Rockwell Automation AK DBU - Dynamic Braking Unit 600/690 VAC - Manual (Page 46)

C-8 Design Information
Calculate Maximum Dynamic Brake Resistance:
V
d
= DC Bus Regulation Voltage
R = Minimum Brake Resistance Value
Record the Maximum Dynamic Brake Resistance R
db1
in above table.
The choice of the Dynamic Brake resistance value should be less than
the value calculated in this step.
If the value is greater, the drive can trip
on DC bus overvoltage.
Step 5 Calculate Required Joule Rating
(joules = Watt-Seconds):
Drive Efficiency = 0.975
Step 6 Select a Resistor
Select a resistor bank from the following tables or from your resistor
supplier that has all of the following:
• a resistance value that is less than the value (R
db1
in Ohms)
calculated in Step 4 , but as close as possible below this value.
• a resistance value that is greater than the minimum resistance for the
DBU listed in Table C.A ,
• a power value that is greater than the value calculated in
Step 3
(P
av
in watts),
• a watt-second value greater than the value calculated in
Step 5 .
Line Voltage V
d
P
max
R P
max
x R R
db1
600V AC 950V DC 268,000 W 3.2 Ohms 857375 Ohm
690V AC 1090V DC 305,000 W 3.7 Ohms 1128695 Ohm
R
db1
oooooooo
·
[ ]
ooooooooo [ ]
-------------------------------- =
P
b
2
-----
⎝ ⎠
⎛ ⎞
t
3
t
2
–
( ) ×
watt-seconds =
Watt-second losses
P
b
2
------x t
3
t
2
– ( ) x 1 motor efficiencyxdrive efficiency ( ) – [ ] =
Total watt-seconds watt-seconds watt-second losses – =