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 frequency
converter 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 frequency converters,
R at 160% braking torque is written as:
REC
NB!:
The resistor 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 frequency converter cuts out for safety reasons.
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 converter).

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.
Over voltage control (OVC) (exclusive 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
frequency converter is avoided. In this situation the ramp-down time is extended.
Introduction to FC 300
,
REC
is
VLT
MG.33.B4.02 - VLT is a registered Danfoss trademark
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