Danfoss VLT 3032 Service Manual page 37

"OVERVOLTAGE" TRIPS DUE TO
REGENERATIVE APPLICATIONS
(CONTINUED)
"OVERTEMP" TRIPS
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 sever 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. In applications with very high inertia loads, or in the case of
such loads as flywheels, the only solution may be that of adding a
Dynamic Brake option.
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 for your application.
Trips due to "OVERTEMP" can be due to internal or external over
temperature conditions. Two temperature sensors are located inside
of the drive. One located on the Interface Board monitors the ambient
temperature within the drive itself. This normally open switch closes at
a temperature of 73 to 87 degrees centigrade. The switch will reset at
a temperature between 53 to 67 degrees centigrade allowing a fault
reset to be possible. A fault occuring as a result of the ambient
temperature switch results in both the "IOT" and the "OTT" LED being
illuminated on the Interface Board and the Control card will display
"OVERTEMP". The second sensor located on the heatsink monitors
the surface temperature of the heatsink. This sensor is a NTC device
which changes in resistance value based on temperature. As the
temperature rises the resistance decreases, and conversely as the
temperature decreases the resistance increases. The sensors
resistance must be between 787 ohms and 105 K ohms to be free of a
fault condition. Testing this sensor is explained on page 27. A fault
occuring as a result of the NTC sensor results in the "OTT" LED on the
Interface Board being illuminated and the Control Card will display
"OVERTEMP". These temperature faults can be caused by high ambient
temperatures, clogged fan filters or inoperative cooling fans. The
heatsink and the door fans operate on 24VDC supplied by the Interface
Board.
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