Electric Motors for Energy Efficient HVAC Applications.
The essence of HVAC is moving fluids from location to location. Whether that fluid be air, water or refrigerant, controlled movement of that fluid is critical to effective and efficient HVAC operation. Most HVAC devices that affect fluid movement are rotational devices such as pumps, fans and compressors, and the vast majority of these are rotated by electric motors. Because most systems operate at both full-load and part load, means of modulating the flow impelled by these devices are often required. Before variable speed motors became common, flow modulation devices might be incorporated into the fluid side of the systems. Examples include the pre-rotation vanes on centrifugal compressors, discharge dampers on centrifugal fans, pressure controlled bypasses for pumping systems and slide valves for screw compressors.
Initial efforts to provide variable speed capability for large electric motors included the mechanical variable speed drives that used variable pitch sheaves to change drive ratios and eddy current clutches that use controlled slip to vary the output speed with fixed input speed. Motors for these systems were constant speed, and the only selection criteria might be the efficiency and power factor implications of operating these motors at lower torque levels because of decreased load.
Electric motors for HVAC applications fall into two general classes, determined by the type of power, single-phase or three-phase, supplied to the motor. Single-phase motors are generally smaller, with a maximum size of 5 hp (3.7 kW), while larger motors are polyphase (usually three-phase motors). In general, three-phase motors should be specified for all loads, if motors are available in that size and if three-phase power is available at the site. For HVAC applications, most motors, both single- and three-phase, are induction motors, although other types of motors, such as synchronous ac motors are sometimes used. Recently a variation of the brushless dc permanent magnet motor, referred to as an electronically commutated motor (ECM), has seen wide market penetration.
With modern emphasis on energy efficiency, electric motor and motor control selection has become a critical task. Not only are issues of energy efficiency important, but also durability, maintainability and impact on system power factor should be considered in motor selection.
Induction motors rely on electric currents induced in the rotor by the stator coils to generate magnetic fields that interact with the stator coil magnetic fields to produce forces that rotate the rotor. The advantage of the induction motor is that there is no physical electrical connection between an electrical source and the rotor, meaning that the motor is brushless, and without slip rings, both of which create frictional losses and wear points that ultimately require maintenance and parts replacement. Most motors for HVAC use, furthermore, can be characterized as "squirrel cage" motors, because of the construction of their rotors. In a squirrel cage motor, the rotor is constructed of circular steel plates that are stacked together to form a cylinder. Grooves are cut into the circumferential face of the cylinder and are fitted with copper or aluminum conductors. The motors are called "squirrel cage" because the pattern of the conductors on the face of the cylinder resembles the rotating exercise cylinder often found in the cages of pet rodents. This construction is used because it is much more cost-effective and robust than wire round rotors.
A characteristic of induction motors is what is referred to as "slip." "Slip" is the difference in rotational speed between the "ideal" stator magnetic field rotation and the rotor in operation. "Slip" represents the rotation of the rotor with respect to the stator magnetic field. If the rotor rotated at precisely the speed that the stator magnetic field rotated, there would be no movement of the rotor within the stator magnetic field, no current would be induced in the rotor conductors, which would result in no magnetic field generated by the rotor and no force to cause rotor rotation. Because the rotational speed of the motor is not the same as the rotational speed of the stator magnetic fields, generated according to the formula below, they are also often called "asynchronous" motors. (1)
The starting condition for an induction motor represents 100% slip, as the stator magnetic fields will be oscillating according to the formula below and the rotor is stationary, resulting in the largest induced currents in the rotor conductors. The stator magnetic field rotation speed, often called the "ideal" speed, is a function of the frequency of the AC line current and the number of poles on the stator:
[N.sub.S] = 120F/p (1)
[N.sub.s] = the magnetic field rotation speed,
F = the frequency of the incoming ac power, and
P = the number of poles on the motor.
Often, induction motors are wired so that the number of poles can be changed. Motors may have three input power taps for low, medium and high speed, each tap is wired internally to aggregate the stator windings into a specific number of poles, so that the different taps cause the motor to operate with a different number of poles and thus at a different speed. Given that the number of poles will be an even integer, the possible "ideal" speeds for a motor with a 60 Hz power supply are 3,600 rpm (with two poles), 1,800 rpm (with 4 poles) and 1,200 rpm (with six poles). The rated speed of induction motors will usually be 2% to 5% less than the "ideal" speed, so commonly rated speeds for induction motors might be 3500 rpm, or 1750 rpm. Induction motors fall into the following National Electrical Manufacturers Association (NEMA) classifications: (2)
* NEMA Design A: High torques, low slip, high locked amperesused for pumps and fans if starting current inrush is not an issue.
* NEMA Design B: Normal torques, normal slip, normal locked amperes--used for pumps and fans if starting current limitation is desired.
* NEMA Design C: High torques, normal slip, normal locked amperes--used for hard to start loads such as positive displacement pumps.
* NEMA Design D: High locked-rotor torque, high slip--not normally used for HVAC.
