Electric AC induction motors run at a fixed speed and are ideally suited to applications where a constant motor output speed is required. However over half of all motor application have some kind of varying demand, such as fans, pups, winding reels and precision tools. Historically, processes would have been controlled using opening and closing dampeners and valves, or changing output speeds with gears or pulleys, all whilst the motor speed remained constant.

In the 1980’s, variable speed drives (VSDs) started appearing on the market offering an alternative method of controls. A VSD also referred to as a ‘drive’ or ‘inverter’ is an electronic power controller that is able to adjust the electrical supply to an AC induction motor with a corresponding change in the motor’s speed and torque output.

By implementing this type of control, a very close match between motor speed and the process requirements of the machine it is driving may be achieved.

Controlling the speed of a motor provides users with improved process control, reduced wear on machines, increased power factor and large energy savings.

Like a good wine, VSD technology has matured, and is now extremely versatile and offers a high degree of motor control. Motor speeds can be accurately varied from zero rpm through over 100% of the rated speed whilst the torque is also adjusted to suit.

Different options are available to suit a variety of applications and budgets. Basic VSD designs are used in simple applications such as fan and pump control whereas more complex versions are available for very precise speed and torque control, for example in multiple winders or materials forming applications.

VSD’s range from 0.18kW through to several MW in size; and are connected to the motor’s electrical supply.

In many applications, variable speed control can lead to substantial reductions in energy use. The use of VSD technology is particularly effective in fan and pump applications where an exponential relationship exists between the machine speed and the energy used.

Surely a 10% reduction in speed should equate to a 10% reduction in energy usage? This is correct when we are talking about a conveyor or a mixer, but fans and pumps are different.

The relationship between the speed and power of a fan or pump is called Cube Law. If you reduce motor speed by 10%, achieve a 27% reduction in energy consumption. Reduce motor speed by 20%, achieve a 48% reduction in energy consumption.