1. Dr. Unnikrishnan P.C. Professor, EEE
EE305 Power Electronics
Dr. Unnikrishnan P.C.
Professor, EEE

2. AC- AC Voltage Controllers Single Phase AC Voltage Controllers
ON-OFF Control
Phase Control

3. Single Phase AC Voltage Controllers

4. Principle of ON-OFF Control
For n ON cycles and m OFF cycles,
the total period is (n+m)2.
The rms value of the output voltage is

5. Phase Control (1 Phase half wave controlled ACVC or Unidirectional Controller)

6. Principle of Phase Control
Positive Half Cycle: T Conducts from  < t < 
At t =  , T turned OFF
Negative Half Cycle: D Conducts from  < t < 2

7. Disadvantages of 1 Phase Half wave controlled ACVC
Average value of the load voltage is not zero.
This introduces a DC component in the load current
Resulting a DC component in the supply intake current introducing harmonics in the AC lines

8. Single Phase Full wave ACVC- Resistive Load
Positive Half Cycle: T1 Conducts from  < t < 
At t =  , T1 is self commuted
Negative Half Cycle: T2 Conducts from (+ ) < t < 2
At t = 2 , T2 is self commuted

9. Full wave AC Voltage Controller with R-Load (Bidirectional ACVC)
For T2 to be turned ON, T1 is to be commuted and vice versa.
No DC component in the load current.
Thyristors do not have a common cathode. So the firing circuits of T1 and T2 must be electrically isolated.

10. Full wave AC Voltage Controller with R-Load (Bidirectional ACVC)
Thyristors connected with a common cathode
Positive Half Cycle: T1 triggered and Conducts from  < t <  . Load current flows through T1, D1 and R. At t =  , T1 is self commuted
Negative Half Cycle: T2 T1 triggered and Conducts from (+ ) < t < 2. Load current flows through T2, D2 and R. At t = 2 , T2 is self commuted

11. Analysis
The rms value of the load voltage is

12. Analysis ……….
Power Factor :

13. Analysis ……….
T1 conducts for half cycle and T2 conducts for another half cycle.
Average value of thyristor current is