VFD (Variable Frequency Drive)

A VFD takes fixed-frequency AC power from the building (60 Hz in the US) and converts it in three stages. A rectifier turns the incoming AC into DC, a DC bus with capacitors smooths it, and an inverter uses fast-switching transistors to build a new AC output at the frequency and voltage the motor needs. Because an AC induction motor's speed is proportional to the frequency it receives, changing the frequency changes the speed.

The drive gets a speed command from the building automation system, often as a 0-10V or 4-20mA signal or over a network protocol such as BACnet, and reports back its status, speed, current and fault codes.

Most HVAC fans and pumps are sized for the hottest or coldest day, so they run below full load most of the time. Without a drive, flow is reduced by closing dampers or throttling valves while the motor keeps running at full speed. A VFD slows the motor instead.

For centrifugal fans and pumps, the affinity laws say that power varies roughly with the cube of speed. In theory, a fan running at 80 percent speed needs about half the power it uses at full speed. Real savings are smaller because of motor and drive losses and minimum pressure setpoints, but the effect is large enough that energy codes based on ASHRAE 90.1 require variable speed control on many fans and pumps.

In air conditioners and heat pumps, an inverter compressor is a compressor driven by a variable frequency drive, which is why these products are often called inverter units. In larger buildings, VFDs are found on:

• Supply and return fans on variable air volume air handling units
• Chilled water, condenser water and hot water pumps
• Cooling tower fans
• Compressors in variable speed chillers
• Domestic water booster pumps
• Exhaust fans and kitchen hood fans with demand control

VFDs are reliable but sensitive to their environment. Heat, dust and moisture shorten their life, so drive cooling fans and filters need regular attention. Common problems include:

• Overcurrent, overvoltage and undervoltage trips, often caused by power quality problems or aggressive acceleration settings
• Harmonic distortion fed back into the building's electrical system, addressed with line reactors or filters and assessed against IEEE 519
• Motor bearing damage from shaft currents, prevented with shaft grounding rings or insulated bearings
• Drives left in bypass or hand mode after a repair, so a fan that should modulate runs at full speed around the clock and wastes energy without anyone noticing
• Capacitor and cooling fan wear in older drives

Useful data points include speed command and actual speed or frequency, output current and power, run hours, hand, off, auto and bypass status, and fault history. Teams also keep the drive model, rated horsepower, parameter settings and the motor it controls, so a replacement drive can be set up correctly.

In a digital twin such as Syyclops, drive readings from the BMS and work orders can be attached to the VFD as an asset in the building model, so speed and fault history are visible next to its maintenance records.