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

FC 300 Design Guide

Selection of Brake Resistor

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To select the right brake resistor, it is necessary to know how often to brake and by
how much the power braking is effected.
The resistor intermittent duty (S5), which is often used by motor suppliers when stating the permissible
load, is an indication of the duty cycle at which the resistor is working.
The intermittent duty cycle for the resistor is calculated as follows, in which T = cycle time in
seconds and t is the braking time in seconds (of the cycle time): The max. permissible load
b
on the brake resistor is stated as a peak power at a given intermittent duty cycle. Therefore,
determine the peak power for the brake resistor and the resistor value.
Dutycycle = T /T
b
The max. permissible load on the brake resistor is stated as a peak power at a given ED. Therefore,
determine the peak power for the brake resistor and the resistor value.
The shown example and formula apply to FC 302.
The brake resistance is calculated as shown:
As can be seen, the brake resistance depends on the intermediate circuit voltage (UDC).
With FC 302 adjustable frequency drives with a line voltage of 3 x 200-240 V, the brake will
be active at 390 V (UDC). If the adjustable frequency drive has a line voltage of 3 x 380-500
V, the brake will be active at 810 V (UDC), and if the adjustable frequency drive has a line
voltage of 3 x 525-600 V, the brake will be active at 943 V (UDC).
NOTE
Check that the brake resistor can cope with a voltage of 430 V, 850 V or 930 V
- unless Danfoss brake resistors are used.
48
Introduction to FC 300
MG.33.B3.22 - VLT is a registered Danfoss trademark
P = P x M x η
PEAK MOTOR BR(%)
x η [W]
MOTOR VLT