Variable Frequency Drives (VFDs) are electronic power controllers that allow for precise control of the speed of alternating current (AC) induction motors that drive a variety of rotating machines including fans, pumps and compressors. These motors are used in most heating, ventilation and air-conditioning (HVAC) systems and account for a significant portion of the total HVAC energy consumption. More efficient operation of these motors using VFDs can result in significant energy savings.


How VFDs Work

The speed and torque of an AC induction motor is proportional to the frequency and voltage of its power supply. Since most electricity supplies are at fixed frequency and voltage, the speed and torque of induction motors connected directly to a conventional electricity source will also be constant. When used to drive an HVAC fan, for example, this means the fan will run at constant speed and torque as well.

VFDs intercept the incoming electricity supply and modify the incoming voltage and AC frequency based on the load requirements. In this way, the motor speed and torque can be adjusted to match the load / demand it is servicing with high precision. Continuing with the HVAC fan example, if the demand on the HVAC system were to change, the speed of the fan could be altered by the VFD controlled motor to match the change in demand.

VFDs can also be used to run machinery at higher than rated speeds for short periods of time. This has additional advantages in terms of energy savings but must be done with appropriate caution.


How Do VFDs Reduce Energy Consumption?

In the absence of VFDs, the fixed AC motor speed and torque mean that the equipment these motors drive operate at constant speed and torque as well and system control is achieved through dissipating (wasting) energy by mechanical means. In the case of the HVAC fan, if the air supply demand decreases, and the energy input from the fan remains constant, then in order to reduce the supply of air to the space being conditioned the energy in the flow must be dissipated using vents or valves or air must be diverted and dumped elsewhere. Either way, energy is being wasted to deliberately reduce the supply of air and the efficiency of the system is reduced.

With a VFD, the output of the system (air supply in this case) is controlled by directly changing the speed or torque of the motor. At low demand the motor runs slowly and the power consumption decreases in proportion to the demand resulting in increased efficiency and energy savings.

In addition, since the fan runs at constant speed it must be sized to deal with the maximum possible demand. This demand would typically only occur for a few hours of the year and so for the rest of the time the system is oversized and operating at reduced efficiency.

Since the VFD can be used to run the system at higher than rated speeds, the whole system can be sized according to more common conditions and the extreme demand conditions can be serviced by running the system at over-speed. The overall energy efficiency of the system is thus further improved.

Where to Use VFDs for Best Energy Savings

VFDs are best suited to variable torque loads, which include components such as fans and pumps, which often operate at part load. In these components power consumption is proportional to the operating speed cubed and significant energy savings can be realized by reducing the motor speed.


In HVAC systems, VFD controlled fans have been shown to significantly reduce energy consumption by up to 70%.

By Michael Owen, PhD

Michael is a mechanical engineer working in academia. His research encompasses various aspects of fluid dynamics and heat transfer with a strong emphasis on industrial heat exchangers.