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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 (par. 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 (par. 2-10 = [2]).
•
DC brake: An over-modulated DC current added
to the AC current works as an eddy current brake
(par. 2-02 ≠ 0 sec. ).
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. For more information see the Brake
Resistor Design Guide, MG.90.OX.YY.
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 intermittent 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.
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
b
/T
T = cycle time in seconds
t
b
is the braking time in seconds (of the cycle time)
ta tc
tb
to ta tc
tb
to ta
130BA167.10
Load
Time
Speed
Cycle time (s)
Braking duty cycle at 100%
torque
Braking duty cycle at over torque
(150/160%)
200-240 V
PK25-P11K 120 Continuous 40%
P15K-P37K 300 10% 10%
380-500 V
PK37-P75K 120 Continuous 40%
P90K-P160 600 Continuous 10%
P200-P800 600 40% 10%
525-600 V
PK75-P75K 120 Continuous 40%
525-690 V
P37K-P400 600 40% 10%
P500-P560 600
40%
1)
10%
2)
P630-P1M0 600 40% 10%
Table 3.8 Braking at High overload torque level
1) 500 kW at 86% braking torque
560 kW at 76% braking torque
2) 500 kW at 130% braking torque
560 kW at 115% braking torque
Introduction to FC 300 FC 300 Design Guide
MG.33.BD.02 - VLT
®
is a registered Danfoss trademark 43
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