– This will determine the maximum allowable resistance value of the DB resistor.
• Calculate the average power dissipation
– This will determine the average power dissipation capacity needed in the DB resistor.
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.
Gather the Information First
• The nameplate power rating of the motor in watts, kilowatts, or horsepower. • The nameplate speed rating of the motor in rpm, or rps. • The motor inertia and load inertia in kilogram-meters2, or lb-ft2. • The gear ratio, if a gear is present between the motor and load, GR. • The motor shaft speed, torque, and power profile of the drive application.
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.
Bus Caps Bus Caps
Using a Dynamic Brake or Chopper
• In general, the motor power rating, speed, torque, and details of the regenerative duty cycle need to be known.
SCR G ate
- DC Bus
FW D
V o lta ge D ivid e r
To V oltage C ontrol
S ign a l Com m on
FW D
To V oltage C ontrol
V o lta ge D ivid e r
Fuse
C rowbar SCR
To V oltage C ontrol
• Generally, a dynamic brake can be used whenever regenerative energy is dissipated on an occasional or periodic basis.
• If the drive will consistently be regenerating, serious consideration should be given to returning the power to the AC utility.
• Third, check to see that the peak temperature of the dynamic braking resistor does not exceed its capacity.
– Plot the Average Load and Peak Load on the curve in the sizing guide. This is necessary only on the dynamic brake modules!

t
P(t)
t
0
t1 t2 t3
t4
t1+ t4
t
-Pb
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.
How to Choose a Dynamic Brake / Chopper
• Calculate the total inertia of the system • Calculate the peak regenerative power you need to dissipate.
• Dynamic brake modules should only be used on drives 75HP and smaller.
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.
How to Size A Dynamic Brake Module
Fuse
To V oltage D ivider
D yn a m ic B ra k e
R e sisto r
To V oltage D ivider
Chopper T ra n sisto r
C h o p p e r T ra n sisto r V o lta ge C o n tro l
To C row bar
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.
Step 1 - Total Inertia
JT Jm GR 2 xJ L
JT = total inertia reflected to the motor shaft, kilogram-meters2 (kg-m2) or pound-feet2 (lb-ft2) Jm = motor inertia, kilogram-meters2 (kg-m2) or pound-feet2 (lb-ft2) GR = the gear ratio for any gear between the motor and load, dimensionless. 2:1 = 0.5 JL = load inertia, kilogram-meters2 (kg-m2) or pound-feet2 (lb-ft2)
Sizing Dynamic Brake Resistors and Chopper Modules
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.
Dynamic Brake Module Circuit
+ DC Bus
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.
Example Speed, Torque, and Power Profile
(t)
0
t1 t2 t3
t4
t1+ t4
t
t
T(t)
0
t1 t2 t3
t4 t t1+ t4