October 25, 2017 by Ian McTeer
Everyone involved in HVAC and refrigeration works with electric motors almost every day. Whether designing new products, specifying equipment, installing, maintaining or repairing a variety of heating, ventilation or cooling products, an electric motor and its needs are regularly the centre of attention. Even at home, the ubiquitous electric motor is indispensable.
As I sat down to write this article, I performed a quick mental survey of the electric motors just in my basement: furnace (three motors, including condensate pump); HRV (three motors); upright freezer (one motor); sump pump ( one motor); propane stove (one motor); desktop computer (one motor); printer (one motor). By the time the refrigerator, vacuum cleaner, washer/drier, range hood, outdoor unit, submersible pump, and several other devices are counted, I am employing north of 25 motors nearly every day.
CREATING A WORLD IN MOTION
The great British scientist, Michael Faraday, experimented with electricity and magnetism and, in 1821, he set in motion a copper conductor rotating around a magnet sitting in a bath of mercury. Faraday had converted chemical energy from a battery into mechanical motion creating the threshold to our world in motion. Years later in 1883, Nicola Tesla invented the first induction motor with rotating magnetic fields thus starting a second industrial revolution still evident today. Indeed, today’s global market for electric motors is close to 100 billion US dollars and is expected to grow to 141 billion by 2022 according to Sherry James of Grand View Research.
HVAC runs on electric motors, which are used in several general classifications:
Compressor drives: hermetic direct drive and external drive
Fan drives: condensers, evaporators, induced draft, air circulation
Pump drives: condensate pumps, hydronic system pumps, chilled water pumps, geothermal system pumps, and cooling towers.
Miscellaneous drives: deep vacuum pumps, recovery machines, ice makers, EEV stepper motors, among others.
THE WAY WE USED TO BE
Before the advent of electronically controlled motors, there were two general classes used to identify electric motors: single phase induction motors and three phase induction motors. Residential installers and technicians typically encounter single phase induction motors classified by the method used to start them:
• Split phase
• Capacitor start
• Permanent split capacitor
• Capacitor start, capacitor run
The 115-volt split phase AC motor found a home in many belt-driven low starting torque applications (like fossil-fueled furnace fans) long before I ever set foot on the scene. The motor had two windings, the main winding and the high resistance auxiliary winding rotated several degrees in magnetic position from the main winding. Wired in parallel, the auxiliary winding would, during start-up, produce a magnetic field out of phase with the magnetic field produced by the main winding. Thus, the two windings together created a rotating field causing the rotor to revolve. The rotor had no windings, but magnetism built-up around the rotor bars. Once the rotor achieved approximately 75 per cent of running speed, a centrifugal switch would open and take the start winding out of the circuit.
Another single phase motor commonly used throughout the industry in low torque starting applications is the shaded pole motor. The stator core has a pair of permanently short circuited (shading) coils placed over a portion of the main field coil causing an unbalance of magnetic forces so that the motor is self-starting (see Figure 1).
Undoubtedly, the workhorse motor of the residential HVAC industry to this day is the permanent split capacitor motor (PSC). Often found in direct drive furnace fan applications and in hermetic compressors, the PSC motor has no centrifugal switch to take the start winding out of the circuit. The motor could easily operate on the run winding alone; however, it will run more smoothly and efficiently when the start winding and capacitor remain in the circuit. Smoother operation results from the out-of-phase force generated by the start winding. This is especially useful when used with the pulsating load of a hermetic compressor.
A run capacitor is connected between the start and run compressor terminals and is in series with the start winding. It unbalances magnetic lines of force for improved starting torque and reduces the current drawn by the motor by improving the power factor. The run capacitor in an AC motor circuit constantly charges and discharges effectively producing another phase by maintaining a slight lead/lag between voltage and current in the rotating motor.
In hermetic applications, PSC motors sometimes struggle to start in low voltage situations, just a five per cent voltage drop could cause starting difficulty and overheating. Because the starting torque is low, a hermetic PSC can only start when the system pressure is balanced. A non-bleed TXV must not be used with this motor without a properly sized start assist kit that includes a start capacitor and start relay. The start capacitor must be taken out of the circuit just as the motor achieves running speed, otherwise the compressor may be damaged.
NOTE: The starting capacitor does NOT send a “Jolt of Juice” to the compressor motor. It is specifically designed to shift the phase by increasing in-rush current in the start winding in relation to the run winding thus providing necessary starting torque needed by the motor to overcome the unbalanced refrigeration side pressures created by the TXV locking up at the end of the previous cycle.
NOTE: The voltage sensing starting relay is position sensitive. The “pick-up” and “drop out” voltages must match the requirements of the compressor manufacturer. Using any old starting relay could easily damage the compressor motor windings. Some aftermarket starting relays are current sensing. Once the current draw of the compressor motor reaches a pre-determined level, the relay drops out the start capacitor. Technicians should consult with the equipment manufacturer for more information about using a current sensing relay when replacing an OEM voltage sensing starting relay.
PSC motors, when used in residential air handling applications, replaced the older less efficient belt drive blowers by directly attaching the blower wheel to the rotor shaft. PSC powered blowers typically feature multiple winding motors that provide a selection of running torques, which, when driving the load of a blower wheel, allows the technician to select a different fan speed for specific heating and cooling cfm requirements.
