Allen-Bradley 20A User Manual

20 Rockwell Automation Publication PFLEX-AT001L-EN-P - September 2017
Chapter 2 Determining Dynamic Brake Requirements
Note that this is 8.1% of rated power and is less than the maximum drive limit of
150% current limit. This calculation is the result of
Step 2 – Peak Braking Power
on page 15 and determines the peak power that must be dissipated by the
Dynamic Brake Resistor.
This is the result of calculating the average power dissipation as outlined in
Step 4. Verify that the sum of the power ratings of the Dynamic Brake Resistors
chosen in Step 3 is greater than the value calculated in Step 4.
For an internal resistor, refer to
Appendix A to determine the continuous power
rating of the resistor in the given drive you are using. Skip this calculation if an
external dynamic brake resistor will be used.
In this case, a 10 Hp PowerFlex 70 drive has an internal resistor rated for 40
continuous watts. Because
P
av
= 114.1 watts, and is greater than the resistor’s
continuous watts rating, the drive will eventually trip on a Resistor Over Heated
fault. Calculate the minimum cycle time (in seconds) using the formula in
Chapter 3, Step 2B.
Recalculate the average power dissipation.
If the cycle cannot be adjusted, the decel time must be extended or the system
inertia lowered to reduce the average load on the resistor. Another option is to
use an external resistor.
Calculate the Percent Average Load. You will need this number to calculate the
Percent Peak Load.
Peak Braking Power P
b
J
T

b

b

o
–
t
3
t
2
–
-------------------------------------------
==
P
b
0.261 186.92 186.92 0–
15
------------------------------------------------------ 607.9 watts==
Average Braking Power P
av
t
3
t
2
–
t
4
------------------
P
b
2
------

b

o
+

b
------------------------==
P
av
15
40
----


607.9
2
----------


186.92 0+
186.92
---------------------


113.9 watts==
607.9
2
---------- 15


40
-------------------------- - 113.9 seconds=
P
av
15
114.1
----------


607.9
2
----------


186.92 0+
186.92
---------------------


40 watts==

Allen-Bradley 20A Specifications

General

Brand	Allen-Bradley
Model	20A
Category	Controller
Language	English