting and maintaining rotation of the rotor. Single-phase motors do not have this magnetic field arrangement; therefore, they require an auxiliary method to start the rotation. Generally, this is accomplished by a separate winding set at an angle of 90° from the main winding. This auxiliary winding is connected in series with a capacitor to provide a starting torque to initiate rotor rotation. After the motor starts, the auxiliary winding is disconnected by an internal switch. This type of motor is referred to as a capacitor-start motor.
14% of the population of motors power centrifugal fans
33% are used for material handling
34% are used for positive displacement pumps
18% are used for centrifugal pumps
32.5% are used in variable torque applications
67.5% are used in constant torque applications
In permanent split-capacitor induction motors, the capacitor is not disconnected after motor startup.
The difference between a synchronous and an asynchronous induction motor is dictated by the amount of full-load motor slip resulting from the motor design.
Table 1 Electric motor application statistics
Brake horsepower output required Torque required for the application Operating cycles (frequency of starts and stops) Speed requirements
Operating orientation (horizontal, vertical, or tilted) Direction of rotation Endplay and thrust limitations Ambient temperature
Environmental conditions (water, gasoline, natural gas,
corrosives, dirt and dust, outdoors)
Power supply (voltage, frequency, number of phases)
Limitations on starting current
Electric utility billing rates (demand, time of day)
Potential application with variable frequency drives
Table 3 Electric motor selection criteria
Referring to Eq. 1 above, a motor having one pole pair (two poles) would have a synchronous speed in the United States (where the applied voltage frequency is 60 Hz) of 3600 rpm. Four poles would result in a synchronous speed of 1800 rpm, and six poles would result in a synchronous speed of 1200 rpm. However, induction motors experience “slip,” which results in a lower operating speed. These motors, referred to as asynchronous induction motors, can experience full-load slip in the range of 4% for small motors and 1% for large motors. Thus, a four-pole asynchronous motor would operate at 1750 rpm.
Induction motors can be further classified according to the design of their rotors. When these rotor components are formed into a cylindrical shape resembling a squirrel cage, the motor is referred to as a “squirrel cage” motor. This type of motor is relatively inexpensive and reliable; therefore, it finds wide use in commercial and industrial applications. In motors where starting current, speed, and torque require close control, the rotor is comprised of copper windings much like the stator. This requires an external source of power for the rotor, which can be accomplished with slip
Table 2 Electric motor facts
It is estimated that 60% of all electric power produced in the United States is used by electric motors
90% of all motors are under 1 hp
8% of all motors are in the 1-5 hp range
2% of all motors are over 5 hp
70% of the electric use by motors is by motors over 5 hp; 22% is by motors in the 1-5 hp range; and 8% is by motors under 1 hp rings and brushes. As might be expected, this increases the first cost of the motor. Maintenance costs are also higher. These wound-rotor motors are generally sold in sizes of 20 hp and up.
When an engineer is considering an electric motor for a given application, it is important to match the enclosure of the motor to the type of operating environment involved. There are tradeoffs to be considered in this selection. A more open motor will stay cooler, which will improve its efficiency and service life. A closed motor will be less subject to contamination by a wet or dirty environment, but it may be less efficient. National Electrical Manufacturers Association (NEMA) Standard MG-1-1978 describes 20 different types of enclosures which address these two tradeoffs. Generally, in commercial and industrial applications, the enclosures fall in one of three categories: open drip proof (ODP), totally enclosed fan cooled (TEFC); and explosion proof (EXP). Fig. 1 is an example of a TEFC motor enclosure. Table 5 describes some of the other basic nomenclature used to describe motor enclosures.
Proper motor selection also includes an understanding of temperature ratings, insulation classifications, and service factors. National Electrical Manufacturers Association Standard MG 1-1998 relates the allowable temperature rises for each insulation classification. The most common insulation class is class B, which allows for a temperature rise of 80°C for Open or TEFC motors with service factors of 1.0, and 90°C for motors operating at a service factor condition of 115% rated load. When applying adjustable speed drives (ASDs) to motors, it is important to ensure that the insulation class is rated for ASD operations. In general, Class F will meet this requirement. Most premium-efficiency motors should also have this classification of insulation. At times, consideration may be given to selecting motors (especially large motors, 200 hp and up) with heaters in the motor windings. These heaters are energized when the motor is de-energized and they keep the motor windings warm to inhibit moisture wicking into the winding interstitial areas.
