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Comsats Institute of Information Science Technology AbbottabadExternal Pulse Commutation

TEACHER:SIR MOHSIN SHAHZAD

MEMBERS:ABDUS SHAKOOR SP09-BEE-052TOUSEEF AHMAD SP09-BEE-008

Comsats Institute of Information Science Technology Abbottabad

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Presentation Outlines

Concept of commutation Types of Commutation Comparison of Commutation External pulse commutation

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Comsats Institute of Information Science Technology AbbottabadCommutation

Commutation is defined as a process turn off a thyristor.

There are two types of commutation techniques.

Commutation

Natural Forced

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Types of Commutation

Forced commutation: It requires external components to store energy and it is used to apply reverse voltage across the SCR or reduce anode current below holding current of the SCR to turn it off.

Natural Commutation: It does not need any external components.it uses supply (mains) voltage for turning off the SCR.

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Comparison of Natural and forced Commutation

Comsats Institute of Information Science Technology Abbottabad

6External Pulse Commutation

This is the voltage commutation scheme. The main objective is to apply a reverse voltage across the

conducting thyristor. The reverse voltage is applied to the load current carrying

SCR from an external pulse source therefore it is called “External pulse commutation”.

EPS

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Explanation

When thyristor is conducting ,the drop across the transformer is negligibly small. The supply voltage appears across the load resistance.

When commutation of SCR is desired a pulse of duration equal or slightly greater than the turn off time specification of the thyristor is applied.

Earlier to the commutation, the capacitor voltage remains at a small value of about 2V.

When a pulse of voltage from the pulse generator is applied to the primary of the pulse transformer, the voltage Induced in the secondary appears across T1 as a reverse voltage (-Vp) and turns it off.

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Explanation (continue)

The capacitor then discharge through the load at load current IL .

A negative voltage is maintained turn off time toff.

As capacitor voltage falls below supply voltage V, the SCR is forward biased.

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MATHEMATICAL ANALYSIS

The turn off time depends upon the current load in this commutation.

Comsats Institute of Information Science Technology Abbottabad

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References

Power Electronics by Asfaq Ahmad