Rockwell Automation 1336_S_F_T Allen-Bradley Dynamic Braking - Manual (Page 3)

Heavy Duty Dynamic Braking 3
1336-5.64 — July, 2005
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 Dynamic Brake
Module Works A Dynamic Brake Module consists of a Chopper Module (a chopper
transistor and related control components) and a Dynamic Brake Resistor.
Figure 1 shows a simplified schematic of a Dynamic Brake Module. The
Chopper Module is shown connected to the positive and negative DC Bus
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 condition exists within the Chopper Module, such as a
shorted Chopper Transistor or open Dynamic Brake Resistor. When an
abnormal condition is sensed, the Chopper Transistor Voltage Control will
fire the Crowbar SCR, shorting the DC Bus and melting the fuse link. 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
Dynamic Brake 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 regulates the voltage of the DC Bus
during regeneration. The average values of DC Bus voltages are:
• 375V DC (for 230V AC input)
• 750 V 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.