SCR COMMUTATION PART 2 NATURAL COMMUTATION FORCED COMMUTATION

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POWER
ELECTRONICS
SCR COMMUTATION
PART 2

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To turn
Thyristor OFF:
Either thyristor
current should be
reduced to lesser
than holding
current
OR
Voltage across it

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Types of Commutation
•Class A
•Class B
•Class C
•Class D
•Class E
•Class F

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CLASS A Commutation
• Or self commutation, or resonant commutation, or
load commutation.
• In this commutation, the source of commutation voltage is
in the load.
• This load must be an under damped R-L-C supplied with a
DC supply so that natural zero is obtained

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CLASS B COMMUTATION
• Self Commutation type
• Turn Off time depends upon L & C
• SCR turns OFF automatically after certain time

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CLASS C COMMUTATION
• Initially both SCRs are OFF
• First Load SCR is triggered
• Then its turned OFF with the help of other another SCR

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CLASS D
COMMUTATION
• Also known as
Auxiliary
commutation

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STEPS
1. Turn ON auxiliary SCR
This will charge capacitor with pper plate POSITIVE
2. Now turn ON main SCR, this will pass main load
current & also lead to discharge of capacitor and then
charging of it with reverse polarity, that is upper plate
NEGATIVE, via L & diode D
3. Now when we wish to turn OFF main SCR, then turn
ON aux SCR
4. This will put reverse charged capacitor across main
SCR
5. This will lead turn OFF main SCR
6. Above steps are then repeated

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CLASS E COMMUTATION
• Turning OFF SCR by applying REVERSE voltage
• Either reverse voltage is applied across SCR
• Or, Big positive pulse is applied at the cathode of SCR
• So that it reverse biases
• And Turns OFF

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CLASS F COMMUTATION
•Also known as Natural
Commutation
•No circuitry required to turn SCR OFF
•This happens only in AC supply
•When negative half pulse arises,
•It automatically reverse biases SCR

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Summary
• Commutation is the process of Turning OFF of an SCR
• Types of commutation : A,B,C,D,E & F
• SCR can either be turned off by applying reverse
voltage or making current lesser than holding current
• Class A & B are current commutations
• Class C,D,E & F are voltage commutation
• Class F is natural commutation as no external circuitry
is required to turn it OFF