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Multiple PWM
The multiple PWM has numerous outputs that are not the same in value but the time period over which
they are produced is constant for all outputs. Inverters with PWM are able to operate at high voltage
output.
Figure: 5.30 Block diagram of multiple PWM technique
The waveform below is a sinusoidal wave
produced by a multiple PWM
Figure: 5.31 Waveform of multiple PWM technique
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Voltage and Harmonic Control
A periodic
waveform that has frequency, which is a multiple integral of the fundamental power with
frequency of 60Hz is known as a harmonic. Total harmonic distortion (THD) on the other hand refers to
the total contribution of all the harmonic current frequencies.
Harmonics are characterized by the pulse that represents the number of rectifiers used in a given circuit.
It is
calculated as follows
h=(n×P)+1or
−
1
Where
n
− is an integer 1, 2, 3, 4….n
P
− Number of rectifiers
Harmonics have an impact on the voltage and current output and can
be reduced using isolation
transformers, line reactors, redesign of power systems and harmonic filters.
Operation of sinusoidal pulse width modulation
The sinusoidal PWM (SPWM) method also known as the triangulation, sub harmonic, or sub oscillation
method, is very popular in industrial applications. The SPWM is explained
with reference to Figure,
which is the half-bridge circuit topology for a single-phase inverter.
Figure: 5.32 schematic diagram of Half bridge PWM inverter
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For realizing SPWM, a high-frequency triangular carrier wave is compared with a sinusoidal reference of
the desired frequency. The intersection of and waves determines the switching instants and commutation
of the modulated pulse. The PWM scheme is illustrated in Figure, in which v
c
the peak value of triangular
carrier
wave and v
r
is that of the reference, or modulating signal. The figure
shows the triangle and
modulation signal with some arbitrary frequency and magnitude. In the inverter of Figure the switches
and are controlled based on the comparison of control signal and the triangular wave
which are mixed in a
comparator. When sinusoidal wave has magnitude higher than the triangular wave the comparator output
is high, otherwise it is low.
