AC VOLTAGE CONTROLLERSChapter 11Power Electronics Circuits, Devices and Applications By Muhammad H. Rashid1AC Voltage Control with PWM e showed that the input power !actor o! controlled recti!iers can "e improved "y PM type o! control. #he naturally commutated thyristor controllers introduce lower$order harmonics in load and the supply side. #he per!ormance o! ac volta%e controllers can "e improved "y PM control.2 #he !ollowin% circuit shows a sin%le$phase ac volta%e controller with PM.3 #he switches &1and &'are turned o!! several times durin% the positive and ne%ative hal!$cycles. &(1and &('provide the !reewheelin% paths !or the load current.4Design of AC Voltage-Controller Circits )n desi%n o! the AC volta%e controllers, the ratin%s o! power devices must "e selected !or the worst$case conditions. #his occurs when the converter delivers the ma*imum rms value o! output volta%e +o.5 #he input and output !ilters must also "e desi%ned !or worst$case conditions. #he output o! a power converter contains harmonics, and the delay an%le !or the worst$case condition o! a particular arran%ement should "e determined.6Effects of Sorce an! Loa! "n!ctances )n derivation o! the output volta%es, we have assumed that the source has no inductance. #he e!!ect o! the source inductance would "e to delay the turn$o!! o! thyristors. #hyristors would not turn o!! at ,ero crossin% o! input volta%e, and %ate pulses o! short duration may not "e suita"le.7 #he harmonic contents o! the output volta%e would also increase. #he load inductance also plays an important role on the per!ormance o! the power controllers. Althou%h the output volta%e is a pulsed wave!orm, the load inductance tries to maintain a continuous current !low.8 #his is shown in the !ollowin% !i%ure.9 )n addition, the input power !actor o! the converter depends on the load power !actor.10