Application Guide

63-706251
Minor Faults
VFD Overheat
This fault is usually caused by problems with cooling air circulation over the VFD heat sink or within the VFD chassis. It may
also be caused through a combination of high motor load and high ambient temperature. The outcome of the above is that the
VFD has sensed that its internal components have become overheated and has closed down, in order to prevent thermal
damage.
1.
Check for correct rotation of fans, both mounted on the VFD and any external cooling fans within the VFD enclosure.
2.
Clean any air filters and heat sinks.
3.
Check for any loose internal connections associated with remote mounted temperature sensors.
Motor thermal trip
The VFD has the capacity to monitor the motor load via the motor current and make an estimate of the degree of heating of the
motor caused by this load. There is usually at least one parameter that allows the user to match the overload to the motor load
conditions:
— Adjust the motor overload parameter to match the motor load condition, or
— Reduce the actual load on the motor until the overload condition is removed.
IMPORTANT
There may be an intermittent overload condition not present during inspection. Adjusting the motor overload
parameter can result in a damaged motor.
The VFD may also have the ability to actually measure the motor temperature via sensors mounted within the motor body. If
this option is available, there is usually no adjustment available. In this situation the only option is to reduce the motor load
condition. It s also possible that the motor temperature sensors have become defective.
Loss of Control Loop
If the VFD is using a 4-20 mA source for speed reference, the VFD will normally monitor for a reference value below 4 mA. If
this occurs, the VFD may respond by tripping, as it understands that the control loop has been lost and is therefore out of
control. This will usually be caused by broken external cabling or a control instrument failure
DC bus over voltage
This problem can have several causes:
— Regeneration from high inertia loads.
— Excessively fast deceleration speeds.
— Open circuiting contactors or other types of switches between the motor and VFD.
— Regeneration from high inertia loads.
Under normal stable operation, the flow of energy is from the power supply through the VFD, to the motor and then out to the
load. There can be situations where energy flow is reversed and the motor becomes a generator. This would occur if the load
tries to
over-run
the motor, or the VFD tries to decrease the motor speed rapidly, and the load inertia is particularly high. In
either case the
regenerated energy
flows back into the VFD. As it cannot flow past the bridge rectifier, it will be stored within the
DC bus capacitors, causing a voltage rise. If this voltage rises to 740 volts the VFD will begin to ignore the speed reference and
attempt to limit the DC bus voltage rise by reducing or stopping the rate of deceleration.
If this control loop is inadequate for the load, the DC bus voltage will continue to rise and at 800V (on a 400V VFD) the VFD will
trip, to prevent over voltage damage occurring.
Extending the deceleration ramp time and or installing a braking unit would be options to prevent tripping. Also, it might be
possible to use coast to stop, if the problem only occurred when the VFD was trying to bring the load to a halt.
Excessively fast deceleration speeds
If the deceleration rate is sufficiently short, it is almost guaranteed that an over voltage trip will occur, irrespective of the
magnitude of the load. A motor without any connected load can regenerate a substantial amount of energy if attempts are
made to bring it to a halt in one or two seconds. Simply extending the deceleration ramp times usually accommodates most
problems in this area