Control Of Mechanical Brake - Danfoss FC 300 Design Manual

FC 300 Design Guide
NB!
Do not touch the brake resistor as it
can get very hot while/after braking.
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 uti-
lized.
- The electronics of the frequency converter will not be overheated if the brake resistor is over-
loaded.
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 pro-
tecting 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.

Control of Mechanical Brake

For hoisting applications, it is necessary to be able to control an electro-magnetic brake. For controlling
the brake, a relay output (relay1 or relay2) or a programmed digital output (terminal 27 or 29) is re-
quired. Normally, this output must be closed for as long as the frequency converter is unable to 'hold'
the motor, e.g. because of too big load. In par. 5-40 (Array parameter), par. 5-30, or par. 5-31 (digital
output 27 or 29), select mechanical brake control [32] for applications with an electro-magnetic brake.
When mechanical brake control [32] is selected, the mechanical brake relay stays closed during start
until the output current is above the level selected in par. 2-20 Release Brake Current. During stop, the
mechanical brake will close when the speed is below the level selected in par. 2-21 Activate Brake Speed
[RPM]. If the frequency converter is brought into an alarm condition, i.e. over-voltage situation, the me-
chanical brake immediately cuts in. This is also the case during safe stop.
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Introduction to FC 300
MG.33.B7.02 - VLT is a registered Danfoss trademark