All News

VFDs Put a Different Spin on Energy Savings

Published:

Fan systems may be critical for indoor comfort, but when they work harder than necessary, they're still wasting energy. That energy waste flows right onto your utility bills. Although there are a number of ways to improve fan system performance, variable frequency drives (VFDs) put a different spin on energy savings. They adjust fan speed to match only what's needed.

How VFDs work

Motors need to get out more. They typically operate at only one speed. However, the equipment they operate (such as fans in an HVAC system) typically run at variable speeds. That's where VFDs come in.

VFDs convert alternating current (AC) to direct current (DC), and then back to AC again at a frequency that drives the motor at the desired speed. Although this makes the motor run slightly less efficiently, the significant reduction in power draw at lower speeds decreases overall energy use.

VFDs are ideal for facilities with long operating hours and frequently changing load requirements. VFDs offer the greatest opportunity for energy savings when driving these fluctuating loads, because:

  • Horsepower varies as the cube of speed
  • Torque varies as the square of speed

In other words, if you reduce motor speed by 20% you can reduce motor energy use by up to 50%.

An odd couple

In addition to energy savings, VFDs also minimize wear and tear on motors, reducing maintenance costs and increasing equipment life.

Despite these benefits, VFDs and motors are, in many ways, an odd pairing. VFDs are motionless, adjustable and the product of modern engineering. Motors, on the other hand, are rotating devices, not easily controlled and nearly two centuries in the making. A number of compatibility issues must be evaluated and resolved before the two can work together successfully:

  • Ratings. VFDs are often matched based on horsepower, but this is only an approximate guide and may result in trip outs due to overload and other problems. Select VFDs based on ratings such as input and output voltage, amps, phase, speed (RPM) and frequency (Hz).
  • Power quality. In some cases, VFDs cause harmonic distortion in a motor's electric currents, which can result in excessive heating. Filters, reactors, multi-pulse drives and other solutions are available to reduce harmonic distortion. Perform a harmonics analysis of the fan system to select the right option.
  • Ventilation. Conventional motors are designed with a fan attached to the rotor shaft. The fan cools the motor when it runs at full speed. Slowing down a motor with a VFD will decrease air flow, which can cause overheating. An auxiliary cooling fan or blower may be required.

Inverter-rated motors work best with VFDs. Work with a qualified electrician or electrical engineer to ensure variable frequency drives are properly selected and installed.