Siemens 6SR41 series Product User Manual page 144

Application and Operation
6.14 Cell Bypass
4. NXG control detects cell fault(s)
5. NXG control monitors motor back EMF until it is within "safe" voltage
6. Control issues commands to bypass faulted cell(s)
7. Brief delay to allow mechanical contactors to fully close
8. Spinning load algorithm is performed (OLVC, CLVC) to detect motor voltage/frequency/
9. VFD output is enabled, synchronized to detected motor position/speed
10. VFD output ramps back to motor speed prior to cell fault
Note
Maximum speed is a function of remaining VFD output voltage capability.
When the Perfect Harmony was first introduced, its most salient attributes were improved
power quality at both the utility interface and the motor interface. A third attribute is now
recognized, which offers extremely high reliability by utilizing the inherent redundancy of
these drives. Cell Bypass is the feature that allows this third attribute to be realized.
The Cell Bypass option is implemented by adding a contactor to the output of each cell as
shown in Figure "Typical Power Cell with Bypass Contactor".
When NXG control interrogates the cells and detects that a cell has faulted, a command is
sent to close the appropriate contactor. This disconnects the cell output from the circuit. The
drive can then be restarted and operation can continue at reduced capacity. The amount of
reduction in capacity that can be tolerated will depend on the application, but in most cases a
reduction in capacity is preferable to a complete shutdown. Neutral Point Shift is a feature
that was developed to minimize the reduction in capacity after a bypass. Another related
feature is Fast Bypass. This feature is designed to quickly bypass a cell and get the drive
running again in less than ½ second. Neutral Point Shift and Fast Bypass are discussed in
the following sections.
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Operating Instructions, Version AE 12/2009, A5E01454341C
Product User Manual