Starting a constant speed induction electric motor is always a problematic event, especially for single-phase motors. Because of the configuration of the stator poles in a three-phase motor and the 120 degree phase shift of each of the electric phases, the magnetic fields generated in the stator coils inherently induce rotation in the rotor. The only problem with starting a constant speed three-phase electric motor is that when the rotor isn't moving, the relatively high rotational speed between the stator coil magnetic fields and the rotor generate large currents in the rotor, causing a large current draw in the stators. This phenomenon is captured in one of the ratings for an electric motor, the LRA, or locked rotor amps, which is typically several times as large as the FLA, or full load amps, the measure of the current draw of the motor when running at full load.
Electrical system design must take into account the starting inrush current to avoid circuit overload issues when the motor is starting, Single-phase induction motors, on the other hand, are not inherently self-starting. The oscillating magnetic field of the single-phase stator, while it generates current and magnetic fields in the rotor, does not induce rotation in the rotor. To initiate rotation of the rotor, a second, out of phase, magnetic field is required. The secondary stator field can be created by the primary stator coil inducing phase shifted currents and magnetic fields in a secondary stator winding or can be created by a secondary stator coil powered in series with a capacitor to create a phase shift in the current flowing through that winding.
The first single-phase motor starting strategy is embodied in the shaded pole motor, and the second in the permanent split capacitor (PSC) motor which, between the two, represent the vast majority of singlephase HVAC motors in use in HVAC applications.
Because the starting system components of both of the types of motors remain engaged during normal operation, the efficiency of these motors is reduced. Typically, they will operate with a maximum efficiency of no more than 65% at full load and 40% or less at part load. Smaller versions of these motors may have full load efficiencies as low as 40%. (3)
One characteristic of induction motors is that they have a trailing power factor, which is worse during starting and during part load (low torque) operation. Lagging and lagging power factor are metrics of the displacement of the sinusoidal current waveform from the voltage waveform. Capacitative loads tend to be leading, while inductive loads tend to be lagging. Harmonic power factor, discussed later, is the deformation of the sinusoidal current waveform to some other shape.
The most common single-phase motors, the shaded pole and the permanent split capacitor motors, tend to have worse power factors than large three-phase induction motors. For locations with heavy motor loads, power factor correction may be required to avoid utility penalties. Application of electronic variable speed ac drives to larger induction motors can significantly reduce power factor issues, both at motor start-up and when running at low part loads. Equipping all large induction motors with variable speed drives will completely overcome power factor issues for the motors; however, when ac drives are used, power factor correction capacitors should not be used to correct power factor issues of other loads, because drive dynamics could damage the power factor capacitors. (4)
Electronically Commutated Motors
Most engineers would consider HVAC systems to be well-served by modern three-phase electric motors. These motors are available with very high efficiency (95+%), compatibility with variable frequency drives for part load operation, soft-start capability to reduce starting current inrush, and with very high reliability and durability. Single-phase motors, on the other hand, are often the problem child for HVAC systems. The inefficiency and unreliability of these motors have rendered some HVAC systems that rely on small motors much less attractive than systems that use larger three-phase motors. In the last few years, however, advances in solidstate power electronics have brought a new single-phase motor type into the market-place. This motor type, the ECM, has efficiency, durability, variable speed and starting characteristics that rival three-phase motors, and offers new viability to previously discounted HVAC system applications.
The ECM is a relatively new technology that is now having a big impact on the design and configuration of HVAC systems and components. The longer life, greater reliability, enhanced energy efficiency, and most importantly, the variable speed capability of this technology has not only given engineers new alternative systems for consideration, but also has increased the relevance of some existing systems. (5) PSC motors in HVAC applications generally range from 35% to 50% efficiency, while ECMs will range from 70% to 80%. (3)
ECMs use permanent magnet rotors and inverterdriven power supply to the stator windings to start the motor and control its speed. Technically, these motors are not considered ac motors, but are brushless dc motors. The power supply to the stator coils is not a typical sinusoidal alternating current wave-form, but is bidirectional direct current with a wave-form that can be precisely configured to maximize the efficiency and smooth operation of the motor. A sensor device (often a Hall Effect sensor), detects the rotor position, so that the dc wave form supplied to the stator coils can be configured optimally for power and energy efficiency.
The technology for this motor design traces its roots to the development of computer accessories such as tape and disk drives, using so-called "stepper motors" that require precise control of the stator power supply to generate precise movement of the devices driven by the motor. As power electronics became more advanced and more economical, these usage of these motors expanded far beyond the precision-oriented applications they initially served.
One disadvantage of ECMs is that, like other nonlinear loads, such as computer power supplies, compact fluorescent lamps, LED lighting power supplies, and other rectifier applications that generate harmonic distortions to the standard alternating current waveform. The harmonic currents produced by these loads produce no useful work and are, therefore, reactive in nature, in the same way that inductive and capacitive loads are reactive, reducing the power factor by reducing the ratio of useful power to apparent power.
