Mechanical Brake Control; Extreme Running Conditions - Danfoss VLT Series Design Manual

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VLT HVAC Drive Design Guide
Over voltage control (OVC)
function in par. 2-17. This function is active for all units. The function ensures that a trip can be
avoided if the DC link voltage increases. This is done by increasing the output frequency to limit
the voltage from the DC link. It is a very useful function, e.g. if the ramp-down time is too short
since tripping of the frequency converter is avoided. In this situation the ramp-down time is ex-
tended.

2.14. Mechanical brake control

2.14.1. Cabling
EMC (twisted cables/shielding)
To reduce the electrical noise from the wires between the brake resistor and the frequency con-
verter, the wires must be twisted.
For enhanced EMC performance a metal screen can be used.

2.15. Extreme running conditions

Short Circuit (Motor Phase – Phase)
The frequency converter is protected against short circuits by means of current measurement in
each of the three motor phases or in the DC link. A short circuit between two output phases will
cause an overcurrent in the inverter. The inverter will be turned off individually when the short
circuit current exceeds the permitted value (Alarm 16 Trip Lock.
To protect the drive against a short circuit at the load sharing and brake outputs please see the
design guidelines.
Switching on the Output
Switching on the output between the motor and the frequency converter is fully permitted. You
cannot damage the frequency converter in any way by switching on the output. However, fault
messages may appear.
(exclusive brake resistor) can be selected as an alternative brake
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MG.11.B2.02 - VLT is a registered Danfoss trademark
2. Introduction to VLT HVAC Drive
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