Question

explain the following, enhance will sketches and describe the output voltage.

- Sinusoidal PWM

- Full bridge inverter single phase

- Bipolar PWM single phase inverter

- Uni-polar WM single phase inverter

- three phase inverter

Answer 1

Sinusoidal Pulse Width Modulation: In this technique there are multiple numbers of output pulse per half cycle and of different width of pulses . The width of each pulse is varying in proportion to the amplitude of a sine wave evaluated at the centre of the same pulse. The gating signals are generated by comparing a sinusoidal reference with a high frequency triangular signal. The rms ac output voltagee and wave forms are as shown in figure

. Features of SPWM on  comparing various PWM Techniques:

 Utilization of Dc power supply that is to deliver a higher output voltage with the same DC supply.

 Linearity in voltage and current control.

full bridge Single-phase Inverter:

A device that converts DC power into AC power at desired output voltage and frequency is called an inverter. For Full bridge inverter when T1, T2 conduct, load voltage is Vs and T3, T4 conduct load voltage is –Vs. Frequency of output voltage can be controlled by varying the periodic time T. During inverter operation it should be ensured that two thyristors in the same branch should not conduct simultaneously as this would lead to a direct short circuit of the source. For resistive load voltage and load current would always be in phase with each other. Due to this reason diodes will not come into action. The ideal waveform of the experimental setup is shown in Figure below:

z

The output voltage equation is given as

Bipolar PWM: you have only 2 levels at the output +vdc and -vdc. So THD performance is poor and filter requirement is more. However, due to the absence of zero state there is no chance for the flow of leakage current.

Only two voltage levels are there in the output

1. It requires only one sinewave
2. It is easier to implement
3. The harmonic performance is poor
4. requires large filter at the output
5. Switching losses are more
6. Not much of use in the modern era.
7. However, better suited for transformerless full bridge PV inverter.

Unipolar PWM: You have 3 levels in the output +vdc zero and -vdc. So THD performance is better. filter requirement is less. But, due to the presence of zero state it opens a path for leakage current.

1. Three voltage levels are there in the output
2. It requires two 180 degree phase shifted sinewaves as modulating waves
3. It is complicated to implement
4. The harmonic performance is better
5. Filter requirement is less at the inverter output
6. Switching losses are less
7. Most extensively used
8. However, less suited for transformerless full bridge PV inverter

Inverter converts fix DC voltage in to equivalent sinusoidal waveform noticeably by performance.

Main idea behind this conversion is fact that sine wave has higher magnitude at its center (90 degree) and its reducing as go far from center both side (toward 0 degree and 180 degree). Thus supplying dc voltage of varying width such that it has high width around center and smaller at far from center resembles sine shape. This DC voltage can be switched from either from 0 to Vdc or from Vdc to –Vdc to produce same effect which call unipolar and bipolar technique respectively. Name unipolar is given as switched DC voltage applied to the load remain positive during positive cycle and negative during negative cycle. While in bilopar method this voltage switches in both the direction throughout the cycle. Thus bipolar method produces twice the voltage stress on load than maximum AC voltage that can be produce by the inverter. Thus unipoar method is more preferable.