Rockwell Automation AK DBU - Dynamic Braking Unit 600/690 VAC - Manual (Page 45)

Design Information C-7
In order to select the appropriate Dynamic Brake Resistor for your
application, the following data must be calculated.
Peak Regenerative Power
(Expressed in watts)
This is used to determine the maximum resistance value of the Dynamic
Brake Resistor. If this value is greater than the maximum imposed by the
peak regenerative power of the drive, the drive can trip off due to
transient DC bus overvoltage problems.
Power Rating of the Dynamic Brake Resistor
The average power dissipation of the regenerative mode must be
estimated and the power rating of the Dynamic Brake Resistor chosen to
be greater than the average regenerative power dissipation of the drive.
(See
Step 3 on page C-6 ).
Protecting External Resistor Packages
Step 4 Calculate the Maximum Dynamic Brake Resistance Value
The maximum allowable Dynamic Brake resistance value (R
db1
) must be
calculated.
R
db1
= Maximum allowable value for the dynamic brake resistor
(ohms)
V
d
= DC bus voltage used for calculating maximum power.
(950V DC for 600V AC, or 1090V DC for 690V AC)
P
b
= Peak breaking power calculated in Step 2 (watts)
Selecting the Resistor
!
ATTENTION: The DBU and most drives do not offer protection for
externally mounted brake resistors. Risk of fire exists if external
braking resistors are not protected. External resistor packages must be
self-protected from overtemperature or circuit equivalent to the one
shown in
Figure 1.5 must be supplied.
R
db1
0.95 V
d
( ) × 2
P
b
----------------------------
P
max
R ×
P
b
--------------------- = =