Basics of variable-frequency motor drives

DC motors have historically had advantages in that their speed and direction of rotation could be easily controlled by adjusting or switching the supply voltage. This has made them well-suited for elevators, ski-lift drives and similar applications requiring smooth and reliable operation.

Compared to equivalent dc motors, three-phase induction motors are relatively lightweight and inexpensive to manufacture and maintain. The big problem was that until recent decades, speed could not be easily controlled. You could reduce the supply voltage to slow the motor, but that was equivalent to substituting a less powerful motor and overloading it. The result was increased power consumption, temperature rise and shortened life. Similarly, unlike a dc motor where rotation can be reversed merely by switching wires, a single-phase ac motor would require an additional reverse winding or similar expedient.

In the early 1960s, elegant new technology emerged that enabled ac motors to easily regulate speed. Variable-frequency drives (VFDs) — also correctly called variable-speed drives, adjustable-speed drives, adjustable-frequency drives, microdrives and motor drive inverters — can be equipped with an operator interface to monitor and control an induction (asynchronous) or synchronous ac motor.

The VFD can refer to the control device alone or to the motor and driven equipment as well. Considering its great functionality and high-power capability, the VFD is simple and robust. Failure modes are most likely to involve power supply, cabling or driven machinery.

Troubleshooting is straightforward, although caution is advised, due to the industrial-level voltages and arc-flash hazards typical of the environment. Beware of stored voltage that can linger in capacitors long after power is disconnected. Extreme care must be exercised to make sure the power supply is locked out and there is no possibility of backfeed so the motor and driven equipment cannot start up unexpectedly.

VFDs involve fairly simple circuitry. The supply power should be provided with a fail-safe disconnect. It should also come in through conductors that are of correct ampacity with short-circuit and ground-fault protection and with separate overload protection for the motor.

Most such systems are three-phase, 480-V ac powered, although single-phase is possible. The three branch-circuit conductors (plus equipment-grounding conductor) enter the enclosure and terminate at the input to the rectifier section. A diode network provides full-wave rectification. The critical function of filtering is via large electrolytic capacitors with or without added inductance.

The output moves through the dc bus to the inverter section, where a suitable ac output is synthesized to power the motor. The inverter regulates voltage and frequency to the motor by means of pulse-width modulation (PWM). Control, either automatic or manual, comes from outside. A human at the operator interface can command the motor to reverse, accelerate or decelerate, start or stop. Also, torque parameters can be monitored or altered. Appropriate signals are sent to the VFD inverter section, which is powered by a buffering dc voltage that is actually 1.414 times the ac line peak-to-peak voltage. Switching is accomplished in the inverter section by insulated-gate bipolar transistors (IGBTs) which, depending on the VFD capacity, may be capable of driving 500 hp motors.

System malfunction can manifest as overheating, usually a prelude to motor failure. In a large motor, motor failure is expensive and downtime must be factored in as well. These factors can be mitigated by taking periodic temperature readings so harmful trends get spotted early.

Cabling from the VFD to the motor can be problematic. The thing to look at is whether symptoms have developed gradually or are present at startup. Increased impedance or harmful signal reflections reduce overall system efficiency. In a harsh factory environment, terminations or the cable may become compromised. Alternatively, in the design-installation process, the cable may have been too long or routed close to a non-linear load.

Design, maintenance and installation in a VFD as in industrial electronics in general profoundly affects performance. But once in place and run in, VFDs usually work for years in a trouble-free manner.