Rockwell Automation 1336_F_E_T_S SERIES A CHOPPER BRAKE MODULE - Manual (Page 3)

Heavy Duty Dynamic Braking 3
1336-5.65 — March, 2007
the necessary negative DC current, the DC bus voltage will increase and
cause a Bus Overvoltage trip at the drive.
Expensive bridge configurations use SCRs or transistors that can transform
DC regenerative electrical energy into fixed frequency utility electrical
energy. 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 energy. This is
generally referred to as Dynamic Braking.
How the Chopper Module
Works Figure 1 shows a simplified schematic of a Chopper Module with Dynamic
Brake Resistor. The Chopper Module is shown connected to the positive
and negative conductors of an AC PWM Drive. The two series connected
Bus Caps are part of the DC Bus filter of the AC Drive.
A Chopper Module contains five significant power components:
Protective fuses are sized to work in conjunction with a Crowbar SCR.
Sensing circuitry within the Chopper Transistor Voltage Control determines
if an abnormal conditions exist within the Chopper Module, such as a
shorted Chopper Transistor. When an abnormal condition is sensed, the
Chopper Transistor Voltage Control will fire the Crowbar SCR, shorting
the DC Bus, and melting the fuse links. This action isolates the Chopper
Module from the DC Bus until the problem can be resolved.
The Chopper Transistor is an Insulated Gate Bipolar Transistor (IGBT). The
Chopper Transistor is either ON or OFF, connecting the Dynamic Brake
Resistor to the DC Bus and dissipating power, or isolating the resistor from
the DC Bus. There are several transistor ratings that are used in the various
Chopper Module ratings. The most important rating is the collector current
rating of the Chopper Transistor that helps to determine the minimum ohmic
value used for the Dynamic Brake Resistor.
Chopper Transistor Voltage Control (hysteretic voltage comparator)
regulates the voltage of the DC Bus during regeneration. The average values
of DC Bus voltages are:
• 375V DC (for 230V AC input)
• 750V DC (for 460V AC input)
• 937.5V DC (for 575V AC input)
Voltage dividers reduce the DC Bus voltage to a value that is usable in signal
circuit isolation and control. The DC Bus feedback voltage from the voltage
dividers is compared to a reference voltage to actuate the Chopper
Transistor.
The Freewheel Diode (FWD), in parallel with the Dynamic Brake Resistor,
allows any magnetic energy stored in the parasitic inductance of that circuit
to be safely dissipated during turn off of the Chopper Transistor.