The Power Circuitry - Siemens 6SR41 series Product User Manual

Theory

3.2 The Power Circuitry

3.2 The Power Circuitry
Note
The examples used in this section refer to drives having 750 V cells. High-Voltage cell
systems (1375 V) and GenIIIe (690 V) will have different values.
The Harmony series drives achieve this uncompromising performance by employing well-
proven technology in a modular configuration. Medium voltage levels are obtained by adding
together the outputs of multiple low-voltage power cells. The low voltage power cells are
simplified variations of standard PWM motor drives for low-voltage service, which have been
built in high volume for many years.
Figure "Topology of Perfect Harmony VFD (3 Cells)" shows a typical power circuit topology
for a 4160 Volt Perfect Harmony series drive, using 750 VAC cells. Each motor phase is
driven by 3 power cells connected in series. The groups of power cells are wye connected
with a floating neutral. Each cell is powered by an isolated secondary winding of an integral
isolation transformer. The nine secondaries are each rated for 750 VAC at one ninth of the
total power. The power cells and their secondaries are insulated from each other and from
ground for full output voltage rating.
For higher output voltages, additional power cells and isolated transformer secondaries
would be added as required.
Each cell is a static power converter. It is capable of receiving input power at 750 VAC 3-
phase, 50/60 Hz and delivering that power to a single-phase load at any voltage up to
750 VAC and at any frequency up to the rated maximum frequency defined in
Chapter "GenIV Specifications".
Note
For output frequencies greater than 167 Hz, the VFD power cell current output may be de-
rated due to switching losses. Consult the factory for information applicable to the specific
application requirements.
With three 750 VAC power cells in series per phase, a Perfect Harmony series drive can
produce as much as 2425 VAC line-to-neutral, or a maximum VAVAILABLE of 4200 Volts.
Note
Over-modulation is used to allow 750 Volt cells to achieve nominal motor ratings (e.g.,
4,160 V with 9 cells).
It should be noted that it is possible to connect as many as eight power cells in series using
the Harmony control. VAVAILABLE determines the maximum voltage that can be delivered
from the VFD output. The actual voltage delivered is fully adjustable. As the Harmony VFD
topology is based on multi-level output capabilities, the result is true adjusted voltage. The
advantages of utilizing the VAVAILABLE capability of the VFD become apparent when the
patented advanced cell bypass option is applied for high availability or redundant
applications.
Other cell voltages are available, which will change the number of cells needed for a given
output voltage. However, the basic principle is unchanged.
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Operating Instructions, Version AE 12/2009, A5E01454341C
Product User Manual