30-07-2012, 03:19 PM
A.C. VOLTAGE CONTROLLER
A.C. VOLTAGE CONTROLLER.doc (Size: 913.5 KB / Downloads: 42)
INTRODUCTION:
A.C. voltage controllers are thyristor based devices which convert fixed alternating voltage directly to variable alternating voltage without a change in frequency. some of the main application of ac voltage controllers are for domestic and industrial heating, transformer tap changing, lighting control, speed control of single- phase and three-phase ac drives and starting of induction motors. earlier, the devices used for these applications were auto-transformers, magnetic amplifiers, saturable reactor etc. but these devices are now replaced by thyristor and triac based ac voltage controllers because of their high efficiency, flexibility in control ,compact size and less maintenance.AC voltage controllers are also adaptable for closed control systems. since the ac voltage controllers are phase-controlled devices, thyristors and triacs are line commutated and as such no complex commutation circuitry is required in these controllers. the main disadvantage of ac voltage controllers is the introduction of objectionable harmonics in the supply current and load voltage waveforms, particularly at reduced output voltage levels.
TYPES OF AC VOLTAGE CONTROLLER:
The ac voltage controllers use two different techniques to vary the load voltage.the techniques are:-
• on-off control.
• phase angle control.
CIRCUIT OPERATION
A fully-controlled rectifier circuit contains only controlled-rectifiers, whereas a semi-controlled rectifier circuit is made up of both controlled and uncontrolled rectifiers. Due to presence of diodes, free-wheeling operation takes place without allowing the bridge output voltage to become negative. In a semi-controlled rectifier, control is effected only for positve output voltage, and no control is possible when its output voltage tends to become negative since it is clamped at zero volt. This page describes the operation of a single-phase half-controlled rectifier.
A semi-controlled full-wave bridge rectifier can be configured in a few ways. They are shown below.
Configuration 1
The circuit contains two SCRs and two diodes. When source Vs is positive, SCR S1 can be triggered at a firing angle called and then current flows out of the
source through SCR S1 first,
then through the load and returns via diode D2.
If then SCR S1 and diode D2 conduct during < wt < . When < wt < 2, Vin is negative and SCR S2 is normally triggered when wt = + . During < wt < , the output of the bridge circuit would have been negative if we had used a fully-controlled bridge rectifer and if the current flow was continuous.
But here we have two diodes D1 and D2 instead of two SCRs. When the output of the bridge tends to becomes negative just after wt exceeds , diode D2 tends to get forward-biased and it starts conducting.
Then diode D1 is reverse-biased and it stops conducting. During < wt < ( + ) , the devices in conduction are SCR S1 and diode D2 and the output of the bridge is clamped at zero, assuming that the on-state drops across devices in conduction is zero. During ( < wt < 2 , the devices in conduction are SCR S2 and diode D2. SCR S2 and diode D1 would conduct during 0 < wt < .
in this circuit SCRs as the devices in the top-half and diodes as the devices in the bottom-half. Instead, it it is possible to use SCRs as the devices in the bottom-half and diodes as the devices in the top-half.