1. Variable speed drives

Motors turn at almost constant speed - as seen in the previous section. But most often the loads being driven may not require the full load power that the motor can supply. This power shortfall means that energy is being wasted and so excessive greenhouse gases are being produced.

If we could control the speed of the motor so that it more closely matched the load’s requirements then motor’s running cost would be lower and greenhouse gases output would be decreased.
A variable speed drive (VSD) or variable frequency drive (VFD) as it is sometimes known, achieves this energy efficiency as seen in the two (2) examples below.

Example A:

A plant process manager may want to slow a pump or a fan so that the flow rate can be dropped. One way is to use dampers or bypass valves. However, these mechanical flow solutions still maintain a motor running at its rated constant speed, that is, the motor is running flat out at its maximum continuously.

Example B:

The Melbourne Coroner's Court installed some VSDs in its airconditioning plant. A 32 kW motor was installed to meet a load of 22 kW. Once the VSD was fitted the power dropped from the maximum of 22 kW to 10.2 kW, that is a 54% power saving.

Source: Energy Toolbox (Victoria)

Variable speed drives (VSDs) allow loads, such as fans and pumps, driven by AC induction motors to operate over a wide range of speeds.

VSDs are also known by other names such as:

• variable frequency drives
  

• adjustable speed drives
  

• variable frequency inverters
  

• frequency converters.