M3452 vR8 EIP/PDP
54
6.5. DC BUS LINK LENGTH LIMITS
The distance that the chopper is mounted from the main DC bus filter capacitors within
the drive is limited by the amount of inductance in the connection. During switching,
the inductance in the DC bus between the chopper and capacitors stores energy that
must be absorbed by the snubbing circuit in the chopper.
In general, this distance should be kept to a maximum of 30 feet total (10m), or 15 feet
(5m) for the DC+ link and 15 feet (5m) for the DC- link.
The values listed in Table 6-7 are the maximum inductance allowed in the DC bus link
to and from the filter capacitors in the drive and the chopper connections.
Table 6-7: Maximum Inductance for DC Link Cable
The distance between the DC bus filter capacitors and the braking chopper can be
increased by using lowering the inductance of the buswork or cables. Typically this
means using buswork or cable with a higher cross sectional area. The inductance of
the buswork can be calculated from the length and inductance/foot published by the
cable manufacturer. There are also standard tables to help this calculation.
If there is an extreme distance with inductance that cannot be removed from the DC
bus, additional bulk capacitance can be added to decouple the DC bus inductance
from the system, or the chopper can be repositioned to minimize the distance from the
DC bus filter capacitance.
Figure 6-1: DC Link
AC
Drive
DC+
DC-
Braking
Chopper
Res
Res
DC+
DC-
Braking
Resistor
Res
Res
DC Bus
Link
Resistor
Link
6.6. RESISTOR LINK LENGTH LIMITS
The distance that the resistor is mounted from the chopper is not a concern for the
chopper as the components used will not be affected by this inductance.
Some ceramic or wirewound resistors can have significant inductance, grid or plate
resistors do not. In very extreme cases, the inductance of the resistor and connecting
cables may limit the risetime of the current to the resistor, but this will not have an
impact on chopper operation.
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