Dakota Supply Group - Index

Dakota Supply Group - dakotasupplygroup - Index

AUTOMATION 42
Tech Talk
VFD Use With Single-Phase Power Systems
Most commercial and industrial electrical
equipment requires three-phase power based on
the electrical demands of heavy manufacturing
populations and general industry. In rural areas or
commercial office buildings that were not originally
designed to support heavy manufacturing, utilities
do not install three-phase power because the cost is
significantly more than single-phase installation.
For many years, people have been using different
technology to generate three-phase power from
single-phase sources. Common technologies
include rotary phase converters, static phase
converters and variable frequency drives (VFDs).
As initial investment costs of VFDs have lowered
and product reliability has increased, more users
are turning to VFDs as the best solution to convert
single-phase motor applications to three-phase,
because their process demands have changed
requiring variable speed control. Common
applications utilizing VFDs with single-phase
input are submersible well pumps, centrifugal
pumps, irrigation systems, fountain systems and
pump jacks.
Using VFDs specifically designed for single-phase
use is not always practical, as manufacturers may
offer single-phase models only in low horsepower
ratings. Therefore, it may be necessary to apply a
three-phase VFD in larger capacity single-phase
situations. When applying single-phase power to a
three-phase VFD, there are several constraints that
need to be considered.
Standard Pulse-Width-Modulated (PWM) VFDs
use a six-pulse diode rectifier because of its
simplicity and low cost structure. The six-pulse
rectification results in 360 Hz DC bus ripple when
used with a three-phase 60 Hz supply. However,
under single-phase use, the DC bus ripple becomes
120 Hz and the VFDs DC bus circuit is subject
to higher stress in order to deliver equivalent
power. Additionally, input currents and harmonics
increase beyond those encountered with threephase
input. Input current distortion of 90% THD
and greater can be expected under single-phase
input, compared to approximately 40% with threephase
input. Therefore, single-phase use requires
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the three-phase VFD power rating be reduced
(derated) to avoid over stressing the rectifier and
DC link components.
Drive HP, Input Current
And Output Current
When using a three-phase VFD with singlephase
input, derating the drive’s output current
and horsepower will be necessary because of the
increase in DC bus ripple voltage and current. In
addition, the input current through the remaining
two phases on the diode bridge converter will
approximately double, creating another derating
consideration for the VFD. The increase in input
current is due to the conversion of three-phase to
single-phase power (√ 3 factor) and the reduced
overall power factor. Input current harmonic
distortion will increase beyond that with a threephase
supply, making the overall input power
factor low. An overall power factor of 0.7 is typical
with single-phase input on units incorporating
the recommended DC link choke. A power factor
of 0.9 is typical with units incorporating DC link
chokes with three-phase input. Input current
distortion over 100% is likely under single-phase
conditions without a DC link choke. Therefore,
DC link chokes are always required.
Be sure to oversize the drive as necessary to
accommodate for operating the motor into its
service factor. The selected drive ratings must
meet or exceed both the motor nameplate HP and
the motor nameplate full load amperage (FLA),
along with satisfying any service factor HP and
FLA requirements. Selecting a drive that meets
only one of these two requirements may result in
poor performance, premature drive failure and
void the factory warranty.
Input Frequency And
Voltage Tolerance
Operation at input frequencies other than 60Hz
will require further review by the factory. The AC
supply voltage must be within the required voltage
range of 240/480Vac +10% to –5% to maximize