Power Electronics Pulse Width Modulation - Power Electronics

What is Pulse Width Modulation in Power Electronics?

Pulse Width Modulation (PWM) is a method used for reducing the overall harmonic distortion (THD) in a load current. After modulating its pulse width, it makes use of a pulse wave in rectangular/square form which leads to a variable average waveform value f(t). Time period for modulation is given by T and hence, waveform average value can be given by

formula.PNG

square_wave_of_inverter.jpg

Sinusoidal Pulse Width Modulation

Switches can be turned ON and OFF easily in a simple source voltage inverter. At the time of each cycle, switch will be turned on or off once and this will lead to a square waveform. When the switch is turned on many times, a enhanced waveform will be obtained.

Sinusoidal PWM waveform can be achieved by comparing the required modulated waveform with a triangular waveform of high frequency. Irrespective of whether the signal voltage is smaller or larger than that of the carrier waveform, resultant output voltage of the DC bus will be either negative or positive.

sinusoidal.jpg

Sinusoidal amplitude can be given as Am and carrier triangle can be given as Ac. For sinusoidal PWM, modulating index m will be given by Am/Ac.

Modified Sinusoidal Waveform PWM

A modified sinusoidal PWM waveform will be used for controlling the power and to optimize the power factor. Main idea is to shift the late current on the grid to the voltage grid by changing the PWM converter. As a result, power efficiency will be improved and also optimization in power factor will be improved.

modified_sinusoidal_waveform.jpg

Multiple PWM

Multiple PWM will have many outputs which will not have similar value but the time period over which they are created will be constant for all outputs. Inverters with PWM can operate at high voltage output.

multiple_pwm.jpg

Below waveform is a sinusoidal wave produced by a multiple PWM

sinusoidal_waveform.jpg

Voltage and Harmonic Control

A periodic waveform with frequency being a multiple integral of the fundamental power with frequency of 60Hz can be referred to as harmonic. Total harmonic distortion (THD) on the other side will refer to the total contribution of all the harmonic current frequencies.

Harmonics can be characterized by the pulse which represents the number of rectifiers that are used in a given circuit. It can be calculated as shown below

h=(n×P)+1or−1

Where n − is an integer 1, 2, 3, 4….n

P − Number of rectifiers

It is reviewed in the table below

Harmonic

Frequency

1st

60 Hz

2nd

120 Hz

3rd

180Hz

4th

240Hz

5th

.

.

49th

300Hz

.

.

2940Hz

Harmonics will be impacting the voltage and current output and can be decreased by making use of isolation transformers, line reactors, redesign of power systems and harmonic filters.

Series Resonant Inverter

Resonant inverter refers to an electrical inverter whose operation will be dependent on resonant current oscillation. Here, switching device and the resonating component will be connected in a series to each other and this will lead to the natural features of the circuit i.e. current which passes through the switching device will drop to zero.

Series Resonant Inverter gives a sinusoidal waveform at very high frequencies in the range of 20kHz-100kHz. Thus, it is appropriate for many applications which are in need of fixed output like induction heating and fluorescent lighting. It is basically small in size as its switching frequency is high.

Resonant inverter will have many configurations and therefore it is categorized into two groups

  • Those with unidirectional switches
  • Those with bidirectional switches

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