SCR COMMUTATION PART 1 NATURAL COMMUTATION ARTIFICIAL COMMUTATION CLASSES OF COMMUTATION

1. POWER ELECTRONICS
SCR COMMUTATION
PART -1

2. COMMUTATION
?

3. COMMUTATION means
what?
•It is a process to TURN OFF SCR
•No more current should pass through
it
•It should act as an OPEN switch

4. • Forward Voltage Triggering
• Gate Triggering
• dv/dt Triggering
• Temperature or Thermal
Triggering
• Light Triggering
Methods
to TURN
ON an
SCR

5. Conditions to keep SCR in ON position
•Anode to cathode current should
remain more than Holding current
•SCR must remain in FOREWARD bias
means Va>Vk

6. Conditions to turn Off SCR
•If foreward Current becomes less than
Holding current, means : IAK < I H
•Sufficient reverse voltage must be
applied across the SCR for sufficient time
as to regain its forward blocking state

7. Commutation
Method
Natural Forced

8. Natural Commutation
• Its NOT possible in DC,Occurs naturally in AC
• It turns OFF automatically
• Either load current becomes lesser than
holding current value
• or SCR reverse biases

9. FORCED COMMUTATION
• In DC, SCR do NOT turn OFF naturally
• So extra circuit is required to forcefully
turn OFF it
• Hence, it is called Forced Commutation
• Most widely used in all applications

10. Commutation
may also be classifies as
Voltage
Commutation
If Reverse voltage is
applied across SCR
Current
Commutation
If thyristor current is
forced to reduce
below holding current

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

12. Remember
To turn Thyristor
OFF:
Either thyristor current
should be reduced to lesser
than holding current
OR
Voltage across it must be
reversed