Because non-linear loads are neither leading nor lagging shifts of the current waveform with respect to the voltage waveform, they cannot be mitigated by application of an opposite displacement reactive load. Instead, passive or active harmonic filters may be added to the power network to mitigate sinusoidal waveform distortion and improve power factor. In most buildings, however, the installed power of ECM motors does not constitute the greatest source of harmonic distortion in the electrical supply system. The need for harmonic power factor correction will be driven more by other solid state non-linear loads, described above, that are becoming more pervasive in consumer products.
In addition to offering efficient variable speed operation, ECMs tend to be much more reliable and have greater longevity than other types of single-phase motors. Longer motor life is related to the following characteristics of the motor:
* Soft start reduces stress of the bearings caused by very high starting acceleration forces.
* Soft start reduces staring current inrush and thus thermal stress on the windings.
* The rotor on almost all solid start motors is a permanent magnet, eliminating rotational stresses and movement for the motor windings.
* Most ECMs incorporate ball or roller bearing that are inherently longer-lived than sleeve bearings used in most PSC and shaded pole motors.
The reliability, energy efficiency and variable speed capabilities of ECMs have given new life to several HVAC system types and components that were previously discounted by issues with induction motors. The system most improved by these motors is the fan coil system. In climate zones or building occupancies where air side economizer is not very beneficial, fan coils with ECMs demonstrate significant energy savings over central VAV systems. (6) For these systems, when ventilation air is provided by a separate dedicated outdoor air system, low-noise soft-start motors enable the fan coils not only to be modulated with load, but to be de-energized when no load exists, further reducing overall fan energy for the system.
Most multi-split variable refrigerant flow (VRF) systems include fan coils with ECMs. The variable speed, soft start, and enhanced reliability that these motors bring to hydronic fan coils are equally attractive for refrigerant fan coils. One component whose viability is improved by ECMs is the parallel-flow fan-powered terminal. Because the lifetime of small induction motors is more a factor of the number of starts, rather than the hours of operations, and because of the starting noise of these motors, series flow fan-powered VAV terminals were often preferred to parallel flow terminals, except ins climates with very few hours of heating operation per year.
Series flow terminals, however, general entail significant additional fan energy consumption per year, compared with parallel flow terminals, both because the local fan generally twice as large for the series flow terminals, and because continuous operation of the very inefficient fractional horsepower induction motors can add significantly to the annual fan energy of a VAV system. Soft starting ECMs can be quietly energized to provide only the airflow necessary for heating and very low-flow VAV damper operation, greatly minimizing terminal fan energy. Finally, systems that incorporate small circulation pumps, such as small scale radiant systems and small evaporative condensers have been greatly improved by the availability of reliable variable-speed single-phase motors.
Electric motors are key components for the successful and energy efficient operation of HVAC systems. Historically, the motor of choice, large or small, for these systems has been the induction motor. For larger, threephase motors, solid state "soft" starters and variable frequency drives have enabled this motor type to be the motor of choice for the most systems with the highest energy efficiency aspirations. For smaller, single-phase motors, ECMs, now offer the energy efficiency and longevity advantages previously only available in large motors. Improvements in efficiency and reliability of these motors have also increased the attractiveness of systems and components previously burdened by the shortcomings of single-phase induction motors. These systems and components include fan coils, both refrigerant and electric, parallel fan-powered terminals and small circulating pumps.
(1.) Peltota, M. 2004. "Minimizing ac induction motor slip." Electrical Construction and Maintenance (4), Penton, NY, NY.
(2.) The Engineering Toolbox. "Motor Control Design Circuit." (www.engineeringtoolbox.com/nema-a-b-c-d-design-d_650.html).
(3.) Navigant Consulting. 2013. Energy Savings Potential Opportunities for High Efficiency Electric Motors in Residential and Commercial Equipment, U.S Department of Energy Office of Energy Efficiency and Renewable Energy, USDOE Office of Technical and Scientific Information, p. 5.
(4.) Peltola, M. 2003. "Improving power factor with variable speed ac drives." Electrical Construction and Maintenance (7), Penton, NY, NY.
(5.) Roth, K., Chertok, A., Dieckmann, J. "Electronically commutated permanent magnet motors." ASHRAE Journal, March, 2004 pp. 75-76.
(6.) Multiple energy modeling analyses performed by or under the direction of the author.
BY DANIEL H. NALL, P.E., BEMP, HBDP, FAIA, FELLOW/LIFE MEMBER ASHRAE
Daniel H. Nall, P.E., FAIA, is vice president at Syska Hennessy Group, New York.
Caption: FIGURE 1 Cutaway view of a totally enclosed fan cooled (TEFC) three-phase induction motor. (Photo by S.J. de Waard, own work. CC BY-SA 3.0, https://commons.wikimedia.org/w/index. php?curid=17313755.).
Caption: FIGURE 2 Configuration of a permanent magnet dc motor, termed ECM motor when used with a variable frequency pulsed dc power supply. (Photo courtesy of Maxon Motors.)
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|Title Annotation:||COLUMN: ENGINEER'S NOTEBOOK|
|Author:||Nall, Daniel H.|
|Date:||May 1, 2016|
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