Rockwell Automation Publication DRIVES-AT005D-EN-P - May 2022
11
Chapter 1 DC Bus Wiring Guidelines
Precharge
Precharge is the process through which the DC bus voltage of a drive is gradually increased. During this increase in DC bus voltage, the DC bus
filtering capacitors are charged in a controlled manner. The precharge assembly can be part of the drive design or it can be externally provided and
controlled.
If an external voltage source is used to power the logic boards of the drives, take precautions to control the precharge sequence. We recommend
using the ‘Precharge Enable’ digital input on the drive for common bus operation. The logic input can be coordinated through a PLC or system-level
control to sequence the precharge. The sequencing lets charge time constants for various horsepower drives settle out before the precharge is
completely done. Generally, a three second delay is acceptable after power has been applied.
When multiple drives are connected through disconnects to a common DC bus, it’s necessary to provide an input to the drive that enables the
precharge to finish. Often, an auxiliary contact on the drive disconnect switch controls this input.
Figure 3 - Common DC Bus Example
If ‘Precharge Enable’ is selected as a digital input, it must be energized to let the initial bus precharge complete. If it’s de-energized, it’s treated as a
coast-to-stop command and it forces the drive to the initial bus-precharge state. Fuse failure is probable unless coordination of precharge circuits
in individual drives is implemented.
WARNING:
An external source of power can be present. To avoid an electric shock
hazard, verify that the AC power supply has been removed before any maintenance is
performed.
IMPORTANT
The 'Precharge Enable' digital input is only available on PowerFlex 750-Series
and PowerFlex 755TM common bus inverter drives. The Kinetix 5700 servo
drives do not have a 'Precharge Enable' digital input.
ATTENTION:
The bus capacitors in the individual drives act as a low-
impedance voltage source. Extra care is needed when connecting
individual drives to an energized bus.
M1
M2
L1
L2
L3
DC+
DC–