AC VOLTAGE CONTROLLERSChapter 11Power Electronics Circuits, Devices and Applications By Muhammad H. Rashid1Introduction he power !low into a load can "e controlled "y varyin# the rms value o! the load volta#e. his can "e accomplished "y thyristors, and this type o! power circuit is $nown as ac volta#e controllers.2 he most application o! ac volta#e controllers are% &ndustrial heatin# 'n(load trans!ormer tap chan#in# )i#ht controls *peed control o! induction motors AC ma#net controls3 +or power trans!er, two types o! control are normally used% 'n(o!! Control Phase an#le control &n on(o!! control, thyristor switches connect the load to the ac source !or a !ew cycles o! the input volta#e and then disconnected !or a !ew cycles. &n phase control, thyristor switches connect the load to the ac source !or a portion o! each cycle.4 he ac volta#e controllers can "e classi!ied into two types% *in#le(Phase Controllers hree(Phase Controllers Each type can "e su"divided into% ,nidirectional or Hal!(-ave Control Bidirectional or +ull(-ave Control *ince the input volta#e is ac, thyristors are line commutated.5 ypically phase control thyristors which are cheaper are used. +or applications up to .// H0, R&ACs are used.6Principle of On-Off Control he principle o! on(o!! control can "e e1plained with the !ollowin# sin#le(phase !ull(wave controller.78 his type o! control is applied in applications which have hi#h mechanical inertia and hi#h thermal time constant. ypical e1amples are industrial heatin# and speed control o! motors. &! the input volta#e is connected to load !or ncycles and is disconnected !or m cycles, the output load volta#e is !ound !rom%9 2ote that $ is called the duty cycle, and the power !actor and output volta#e vary with the s3uare root o! $.k Vn mnV Vt d t Vm nnVs s rms os rms o=+=+=2 / 1202 2) ( sin 2) ( 2 10Principle of Phase Control he principle o! phase control can "e e1plained with the !ollowin# circuit.11 Due to the presence o! diode D1, the control ran#e is limited. he rms output volta#e can only "e varied "etween 4/.4 to 1//5. he output volta#e and input current are asymmetrical and contain a dc component.12 his circuit is a sin#le(phase hal!(wave controller and is suita"le only !or low power resistive loads, such as heatin# and li#htin#. *ince the power !low is controlled durin# the positive hal!(cycle o! input volta#e, this type o! controller is also $nown as unidirectional controller.13 he rms value o! the output volta#e is !ound !rom% he avera#e value o! the output volta#e is%2 / 12 / 1 2 222 2)]22 sin2 (21[)]} ( sin 2 ) ( sin 2 [21{ + =+ = s os s oV Vt d t V t d t V V) 1 (cos22)] ( sin 2 ) ( sin 2 [212 =+ = sos s dcVVt d t V t d t V V14Single-Phase idirectional Controllers !ith Resisti"e Loads he pro"lem o! dc input current can "e prevented "y usin# "idirectional or !ull(wave control.1516 he !irin# pulse o! 1and 6are 17/ de#rees apart. he rms value o! the output volta#e is% By varyin# 8 !rom / to 9, :ocan "e varied !rom :sto /.2 / 12 / 12 222 sin(1) ( sin 222+ == s os oV Vt d t V V17