Application and Operation
6.14 Cell Bypass
Product User Manual
144 Operating Instructions, Version AE 12/2009, A5E01454341C
6.14.3 Neutral Point Shift during Bypass
Since the cells in each phase of a Perfect Harmony Drive are in series, bypassing a cell has
no effect on the current capability of the drive, but the voltage capability will be reduced.
Usually the required motor voltage is roughly proportional to speed, so that the maximum
speed at which the drive can fulfill the application requirements will also be reduced.
Therefore, it is important to maximize the motor voltage available after one or more cells
have been bypassed. The following Figures illustrate the voltage available from a Perfect
Harmony drive, where the cells, represented by circles, are shown as simple voltage
sources. Figure "Drive Output after Loss of 3 Cells" shows a 15-cell drive in which no cells
are bypassed. With 100% of the cells in use, 100% of the original voltage is available. The
voltage commands to the three phase groups of cells will have phase A displaced from
phase B by 120°, and from phase C by 120°.
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Figure 6-17 A Simplified Diagram of a 15 Cell Drive
When cells are bypassed in one of the drive phases, the output voltage will tend to become
unbalanced, as illustrated in Figure "Drive Output with 2 Cells Bypassed". One possible
remedy is to bypass an equal number of cells in all three phases, even though some may not
have faulted. Figure "Drive Output Re-Balanced by Bypassing Functional Cells" illustrates
this approach. Obviously, this method prevents unbalance but sacrifices possible voltage
capability. In Figure "Drive Output Re-Balanced by Bypassing Functional Cells", 87% of the
cells are functional, but only 60% are in use, and only 60% voltage is available.
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