Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 197)

Rockwell Automation Publication 750-RM002B-EN-P - September 2013 197
Motor Control Chapter 4
Dynamic Braking When an induction motor’s rotor is turning slower than the synchronous speed
set by the drive’s output power; the motor is transforming electrical energy
obtained from the drive into mechanical energy available at the drive shaft of the
motor. This process is referred to as motoring.
When the rotor is turning faster than the synchronous speed set by the drive’s
output power, the motor is transforming mechanical energy available at the drive
shaft of the motor into electrical energy that can be transferred back into the
utility grid. This process is referred to as regeneration.
On most AC PWM drives, the AC power available from the fixed frequency
utility grid is first converted into DC power by means of a diode rectifier bridge
or controlled SCR bridge, before being inverted into variable frequency AC
power. These diode or SCR bridges are very cost effective, but can handle power
in only one direction, and that direction is the motoring direction. If the motor is
regenerating, the bridge is unable to conduct the necessary negative DC current,
and the DC bus voltage increases until the drive trips on a Bus Overvoltage fault.
There are bridge configurations, using either SCRs or Transistors that have the
ability to transform DC regenerative electrical energy into fixed frequency utility
electrical energy but are expensive. A more cost effective solution is to provide a
Transistor Chopper on the DC bus of the AC PWM drive that feeds a power
resistor, which transforms the regenerative electrical energy into thermal heat
energy, which is dissipated into the local environment.
This process is generally called Dynamic Braking, with the Chopper Transistor
and related control and components called the Chopper Module, and the power
resistor called the Dynamic Brake Resistor. The entire assembly of Chopper
Module with Dynamic Brake Resistor is sometime referred to as the Dynamic
Brake Module.
Chopper Modules are designed to be applied in parallel if the current rating is
insufficient for the application. One Chopper Module is the designated Master
Chopper Module, while any other Modules are the designated Follower Modules.
Two lights have been provided on the front of the enclosure to indicate Chopper
Module operation – the DC Power light and the Brake On light. The DC Power
light is lit when DC power has been applied to the Chopper Module. The Brake
On light is lit when the Chopper Module is operating or chopping and is a
flickering type of indication.
Update: As of December of 2010, Rockwell Automation no longer has a
Chopper Module product as well as a Dynamic Braking Module product. The
light configuration stated above was specific to the Rockwell Automation
product.