Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 198)

198 Rockwell Automation Publication 750-RM002B-EN-P - September 2013
Chapter 4 Motor Control
How it Works
There are two different types of control for dynamic braking, hysteretic control
and PWM control. Each used by themselves in a standard stand alone product
has no advantage over the other. The preferred control is the PWM method
when the application is common DC bus. This advantage is described below.
Hysteretic Control
The hysteretic method of dynamic braking uses a voltage sensing circuit to
monitor the DC bus. As the DC bus voltage increases to the V
dc_on
level the
brake IGBT is turned on and is left on until the voltage drops to the V
dc_off
level,
which is not so desirable in common DC bus applications—see below. Some
PowerFlex drives allow the V
dc_off
level, [DB Threshold], to be adjusted if the
application required it. Setting this level lower makes the dynamic braking more
responsive but could lead to excessive DB activation.
t
t
V
dc
V
dc_on
V
dc_off
on
off
Switched from 50 Hz to 100 Hz
V
dc