Dear Janardhan,

Hope you are well and every thing is fine with you.

Whenever you have two nodes a and b with a voltages Va and Vb referenced to some reference point or ground. In order to compare between these voltages define for them the common mode voltage Vcm and the difference voltage Vd

such that:

Va= Vcm +Vd/2

and Vb= Vcm - Vd/ 2,

Then one can get the common mode voltage in terms of Va and Vb:

From the above relations one gets

Vcm= (Va +Vb)/2 which is the average value of the two voltages

and Vd= Va-Vb

which is the difference between the two voltages.

This means that the two nodes a and b has a common voltage level and when we refer Va to it it will be higher with a voltage Vd/2 and the other will be lower than it by Vd/2.

If you want to keep Vcm consatnt then you have to keep the average of Va and Vb consatnt.

Please apply the concepts of my answer to draw the output waveform you will be found it alternating around the common mode voltage. So, you have to filter it out to get a pure AC voltage.

Best wishes

Dear Janardhan Gurram,

I am fully agree with Respected Abdelhalim Zekry,

but want to share some knowledge which will be helpful for you,

Actually,

Transformerless inverters use a computerized multi-step process and electronic components to convert DC to high frequency AC, back to DC, and ultimately to standard-frequency AC. ... As such the individual transformer within the home-scale inverter becomes redundant.

The main function of an inverter is to change direct current (DC) to alternating current (AC). The input voltage, output voltage and frequency, in fact the power on the whole all depend largely on the design of the specific device and/or the circuitry.

Review of Common-Mode Voltage in Transformerless Inverter Topologies for PV Systems. When a galvanic connection between the grid and the PV array is made, a common-mode voltage exists which generates common-mode currents. ... Some topologies available in the market touch more or less such a crucial requirement.

This link will hell you for more information related to common mode voltage and transformerless inverter.

https://link.springer.com/chapter/10.1007/978-3-642-27509-8_49

Hope you understand about it if any question then please feel free to ask me.

Thanks.

Abdelhalim abdelnaby Zekry & Zia Hameed

Thank you sir

I well understood your wordings , so that I am able to ask you another question that,

Why the use of Transformer in Inverter would eliminate or maintain constant common mode voltage (eliminate or maintain constant-I am not sure ).

Dear Janardhan,

welcome again!

The common mode voltage can be eliminated from the voltage between the nodes a and node b by using a DC blocking capacitor or by using a transformer. A transformer can not only step up or step down the the AC voltage but also prevent the DC or common mode voltage from passing through it because the out put voltage is proportional to rate of change of the flux linkage dPhi/dt. So if the magnetic flux is consatnt with time the output voltage will be zero. A DC current results in a DC flux which can not induce any voltage. So, transformers can not transfer DC current.

So is we cancel the common mode or the DC voltage from difference Va-Vb the output will be purely alternating with symmetrical amplitude = Vs/2.

Best wishes

Dear All

I respect the great answers from Prof.Dr. Abdelhalim Zekry , he added a reference answer for all.

Hi

Attached paper on topic of Common Mode-Voltage (CMV).

What is CMV and how it reduce, is nicely explain in INTRODUCTION -> 2nd para.

And also nice explained equations of CMV.

I hope it is helpful to you.

Thank you.

thanks for this complete and helpful answers Abdelhalim abdelnaby Zekry , Mothanna Sh. Aziz