Industrial Electronics Laboratory Technical Note Author: Alex Florisca Tutor: Steve Burrow Date: 29/03/2010 1
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Industrial Electronics Laboratory Technical Note
Author: Alex Florisca Tutor: Steve Burrow
Date: 29/03/2010
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ABSTRACT
This technicalnote provides thewritten results
and conclusionsto the thyristorlaboratoryexercise. Thebasiccharacteristics of thyristors areanalysed first.
The experimentsinclude triac andbridge rectifierconfigurations.For each circuit,an analysis of circuit operationis performed,with an emphasisof the effect thishas on an RLload.
INTRODUCTION
Objective
The purpose of this document isto provide anintroduction tothyristors by useof practical
means. Fiveexperimentshave beencarried out andthis technicalnote provides ananalysis of the
results and theconclusionsdrawn.
Background
Thyristors are afamily of semiconductordevices with fourlayers of alternating N andP-type material.
Their operationcan be thoughtof as a latched
diode. A currentis required tostart conducting,after which thethyristorscontinues toconduct as longas it is forwardbias, regardlessof weather theactivating signalis present or not.As a generalanalogy, this canbe compared to apower button ona computer.When this ispressed (andreleased), thecomputer turns
on/off andremains in thisstate thereafter.
The button presscan be thoughtof as theactivating
current, whichthe thyristorsneeds to startconducting, andthe action of turning on/off is
representative of the current flowfrom anode tocathode within athyristors device.
Figure 1: Thyristor(Silicon Controlled
Rectifier)
Thyristors arealso often knownas SiliconControlledRectifier. This isone of thecomponentsunder thethyristor family;however theterms havebecomesynonymous inmany sources.Other devicesinclude:
• Shockley Diode• Silicon
ControlledSwitch
• Gate turn-off (GTO) thyristor
EQUIPMENT
The experimentsrequired asignificantnumber of
measurements of current, voltageand power, bothRMS and averagevalues. In orderto do thisaccurately, thefollowingequipment wasused:
• PowerTekISW8000 Watt-Meter
• Thyristor Kit• Digital
Oscilloscope
The wattmeterwas used formeasuring RMSvalues of current,voltage andpower. Theanalogue meterswere used formeasuring theaverage currentand voltageacross the load.
The digital
oscilloscope wasused to displaywaveforms forthe input andoutput voltages.
The reactive loadmentioned above
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consisted of twoparts:
1. A variableresistancewire-woundrheostatwith amoveableslider
2. A variableinductorwith aremovablecore madefromlaminatedsteel
The thyristor kitcan be seen inFigure 2 . Itconsists of fourthyristors, firedas two pairs. T1and T3 are firedby the samefiring angle/pulse(TP1) and T2 and
T4 are fire by TP2). Howeverthe two firingangle pulses (TP1and TP2) areseparated by180 o . Thepotentiometer
labelled “Firingangle” controlsthe timing of thegate currentpulse used to fireeach pair of thyristors and
hence the firingangle to bevaried from 0 o to180 o in the caseof T1 and T3 andfrom 180 o to 360 o
in the case of T2and T4.
Figure 2: ThyristorKit
EXPERIMENTC: REACTIVELOADS
Aim
The aim of thisexperiment is tounderstand howthe impedancevaries when
using a thyristoras a power-switching device.In order to dothis, theinductance andresistance of the
load weremeasured overfour conditions,varying theconfiguration of the load.
Setup
Figure 3: ExperimentC Setup
The experimentsetup is shown inFigure 3 . It isimportant to notethat none of thethyristors are yetconnected into
the circuit, onlythe 50V ACsupply. Thewattmeter wasused to measurevalues for the
RMS current, realpower, reactivepower and thepower factor forall fourconditions of resistance andinductance. Thepower trianglewas then used tocalculate valuesfor theresistance,inductance and
impedancephase angle. These results areshown in Table 1 .
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ResistorState
InductorState
R L θ
Lowresistance
Coreremoved
44.72 0.02 8.11
Maxresistance
Coreremoved
85.79 0.03 8.11
Lowresistance
Core fullyinserted
44.98 0.19 52.41
Maxresistance
Core fullyinserted
85.18 0.20 35.9
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Table 1: LoadConfigurations
Results
The load used inthis experiment
has both reactiveand resistiveelements andtherefore theapparent powervaries dependingon the influenceof thesecomponents. Thevalues obtainedfor the currentare as expected;large currentflows when theimpedance islowest and smallcurrent flowswhen impedanceis largest. Thetrue power takesis equivalent to
purely resistivepower, and istherefore higherat lowerimpedance (coreremoved). Theopposite is truefor the reactivepower (VAR). Thepower factor issimply the ratioof real power toapparent power,and indicates thepower efficiencyof a circuit. Inthis experiment,it can be seen
that the inductor(with coreinserted) reducesthe efficiency of the circuit (powerfactor decreases)
by drawing morecurrent in orderto store energy.
