Rockwell Automation AK DBU - Dynamic Braking Unit 600/690 VAC - Manual (Page 50)

C-12 Design Information
Step 4 Calculate the Maximum Dynamic Brake Resistance
R
db1
= (P
max
x R) / P
b
R
db1
= (268 000 x 3.2) / 123 700 = 6.93 Ohm
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)
(P
b
/2) x (t
3
- t
2
) = (123 700/2) x 2 = 123 700 watt-seconds
Drive Efficiency = 0.975, Motor Efficiency = 0.86
Watt-second losses = [P
b
/2 x (t
3
- t
2
)] x [1 – Motor Effic. x Drive Effic.)]
Watt-second losses = 123 700 x [1 – 0.86 x 0.975] = 20 000
Calculate total watt-seconds
Step 6 Select a Resistor
From Table C.B resistor PF6F5R4K39 with 6.5 ohm, 4394 watts and
256 400 watt-seconds should be selected based on the following data:
• Maximum Dynamic Brake Resistance Value (R
db1
) of 6.9 ohms.
The selected resistor must be less than (R
db1
) but as close as possible
below 6.9 ohms.
• Minimum Brake Resistance (R) of 3.2 ohms.
See Table C.A . The selected resistor must be greater than (R).
• Average Braking Power (P
av
). The power rating of the selected
resistor must be greater than 4120 W or 4.12 kW.
• Watt-seconds. The watt-seconds for the selected resistor must be
greater than 103 700 Watt-seconds or 103.7 kW-seconds.
Line Voltage V
d
P
max
R P
max
x R R
db1
600V AC 950V DC 268 000 W 3.2 Ohms 857 375 Ohm
690V AC 1090V DC 305 000 W 3.7 Ohms 1 128 695 Ohm
Total watt-seconds 123700 20000 103700 = – =