Control With Brake Function - Danfoss VLT AutomationDrive FC 300 Design Manual

FC 300 Design Guide
Danfoss recommends the brake resistance R
i.e. one that guarantees that the adjustable
frequency drive is able to brake at the highest
braking torque (M ) of 160%.
br
η is typically at 0.90, while η
motor
typically at 0.98.
For 200 V, 500 V, and 600 V adjustable frequency
drives, R at 160% braking torque is written as:
REC
NOTE
The brake circuit resistance selected should not be higher than that recommended by Danfoss.
If a brake resistor with a higher ohmic value is selected, the 160% braking torque may not be
achieved because there is a risk that the adjustable frequency drive cuts out for safety reasons.
NOTE
If a short circuit in the brake transistor occurs, power dissipation in the brake resistor is only
prevented by using a line switch or contactor to disconnect the AC line for the adjustable frequency
drive. (The contactor can be controlled by the adjustable frequency drive).

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 is dissipated outside the control panel, i.e. where the energy can be utilized.
-
- The electronics of the adjustable frequency drive 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 adjustable frequency drive.
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.
Over-voltage Control (OVC) (excl. brake resistor) can be selected as an alternative brake 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
adjustable frequency drive is avoided. In this situation, the ramp-down time is extended.
NOTE
Monitoring the brake power is not a safety function; a thermal switch is required for this
purpose. The brake resistor circuit is not ground leakage protected.
Introduction to FC 300
,
REC
is
VLT
MG.33.B3.22 - VLT is a registered Danfoss trademark
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