Introduction; Figure 2-1 Harmony Topology - Siemens 6SR41 series Product User Manual

2.2

Introduction

The Perfect Harmony VFD is based on a patented (U.S. patent #5,625,545) multi-level
output topology. 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.
The Harmony series drives achieve this uncompromised performance by employing time-
proven technology in a simple configuration. Figure "Harmony Topology" shows a typical
power circuit topology for a 4160 Volt Perfect Harmony series drive, using 750 VAC power
cells. In this configuration, 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 associated
transformer secondaries are insulated from each other and from ground for 5 kV class
service or "according to the output voltage rating of the drive."
Figure 2-1 Harmony Topology
For higher output voltage capabilities, Figure "Harmony Topology" would be extended to
have up to 8 power cells in series in each phase, with additional secondary windings
(number of secondaries equals number of power cells) on the integral isolation transformer.
Each power cell is simply 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 a variable
frequency from 0.5 to the maximum rated output fequency of the drive.
Product User Manual
Operating Instructions, Version AE 12/2009, A5E01454341C
Overview
2.2 Introduction
19