Motor efficiency is the ratio of mechanical power output to the electrical power input, usually expressed as a percentage. Energy-efficient motors use less energy. Because they are manufactured with higher quality materials and techniques, they usually have higher service factors and bearing lives, less waste heat output, and less vibration, all of which increase reliability. This is often reflected by longer manufacturer’s warranties.
To be considered energy-efficient, a motor’s performance must equal or exceed the nominal full-load efficiency values provided by the National Electrical Manufacturer’s Association (NEMA) in their publication MG-1. The Energy Policy Act of 1992 (EPACT) required most general purpose motors between 1 and 200 horsepower for sale in the U.S. to meet these NEMA standards by October 24, 1997.
What Efficiency Value Should I Use?
When comparing two motors, be sure to use a consistent measure of efficiency. "Nominal" efficiency is best. This value is obtained through standardized testing. "Minimum" or "guaranteed" efficiency is slightly lower to take into account typical variations in efficiency within a population of motors.
When Should I Consider an Energy-Efficient Motor?
Assuming 6 cents per kWh electricity cost and a payback criteria of 2 years, most motors should be replaced with an energy-efficient model if they operate over 4,000 hours per year. In general, energy-efficient motors should be considered in the following circumstances:
New installations, both separate and as part of packages such as HVAC systems
When major modifications are made to a facility or a process
Instead of rewinding older, standard-efficiency motors
As part of a preventive maintenance or energy conservation plan