Application Guide

63-7062 52
Switches on output of VFD
Isolating switches or contactors will often be located in the output cables from the VFD. As when switching an inductive load,
an arc develops at the point where the contact first breaks. This arc can cause high voltage spikes, as high as 3-4 kV, on the
cables back to the VFD. The high voltage spike can completely disrupt the DC bus voltage monitoring circuit, causing a false
high DC bus voltage trip.
To ensure optimum reliability, the switch on the output of the VFD must be interfaced with the VFD controls circuit, by a late
make early break auxiliary contact. When programmed correctly, the VFD will see the opening of this switch and turn off - into a
safe controlled state, thus preventing the trip.
Loss of input phase
A VFD does not usually have the ability to monitor the input power supply integrity. If the VFD is single phase input, loss of a
phase will cause the VFD to close down safely and possibly log a power supply loss. If one phase of a three-phase input VFD
is lost, there is usually sufficient capacity within the input bridge rectifier to cope with it. Heavy loading of the motor causes
overheating of the input bridge rectifier and ultimately cause a trip on
VFD overheat
.
Some VFDs monitor the phases feeding the input bridge rectifier. If one phase voltage deviates from the acceptable limit, the
VFD trips and record
phase loss
.
Major Faults
Instantaneous over current
This fault may be caused for various reasons:
— Motor phase to phase short circuit.
— Short circuit between two or more motor cables.
— Closing switches between VFD and motor while VFD still running.
— Acceleration rate too fast.
With short circuits, the VFD detects a low resistance between two or more phases that allows high current to flow at a level
beyond the normal absolute rating of the VFD. High speed protection circuits have taken effect to protect the output power
transistors of the VFD.
Earth fault
This fault will be reported if either the motor or motor cables have become shorted to earth. Most VFDs monitor the current
balance between all three-phases and at maybe 40% imbalance on one phase, the VFD will trip and report an
earth fault
.
NOTE: With phase-to-phase and phase-to-earth faults, the speed of VFD response usually prevents any major damage from
occurring in the fault area.
In some cases, there can be little or no apparent damage and even with a simple high voltage
megger
test the fault may not
show itself. There can be situations where a high voltage
flash test
may have to be undertaken on the motor to identify the
problem.
NOTE: The VFD should be disconnected from the motor when a megger test or high voltage test is to be undertaken. Failure
to do so may cause damage to output transistors.
Open circuit output phase.
Many VFDs do not allow the motor to run with an open circuit on one of the phases to the motor. If the currents flowing in each
phase do not balance to a reasonable level, the VFD will trip.
DC bus fuse open circuit
Most VFDs have a fuse located in the DC bus circuit. This fuse is usually a high speed semi-conductor protection fuse and is
designed to protect the input bridge rectifier in the event of a major failure on the VFD output stage. If the fuse has become
open circuit it should not be replaced without first checking the VFD power devices. It is unlikely that the fuse will have blown
without some major component having failed elsewhere. See Fig. 57 for bridge rectifier and output power transistor testing
information.