The service factor rating for a motor addresses the capacity at which an electric motor can operate for extended periods of time at overload conditions. As an example, if the service factor is 1.0, the motor cannot operate above full-load capacity for a significant period of time without damaging the motor. Service factors of 1.15, 1.25, or 1.35 indicate that a motor can operate at 1.15, 1.25, or 1.35 times its rated full load, respectively, for extended periods of time without failure. It should be noted, however, that this does not mean that the motor’s service life is not affected. Insulation life can be reduced by as much as 50% operating under these conditions.
Table 4 Classification of common motor types
| a.c. | Induction | Squirrel cage Wound rotor | Three-phase, general purpose, > 0.5 hp, low cost, high reliability
Single-phase, < 0.5 hp, high reliability > 20 hp, special purpose for torque and starting current regulation, higher maintenance requirement than for squirrel cage Very large sizes, high efficiency and reliability, higher maintenance requirement than for squirrel cage |
| a.c. | Synchronous | Reluctance
Brushless permanent magnet (overlaps d.c. also) |
Standard, small motors, reliable, synchronous speed Switched, rugged high efficiency, good speed control, high cost High efficiency, high performance applications, high reliability |
| d.c. | Wound rotor | Limited reliability, relatively high maintenance requirements | Series, traction, and high torque applications Shunt, good speed control
Compound, high torque with good speed control Separated, high performance drives; e.g., servos |
When evaluating motor replacement options, the frame size for a motor is also a matter of consideration. U-frame and T-frame designations are typical with electrical motors. New high-efficiency motors with U-frames are not always interchangeable with older style U-frame motors. It is important to check the frame size and determine if a conversion kit is required. The method of attachment or integration of the frame with the motor is also important. The author has often experienced owner preferences in selecting motors that will be used to drive centrifugal fans with a belt drive system. In one case, the owner had experienced structural failures with motors in which the motor housing was welded to the base frame.
Fig. 1 Premium-efficiency motor
Table 5 Examples of motor enclosure nomenclature
Open-type—full openings in frame and endbells for maximum ventilation, low cost
Semi-protected—screens on top openings to keep falling debris out, protected with screens on all openings (older style motors) Drip-proof—upper parts are covered to keep drippings out falling at an angle not over 15° from vertical
Splash-proof—baffled at bottom to keep particles out coming from an angle of 100° from vertical
Totally enclosed—explosion proof, nonventilated, or separately ventilated for hazardous atmospheres
Fan-cooled—totally enclosed motor with fan built in to ventilate motor
Now that we have covered some of the basics in motor types and selection, the discussion will turn to addressing three questions that consulting engineers often have to consider when evaluating options for motor replacement: what motor efficiency is most cost effective? Should motor rewinding be considered if an existing motor has failed and requires replacement? Are there other options to consider that might save energy?
The importance of motor efficiency is certainly obvious considering the energy consumption trends referenced at the introduction of this chapter. Improvements in electric motor efficiency have been recognized as a means of reducing energy consumption in all sectors of the economy.
The efficiency for motors is generally defined by Eq. 2:
This particular owner preferred motors in which the base and motor housing was a single integrated piece.
In motor replacement or in selection of new motors, the supply voltage should be coordinated with the selection. Most three-phase motors are designed to operate at 460 V and 60 Hz in the United States. Many commercial or institutional facilities are served with 208- or 230-V services. Residential motors and small fractional horsepower industrial motors would be served with 120-V power. It should be noted that even though a motor might be rated to operate at either 208 or 230 V, the operation at the lower voltage will result in a lower efficiency and shorter service life. Off-voltage operation can adversely affect electric motor performance and should be avoided.
Associated with the power supply to the motor is the motor’s contribution to the overall power factor for the facility using the motor. Utilities generally penalize customers with poor power factors, because this translates into reduced availability of transformer capacity and inefficient use of power. The typical threshold level for power factors may be in the range of 85%-95% for some utilities. When specifying electric motors, the power factor should be included, as well as the other factors indicated above.
In October of 1992, the U.S. Congress signed into law the Energy Policy Act (EPAct) that established energy efficiency standards for general-purpose, three-phase alternating current industrial motors ranging in size from 1 to 200 hp. In October of 1997, the EPAct became effective.[3] Table 6 illustrates a sample of required full-load nominal efficiencies for general purpose motors. Note that the power factors are not given in this table. Consideration should be given to selecting the highest possible power factor when selecting any motor. There are several standards available for testing motors. However, for the same motor, the tests performed using these standards may result in differing efficiencies. The generally recognized standards for testing are the IEEE 112 Method B and CSA C-390-93 standards. These two testing standards typically provide the same results when applied to the same motor.
