Over voltage control (OVC) (exclusive brake resistor) can be selected as an alternative brakefunction in par. 2-17. This function is active for all units. The function ensures that a trip can beavoided if the DC link voltage increases. This is done by increasing the output frequency to limitthe voltage from the DC link. It is a very useful function, e.g. if the ramp-down time is too shortsince tripping of the frequency converter is avoided. In this situation the ramp-down time is ex-tended.2.14. Mechanical brake control2.14.1. CablingEMC (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 conditionsShort Circuit (Motor Phase – Phase)The frequency converter is protected against short circuits by means of current measurement ineach of the three motor phases or in the DC link. A short circuit between two output phases willcause an overcurrent in the inverter. The inverter will be turned off individually when the shortcircuit 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 thedesign guidelines.Switching on the OutputSwitching on the output between the motor and the frequency converter is fully permitted. Youcannot damage the frequency converter in any way by switching on the output. However, faultmessages may appear.VLT® HVAC Drive Design Guide 2. Introduction to VLT HVAC DriveMG.11.B2.02 - VLT® is a registered Danfoss trademark 472
