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

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

Single Phase AC Voltage Controllers

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

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

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

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

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

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.

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

Analysis The rms value of the load voltage is

Analysis ………. Power Factor :

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