Rectifier Configuration; Power Cell Bypass Option - Siemens 6SR41 series Product User Manual

VFD Rating (KVA) = 1.732 * VAVAILABLE * Power Cell Continuous Current Rating.
Note
The VFD integral isolation transformer and cell frame are chosen in agreement with the load
ratings, i.e., site conditions, cable length, and motor nameplate data (power factor efficiency,
frequency, and service factor).
2.3.4

Rectifier Configuration

Each Power Cell Input is configured as a 6-Pulse uncontrolled diode rectifier. The Input
transformer includes a dedicated secondary winding for each power cell, where the
transformer secondaries are arranged in a delta and extended delta configuration with
varying degrees of phase shift. Typical Perfect Harmony VFD configurations consist of a
minimum of 3 to a maximum of 8 series power cells per output phase.
Effective Rectifier Configuration is:
● 3 Cells/Phase = 18 Pulse
● 4 Cells/Phase = 24 Pulse
● 5 Cells/Phase = 30 Pulse
● 6 Cells/Phase = 36 Pulse
● 7 Cells/Phase = 42 Pulse
● 8 Cells/Phase = 48 Pulse
2.3.5

Power Cell Bypass Option

As an option, each power cell in the drive can be equipped with a bypass contactor. This
contactor will be automatically energized by the VFD master control if the associated power
cell malfunctions. Once the contactor is energized, the damaged cell is no longer electrically
part of the inverter system, which allows the VFD to resume operation.
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 output 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 Harmony control system maximizes output voltage capability
using a patented technique (US Patent 5,986,909) defined as "Neutral Point Shift".
See also
Cabinet Details (Page 61)
Product User Manual
Operating Instructions, Version AE 12/2009, A5E01454341C
Overview
2.3 Perfect Harmony Features
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