Brake Functions In Automationdrive Fc; Mechanical Holding Brake; Dynamic Braking; Selection Of Brake Resistor - Danfoss FC 300 Design Manual

FC 300 Design Guide

3.8 Brake Functions in AutomationDrive FC 300

Braking function is applied for braking the load on the motor shaft, either as dynamic braking or static braking.

3.8.1 Mechanical Holding Brake

A mechanical holding brake mounted directly on the motor shaft normally performs static braking. In some applications the static holding torque is working
as static holding of the motor shaft (usually synchronous permanent motors). A holding brake is either controlled by a PLC or directly by a digital output
from the frequency converter (relay or solid state).
NB!
When the holding brake is included in a safety chain:
A frequency converter cannot provide a safe control of a mechanical brake. A redundancy circuitry for the brake control must be included
in the total installation.

3.8.2 Dynamic Braking

Dynamic Brake established by:
• Resistor brake: A brake IGBT keep the overvoltage under a certain threshold by directing the brake energy from the motor to the connected
brake resistor (. 2-10 = [1]).
• AC brake: The brake energy is distributed in the motor by changing the loss conditions in the motor. The AC brake function cannot be used in
applications with high cycling frequency since this will overheat the motor (. 2-10 = [2]).
DC brake: An over-modulated DC current added to the AC current works as an eddy current brake (. 2-02 ≠ 0 s).
•

3.8.3 Selection of Brake Resistor

To handle higher demands by generatoric braking a brake resistor is necessary. Using a brake resistor ensures that the energy is absorbed in the brake
resistor and not in the frequency converter.
If the amount of kinetic energy transferred to the resistor in each braking period is not known, the average power can be calculated on the basis of the
cycle time and braking time also called intermitted duty cycle. The resistor intermittent duty cycle is an indication of the duty cycle at which the resistor
is active. The below figure shows a typical braking cycle.
NB!
Motor suppliers often use S5 when stating the permissible load which is an expression of intermittent duty cycle.
The intermittent duty cycle for the resistor is calculated as follows:
Duty cycle = t /T
b
T = cycle time in seconds
t is the braking time in seconds (of the cycle time)
b
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