Dynamic Braking; Selection Of Brake Resistor - Danfoss VLT AutomationDrive FC 300 Design Manual

Product Introduction

3.8.2 Dynamic Braking

Dynamic braking is accomplished internally within the adjustable frequency drive and is used to slow down the motor to an
eventual stop. Dynamic braking is applied using of the following methods:
•
Resistor brake: A brake IGBT keeps the overvoltage under a certain threshold by directing the braking energy from
the motor to the connected brake resistor (2-10 Brake Function=[1])
•
AC brake: The braking 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 Brake Function=[2])
•
DC brake: An overmodulated DC current added to the AC current works as an eddy current brake (2-02 DC Braking
Time≠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 adjustable frequency drive. For more information, see Brake Resistor
Design Guide.
If the amount of kinetic energy transferred to the resistor in each braking period is not known, the average power can be
calculated based on the cycle time and braking time (intermittent duty cycle). The resistor intermittent duty cycle is an
indication of the duty cycle at which the resistor is active. Figure 3.32 shows a typical braking cycle.
NOTICE!
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 s
t is the braking time in s (of the cycle time)
b
MG34S222
Design Guide
Danfoss A/S © Rev. 2014-02-10 All rights reserved.
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