Siemens 6SR41 series - Page 147

Application and Operation
6.14 Cell Bypass
Product User Manual
Operating Instructions, Version AE 12/2009, A5E01454341C
145
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Figure 6-18 Drive Output with 2 Cells Bypassed
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Figure 6-19 Drive Output Re-Balanced by Bypassing Functional Cells
A better approach is illustrated in Figure "Drive Output Re-Balanced by Adjusting Phase
Angles (Neutral Shift)". This method takes advantage of the fact that the star-point of the
cells is floating, and is not connected to the neutral of the motor. Therefore the star-point can
be shifted away from the motor neutral, and the phase angles of the cell voltages can be
adjusted, so that a balanced set of motor voltages is obtained even though the cell group
voltages are not balanced.
Siemens calls this approach Neutral-Shift, and has a US Patent (5,986,909) that covers it.
This approach is equivalent to introducing a zero-sequence component into the voltage
command vectors for the cells. In Figure " Drive Output Re-Balanced by Adjusting Phase
Angles (Neutral Shift)", the full remaining 87% of functional cells are in use, and 80% of the
original voltage is available. The phase angles of the cell voltages have been adjusted so
that phase A is displaced from phase B and from phase C by 132.5°, instead of the normal
120°.