TODAY’S HVAC MOTORS
New energy performance standards proposed by NRCan for gas furnace fans, set to be law by July 2019, mean the PSC motor will be steamrollered out of existence. Barely able to convert a paltry 65 per cent of its electrical input into mechanical work, PSC motors suffer from asynchronous alignment, that is, the rotor constantly lags the magnetic field in the stator. Known as slippage, it means that a six pole PSC motor, for example, should turn at 1200 rpm:
Synchronous motor rpm = AC Frequency in Hz x 120 ÷ number of poles in the motor
Thus, 60Hz x 120 = 7200/6 should result in 1200 rpm; however, slippage means the motor turns at 1075 rpm. A considerable amount of waste heat is the byproduct of slippage. PSC motors start abruptly adding stress to internal motor components and noise to the air handling system. Multi tap PSC motors must be carefully matched to the application as only minor changes in speed are possible.
Electronically Commutated Motor (ECM)
In the last two decades, several electric motor manufacturers, including Genteq and Emerson introduced fractional horsepower variable speed ECMs touting a wide range of benefits such as:
• Incorporating a separate motor that is wound like an industrial three-phase motor with a computer that “decides” which winding to power at exactly the right time thus optimizing efficiency.
• The rotor consists of three permanent magnets so no energy is wasted in creating an armature magnetic field; rotor losses are very low; goodbye slippage.
• Emerson’s serial port communicating ECM uses 10 poles
• The motor computer, housed in a replaceable module, can control torque over a wide range of applications; provides a much wider range of airflows between lowest and highest settings.
• The motor computer “knows” the rotor position allowing control without the need for mechanical brushes and commutators.
• Motor starts softly, ramps to proper speed.
• Under standard residential operating conditions, a variable speed ECM is typically 40 per cent more efficient than a PSC motor.
• ECMs used with furnace fans are optimized to provide constant air volume should the external static pressure change for some reason.
Constant Torque Motor (or X13 Motor)
Think of CTMs as the next generation PSC motors. The non-ramping CTM motor programming is optimized around providing a consistent rotational force. Should the system external static pressure change due to a restricted air filter, for example, then the motor program will maintain its programmed torque although airflow will decrease, but not as drastically as a PSC motor in similar circumstances.
CTMs reduce power consumption compared to a PSC motor, have soft start capability, and provide multiple speed taps providing a level of torque suitable for varied applications. The motor interface, simpler than that of an ECM, receives analog 24-volt turn-on instructions to the appropriate motor speed tap by the furnace or air handler control board.
Many high efficiency heat pumps and cooling units feature a variable speed hermetic scroll compressor powered by an inverter or variable frequency drive that alters AC power by changing it into DC power. Then, using diodes and capacitors, the converted DC power feeds the appropriate motor windings in the compressor at the right time in a similar fashion to the ECM fractional horsepower motor. Inverter drives are continuously variable, brushless DC motors driven by a switching frequency typically ramping some motors from 900 rpm to as high as 6000 rpm based on load. Variable speed drives incorporate protection and monitoring functions that continuously adjust compressor operation preventing undesirable operating conditions and situations that could damage the compressor.
NOTE: ECM motors and inverter drive compressors could cause personal injury or death due to electrocution. Be sure to understand and follow the manufacturer’s instructions before touching electrical components.
IT IS ALWAYS SOMETHING
I said it once before: air is a commodity, it has weight. One cubic foot of air weighs approximately 0.075 pounds. A blower moving 1000 cubic feet of air per minute will move 0.075 x 1000 = 75 pounds of air per minute or 4500 pounds (2.05 metric tons) per hour. Suppose a dirty air filter restricts air entering the blower by 50 per cent or 37.5 pounds per hour. A PSC motor simply unloads, current draw decreases. However, a constant volume ECM will ramp up in a futile attempt to maintain programmed airflow. ECM powered fans, starved of air, will simply move whatever they can get at a higher velocity. Motor wattage increases considerably, blowing AHRI rated SEER and HSPF numbers out the window.
ECMs, often promoted as a method of easily overcoming airflow problems, typically operate well beyond rating plate maximum external static pressure in second-rate duct systems. Consistent high static operation of an ECM rated for a total ESP of 0.5″ often results in control module failure. My recommendation: do not specify an ECM unless the duct system warrants.
The introduction of ECMs and Inverter Drives created some significant concerns about power quality for electric utilities maintaining the grid. In HRAI’s National Review in 2012, it was noted that “The need to control the speed of a motor by manipulating its frequency or voltage has introduced a host of electronic components that introduce waveform distortion (harmonics) into the power distribution systems.”
It will be a long time before expensive ECM12s and Inverter drives begin to surpass the enviable record of low cost, low maintenance and highly reliable PSC motors. Undoubtedly, this technology will prevail and we must become accustomed to it.
I like comedian Dave Barry’s amusing technological insight, “…scientists have developed the laser, an electronic appliance so powerful that it can vaporize a bulldozer 2,000 yards away, yet so precise that doctors can use it to perform delicate operations to the human eyeball, provided they remember to change the power setting from [bulldozer] to [eyeball].”
CHECK OUT HPAC OCTOBER DIGITAL EDITION FOR ECM SERVICE AND PERFORMANCE TIPS FROM IAN.