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Voltage control techniques for inverters
Pulse width modulation techniques
PWM is a technique that is used to reduce the overall harmonic distortion (THD) in a load current. It
uses a pulse wave in rectangular/square form that results in a variable average waveform value f(t), after
its pulse width has been modulated. The time period for modulation is given by T. Therefore, waveform
average value is given by
Figure: 5.27 Square waveform used for PWM technique
Sinusoidal Pulse Width Modulation
In a
simple source voltage inverter, the switches can be turned ON and OFF as needed. During each
cycle, the switch is turned on or off once. This results in a square waveform. However, if the switch is
turned
on for a number of times, a harmonic profile that is improved waveform is obtained.
The sinusoidal PWM waveform is obtained by comparing the desired modulated waveform with a
triangular waveform of high frequency. Regardless of whether the voltage
of the signal is smaller or
larger than that of the carrier waveform, the resulting output voltage of the DC bus is either negative or
positive.
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Figure: 5.28 Sinusoidal PWM waveform
The sinusoidal amplitude is given as A
m
and that of the carrier triangle
is give as A
c
. For sinusoidal PWM,
the modulating index m is given by A
m
/A
c
.
Modified Sinusoidal Waveform PWM
A modified sinusoidal PWM waveform is used for power control and optimization of the power factor.
The main concept is to shift current delayed on the grid to the voltage
grid by modifying the PWM
converter. Consequently, there is an improvement in the efficiency of power as well as optimization in
power factor.
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