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Thyristor Commutation Techniques

There are two methods for turning off a thyristor from its on state, called commutation: natural commutation and forced commutation. Natural commutation occurs automatically when an AC current passes through the thyristor and reaches zero, turning it off. Forced commutation is required in DC circuits and can be achieved through methods like self commutation using inductance and capacitance, impulse commutation using a capacitor reversal voltage, resonant pulse commutation using an LC circuit pulse, complementary commutation alternating two SCRs, or external pulse commutation using an external generated pulse.

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174 views10 pages

Thyristor Commutation Techniques

There are two methods for turning off a thyristor from its on state, called commutation: natural commutation and forced commutation. Natural commutation occurs automatically when an AC current passes through the thyristor and reaches zero, turning it off. Forced commutation is required in DC circuits and can be achieved through methods like self commutation using inductance and capacitance, impulse commutation using a capacitor reversal voltage, resonant pulse commutation using an LC circuit pulse, complementary commutation alternating two SCRs, or external pulse commutation using an external generated pulse.

Uploaded by

Imran Hussain
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Thyristors Commutation Techniques

Power Electronics Circuit, Devices and Applications


By M. H. Rashid (2nd Edition), Chapter 3
Commutation?

• Turning a thyristor off from the on state is called ‘commutation’.

• An SCR can be commuted by two methods:

• Natural Commutation

• Forced Commutation
Natural Commutation

• When an SCR turns off by the occurrence of 𝑰𝑨 < 𝑰𝑯

• This condition is automatically achieved when an AC current


passes through SCR.

• Zero-crossing point of AC current automatically turns off SCR.

• This is also called line commutation.


Forced Commutation
• In DC circuits, commutation is achieved by applying other circuits that
cause SCR to turn off.
• Common methods are:
• Self Commutation
• Impulse Commutation
• Resonant Pulse Commutation
• Complementary Commutation
• External Pulse Commutation
Self Commutation

• Achieved by using inductance and capacitance in series with load.


Impulse Commutation

• Achieved by reversal voltage of capacitor by alternatively firing


T2 and T3.
Resonant Pulse Commutation

• Consists of series LC circuit that generates resonant pulse to


commute T1.
Complementary Commutation
• Current is transferred between two loads by alternately
commuting two SCRs.

• One SCR becomes on that commutes other SCR and vice versa.
External Pulse Commutation
• An external pulse (generated by another circuit) is used to turn off
an SCR.

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