Energy Efficiency in Electrical Motors
Motor energy efficiency is a measure of the effectiveness with which a motor can convert electrical energy into output mechanical energy on its shaft. The motor efficiency is defined as the ratio of motor power output to electrical power input at the terminals. The difference between the input and output power is due to electrical and mechanical losses.
Electrical motors of higher power rating have higher energy efficiency ratings, however this does not mean that one should use a higher rated motor for a given operating power. On the other hand, if the electrical motor is operated at a power level lower than its rating, it works inefficiently and at a lower power factor.
Losses in Electrical Motor Losses are made up of: I2R losses in the windings; Magnetic Core Losses (Eddy current and Hysteresis Losses); Friction and Windage Losses and Stray Load Losses. At machine design stage, the energy efficiency of electrical motors can be improved mainly by: reducing the losses in the windings; using better magnetic steel; improving the shape of the steel stator core and rotor magnetic circuits; using smaller air gaps; improving the aerodynamics and cooling of the motor and improving manufacturing tolerances.
Electrical Motors with improved energy efficiency are called ‘High Efficiency Motors’ or HEMs. The standard which covers the efficiency class of Electric Motors is IEC 60034-30-1:2014. The classes of efficiency are as follows:
- Super-Premium efficiency IE4
- Premium efficiency IE3
- High efficiency IE2
- Standard efficiency IE1
The standard covers single and three phase motors from 120 W to 1000 kW and is applicable to single speed motors which can:
- Operate at 50 or 60 Hz
- Have 2, 4, 6 or 8 poles
- Have a rated voltage from 50 V up to 1000V
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Apart from making use of the more efficient type of electric motors, there are other measures than can be applied in industrial applications. After having optimised as much as possible the mechanical set-up for increased efficiency, the use of power converters can be applied. Three phase variable voltage/frequency inverters are recommended for applications of variable speed operation of Electric Motors. However inverters can affect the power quality of the supply (Figure 1) and proper filtering must be designed and implemented so as to reduce the supply harmonics generated by the inverter.
Figure 1: Three-Phase Inverter Supply Current
The methodology to determine how to increase the efficiency of an existing or planned electrical drive system has to take a lot of factors into account: the motor; the mechanical system; the power converter and the control mechanism. To optimise the system as much as possible it is recommended that electrical drive engineers are consulted to make the system as efficient as possible without impacting negatively on the electrical supply or plant itself.