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WARNING
Hazardous
voltage
Section 6
(continued)
Ungrounded secondary
Grounding of the transformer secondary is essential to the safety of personnel as well as the safe operation of
the drive. Leaving the secondary floating can permit dangerously high voltages between the chassis of the drive
and the internal power structure components. In many cases this voltage could exceed the rating of the EMC
filter and input MOV protection devices of the drive causing a catastrophic failure. In all cases, the input power to
the drive should be referenced to ground. If the transformer can not be grounded, then an isolation transformer
must be installed with the secondary of the transformer grounded.
In this configuration the line to ground voltage from the incoming power supply may exceed the voltage rating
of the line to ground EMC filter components and line to ground MOV protection. This can cause catastrophic
controller failure if the line to ground EMC filter and input MOV protection devices are not disconnected. Refer
to Disconnecting EMC Filter and MOVs for details on line to ground voltage limitations and disconnecting these
devices.
WARNING
If a power system with an ungrounded secondary is used, the line to ground EMC filter
components and line to ground MOV protection must be disconnected or damage to the
controller can result.
Resistance grounding and ground fault protection
Connecting the Wye secondary neutral to ground through a resistor is an acceptable method of grounding.
Under a short circuit secondary condition, any of the output phases to ground will not exceed the normal
line to line voltage. This is within the rating of the MOV input protection devices on the drive. The resistor is
often used to detect ground current by monitoring the associated voltage drop. Since high frequency ground
current can flow through this resistor, care should be taken to properly connect the drive motor leads using
the recommended cables and methods. In some cases, multiple drives on one transformer can produce a
cumulative ground current that can trigger the ground fault interrupt circuit.
POWER SUPPLY AND WIRING