A motor that meets or exceeds the minimum efficiencies specified by the Consortium for Energy Efficiency (CEE) is referred to as a CEE premium-efficiency motor. These motors generally have efficiencies that exceed those of EPAct motors. Table 7 illustrates a comparison between EPAct motor efficiencies and CEE premium motor efficiencies for several motor sizes. Even though the price of CEE premium-efficiency motors exceeds EPAct motors by as much as 20%, energy and demand savings can result in simple paybacks of 2 to 3 years.
Table 6 Required full-load nominal efficiencies for general purpose motors.
| Motor hp | Open motors (%) | Enclosed motors (%) | ||||
| 6-pole | 4-pole | 2-pole | 6-pole | 4-pole | 2-pole | |
| 1 | 80.0 | 82.5 | 80.0 | 82.5 | 75.5 | |
| 5 | 87.5 | 87.5 | 85.5 | 87.5 | 87.5 | 87.5 |
| 7.5 | 88.5 | 88.5 | 87.5 | 89.5 | 89.5 | 88.5 |
| 10 | 90.2 | 89.5 | 88.5 | 89.5 | 89.5 | 89.5 |
| 15 | 90.2 | 91.0 | 89.5 | 90.2 | 91.0 | 90.2 |
| 20 | 91.0 | 91.0 | 90.2 | 90.2 | 91.0 | 90.2 |
| 100 | 94.1 | 94.1 | 93.0 | 94.1 | 94.5 | 93.6 |
| Table 7 Comparison of Energy Policy Act (EPAct) to premium-efficiency motors | |||
| Horsepower | Average EPAct efficiency at 75% load (%) | Average premium efficiency at 75% load (%) | Ratio of premium-efficiency motor cost to EPAct motor cost |
| 1 | 82.4 | 85.2 | 1.20 |
| 5 | 88.5 | 90.5 | 1.20 |
| 10 | 91.1 | 91.9 | 1.09 |
| 20 | 92.3 | 93.5 | 1.08 |
| 50 | 93.9 | 94.8 | 1.14 |
| 75 | 94.5 | 95.7 | 1.09 |
| 100 | 94.9 | 95.7 | 1.17 |
| 150 | 96.1 | 95.9 | 1.24 |
| 200 | 95.3 | 96.3 | 1.14 |
The EPAct of 2005 required that federal agencies select and purchase only premium efficient motors that meet a specification set by the Secretary of Energy. On August 18, 2006, the DOE set forth the specifications developed by the Federal Energy Management Program to be used for purchasing. These standards are consistent with those recommended by the NEMA and the CEE. Tables 8 and 9 illustrate these new standards. In order to meet Energy Star requirements, the efficiencies in Tables 8 and 9 must also be met.
In evaluating motors in the field for general performance; in particular, to determine if the motor is overloaded or underloaded, measurements of applied voltage and amperes can provide valuable information. This data can also be used to evaluate annual energy consumption for a motor if an estimate of operating periods can be made. Eq. 3 illustrates a method for estimating the power consumption of a motor in kilowatts1[4]:
Three-Phase Power, kW
When replacing or rewinding motors—premium-efficiency motors (motors whose efficiencies exceed EPAct requirements)—the economics for the decision making will obviously depend on several factors such as motor type, operating regime (hours/year), and cost. Consortium for Energy Efficiency premium-efficiency motors[2] with operating regimes in the range of 4000 h per year will experience simple paybacks of 6 years or less compared to EPAct motors. Totally enclosed fan cooled motors in the size range of 2-25 hp will pay back in 3 years or less. Motors in sizes of 10 hp and below are generally replaced when they fail; larger motors are often repaired. When the rewinding or repair option of a standard-efficiency motor is compared to replacement with a premium-efficiency motor, the simple payback can be less than 2 years (for ODP motors in sizes up to 200 hp and for TEFC motors in sizes up to 40 hp). For single-phase and fractional horsepower motors, the simple payback varies significantly (2.5-10 years) when replacement with a premium-efficiency motor is evaluated.
Another area of concern consulting engineers face is in the selection of motors for pumps. Many engineers base their selection of centrifugal pump motor size on nonoverloading criteria. In order to ensure that the pump motor never overloads, the engineer checks the box on the pump selection software that will pick nonoverloading motor sizes. The resulting pump curve will always be to the left of the motor hp curve. The significance of this is that a pump with a duty horsepower of 2.3 hp could have a 5-hp motor selected. This results in the motor operating many hours of the year at part load and consequently, at lower than full-load efficiencies. Consulting engineers should evaluate their decisions to select nonoverloading motors with energy efficiency in mind. With the advent of reliable electronic ASDs, significant energy savings are now available for many applications involving electrically driven pumps and fans. It is very common today to design commercial building air conditioning systems with ASD technology. Variable-air-volume air conditioning systems use ASDs to vary fan speed as the demand for supply air varies with cooling and heating loads. Similarly, in large chilled-water or heating-hot-water distribution systems, ASDs are used to vary the flow rate of water in response to varying loads. In one such application involving two 400-hp circulating pumps on a chilled water system, the author observed a payback of under 3 years for a major retrofit to variable speed pumping. This project also included modifications to the control valves to accommodate the variable flow strategy.
Table 8 Nominal efficiencies for induction motors rated 600 V or less
Table 9 Nominal efficiencies for induction motors rated 5 kV or less
| Horsepower | Form wound | |||||
| Open drip-proof (%) | Totally enclosed fan-cooled (%) | |||||
| 6-pole | 4-pole | 2-pole | 6-pole | 4-pole | 2-pole | |
| 250 | 95.0 | 95.0 | 94.5 | 95.0 | 95.0 | 95.0 |
| 300 | 95.0 | 95.0 | 94.5 | 95.0 | 95.0 | 95.0 |
| 350 | 95.0 | 95.0 | 94.5 | 95.0 | 95.0 | 95.0 |
| 400 | 95.0 | 95.0 | 94.5 | 95.0 | 95.0 | 95.0 |
| 450 | 95.0 | 95.0 | 94.5 | 95.0 | 95.0 | 95.0 |
| 500 | 95.0 | 95.0 | 94.5 | 95.0 | 95.0 | 95.0 |
The application of ASD technology does require care. Consider the effect on power factor when selecting an ASD. Some ASD applications can actually result in an installed system with a power factor better than the original motor power factor.
However, improper selection of an ASD can also result in a lower overall power factor. Another concern deals with the harmonics that an ASD can superimpose on the system. From a consulting mechanical engineer’s perspective, this is an electrical engineering problem! Consulting with the project’s electrical engineer on the effects of harmonics is important. Applying too many ASDs on an electrical system can cause harmful harmonics that can affect the overall system performance. Many ASD suppliers offer a service for evaluating these harmonics and can assist in dealing with this issue.
Another significant issue in the selection of an ASD for an application is having a reasonable estimate of the operating profile for the system.[1] Without this knowledge, energy savings associated with the ASD application can be over- or even underestimated. Once this issue is resolved, and assuming the estimate is favorable, the resulting energy savings are still in question if the ASD system is not maintained properly. The total success of the ASD system hinges on the accuracy and reliability of sensing pressure differentials or flow rates. If the sensor goes out of calibration or ceases to function, the ASD will be rendered useless.
Because of the complex electronics in an ASD system (at least, complex to the consulting mechanical engineer!) and the various sensors and controllers that might be connected to the ASD, lightning and surge protection is another consideration in the overall system design. Again, the consulting electrical engineer should be able to provide insight into the appropriate lightning- and surge-protection systems for the project. The use of fiber optics in the communication system between the sensors and the ASD can eliminate some of the problems from lightning. Surge protection and optical relays can provide additional protection.
One last consideration in the control of electrical motors addresses the method of starting electric motors.
There are many technologies available that can be used to start motors (such as mechanical motor starters, ASDs, and soft-start starters) and improve the life expectancy of these motors. The selection of the motor starter sometimes falls on the mechanical engineer, and sometimes the electrical engineer. Whichever the case, consideration should be given to coordinating the starter selection with the motor selection to ensure the most efficient combination results. The electrical engineer’s role at this point is also significant. Careful coordination is required to verify and make necessary adjustments to the electrical service for the motor, to verify branch circuit wiring capacity, and to coordinate branch circuit protection as well as overload protection and disconnect requirements. Then it would be prudent to check the motor rotation (or else pumps will run backwards!) and alignment.
CONCLUSION
In summary, electric motor selection, replacement, and repairs require careful evaluation in order to provide an efficient, reliable, and easy-to-maintain motor drive system whether in the residential, commercial, or industrial sector. With over 60% of our nation’s industrial energy use represented by the use of electric motors, it is irresponsible not to apply the utmost of care in our practice of motor selection and replacement.
