KFU 2-/4-
217
17.4.1 Dimensioning
of Brake Resistor
The following values must be known for dimensioning:
Peak braking power P
b Peak
in W
Resistance
R
b
in
Operation Time OT in %
Calculation of peak braking power P
b Peak
P
b Peak
= Peak braking power in W
J
= Moment of inertia of drive system kgm
2
n
1
= Speed of drive system before the braking op-
eration in min
-1
n
2
= Speed of drive system after the braking opera-
tion in min
-1
b
2
2
2
1
Peak
b
t
182
n
n
J
P
t
b
= Braking time in s
Calculation of resistance R
b
R
b
= Resistance in
U
d BC
= Switch-on threshold in V
Peak
b
2
BC
d
b
P
U
R
P
b Peak
= Peak braking power in W
The switch-on threshold U
d BC
is the DC link voltage at which the brake resistor is
switched on. The switch-on threshold can be set, as described above, via parameter
Trigger threshold
506
.
Caution!
The resistance of the brake resistor must not be less than the minimum
value R
b min
-10%. The values for R
b min
are listed in chapter "Technical
Data".
If the calculated resistance R
b
of the brake resistor is between two standard series
values, the lower resistance is to be selected.
Calculation of operation time OT
OT = Operation
time
t
b
= Braking
time
cycle
t
b
t
C
D
t
cycle
= Cycle
time
Example:
t
b
= 48 s, t
cycle
= 120 s
t
cycle
t
b
%
40
4
.
0
cycle
t
b
t
C
D
In the case of infrequent short braking operations, typical values of the operation time
OT are at 10 %, for long braking operations (
120 s) typical values are at 100%. In
the case of frequent deceleration and acceleration operations, it is recommended that
the operating time OT be calculated according to the above formula.
The calculated values for P
b Peak
, R
b
and OT can be used by the resistor manufactur-
ers for determining the resistor-specific permanent power.
Warning!
The brake resistor is to be connected according to the specifications and
instructions in chapter "Electrical Installation, Connection of a Brake
Resistor".
