Siemens 6SR41 series - Page 148

Application and Operation
6.14 Cell Bypass
Product User Manual
146 Operating Instructions, Version AE 12/2009, A5E01454341C
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Figure 6-20 Drive Output Re-Balanced by Adjusting Phase Angles (Neutral Shift)
The same neutral-shift approach can be applied to more extreme situations, as is illustrated
by Figure "Drive Output after Loss of 3 Cells" and Figure "Drive Output after Loss of 5 Cells"
2. Figure "Drive Output after Loss of 3 Cells" shows a drive which originally had five cells per
phase, or a total of 15 cells. All five cells remain in phase A, but one cell has faulted in phase
B and two cells have faulted in phase C. Without neutral-shift, all phases would need to be
reduced to match the cell count of phase C to maintain balanced motor voltages. One
functional cell would be bypassed in phase B, and two functional cells would be bypassed in
phase A. Only 60% of the original cells would remain in use, and only 60% of the original
voltage would be available.
However, with the neutral-shift approach shown in Figure "Drive Output after Loss of 3
Cells", only the faulted cells are bypassed. The phase angles of the cell voltages have been
adjusted so that phase A is displaced from phase B by 96.9° and from phase C by 113.1°,
instead of the normal 120°. The star point of the cells no longer coincides with the neutral of
the motor voltages, but the motor voltage is still balanced. The neutral-shift keeps 80% of the
original cells in use, and 70% of the original voltage is available.
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Figure 6-21 Drive Output after Loss of 3 Cells
As another example, Figure "Drive Output after Loss of 5 Cells" shows the same 15-cell
drive. All five cells remain in phase A, but two cells have faulted in phase B and three cells
have faulted in phase C. Without neutral-shift, one functional cell would be bypassed in
phase B, and three functional cells would be bypassed in phase A. Only 40% of the original
cells would remain in use, and only 40% of the original voltage would be available. However,