Saving money with energy efficient motors

Motor energy use, efficiency and loading

Guide

By understanding how an electric motor works you can get a better idea of how energy wastage in your business may occur.

Reducing wear when starting an electric motor

A motor connected directly to an electrical power supply accelerates to a fixed speed. When it starts up, the motor draws a very high current as it accelerates - this is known as the 'motor starting current' and it generates a great deal of heat. This excess heat considerably increases motor wear and reduces its life expectancy. For this reason, motor manufacturers normally give a maximum number of 'starts' per hour.

However, by connecting a motor to a 'soft starter' or a variable-speed drive (VSD) the starting current can be limited. This gives a much smoother start and results in less wear on the motor. To find out more about soft starters and VSDs, see upgrading your system equipment.

Electric motor loading

Only around 75 to 80 per cent of the energy put into an electric motor is used by the load being moved. This includes:

  • heat wastage of around 5 per cent in the motor's drive unit
  • internal losses in the motor of around 8 per cent
  • additional losses through friction if the motor is attached to a transmission system like a gearbox or pulley

The 'loading' of a motor is the actual amount of work it does compared with its maximum rated power output. For example, a motor rated at 90 kilowatts driving an 81 kilowatt load is described as 90 per cent loaded. Modern motors typically operate most efficiently at above 75 per cent loading, peaking at around 90 per cent efficiency.

Electric motor efficiency

The European Minimum Energy Performance Standard (EU MEPS) scheme sets mandatory minimum efficiency levels for electric motors introduced into the European market.

The scheme which came into effect in June 2011 covers two, four and six pole single-speed, three-phase induction motors from 0.75kW to 375 kW, rated up to 1000 Volts. The timeline for changes is as follows:

  • From 16 June 2011 the minimum efficiency requirement changed to IE2 covering 0.75 - 375kW motors.
  • As of January 2015 motors with a rated output between 7.5-375kW need a minimum efficiency class of IE3, or minimum IE2 if fitted with a variable speed drive (inverter).
  • From January 2017 motors with a rated output between 0.75-375kW need a minimum efficiency class of IE3, or minimum IE2 if fitted with a variable speed drive (inverter).

Replacing the voluntary CEMEP scheme, the mandatory EU MEPS scheme defines the following efficiency classes according to the new IEC 60034-30 standard.

  Old CEMEP Class New EU MEPS Class
Super Premium Efficiency - IE4
Premium Efficiency - IE3
High Efficiency EFF1 IE2
Standard Efficiency EFF2 IE1
Low Efficency EFF3 -

Download the REMA guide to the new LV Motor Efficiency Standards (PDF, 658K).

Older motors and motors that have been rewound may be several percentage points less efficient even than standard-efficiency motors. So it makes sense always to specify higher-efficiency models wherever possible.