Running electric motors may account for a significant portion of a farm’s energy usage. Typically, motors are used in areas including refrigeration, ventilation and irrigation, and drive hydraulic equipment, fans and pumps.
High-efficiency motor (HEM) is a term applied to an electric motors whose energy losses have been reduced to a minimum. To be labelled a HEM, a motor must meet the high-efficiency performance criteria set out in AS 1359.5-2004. This standard also specifies the Australian Minimum Energy Performance Standard (MEPS) an electric motor must meet.
HEMs can cost between 25 and 30 percent more than MEPS-compliant motors and typically demonstrate good return on investment in the small motor range (up to 185 kW), where efficiency gains of two to five percent are common.
Over a 10-year operating period for a motor, the purchase price represents only about two percent of the total motor installation and operating costs. Energy and maintenance costs account for the remaining 98 percent. Even a small improvement in motor operating efficiency can produce significant energy and cost savings and provide a rapid return on investment.
HEMs should be considered when:
- purchasing new motors,
- specifying motor-driven equipment,
- repairing and rewinding failed standard-efficiency motors, and/or
- replacing older, lower-efficiency motors, particularly when the existing motor has been rewound or is oversized and under-loaded.
The efficiency of a motor is the ratio of mechanical power output to electrical power input. Mathematically, this is expressed as:
Higher motor efficiencies have been achieved by reducing motor losses (e.g. stator, rotor, core, friction and stray losses), thanks to the use of new materials and better motor design.
The Minimum Energy Performance Standards (MEPS) requirements are set out as minimum efficiency levels. From April 2006, MEPS levels for three-phase electric motors were revised to become more stringent. The ‘high efficiency’ level from 2001 became the MEPS level on 1 April 2006. (Commonwealth of Australia, 2013) The minimum efficiency levels are shown in Figure 1.
- HEMs are generally two to four percent higher in efficiency for large motor sizes.
- For motor sizes below 5.5 kW, HEMs are often four to seven percent higher in efficiency.
Key factors when evaluating quotes
There are a number of definitions of efficiency, as shown below. It is essential that the same definitions be used when quoting and comparing efficiency figures.
Efficiency definitions that should be used are:
- Average expected or nominal efficiency. Identical terms, which refer to the average full-load efficiency value that is obtained through testing of a sample population of the same model of motor. It is commonly used to compare motor efficiencies.
- Guaranteed minimum or expected minimum efficiency. Either all motors or a stated percentage of the motors purchased are guaranteed to have efficiencies that meet or exceed this full-load value. Most national standards set limits on the allowable difference between nominal and guaranteed minimum efficiencies.
Figures that should be avoided when comparing motors:
- Apparent efficiency. This is the product of the motor’s power factor and minimum efficiency. The variability of the power factor makes this type efficiency rating highly unreliable. Specification using this figure should be avoided.
- Calculated efficiency. This term refers to an average expected efficiency based on a relationship between design parameters and test results. Specification using this figure should be avoided.
Commonwealth of Australia, 2013. MEPS requirements. [Online]
U.S. Department of Energy, 2012. Motor systems tip sheet#1: When to purchase premium efficiency motors. [Online]