Variable frequency drives (or VFDs) are widely used throughout industry to provide adjustable speed control of ac motors. Although modern versions are relatively simple to install and operate, VFDs are quite complex, containing a myriad of advanced hardware and software. VFD implementation and operation can often be improved by understanding internal VFD processes. To that end, we present the terminology used to describe VFD internal operations and external interfaces. This lesson contains 40 of the most important VFD terms for today's design engineers.

1. Rectifier/converter: One of the three primary sections of a VFD's main power circuit, and first in terms of power flow. Incoming ac line voltage is rectified to dc voltage in the converter section, which consists of diodes, silicon-controlled rectifiers (SCRs), or insulated gate bipolar transistors (IGBTs) connected in a full-wave bridge configuration.

2. Dc bus: The second primary section of a VFD's main power circuit, chiefly comprised of capacitors that store power rectified by the converter.

3. Inverter: The third and final primary section of a VFD's main power circuit.

The inverter section is comprised of IGBTs that create sinusoidal output current using pulsed dc bus voltage, or pulse width modulation (PWM). VFDs themselves are sometimes called inverters, as the presence of an inverter section is the primary difference between VFDs and dc drives.

4. IGBT: Very fast semiconductor switches that are actuated electronically.

By applying a small positive voltage between the gate and emitter points of the IGBT, current is allowed to flow from a collector point to an emitter point. IGBT switching rates in VFDs range from 2 to 15 kHz. (See carrier frequency.)

5. PWM: A VFD control scheme in which a constant dc voltage is used to reconstruct a pseudo ac voltage waveform using a set of six power switches, usually IGBTs.

Varying the width of the fixed-amplitude pulses controls effective voltage. This pulse width modulation scheme works because the motor is a large inductor that does not allow current to pulse like the voltage.

Sequenced correctly, PWM outputs motor current in a nearly perfect sinusoidal waveform.

Carrier frequency is the rate at which output transistors are gated or turned on.

6. Carrier frequency: In PWM-based VFDs, the rate at which output transistors are gated or turned on, usually 2 to 15 kHz. (See image.) Higher values yield better current waveform, but more VFD losses.

7. Common busing: A method for connecting the dc bus sections of separate VFDs, or operating multiple independent inverter sections from a common dc source. The advantage of this method is that motor-operation sequencing can be used to balance motoring and regenerating so that little or no dynamic braking is necessary.

8. Dynamic braking: In VFDs, this refers to connecting resistors to the dc bus through a transistor. The transistor is closed and shunts power to the resistors only when the dc bus voltage exceeds a predetermined level, which usually occurs when the load is decelerated quickly.