Figure 3-6 Harmony Output Waveforms, 2400 Volt Drive At Full Load - Siemens 6SR41 series Product User Manual

Note
The phase shift of the carrier signals between phases is determined by the number of cells in
the system, the equation being Phase shift = 180 degrees / total number of cells. In this case
(3 ranks or 9 cells), the carrier signal phase shift phase to phase is (180 / 9) = 20 degrees.
This shift of the carriers between phases reduces the number of devices that are switching at
one time. The above is true if no cells are in bypass. If one or more cells are in bypass, the
carrier signals are offset by 180 degrees / total remaining cells.
Figure 3-6
Figure "Harmony Output Waveforms, 2400 Volt Drive at Full Load" shows motor voltage and
current waveforms for a 2400 VAC Harmony drive rated at 1000 Hp. The voltage shown is
between phase A and the motor neutral (not the same as the drive neutral). The motor
current is in phase A during full-load operation. Anyone familiar with such waveforms for
other types of static drives will appreciate how accurately they approximate true sine waves.
A quantitative measure of the waveform quality is its Total Harmonic Distortion, or THD. The
THD of the motor currents with a Harmony series drive is always less than 5 percent.
Figure "Harmony Input Waveforms for a Drive at Full Load" shows the input voltage and
current waveforms for the same drive as in Figure "Harmony Output Waveforms, 2400 Volt
Drive at Full Load", under the same conditions. The perfect sine wave in Figure "Harmony
Input Waveforms for a Drive at Full Load" is the voltage into the special input transformer,
measured between phase A and the neutral of the wye-connected primary. The other
waveform is the current into phase A of the same winding.
The currents drawn from the power source by Harmony series drives are also good
approximations to true sine waves, due to the harmonic cancellation obtained with the
phase-shifted secondary windings of the transformer. The THD of the input currents with a
Harmony series drive is also always less than 5 percent.
Product User Manual
Operating Instructions, Version AE 12/2009, A5E01454341C
Harmony Output Waveforms, 2400 Volt Drive at Full Load
Theory
3.2 The Power Circuitry
39