Danfoss VLT Series Design Manual page 53

VLT ® HVAC Drive Design Guide
Overvoltage control (OVC)
tion 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 if, for example, the ramp-down time is too short
because tripping the adjustable frequency drive is avoided. In this situation, the ramp-down time
is extended.
2.13. Mechanical brake control
2.13.1. Cabling
EMC (twisted cables/shielding)
To reduce the electrical noise from the wires between the brake resistor and the adjustable fre-
quency drive, the wires must be twisted.
For enhanced EMC performance, a metal shield can be used.
2.14. Extreme running conditions
Short Circuit (Motor Phase – Phase)
The adjustable frequency drive is protected against short circuits by means of current measure-
ment 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 adjustable frequency drive is fully permitted.
You cannot damage the adjustable frequency drive in any way by switching on the output. How-
ever, fault messages may appear.
Motor-generated Overvoltage
The voltage in the intermediate circuit is increased when the motor acts as a generator. This occurs
in the following cases:
1.
2.
3.
NOTE
Monitoring the braking energy is not a safety function; a thermal switch is required
for that purpose. The brake resistor circuit is not protected against ground leakage.
(exclusive brake resistor) can be selected as an alternative brake func-
The load drives the motor (at constant output frequency from the adjustable frequency
drive), i.e., the load generates energy.
During deceleration ("ramp-down"), if the moment of inertia is high the friction is low
and the ramp-down time is too short for the energy to be dissipated as a loss in the
adjustable frequency drive, the motor and the installation.
Incorrect slip compensation setting may cause higher DC link voltage.
MG.11.B4.22 - VLT ® is a registered Danfoss trademark.
2. Introduction to the VLT HVAC Drive
2
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