The power factoris also directlyproportional tothe impedancephase angle,which tends toincrease as thereactive powerincreasescompared to realpower. This isdue to the factthat in a purelyreactive circuitthe phase angleis 90 o , whereas ina purely resistive
circuit the phaseangle is 0 o . Thevalues show thatinserting the coreinto the inductorproduces higherimpedance,thereforeincreasing thephase angle andincreasingreactive power.
EXPERIMENTD: CURRENTTRANSDUCERCALIBRATION
Aim
As part of thethyristor kit,there is a currenttransducer,which measuresthe current andproduces aproportionalvoltage. The aimof thisexperiment is tofind out theconstant of proportionalityfor use in futureexperiments.
Method
The setupremained thesame asExperiment C,and the RMSVoltage andCurrent weremeasured whilevarying the load.
Results
The results areshown in Figure 4 where the linearregression showsthe constant of proportionality isaround 1. A moreaccurate value
can be obtainedfrom the linearequation inFigure 4 ,however thiswould not
necessarily bereliable due toequipmentinaccuracies ornon-perfectsinusoidwaveforms. Itwas thereforedecided that theexact value of 1.0226 could beapproximated to1 for the purposeof theseexperiments.
Figure 4: Constant of Proportionality of theCurrent Transducer
EXPERIMENTE: THYRISTORSWITCHING –SINGLEDEVICE
Aim
This experimentis split into twoparts. Firstly
(E1), a thyristorwill beintroduced intothe circuit toobserve itsbehaviour. In thesecond part (E2),
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A diode is thenintroduced andthe results areobserved anddiscussed. Anequation is
developedrelating theaverage loadvoltage to thefiring angle.
E1: SingleThyristor Circuit
The experiment
is set up asshown in Figure 5 and the inputcurrent andoutput voltageare measured.
The result can beseen in Figure 6 ,which shows howthe outputvoltage varieswith inputcurrent. Thegraph shownhere isrepresentative of the inductor withits core fullyinserted. In thiscase, there issignificant
inductance in thecircuit and thethyristor voltagelags behind thecurrent, causingit to drop to anegative before
the currentreaches zero.
This effect ismuch lesssignificant withthe inductor core
removed,because there ismuch lessinductance in thecircuit.
Figure 5: Single Thyristor Experiment
Setup
Figure 6: AC current(orange) and outputvoltage (blue) of a
single thyristorcircuit with load
inductor coreinserted
E2: Free-WheelingDiode Circuit
The voltage spikeseen in Figure 6 may sometimesbe undesirable ina circuit. Anexample of this isa load thatcontains acapacitance. A
change inpolarity in theoutput voltagemay damage thecapacitor andtherefore needsto be eliminated.
This is done witha free-wheeling(or flyback) diode(see Figure 7 ).
These aregenerally used incircuits whereinductive loadsare switched off by siliconcomponents. Anexampleapplication of this is in DC
motor drives.Unfortunately nograph isavailable tovisualize thiseffect, howeverthe only
difference thegraph wouldshow from thatof Figure 6 is thatthe outputvoltage (blue)
would be cut off at zero andwould neverbecomenegative.
Figure 7: Free-wheeling Diode
Experiment Setup
To analyse theeffect of thefiring angle onthe average loadvoltage, anequation wasderived to relatethe two. This wasdone by lookingat the voltagewaveform and
integrating. Sincethe frequency isknown, the firingangle can beconverted from atime constant toan angle. Thethought process
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is illustratedmore clearly inFigure 8 , whichresults inEquation ( 0 ) .
Figure 8: Convertingfiring angle to angle
Given that thevoltage takes theform of asinusoid, it hasbeenapproximated assuch here, wherefor each period:
Thereforeleading to the
followingintegral:
which whensolved, leads toEquation ( 0 ) .
This is anapproximation tothe relationship
of the averageload voltage tothe firing angle,based on aperfect sinusoid.
