What is a Variable Frequency Drive?
How does a VFD Work?
is a VFD? (Part 1)
By: Dave Polka
How Drive Changes
Just how does a drive
provide the frequency and voltage output necessary to change
the speed of a motor? That's what we'll look at next. Fig. 6
basic PWM drive. All PWM drives contain these main parts, with
subtle differences in hardware and software components.
Figure 6, Basic PWM Drive Components
Although some drives
accept single-phase input power, we'll focus on the 3-phase drive.
But to simplify illustrations, the waveforms in the following
drive figures show only one phase of input and output.
The input section of
the drive is the converter. It contains six diodes, arranged
in an electrical bridge. These diodes convert AC power to DC
next section-the DC bus section-sees a fixed DC voltage.
The DC Bus section
filters and smoothes out the waveform. The diodes actually
reconstruct the negative halves of the waveform onto the positive
half. In a 460V unit, you'd measure an average DC bus voltage
of about 650V to 680V. You can calculate this as line voltage
1.414. The inductor (L) and the capacitor (C) work together to
filter out any AC component of the DC waveform. The smoother
the DC waveform, the cleaner the output waveform from the drive.
The DC bus feeds the
final section of the drive: the inverter. As the name implies,
this section inverts the DC voltage back to AC. But, it does
so in a
variable voltage and frequency output. How does it do this? That
depends on what kind of power devices your drive uses. If you
have many SCR (Silicon Controlled Rectifier)-based drives in
your facility, see the Sidebar. Bipolar
Transistor technology began superceding SCRs in drives in the
mid-1970s. In the early 1990s, those gave way to using Insulated
Gate Bipolar Transistor (IGBT) technology, which will form the
basis for our discussion.
Switching Bus With
use Insulated Gate Bipolar Transistors
(IGBTs) to switch the DC bus
on and off at specific intervals. In doing so, the inverter actually
creates a variable AC voltage and frequency output. As shown
in Fig. 7, the output of the drive doesn't provide an exact replica
AC input sine waveform. Instead, it provides voltage pulses that
are at a constant magnitude.
Figure 7, Drive Output Waveform
The drive's control
board signals the power device's control circuits to turn "on" the
waveform positive half or negative half of the power device.
This alternating of positive and negative switches
recreates the 3 phase output. The longer the power device remains
on, the higher the output voltage. The less time the power device
is on, the lower the output voltage (shown in Fig.8). Conversely,
longer the power device is off, the lower the output frequency.
Figure 8, Drive Output Waveform
The speed at which
power devices switch on and off is the carrier frequency, also
known as the switch frequency. The higher the switch frequency,
the more resolution each PWM pulse contains. Typical switch frequencies
are 3,000 to 4,000 times per second (3KHz to 4KHz). (With an
SCR-based drive, switch frequencies are 250 to 500 times per
second). As you can imagine, the higher the switch frequency,
the smoother the output waveform and the higher the resolution.
higher switch frequencies decrease the efficiency of the drive
because of increased heat in the power devices.
Shrinking cost and
Drives vary in the
complexity of their designs, but the designs continue to improve.
Drives come in smaller packages with each generation. The trend
is similar to that of the personal computer. More features, better
performance, and lower cost with successive generations. Unlike
computers, however, drives have dramatically improved in their
reliability and ease of use. And also unlike computers, the typical
drive of today doesn't spew gratuitous harmonics into your
distribution system-nor does it affect your power factor. Drives
increasingly becoming "plug and play." As electronic
power components improve in reliability and decrease in size,
the cost and
size of VFDs will continue to decrease. While all that is going
on, their performance and ease of use will only get better.
Sidebar: What if you
With the large
installed base of SCRs, you might want to know how these operate.
An SCR (originally referred to as a thyristor) contains a control
element called a gate. The gate acts as the "turn-on"
switch that allows the device to fully conduct voltage. The device
conducts voltage until the polarity of the device reverses-and then
it automatically "turns off." Special circuitry, usually
requiring another circuit board and associated wiring, controls
The SCR's output depends on how soon in the control cycle that
gate turns on. The IGBT output also depends the length of time
the gate is on. However, it can turn off anytime in the control
providing a more precise output waveform. IGBTs also require
a control circuit connected to the gate, but this circuitry is
complex and doesn't require a reversal of polarity. Thus, you
would approach troubleshooting differently if you have an SCR-based
This information has
been provided by: ABB Inc. - Drives and Power Electronics