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

Product Introduction

3.8.3 Selection of Brake Resistor

To handle higher demands by generatoric braking, a brake resistor is necessary. Using a brake resistor ensures the energy is
absorbed in the brake resistor and not in the frequency converter. 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. Illustration 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
Load
Speed
ta tc tb to
T
Illustration 3.32 Typical Braking Cycle
380-500 V
N90K-N160
N200-N250
P315-P800
525-690 V
N55K-N315, P355-P400
P500-P560
P630-P1M0
Table 3.18 Braking at High Overload Torque Level
Danfoss offers brake resistors with duty cycle of 5%, 10% and 40%. If a 10% duty cycle is applied, the brake resistors are
able to absorb brake power for 10% of the cycle time. The remaining 90% is used on dissipating excess heat. Make sure the
resistor is designed to handle the required braking time. The maximum permissible load on the brake resistor is stated as a
peak power at a given intermittent duty cycle. The brake resistance is calculated as shown:
U dc 2
R br Ω =
P peak
As can be seen, the brake resistance depends on the intermediate circuit voltage (U
®
VLT AutomationDrive FC 300 Design Guide 90-1200 kW
ta tc tb to ta
Time
Cycle time (s)
600
600
600
600
600
600
MG34S202 - Rev. 2013-08-19
Braking duty cycle at 100% Braking duty cycle at over torque
torque
Continuous
Continuous
40%
40%
40%
40%
P =P xM [%]xη xη
peak motor br motor
).
dc
(150/160%)
10%
10%
10%
10%
10%
10%
[W]
VLT
47
3 3