Variable-frequency drive

Did You Know?

• Pulse-width modulation (PWM) variable-frequency drive projects started in the 1960s at Strömberg in Finland.
• In the United States, an estimated 60–65% of electrical energy is used to supply motors, 75% of which are variable-torque fan, pump, and compressor loads.
• Only about 3% of the total installed base of AC motors are provided with AC drives, but drive technology is adopted in as many as 30–40% of all newly installed motors.

System description and operation

A variable-frequency drive system consists of three main sub-systems: an AC motor, a main drive controller assembly, and a drive/operator interface. The AC motor used is usually a three-phase induction motor, though some single-phase or synchronous motors may be advantageous in certain situations. Motors designed for fixed-speed operation are often used, but those supplied by VFDs must be designed for definite-purpose inverter-fed duty per Part 31 of NEMA Standard MG-1 due to elevated-voltage stresses. The VFD controller is a solid-state power electronics system with three sub-systems: a rectifier bridge converter, a DC link, and an inverter. Voltage-source inverter (VSI) drives are the most common type. Most drives are AC–AC drives, converting AC line input to AC inverter output, though some are DC–AC drives for applications like common DC bus or solar. The basic rectifier converter for VSI drives is a three-phase, six-pulse, full-wave diode bridge. The DC link uses a capacitor to smooth ripple and provide stiff input to the inverter. VSI drives offer higher power factor and lower harmonic distortion than current-source inverter (CSI) and load-commutated inverter (LCI) drives. The insulated-gate bipolar transistor (IGBT), introduced in 1983, has become the dominant inverter switching device in VFDs.

Starting and software behavior

Most VFDs allow auto-starting, which drives the output to a designated frequency after a power cycle, after a fault is cleared, or after an emergency stop signal is restored. One common control method is to enable auto-start and place L1, L2, and L3 into a contactor; powering on the contactor turns on the drive and sets it to a designated speed. Depending on the drive's sophistication, multiple auto-starting behaviors can be developed, such as auto-starting on power up but not from clearing an emergency stop until a reset is cycled. Drive applications are categorized as single-quadrant, two-quadrant, or four-quadrant. Quadrant I involves forward accelerating with positive speed and torque; Quadrant II involves forward braking-decelerating with positive speed and negative torque; Quadrant III involves reverse accelerating with negative speed and torque; Quadrant IV involves reverse braking-decelerating with negative speed and positive torque. Most applications are single-quadrant loads in Quadrant I, such as variable-torque loads like centrifugal pumps or fans and constant-torque loads like extruders. Two-quadrant loads operate in Quadrants I and II where speed is positive but torque changes polarity, as in a fan decelerating faster than natural losses. Four-quadrant loads, such as hoists, elevators, and hilly conveyors, allow speed and torque in any direction.

VFD types and ratings

AC drives are classified by generic topologies. Voltage-source inverter (VSI) drives use a diode-bridge converter with a capacitor bus to supply stiff voltage input to the inverter; the vast majority of drives are VSI type with PWM voltage output. Current-source inverter (CSI) drives use an SCR-bridge converter with a series inductor to supply stiff current input, and can operate with PWM or six-step waveform output. Six-step inverter drives, now largely obsolete, can be VSI or CSI type and are also called variable-voltage inverter drives, pulse-amplitude modulation (PAM) drives, square-wave drives, or DC chopper inverter drives. Load commutated inverter (LCI) drives, a special CSI case, use an SCR-bridge converter with a DC link inductor to supply stiff quasi-sinusoidal six-step current output to an over-excited synchronous machine; low-cost SCR-thyristor-based LCI drives are used in high-power low-dynamic-performance fan, pump, and compressor applications up to 100 MW. Cycloconverter or matrix converter (MC) topologies have no intermediate DC link; cycloconverters use three anti-parallel-connected SCR-bridges, while MC drives are IGBT-based. Doubly-fed slip recovery systems feed rectified slip power through a smoothing reactor to supply power to the AC network via an inverter, controlling speed by adjusting DC current. Most drives use scalar control, vector control (VC), field-oriented control (FOC), or direct torque control (DTC).

Application considerations

The VFD's diode-bridge rectifier converts AC line voltage to DC by superimposing non-linear half-phase current pulses, creating harmonic current and voltage distortion on the AC line input. When VFD loads are small relative to a stiff power system, harmonic effects are often within acceptable limits. In low-voltage networks, harmonics from single-phase equipment like computers and TVs are partially cancelled by three-phase diode bridge harmonics because their 5th and 7th harmonics are in counterphase. However, when the proportion of VFD and other non-linear load is large relative to total load or to the stiffness of the AC power supply, the load can negatively impact the AC power waveform for other customers. Distorted voltage increases losses in fixed-speed AC motors, potentially causing overheating and shorter life, and also affects substation transformers and compensation capacitors. Capacitors can create resonance conditions that magnify harmonic levels. To limit voltage distortion, VFD load owners may need to install filtering equipment, or the utility may install filtering at substations. In high-power installations, harmonic distortion can be reduced by supplying multi-pulse rectifier-bridge VFDs from transformers with multiple phase-shifted windings.