Variable Frequency Drives

Variable Frequency Drives (VFDs) are solid state power electronics which are designed enable induction motors to operate at variable speed as opposed to fixed speed. VFDs are alternatively called adjustable speed drives (ASD) or variable speed drives (VSD) or inverters.

Normally, an induction motor is constrained by design to a few specific speeds (e.g. 1200 rpm, 1800 rpm, 3600 rpm). VFDs offer greater flexibility and large energy savings potential particularly in fan and low head pump applications. Today’s VFDs are typically pulse width modulation (PWM) drives that vary speeds by rectifying incoming AC waveform to DC and then sending out a series of short DC voltage pulses to the motor. By changing the timing and polarity of these pulses, the motor can be operated at any speed.

In industrial refrigeration systems, VFDs are primarily applied to save energy. They are often cost effective on evaporator fan, condenser fan, and screw compressor applications.

In evaporator fan VFD applications, normally one VFD will control all the fans in an evaporator zone. A zone can encompass from one to thirty fan motors.

VFDs are frequently installed with other components:

Input reactors - These are installed on the input to the VFD. They buffer the electrical system from harmonics generated by the VFD and conversely buffer the VFD from harmonics present in the electrical distribution system.

Output filters - These are typically installed at the output from drive. They are designed to protect the motors by eliminating or mitigating a voltage spiking phenomenon that can result in motor insulation failure. They are most often applied where there is some combination of risk factors including:

  • Multiple motors.
  • Long wiring runs (greater than 50 feet) from the VFD to the motor.
  • Older existing motors not originally designed for VFD duty.


Bypasses - These devices provide the flexibility to bypass the VFD and revert to conventional “across the line” starting in the event of a VFD failure. Bypasses typically add considerable expense and their use is normally restricted to high risk applications, where down-time is costly or catastrophic.

Inverter rated motors - Motors designed for the rigors of VFD duty. They are most frequently applied in new construction applications where VFD control is integrated into the original design. These motors are more robust in terms of their ability to withstand voltage spiking. On retrofit applications, it is typically more cost effective to specify an effective output filter than to replace motors.

Download the following document to learn more about protecting motors during VFD applications.


Refrigeration control system - Typically evaporator fan speeds are managed with a PC or PLC based refrigeration control system. The control system monitors air or product temperature and controls the evaporator zone valves and fans. The control system includes an evaporator fan control algorithm which is responsible for the speed reference signal sent to the VFD.