VTdrive - Basic design and operation principles behind variable-frequency drives (VFDs)0 pages

Basic design and operation principles behind variable frequency drives (VFDs)

Although much has been published on variable frequency drives (VFDs), people continue to be

confused by them. The intent of this article is to provide a basic overview of these devices by

evaluating key features as well as their impact on a building' s electrical system.

A VFD is used extensively in modern facilities to save energy on mechanical systems, such as

motors, pumps, etc. Selected to match motor curves to ensure speed and loads are matched, VFDs

can help save motor energy by allowing for variable flow of air, water, etc., based on the demands

and needs of a particular site. This is accomplished by converting the fixed frequency of incoming

alternating current (AC) voltage to direct current (DC) — and then reconverting it back to AC

voltage by varying the frequency at which the insulated gate bipolar transistors (IGBTs) are gated

on and off.

Basic operation

A VFD operates by converting the input sinusoidal AC voltage to DC voltage and then back to AC

voltage. This conversion occurs by using either silicon-controlled rectifiers (SCRs) or IGBTs. The

DC voltage is switched using IGBTs to create an AC output voltage (called the inverter). The

IGBT can switch on and off to create an AC voltage waveform that delivers power to the motor.

The IGBTs create an AC waveform by using pulse width-modulated (PWM) switching. The

frequency at which the switching occurs, which varies from manufacturer to manufacturer, is

called " carrier frequency."

AC power

0-

Reference

Transformer

Variable frequency drive

Diode bridge

rectifier

Filter

Voltage and

frequency

controller

Inverter

Motor

M

A typical 6-pulse VFD has six diodes as a front-end bridge rectifier that converts AC to DC. VFDs

can also have 12 diodes — two sets per phase (2 X 2 X 3 = 12 pulse) — or 18 diodes —

three sets per phase (3 X 2 X 3 = 18 pulse) — and so on.

One set of diodes is supplied by a Delta-Y transformer to create a phase shift on the AC side