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Trips occurring from ground faults are usually the result of short circuits
to earth ground either in the motor or the wiring to the motor. The VLT
detects ground faults by monitoring all three phases of output current
and looking for severe imbalances in those currents. When a "Ground
Fault" trip occurs it is necessary to measure the resistance of the motor
windings and wiring with respect to earth ground. The instrument
normally used for this purpose is a Megohmmeter or commonly referred
to as a "Megger". Many times these resistance readings are taken
with a common Ohmmeter, which is actually incapable of detecting
any shorts other than those that are virtually direct. A Megger has the
capability of supplying higher voltages, typically 500 volts or more, which
enables the Megger to detect breakdowns in insulation or higher
resistance shorts which cannot be picked up through the use of an
Ohmmeter. When making resistance measurements to ground, it is
necessary to disconnect the motor leads from the output of the VLT.
The measurements should then be taken at the point of connection to
the VLT so the motor and all associated wiring and connections are
captured in the test. When reading the results of the Megger test, the
rule of thumb is any reading less than 500 Megohms should be suspect.
Solid, dry wiring connections normally result in a reading of infinity.
Since the VLT monitors output current to detect ground faults, there is
also the possibility that the current sensors and/or the detection circuitry
in the VLT could also be the cause of a ground fault. Tests can be
made on this circuitry to isolate the possibilities. Refer to the Dynamic
Test procedures on "Testing for Current Feedback" page 30. Consult
the factory for additional assistance.
Trips due to "OVERCURRENT" can be caused by short circuits on the
output of the unit or by instantaneous high currents occurring so rapidly
that the unit cannot respond.
Short circuit trips are generally a result of a phase-to-phase short in
the motor windings or in the wiring between the unit and the motor.
Short circuit trips are easily diagnosed by removing the motor leads
from the unit and performing a phase-to-phase resistance test on the
motor leads. This resistance read in ohms will normally be quite low so
it is important to have the ohmmeter set on its lowest resistance scale
to avoid misinterpreting the readings observed.
"GROUND FAULT" TRIPS
"OVERCURRENT" TRIPS