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.
Type of drive | 6-Pulse | 12-Pulse | 18-Pulse |
Height | 4 ft - 11 in. | 7 ft - 6 in. | 7 ft - 6 in. |
Width | 2 ft - 2.5 in. | 2 ft - 6 in. | 2 ft - 6 in. |
Depth | 1 ft - 7.5 in. | 1 ft - 9.5 in. | 1 ft - 9.5 in. |
Approx. cost | $7,000 | $15,900 | $17,100 |
One set of diodes is supplied by a Delta-Y transformer to create a phase shift on the AC side
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