TECHNOLOGY BASICS: What are High Efficiency Motors
Electric motors are devices that convert electrical power into mechanical power. The mechanical power can be used to turn a fan or pump in a HVAC system. The basics of an electric motor system are an outer stationary piece, the stator, and a moving inner piece, the rotor. Typically, electricity is applied to the stator to create a means of torque to make the rotor spin at the desired speed and torque.
Electric motors come in various design types, each having its own set of benefits and drawbacks.
Induction Motors — Induction Motors use an alternating current run through stator coils to create an electromagnetic field that excites current in the rotor. The stator magnetic field rotates around the now current-carrying rotor and creates a torque to rotate the rotor. Induction Motors are very common and are relatively cheap, reliable, and simple.
Permanent Magnet Motors – Permanent Magnet Motors operate by running an alternating current through the stator to create a magnetic field which applies a torque to permanent magnets on the rotor. The use of permanent magnets instead of electromagnetic rotor windings on the rotor – as in Induction Motors – makes Permanent Magnet Motors more expensive and typically heavier than Induction Motors. Permanent Magnet Motors offer the benefits improved efficiency, improved variable speed efficacy, and noise reduction.
Switched Reluctance Motors – Switched Reluctance Motors operate by running direct current through stator winding poles to generate a magnetomotive force pulling on nearby rotor poles. Switched Reluctance Motors are not new technologies, but relatively recent advances are overcoming past limitations to achieve high efficiency. Switched Reluctance Motors are typically lighter and offer some of the highest efficiency and drive compatibility of the motors listed. Due to their design, Switched Reluctance Motors require a drive to operate.
Frame type is used with the frame size basis to describe the shaft and bearing sizes of a motor, as well as mounting bolt hole locations for the motor.
There are three primary standardized motor frame types: Original, U Frame (introduced in 1952), and T Frame (introduced in 1964).
Frame size basis is a 2 or 3 number sequence written before the frame type, for example: 213T. For 3-digit frame size basis the first two digits describe the shaft distance between the center of the shaft and the base of the foot on a foot mounted motor in quarter inch increments. The ‘21’ from our example above refers to 21 quarter inches or 5-1/4 inches. The third digit does not directly relate to a specific distance but it does refer to the motor body length, which affects the distance between mounting bolt locations in the base. A table of the distances can be found here: https://www.engineeringtoolbox.com/nema-electrical-motor-frame-dimensions-d_1504.html. For two-digit frame size basis the two digits refer to shaft height in 1/16th of an inch increments.
U-Frame Motors have Class A insulation ratings, acceptable up to 105 degrees Celsius.
T-Frame Motors have Class B insulation ratings, acceptable up to 130 degrees Celsius.
Motors are rated by their Horsepower (HP), which is dependent on their rotational speed, measured in Revolutions Per Minute (RPM), and the torque, measured in either foot-pounds (ft-lbs.) or Newton-meters (nm).
Motors can be controlled to run at a single speed or at variable speeds.
Single Speed – Single Speed Motors operate at one set point of speed when active. Single Speed Motors are suited for systems with constant loads such as a constant air volume HVAC system or any other system where the motor is either off or operating at full speed.
Variable Speed – Variable Speed Motors are able to operate at multiple speed and torque loads when controlled by a variable speed drive (VSD) or variable frequency drive (VFD). For applications where the speed and torque can vary, the energy consumption can be greatly reduced by switching from a single speed motor to a variable speed motor, particularly if the system often operates at low speed and torque. Variable speed drives may slightly reduce the motor system efficiency at peak load, but the ability to match load demand often leads to significantly lower energy consumption overall. The combination of a variable speed motor and drive typically has a higher cost than a single speed motor.
A few other important attributes of motors include:
Voltage – The supply voltage must meet the voltage required to power the motor; while many motors operate at the equivalent of 240V AC, some only operate with a supply voltage equivalent of 480V AC. Single phase 120 VAC or split phase 240 VAC are more common in light duty and residential applications.
Enclosure – Enclosure type describes the way the motor components are protected and/or cooled. NEMA defines many enclosure types that are pivotal in understanding what conditions the motor can work under. Motor enclosures detail whether the motor can handle wet conditions, if the motor must be in an active airstream to stay cool, or if the motor is blast resistant.
VFD Compatibility – Running at variable speeds or torque loads will likely require a VFD or a VSD. VFD refers to an AC drive only, whereas VSD can mean an AC or DC drive. Certain motors are only compatible with specific drive types so compatibility should be reviewed.