11 Facts about Variable Frequency Drives

Variable Frequency Drives (VFDs) are a fundamental component of energy efficient HVAC systems, but they are often overlooked.

In newer buildings, maintaining them can be complicated.

In older buildings, they might not even be installed.

What does a VFD do?

A type of Variable Speed Drive (VSD), Variable Frequency Drives (VFDs) precisely control the speed of an AC motor. These motors power the pumps, fans, compressors, and other mechanical components of buildings, plants, and factories.

The VFDs vary the frequency and voltage supplied to a motor to control a motor’s acceleration, change of speed, and deceleration, essentially reducing power consumption when the motor is not needed and controlling motor speed to give them a smooth startup. This has the added benefit of reducing mechanical stress and motor wear, thus requiring less maintenance and prolonging equipment life.

How does the VFD save money?

By matching motor speed with cooling demand, the VFD ensures that the equipment operates at the lowest possible motor speeds and also provides flexibility for changing building needs. This optimization results in reduced energy consumption and immediate cost savings, providing a quick return on investment.

By ensuring a smooth startup, the VFD ensures that the inrush current is maintained at or below the motor full-load amp (FLA) rating, thereby reducing motor wear and increasing equipment longevity.  Additionally, the controlled acceleration reduces electrical and mechanical stress.

12 Facts


1 – Older buildings typically do not have VFDs. In fact, despite being commercially available in the 1980’s, many pre-2000 buildings do not have VFDs.

2 – VFDs can be retrofitted to commercial HVAC systems.  They are available from 1/2Hp to 10,000Hp from the smallest HVAC fans to the largest chillers.  In commercial HVAC systems, VFDs are installed in two places:

  • Hot or Cooled Water Applications, such as Boiler Pumps, Condenser Water Pumps and Chiller Motor Pumps, where the flow of water is regulated based upon whether the end temperature is satisfied.
  • Fans, such that dampers are opened or closed depending upon whether the end temperature is satisfied.

3 – Even buildings with VFDs can benefit from a retrofit since older VFDs are not as energy efficient. They tend to have a higher heat transfer which leads to shorter lifespans.


4 – VFDs may need to be repaired or reprogrammed after power surges, which often eradicate their programming.

5 – Many HVAC maintenance contracts do not include VFDs, which typically require a separate line item on the contract.

6 – Not all HVAC companies have experience repairing or reprogramming VFDs, and most VFDs have microprocessors which requires a level of expertise.

7 – VFDs often have filters which need to be replaced. Output filters are used on VFD outputs to extend the life of motors and protect them from voltage wave reflection and insulation breakdown.

Energy Savings

8 – VFDs provide a quick return on investment with energy savings resulting from the decreased demand. Some estimates indicate an energy savings between 35-60% upon installation of a VFD.

9 – VFDs provide a quick ROI with increased lifespan of your motor since VFDs prevent a heavy load from straining the motor on startup.

Building Automation

10 – VFDs provide increased oversight, because they can control a motor at a precise speed, apply a specific amount of torque or even stop at a specified position.

11 – Depending upon the VFD you install, you can obtain diagnostic data on system performance which further helps with energy savings and system maintenance.

CM3 offers site assessments for VFD retrofits and repairs. Please reach out for more information.