Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 200)

200 Rockwell Automation Publication 750-RM002B-EN-P - September 2013
Chapter 4 Motor Control
Common DC Bus Applications
In a common bus configuration when a dynamic braking resistor is installed on
each drive sharing the DC bus, it’s possible that the brake IGBT in some drives
will not turn on, giving the impression that the drive is not functioning correctly
or seeing one drive’s brake IGBT failing consistently while the other drives are
fine. Looking at the below diagram, it shows the DC bus level for two drives on
common bus. The delta between these voltages are exaggerated for clarity. As the
voltage increases, the Drive #1 IGBT turns on and decreases the voltage level
before Drive #2 sees voltage high enough to be told to turn on. This results in
Drive #1 doing all the dynamic braking. Now this situation could be alright as
long as the minimum ohmic value for resistance is not violated and the regen
event isn’t so great that a single resistor can’t handle the power. Of course, if there
is a large regen event where the voltage continues to rise after Drive #1 has turned
on, Drive #2 fires its IGBT when it reaches the voltage limit.
t
t
t
V
dc
V
dc_on
V
dc_off
on
off
DB IGBT
Drive #1
on
off
DB IGBT
Drive #2