KEB combiverty Instruction Manual - Braking Resistors
GB - 3
Braking Resistors
1.
Braking Resistors
The KEB COMBIVERT fitted with an external braking resistor is suitable for a limited 4-
quadrant operation. The braking energy, refeed into the DC-bus at generatoric operation, is
dissipated over the braking transistor to the braking resistor.
Different braking resistors are available for the KEB COMBIVERT. Please refer to the next
page for the corresponding formula and restrictions (valid range).
1.1
Selection of braking resistor:
1. Preset desired braking time.
2. Calculate braking time without braking resistor (t
Bmin
).
3. A braking resistor is necessary if the desired braking time is smaller than the calculated
braking time (t
B
< t
Bmin
).
4. Calculate braking torque (M
B
). Take the load torque into account at the calculation.
5. Calculate peak braking power (P
B
). The peak braking power must always be calculated
for the worst case (n
max
to standstill).
6. Selection of braking resistor:
a) Peak power of the braking resistor P
S
≥ peak braking power P
B
b) P
N
is to be selected according to the cycle time.
The resistance values shall not decrease the min. permissible value of the braking
transistors. The minimum braking resistor can be taken from the technical data of the
inverter documentation.
The maximum cyclic duration of a braking resistor shall not be exceeded. For a longer
cyclic duration time special designed braking resistors are necessary. The continuous
output of the braking transistor must be taken into consideration.
7. Check, whether the desired braking time (tBmin) is attained with the braking resistor.
Restriction: Under consideration of the rating of the braking resistor and the brake power of
the motor, the braking torque may not exceed 1,5times of the rating torque of the motor (see
formula).
When utilizing the maximum possible braking torque the frequency inverter must be
dimensioned for the higher current.
1.2
Braking ramp
The braking ramp is adjusted at the frequency inverter. If it was selected too small, the KEB
COMBIVERT switches off automatically and error message overvoltage (E.OP) or overcurrent
(E.OC) is displayed. The approximate braking time can be determined according to following
formula.