08-16-2017, 10:04 PM
HARMONICS REDUCTION
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HARMONICS REDUCTION
The harmonics present in a dc to ac inverter are very much obvious compared to the harmonics that can be present in an ac to dc converter. This is because of the output of dc to ac inverter being ac. Thus, the filters that are used in dc to ac inverter have different designs compared to the filters used in ac to dc converters. In case of ac to dc converters, the main objective is to improve the output voltage ripple. Thus, passive filters can be easily used in order to improve the output of an ac to dc converter. While, in case of dc to ac inverter, the harmonic reduction is harder and it also includes the use of active filters.
As the output of dc to ac inverters is alternating, it is very important to produce sinusoidal output waveforms. In order to produce such sinusoidal waveforms, filters are implemented which reduce the harmonic effect by removing the third and higher harmonics from the system. The filters used to remove the harmonics from the inverters are more complex and consists of large number of inductors and capacitors to remove the harmonics of higher order. This also results into more costly filters to remove harmonics from the inverter.
Thus, in order to avoid the cost of such expensive and complex filters controlling the width or reducing the number of pulses may result into reduction of harmonics. One such technique is explained below.
Pulse Width Modulation Technique:
a single phase inverter block diagram with a high frequency filter that is used in order to remove the harmonics from the output waveform. Here, vo is the ac output while vin is the input dc voltage.
Non Sinusoidal Pulse Width Modulation:
In case of Non sinusoidal pulse width modulation, all the pulses that have same pulse width are modulated together. The pulse widths of pulses are adjusted together in same proportion on order to remove the harmonics from the system.
Sinusoidal Pulse Width Modulation
Sinusoidal Pulse Width Modulation is a bit different compared to the Sinusoidal Pulse Width Modulation. In case of sinusoidal pulse width modulation, all the pulses are modulated individually. Each and every pulse is compared to a reference sinusoidal pulse and then they are modulated accordingly to produce a waveform which is equal to the reference sinusoidal waveform. Thus, sinusoidal pulse width modulation modulates the pulse width sinusoidally.
NEED FOR PULSE WIDTH MODULATION
An inverter is nowadays commonly used in variable speed AC motor drives to produce a variable, three phase, AC output voltage from a constant DC voltage. Since AC voltage is defined by two characteristics, amplitude and frequency, it is essential to work out a strategy that permits control over both these quantities. The most effective method of controlling the output of the inverter is to incorporate PWM control within the inverters. Pulse Width Modulation (PWM) strategies are required for switching the devices in a VSI appropriately to generate variable voltage, variable frequency.