Danfoss VLT Series Service Manual page 38

"OVERVOLTAGE" TRIPS
DUE TO
REGENERATIVE
APPLICATIONS
When the energy returned, combined with the DC Bus voltage, exceeds the upper
voltage limit, the unit responds in different ways to limit the voltage rise. If the
returned energy is occurring during ramp down (to stop or to a lower speed), the
unit will automatically adjust the decel ramp in an attempt to limit the voltage. In
more severe instances, the ramp may even stop for periods of time to allow the
voltage to dissipate. During these periods while regeneration is occurring, the
words "HIGH VOLTAGE" can be observed flashing in the control card display. If
the returned energy is returned at a high enough level and/or so fast that the unit
cannot respond, the unit will trip on "OVERVOLTAGE".
To prevent a trip from occurring, one solution is to lengthen the decel ramp. Another
solution is to release the motor using the "Motor Coast" function. The "Flying
Start" function is usually employed when using this method.
In very high inertia applications where a short decel time is required, the only
solution may be that of adding a Dynamic Brake Option (only VLT Series 3000
units only).
The Dynamic Brake option combines a power IGBT, the electronics for controlling
it and a resistor bank of sufficient wattage to dissipate the unwanted energy. The
Dynamic Brake option monitors the level of the DC Bus voltage. When the voltage
level exceeds permissible limits, the IGBT is switched on and the excess DC Bus
voltage is dissipated in the resistor bank.
Particular attention must be paid to the proper sizing of the resistor bank. Consult
your local representative or the factory for assistance in selecting the appropriate
Dynamic Brake option and dynamic brake resistors for your application.
Section Five
5 - 5