Ground Fault Trips; Overcurrent Trips - Danfoss VLT Series Service Manual

"GROUND FAULT" TRIPS
"OVERCURRENT" TRIPS
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 using a megger, it is necessary
to disconnect the motor leads from the output of the VLT. The measurements
should then be taken so that 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 3-7. 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's
current limit 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 mis-interpreting the readings observed.
Section Five
5 - 3