Variable Frequency Drives

What is a variable frequency drive?
Heating, ventilation, and air conditioning systems' air supply used to be constant volume into a space. Now a variable air volume control is being used to provide the proper amount of air flow to a space to provide comfort to the occupants. If there is excess flow, the control box partially closes to reduce the flow. This flow change is sensed by components of the main air handling unit so that the fan speed of the unit can be slowed down. Slowing down the fan saves the unneeded air flow and saves the horsepower of the fan, which saves money. The device that controls the fan speed is called a variable speed drive. Variable frequency drives are also used on pumps and cooling towers to match the demand of the space with the proper energy supply. Adding this to a motor-driven system can offer potential energy savings in a system in which the loads vary with time. Variable frequency drives belong to a group of equipment called adjustable speed drives or variable speed drives. The operating speed of a motor connected to a drive is varied by changing the frequency of the motor supply voltage. This allows continuous process speed control. Motor-driven systems are often designed to handle peak loads that have a safety factor. This often leads to energy inefficiency in systems that operate for extended periods at reduced load. The ability to adjust motor speed enables closer matching of motor output to load and often results in energy savings.

Variable frequency drives are used for two main reasons:

  1. To improve the efficiency of motor-driven equipment by matching speed to changing load requirements.
  2. To allow accurate and continuous process control over a wide range of speeds.

Motor-driven centrifugal pumps, fans and blowers offer the most dramatic energy-saving opportunities. Many of these operate for extended periods at reduced load with flow restricted or throttled. In these centrifugal machines, energy consumption is proportional to the cube of the flow rate. Even small reductions in speed and flow can result in significant energy savings. In these applications, significant energy and cost savings can be achieved by reducing the operating speed when the process flow requirements are lower.