A variable frequency drive (VFD) lets a plant run a motor at the speed a process needs instead of full speed all the time. This guide explains how the drive produces its output, where savings come from and what can go wrong, drawing on tip sheets from the U.S. Department of Energy (DOE). Drive and motor manufacturers’ instructions, and qualified electrical staff, remain the authority for any installation.

What a VFD is

DOE describes an adjustable speed drive as a device that controls the rotational speed of motor-driven equipment. VFDs are the most common type: solid-state electronic controllers that adjust the frequency and voltage supplied to an alternating current (AC) motor so it can run over a wide speed range. External sensors monitor flow, liquid level or pressure and signal a controller, which adjusts frequency and speed to match the process.

How the drive builds its output

  1. Rectify. According to DOE, all VFDs rectify the 60-hertz (Hz) fixed-voltage AC supply to direct current (DC).
  2. Invert. An inverter then simulates an AC output of adjustable frequency and variable voltage.
  3. Switch fully on or off. Transistors have very high losses when they create wave shapes other than square waves, so the drive runs them full-on or full-off at a carrier frequency usually between 2 kHz and 20 kHz.
  4. Shape the pulses. Each on-portion of the carrier is a pulse, and its duration is the pulse width. The pulses approximate a sine wave at the much lower fundamental frequency. Each pulse also has a brief rise time, which matters later.

DOE says the most common design sold today is the pulse-width-modulated (PWM) VFD using fast-rise-time insulated gate bipolar transistors (IGBTs), which reduce switching losses and noise.

Where the savings come from

PWM drives are most often used on variable torque loads in the 1 to 1,000 horsepower (hp) range, according to DOE. For centrifugal fans and pumps with no static lift, the affinity laws state that flow varies directly with speed while input power varies with the cube of the speed ratio. Reducing speed, and so flow, by 20% can cut input power by approximately 50%.

DOE’s worked example is an exhaust fan whose motor needs 16.4 kilowatts (kW) to deliver 20 shaft hp at full speed. At half speed the fan gives 50% of rated airflow but needs only one-eighth of full-load power. Even with motor efficiency down to 77.8% and drive efficiency at 86%, the fan and VFD together need 2.8 kW, an 82.9% reduction.

Constant torque loads behave differently: their power varies linearly with speed. DOE lists cranes, hoists, conveyors, extruders, mixers, positive displacement pumps and reciprocating and rotary screw air compressors as examples.

To estimate savings, DOE says to establish the load duty cycle, meaning the share of time spent at each operating point, and the part-load efficiency of drive and motor. System efficiency is the product of drive, motor and driven-equipment efficiencies. Drive efficiency falls with load, more steeply in smaller drives; in DOE’s table of typical PWM values, a 5-hp drive is 95% efficient at full output and 80% at 12.5% load.

What fast switching does to the motor

DOE’s tip sheet on inverter-duty motors says the nonsinusoidal output can increase motor losses, reduce ventilation at low speeds, raise dielectric stress on windings and cause magnetic noise and shaft currents. When motor impedance exceeds cable impedance, the voltage pulse reflects at the motor terminals and forms a standing wave, and longer cables favor larger ones. DOE reports spikes as high as 2,150 volts in a 480-V system operating at 10% overvoltage.

A companion tip sheet says existing general-purpose low-voltage motors may work fine with PWM drives if peaks stay below 1,000 V. Above that, a turn-to-turn short can develop, usually within the first couple of turns of a coil group.

DOE summarizes the motor standard NEMA MG 1-2011: Part 30 general-purpose motors should see no significant loss of service life if peaks are held to 1,000 V with rise times of at least 2 microseconds. Part 31 inverter-duty motors rated 600 V or less should withstand 3.1 times rated line-to-line voltage, or 1,426 peak volts for a 460-V motor, with rise times of at least 0.1 microsecond.

Common problems

  • Long motor leads. DOE says modern IGBT drives almost always need mitigation at cable lengths of 50 feet or more, while reflected waves are generally not a problem under 15 feet. Locate the drive near the motor, or fit line inductors (load reactors) at the drive end or dV/dt filters. Filters add losses, so short cables remain the best option.
  • Bearing fluting. Fast pulses can drive current across bearing rolling elements. DOE says carrier frequencies above 5 kHz are likely to damage bearings without protection, and recommends a 2 kHz to 5 kHz carrier, a dedicated low-impedance ground wire from motor frame to drive case, symmetrical cable, insulated bearings or a shaft grounding brush, and insulated couplings for driven-load bearings.
  • Several motors on one drive. DOE says the potential for damaging reflected waves is especially high in that arrangement, and overshoots are more likely with smaller motors.
  • Wrong motor for the duty. DOE advises inverter-duty motors for extremely low speeds, especially with constant torque, or for running above base speed. For variable torque loads, inverter-ready general-purpose motors are well suited.
  • Overheating at low speed. Larger inverter-duty motors often carry a constant-speed auxiliary blower for cooling at low speeds.