Danfoss offers brake resistors with duty cycle of 5%, 10% and 40% suitable for use with the VLT®
FC102 HVAC drive series. If a 10% duty cycle resistor is applied, this is able of absorbing braking
power upto 10% of the cycle time with the remaining 90% being used to dissipate heat from the
resistor.
For further selection advice, please contact Danfoss.
NB!
If a short circuit in the brake transistor occurs, power dissipation in the brake resistor
is only prevented by using a mains switch or contactor to disconnect the mains for
the frequency converter. (The contactor can be controlled by the frequency con-
verter).
2.13.2. Control with Brake Function
The brake is to limit the voltage in the intermediate circuit when the motor acts as a generator.
This occurs, for example, when the load drives the motor and the power accumulates on the DC
link. The brake is built up as a chopper circuit with the connection of an external brake resistor.
Placing the brake resistor externally offers the following advantages:
-
The brake resistor can be selected on the basis of the application in question.
-
The brake energy can be dissipated outside the control panel, i.e. where the energy can
be utilized.
-
The electronics of the frequency converter will not be overheated if the brake resistor is
overloaded.
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. In par. 2-13, select the function to carry out when
the power transmitted to the brake resistor exceeds the limit set in par. 2-12.
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.
2. Introduction to VLT HVAC Drive
VLT
®
HVAC Drive Design Guide
46
MG.11.B2.02 - VLT
®
is a registered Danfoss trademark
2