Variable Frequency Drives (VFDs)

VFDs in HVAC Systems

One of the best ways to reduce the energy usage of an HVAC systems is to install variable frequency drives (VFDs). VFDs have been used for more than 20 years on fan and pump motors. Energy savings vary from 30 to 50 percent over conventional constant speed applications, resulting in a return on investment of six months to two years.

Today’s VFDs can be installed in practically any HVAC application found in commercial buildings.  These drives are capable of handling motors up to 500 horsepower and the operational noise of these newer units is far lower than the older drives.

Building System Design

Building HVAC systems are sized for peak load conditions which occur less than 5% of the total operational hours of the facility.  Many times, systems are also over-sized to handle potential future loads like new equipment or additional people.  Most older conventional building HVAC systems are designed to operate fans and pumps at a constant speed. The building load however is far from constant so when there is a small demand and the motors and pumps are running at full speed, the system is using more energy than is necessary.

VFD Operation

VFDs work as a throttle to ramp up the motor or pump only as much as needed in order to meet the current load demands of the building.  The most commonly used motor in commercial HVAC systems is the induction motor. With induction motors, the power drawn by the motor varies with the motor’s speed. If the motor is slowed by 25 percent of its normal operating speed, the energy usage is reduced by nearly 50 percent and at a 50 percent reduction in speed, the energy use is reduced by nearly 80 percent.

VFD Benefits

The most significant benefit to using a VFD is energy savings. By ramping up the motors and pumps slowly to meet the building load requirements throughout the year, there sill be a significant drop in energy consumption.

Another benefit of VFDs is reduced wear and tear on the motors. When an induction motor is started, it draws a much higher current than during normal operation. This higher current can be three to ten times the full-load operating current for the motor which creates additional stress in the motor’s windings and other components. In motors that start and stop frequently, the additional wear and tear shortens the life expectancy of the motor.   Conversely, a motor connected to a VFD is ramped up at a controlled rate which will extend the life of the motor.

Variable Frequency Drives (VFDs) offer an effective and efficient way to reduce overall energy usage.