Section Five
5 - 4
Instantaneous overcurrent trips are caused by the current rising so fast on the
output that the unit cannot respond. One example of this situation is in applications
where the unit is running at speed and an output contactor is closed between the
unit and the motor. At the point the contactor is closed, the motor is effectively
seen as a short circuit to the unit. During this time the unit will attempt to gain
control of the motor by employing current limit. If the current limit function is
unable to limit the current to acceptable levels, the result will be an
"OVERCURRENT" trip. This example is not to imply that output contactors should
not be used. In fact, that is quite the contrary as the VLT has been designed to
withstand this type of operation without failure. The important consideration in
applications such as this is that the unit is properly sized to handle the inrush
currents.
A second example of instantaneous overcurrent is that experienced in applications
with windmilling loads. A large fan has not yet been commanded to run; however,
air movement is causing the fan to rotate. When the unit is started it must first
drive the fan to zero speed and then begin the acceleration process from there.
The amount of current required may be so great and rise so rapidly that the current
limit function cannot control the process. The result is an "OVERCURRENT" trip.
However, this situation can also be solved by a VLT feature, "Flying Start". With
the flying start feature employed the VLT will interrogate the motor to determine its
effective frequency and match the VLT output to that same frequency. Flying start
results in a smooth start and full control of the load current.
Regenerative energy is created when the load overhauls the motor. This means
that the motor is being forced by the inertia of the load to rotate at a speed greater
than the command speed. When overhauling occurs, the motor acts as a
generator and the voltage generated is returned to the DC capacitor bank in the
unit.
Regeneration is most commonly found in applications with high inertia loads and
medium to fast decel ramps. However, even an unloaded motor ramped down
fast enough can cause regeneration to occur.
It is most common that regeneration is experienced during ramping, although
loads such as flywheels will generate regenerative energy to some degree on
every cycle.
Since the unit can absorb approximately 15 percent of the motor's rated power in
regenerated energy, this phenomena will go unnoticed in most applications.
"OVERCURRENT" TRIPS
"OVERVOLTAGE" TRIPS
DUE TO
REGENERATIVE
APPLICATIONS
To Next Page
Loading...
Need help?
General
