Installation, Operation and Maintenance Manual
Field Installed Variable Frequency Drives (VFD’s
5
Subject to change without notice.
120.7-IM (1218)
ELECTRICAL HAZARD
Disconnect all power before servicing the
drive controller� WAIT ONE MINUTE until
DC bus capacitors discharge, then meas
ure DC bus capacitor voltage to verify that
DC voltage is less than 45 volts�
ELECTRICAL HAZARD
DO NOT short across any internal
components or touch unshielded
components � Use of electrically insulated
tools is required�
ELECTRICAL HAZARD
Equipment must be properly grounded per
all applicable national and local codes�
Refer to the VFD manufacturers manual
for specific grounding details.
ELECTRICAL HAZARD
Make sure all covers and doors are closed
before applying power or starting the VFD�
Voltage Unbalance
Voltage unbalance occurs when the RMS line voltages on
a 3phase power supply are unequal� Voltages are never
balanced between phases, but if the level of the unbalance
becomes excessive it will create problems for not only
motors but also controls�
The maximum desirable voltage unbalance is 2�0%�
When testing for voltage unbalance, the phasetophase
voltages should be measured rather than the phaseto
neutral voltages since 3phase motors are connected across
phases� Use the following formula to determine the percent
of voltage unbalance:
Percent Voltage Unbalance = 100 x (Maximum Voltage
Deviation / Average Voltage)
Example:
PhasePhase voltages
A8 = 479V
8C = 472V
CA = 450V
Average Voltage = (479 + 472 + 450) / 3 = 467
Maximum Voltage Deviation from Average =
467 450 = 17 (Must always be positive)
Voltage Unbalance = 100 x (17/467) = 3�6%
In this example the percent of voltage unbalance exceeds the
desired maximum of 2%� Additional checks should be made
at the unit disconnect to confirm the values. Use accepted
industry practices to check or test the quality of the power
supply � Often, it is just a matter of repairing malfunctioning
equipment or redistributing loads to improve the unbalance�
If no cause can be located and resolved for the unit power
supply, the building manager or owner should be notified of
the issue to get the proper power supplied to the unit�
It should be noted that the inclusion of a variable frequency
drive (VFD) with an unbalanced power supply may result in
nuisance tripping and 3rd harmonic currents�
