14 Operation principle and hardware description
Benefits of the common DC system
Benefits of the common DC system:
• You can save energy by using the braking energy of one drive in the others - less
energy needs to be taken from the AC power line.
• DC capacitors of all drives form a high-capacity energy storage that can absorb short
braking pulses of individual drives without a need for a brake chopper and resistor.
• If you need brake choppers and resistors, they can be optimized for the whole system.
You do not have to use the chopper of every drive.
• You do not necessarily need to connect every drive to the AC power line.
Challenges of the common DC system
Challenges of the common DC system:
• You cannot operate any of the drives in the common DC system if one of the drives
connected to the AC power line has an active fault. See section Connecting the Ready
and Start enable signals on page 36.
• If you have drives with different type of charging circuits in the system, and you want to
connect them to AC power line, you must add extra contactors to the system and
arrange their control: For power up and charging, you must disconnect the DC links of
the drives which have different type of charging circuits. You can connect the DC links
together only after charging. See sections Charging circuit types on page 15 and
Charging resistance values on page 43.
• You must make sure that the load imbalance between the drives that are connected to
the AC power line is as small as possible. There is always slightly unequal AC input
current distribution due to differences in the input cables, chokes and input bridges’
forward bias characteristics. If the voltage reduction over the input cable, rectifier and
chokes is not the same in all drives, more current will flow through the rectifier which
has the lowest voltage reduction.
• You must make sure that the common DC system complies with the relevant
regulations and directives. The compliance of individual drives does not guarantee or
cover the compliance of the common DC system.
• If you supply the drives from a totally separate DC source, the DC source:
• must be capable of powering the drives when motoring,
• must be protected to prevent regeneration onto the DC from causing any damage,
or from effecting the devices supplying the DC source (for example AVR systems
on generators).
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