Saving money with energy efficient motors

Types of load on electric motors


It is very important that you consider the type of load that is being put on each electric motor, as this can affect the opportunity for energy saving.

The three main electric motor load types are:

  • Variable-torque loads - eg fans and pumps where the speed varies. With this type of load, reducing the speed of the motor by even a small amount can save a lot of energy.
  • Constant-torque loads - eg conveyors, screw and reciprocating compressors, and crushers. With these loads, the amount of power consumed is in direct proportion to the useful work done. So halving the speed will halve the energy consumed.
  • Constant-power loads - eg machine tools and centre winders. With these loads, the power is constant so there will rarely be energy savings when the speed is reduced.

Using a variable-speed drive (VSD)

For variable torque loads in particular, using a VSD in conjunction with a suitable motor allows you to get a better match between the speed of the motor, the speed of the machine it is driving, and the requirements of the process. A VSD converts the incoming electrical supply from a fixed frequency to a variable frequency output. This frequency variation allows the VSD to control the speed of the motor.

You can achieve significant energy and cost savings by using a VSD in certain applications, particularly those that use variable torque such as fans and pumps. For example, you could save up to 50 per cent of the energy used by a fan or pump if you reduce the speed by 20 per cent.