Siemens 6SR41 series Product User Manual page 37

The power cells all receive commands from one central controller. These commands are
passed to the cells over fiber optic cables to maintain electrical isolation.
The transformer secondaries that supply the power cells in each output phase are wound to
obtain a small difference in phase angle between them. This cancels most of the harmonic
currents drawn by the individual power cells, so that the primary currents are nearly
sinusoidal. The power factor is always high - typically 95% at full load.
The schematic of a typical power cell is shown in Figure "Schematic of a Typical Power
Cell". In this example, a 3-phase diode rectifier, fed by the 750 VAC secondary, charges a
DC capacitor bank to about 1012 VDC. The DC voltage feeds a single-phase H-bridge of
IGBTs.
At any instant of time, each cell has only three possible output voltages. If Q1 and Q4 are on,
the output will be +DC bus volts from T1 to T2. If Q2 and Q3 are on, the output will be –DC
bus volts. Finally, if either Q1 and Q3 or Q2 and Q4 are on, the output will be 0 volts.
With 3 power cells per phase, the circuit can produce 7 distinct line-to-neutral voltage levels
(±3036, ±2024, ±1012, or 0 volts). With N cells per phase, (N*2)+1 distinct voltage levels are
available, where N is a maximum of 8. The ability to generate many different voltage levels
allows the Harmony series drives to produce a very accurate approximation to a sinusoidal
output waveform.
Figure "Waveforms for Phase A" shows how these waveforms are generated for the case of
3 cells per phase. First, a reference signal is created for each phase. These signals are
digital replicas of the ideal waveform to be approximated. In Figure "Waveforms for Phase
A",RA illustrates the reference signal for phase A. This reference signal is then compared
with 3 triangular carrier signals. Figure "Waveforms for Phase A" shows conditions when the
output frequency is 60 Hz and the carrier frequency is 600 Hz, so that there are exactly 10
carrier cycles per reference cycle. The 3 carriers are identical except for successive phase
shifts of 60 degrees (based on the number of cells per phase). Phase shift between carriers
in each phase is computed based on the following equation:
Carrier Phase Shift (same phase) = 180 degrees / # Cells per Phase
Product User Manual
Operating Instructions, Version AE 12/2009, A5E01454341C
Theory
3.2 The Power Circuitry
35