Allen-Bradley 20A User Manual

Rockwell Automation Publication PFLEX-AT001L-EN-P - September 2017 19
Determining Dynamic Brake Requirements Chapter 2
Example Calculation
A 10 Hp, 4 Pole, 480 Volt motor and drive is accelerating and decelerating as
depicted in
Figure 2.
• Cycle period t4 is 40 seconds
• Rated speed is 1785 rpm and is to be decelerated to 0 speed in 15.0
seconds
• Motor load can be considered purely as inertia, and all power expended or
absorbed by the motor is absorbed by the motor and load inertia
• Load inertia is 4.0 lb•ft
2
and is directly coupled to the motor
• Motor rotor inertia is 2.2 lb•ft

• A PowerFlex® 70 drive, 10 Hp 480V Normal Duty rating is chosen.
Calculate the necessary values to choose an acceptable Dynamic Brake.
This information was given and must be known before the calculation process
begins. This can be given in Hp, but must be converted to watts before it can be
used in the equations.
This information was given and must be known before the calculation process
begins. This can be given in rpm, but must be converted to radians per second
before it can be used in the equations.
This value can be in lb•ft
2
or Wk
2
, but must be converted into kg•m
2
before it
can be used in the equations.
This was known because the drive is rated at 480 Volts rms. If the drive were rated
230 Volts rms, then
V
d
= 395 Volts.
All of the preceding data and calculations were made from knowledge of the
application under consideration. The total inertia was given and did not need
further calculations as outlined in
Step 1 – Total Inertia .
Rated Power 10 HP= 746 watts 7.46 kW=
Rated Speed 
b
1785= rpm 2==
1785
60
---------
186.98 Rad
s
-------------------- -=
Lower Speed 
o
0= rpm 2
0
60
----
0 Rad
s
----------===
Total Inertia J
T
6.2 12.4= lb•ft
2
0.261 kg•m
2
==
Deceleration Time t
3
t
2
–15 seconds==
Period of Cycle t
4
40 seconds==
V
d
790 Volts=

Allen-Bradley 20A Specifications

General

Brand	Allen-Bradley
Model	20A
Category	Controller
Language	English