2.12.3 Control with Brake Function
The brake is protected against short-circuiting of the brake resistor, and the brake transistor is monitored to ensure that short-circuiting of the transistor
is detected. A relay/digital output can be used for protecting the brake resistor against overloading in connection with a fault in the frequency converter.
In addition, the brake makes it possible to read out the momentary power and the mean power for the latest 120 seconds. The brake can also monitor
the power energizing and make sure it does not exceed a limit selected in par. 2-12
Brake Power Limit (kW)
. In par. 2-13
Brake Power Monitoring
, select
the function to carry out when the power transmitted to the brake resistor exceeds the limit set in par. 2-12
Brake Power Limit (kW)
.
NB!
Monitoring the brake power is not a safety function; a thermal switch is required for that purpose. The brake resistor circuit is not earth
leakage protected.
Over voltage control (OVC)
(exclusive brake resistor) can be selected as an alternative brake function in par. 2-17
Over-voltage Control
. 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 extended.
2.12.4 Brake Resistor Cabling
EMC (twisted cables/shielding)
To reduce the electrical noise from the wires between the brake resistor and the frequency converter, the wires must be twisted.
For enhanced EMC performance a metal screen can be used.
2.13 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 frequency converter 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.
Motor-generated Over-voltage
The voltage in the intermediate circuit is increased when the motor acts as a generator. This occurs in following cases:
1.
The load drives the motor (at constant output frequency from the frequency converter), ie. the load generates energy.
2.
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 frequency converter, the motor and the installation.
3.
Incorrect slip compensation setting may cause higher DC link voltage.
The control unit may attempt to correct the ramp if possible (par. 2-17
Over-voltage Control
.
The inverter turns off to protect the transistors and the intermediate circuit capacitors when a certain voltage level is reached.
See par. 2-10
Brake Function
and par. 2-17
Over-voltage Control
to select the method used for controlling the intermediate circuit voltage level.
Mains Drop-out
During a mains drop-out, the frequency converter keeps running until the intermediate circuit voltage drops below the minimum stop level, which is
typically 15% below the frequency converter's lowest rated supply voltage. The mains voltage before the drop-out and the motor load determines how
long it takes for the inverter to coast.
VLT
®
HVAC Drive Design Guide
2 Introduction to VLT HVAC Drive
MG.11.B9.02 - VLT
®
is a registered Danfoss trademark
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