User’s Manual
63
7.1.2. COMMON BUS APPLICATION NOTE
Bonitron dynamic braking transistor modules are designed to be compatible
with individual stand-alone inverter/drive systems, or systems that
incorporate a common DC bus arrangement. The common DC bus can be
composed of multiple inverter/drive sections tied together where all or some
of the sections use their respective AC input, or there may be a large
independent master DC bus supply feeding the DC inputs of all inverter/ drive
sections. In the case of the large master DC bus supply, it is common to find
multiple rectifier sections in parallel to provide very high power levels. Some
high power systems also include redundant or back up sections as well.
Once power is applied, all Bonitron modules are designed to be sourced from
DC buses that have all the bus capacitors present.
Common DC bus systems composed of separate master DC bus or rectifier
sections have important imbedded differences. It is common to have a main
distributed DC bus, and this is typically where the dynamic braking transistor
modules connect. In this way, the dynamic braking system is always present,
even if some of the inverter/drive sections need to be removed from the bus
for maintenance or other purposes. In emergency situations, it may even be
necessary to “limp” along until repairs or swap outs can occur. Even though
the modules are well suited for use in these systems, the following modes of
operation could arise or exist and are not allowed:
Do not connect the dynamic braking transistor module on the rectifier
side of a DC link choke. The connections must always be made to the
inverter/drive side directly to the DC bus capacitors. During normal
system operation, the choke can cause the braking system to begin
ringing. This ringing causes high voltages that will damage the system.
Do not energize the system with no inverters/drives present on the
distributed DC bus.
Do not energize, operate, or run the system with less than 60% of the
total expected system capacitance present.
Operating the modules in conditions 2 and 3 may make the modules
respond to inbound line transients caused by SCR type rectifiers,
powering up the system, or any number of other sources. Without
sufficient DC bus capacitance, the DC bus will not be filtered, and can
cause ringing that will produce high voltages that will damage the system.
In some drives, the pre-charge contactor may open under fault
conditions, leaving the bulk system capacitance only resistively coupled
to the dynamic braking transistor modules. Do not enable the modules
in this situation. Review the inverter/drive DC bus pre-charge circuit
operation with the drive manufacturer.
If there is the possibility of these situations:
Open the enable input on the R8 option.
Use a properly rated contactor in series with the modules’ control voltage
AC Input. The modules are effectively disabled when they do not have
their control power.
Keep the modules disabled during power up or any other time until all
system capacitances are present.
Disable the modules in the event system pre-charge contactors open.
Always consult Bonitron with any questions or concerns surrounding this
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