Operational manual for dynamic braking unit
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8 Selection
8.1 Guidelines of braking voltage selection
The setting of the braking voltage is low enough to make the DBU works around the rated
voltage and ensure a safe running. Select high braking voltage can avoid misaction of the
braking unit, but too high voltage has impact on the long-term running safety.
8.2 Calculation of the braking resistor and the braking current
(at 100% of the braking torque)
Braking current is the DC current running through the braking unit and the braking resistor
during braking.
Calculating at the standard AC motor of 380V:
P―――― Rated power of the motor (kW)
V―――― DC operating point of the braking unit, usually it is 700V
I―――― Braking current (A)
―――― Conversion efficiency of the mechanical energy during feedback, usually it is 1.0
Calculation basis: the braking resistor needs to absorb all the regenerative electric energy of
the motor.
Absorbed power of the braking resistor (V*I)＝The regenerative electric energy of the motor
(W)=1000*P*
8.3 Calculation and selection of the braking resistor
(at 100% of the braking torque)
The braking resistor reflects the braking torque in the system. If the braking torque is too small,
the DBU will carry out overvoltage protection.
Calculating at the standard AC motor of 380V:
P―――― Rated power of the motor (kW)
P
R
―――― Rated consumed power of the braking resistor (kW)
V――――DC operating point of the braking unit, usually it is 700V
R―――― Equivalent resistance of the braking resistor (Ω)
――――Conversion efficiency of the mechanical energy during feedback, usually it is 1.0
――――Safety coefficient of the power consumption for the braking resistor
=1.4
K
f
――― Braking frequency, the time ratio of the regeneration to the whole working time
Usually, K
f
is:
Winding up and winding down K
f
＝50－60%
Oilfield machines K
f
＝10－20%
Elevators K
f
＝10－15%
Centrifuge K
f
＝5－20%
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