Rockwell Automation 2098-IPD-xxx Ultra5000 Intelligent Positioning Drives Installation Manual - Manual (Page 149)

Publication 2098-IN001E-EN-P — April 2002
Wiring Three Phase Power to a Single Phase Ultra5000 D-3
Adding a Safety Ground to the Isolation Transformer
Provided that the transformer used is an isolating transformer, it is a
good idea to fit a safety ground to the neutral connection. This will do
two things:
1. Prevent the system floating, and thereby avoid any high voltages
that might otherwise occur (e.g., through static), and
2. Provides a solid earth path for fault conditions.
Three Phase Line Filtering
Requirements for EMC
If a three phase line filter is used, it is important that the filter include
a neutral connection as shown in
Figure D.2 . This applies if three
phase is brought directly into the filter (i.e., no isolating transformer
present). The reasons are as follows:
• The normal components in filters of this kind are inductance and
capacitance.
• The inductive elements are normally toroids which are placed
around the three phases of the device. The neutral will carry return
currents due to the unbalanced nature of the loads so it is vital that
the neutral return is included in the toroidal inductors.
(Note: Toroidal inductors are sometimes referred to as common
mode chokes, or ferrite cores.) If the neutral is not included, the
net power current through the choke (Kirchoff's Laws) will not be
zero which causes the chokes to saturate with a reduction in
filtering performance.
• Filters with this design requirement are available (e.g. Schaffner FN
256 three phase and neutral filter). It is important to ensure that the
three phase current rating of the filter adequately covers the
application requirements.
ATTENTION
!
If the supply transformer is an auto transformer, a
safety earth ground should not be added.
A safety earth ground should already be fitted
elsewhere in the system, and fitting another would
create a bolted short.