Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 201)

Rockwell Automation Publication 750-RM002B-EN-P - September 2013 201
Motor Control Chapter 4
Here are two drives with PWM DB control on a common bus. Because one drive
turns on at a certain duty cycle the bus voltage is likely to continue to rise
guaranteeing that the other drive’s IGBT turns on (at a different duty cycle).
How to Select A Chopper Module and Dynamic Brake Resistor
In general, the motor power rating, speed, torque, and details regarding the
Regenerative mode of operation is needed to estimate what Chopper Module
rating and Dynamic Brake Resistor value to use. A rule of thumb to use is that a
Dynamic Brake Module can be specified when regenerative energy is dissipated
on an occasional or periodic basis. When a drive is consistently operating in the
Regenerative mode of operation, consider utilizing equipment that transforms
the electrical energy back to the fixed frequency utility.
The peak regenerative power of the drive must be calculated to determine the
maximum Ohmic value of the Dynamic Brake Resistor and to estimate the
minimum current rating of the Chopper Module. The Rating of the Chopper
Module is chosen from the Brake Chopper Module manual. Once the Chopper
Module current rating is known, a minimum Dynamic Brake Resistance value is
also known. A range of allowable Dynamic Brake Ohmic values is now known.
These values exist from the minimum value set by the Chopper Transistor
current rating to a maximum value set by the peak regenerative power developed
by the drive to decelerate or satisfy other regenerative applications. If a Dynamic
Brake Resistance value less than the minimum imposed by the choice of the
Chopper Module is made and applied, damage can occur to the Chopper
Transistor. If a Dynamic Brake Resistance value greater than the maximum
imposed by the choice of the peak regenerative drive power is made and applied,
t
t
t
V
dc_on
V
dc_off
on
off
DB IGBT
Drive #1
on
off
DB IGBT
Drive #2
V
dc
Drive #2
V
dc
Drive #1