In order to checkthis expression,readings weretaken for theaverage voltageas the firingangle waschanged. Thesevalues werecompared tothose obtainedby using the
equation aboveand the resultsare shown inFigure 9 . As canbe seen thetheoretical linematches theexperimental onefairly closely.Since noreadings areavailable forfiring anglesunder 0.5radians, thetheoretical line isslightly off,although this is
believed to bedue toinsufficientreadings in thatarea.
Figure 9: Firing Anglevs Average Load
Voltage
EXPERIMENTF: THYRISTORSWITCHING INPAIRS (TRIAC)
Aim
This experimentanalyses thebehaviour of acircuit consistingof two thyristorsswitching as apair (triac). Italso looks atsingle andmultiple-pulsefiring and theeffect this has onthe load voltage.
Method
The setup issimilar as before,connecting anextra thyristor ina triaccombination (seeFigure 10 ). Thetwo thyristorsused are T1 and
T4. The load willneed to beconfigured
according to thetwo observationsnecessary: lowresistance, coreremoved and lowresistance, core
inserted. Foreach state,observations aremade using firstsingle firingmode and thenmultiple firing.
Figure 10: TriacExperiment Setup
Results
The result of introducing apair of thyristorsis that firingoccurs on bothhalf cycles, ascan be seen inFigure 11 . The
thyristors workas a pair, onealways being onand the other off.
The blue lineshows the outputvoltage and the
V AVE = f ( t )
T 0
T
∫
0 ≤ θ < α θ = 0α ≤ θ ≤ π θ = Asin(θ )π < θ ≤ 2π θ = 0
V AVE = Asin(θ ) d θ
α
π
∫
V AVE = A(1 + cos(α ))
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orange lineshows the firingpulse. Thisparticularscenario isrepresentative of
a very smallfiring angle. Infact if the anglewere anysmaller, thecircuit wouldattempt to switchon the secondthyristor beforethe first hasturned off. Thisresults in abreakdown of theoutput voltagefor that half cycleperiod. Thiseffect is seen inFigure 12 .
Figure 11: Loadvoltage with smallfiring angle - single
firing mode
Figure 12: Smallfiring angle causingthyristors to switch
at the wrong time
This problem canbe solved withmultiple firingangles. Ratherthan trying toactivate thesecond thyristor
just once,multiple pulses
are sent over aperiod of time,each one of which can triggerthe thyristor.
This eliminatesthe timing errorof having to waitfor the previousthyristor toswitch off. Theeffect can beseen in Figure13 .
Figure 13: Triac withmultiple pulse gate
currents
Typicalapplications for atriac controllerare those wherea steadysinusoidal (or bi-polar) waveformis needed; suchas a variable ACpower supply.
EXPERIMENTG: BRIDGERECTIFIERCIRCUITS
Aim
This experimentis split into twoparts. The aim of
theseexperiments is tobuild anunderstanding of the bridgerectifier circuit.
There are two
types, half controlled andfully controlled,which areanalysed byexperiments G1
and G2respectively.
G1: Half Controlled BridgeRectifier Circuit
Figure 14 showsone configuration
of a half controlled bridgerectifier circuit, inwhich the currentin only two of thefour arms iscontrolled. Theother two armsproviderectification viathe use of free-wheeling diodes,to provide a fullypositive output.With this type of circuit, control isachieved only forthe positiveoutput voltage,and no control ispossible for
negative voltage,as it is clampedat zero.
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.
Figure 14: Half Controlled, Single
Phase, BridgeRectifier Setup
The circuitoperation is asfollows. Whenthe voltagesource V SUP ispositive, thyristor
T1 is triggeredvia firing angleα . The currentthen flowsthrough to theload (in this case
consisting of aninductor and aresistor) andback into thesupply throughdiode D3. Duringnegative cyclesof the supply, T2is triggered atfiring angle α .
Before T2 isactivated, diodeD4 tends tobecome forwardbias and startsconducting. Once
T2 is active,current flows
through T2 andD4. Thisbehaviour canobserved inFigure 15 .
Figure 15: OutputVoltage of a Half
Controlled Rectifier
It is important innormalapplicationswhen using abridge rectifierthat the currentdrawn by theload iscontinuous. Thisis because acontinuous loadkeeps thevoltage on thepower supplyconstant. If the
current drawnpulsates, it wouldintroduceharmonics intothe circuit,potentiallyleading to
voltage spikes.However, it is notalways possibleto keep the loaddrawing constantcurrent so there
are ways toreduce theharmonics, forexample with aninductor or an RCfilter.
The averagevoltage can beexpressed as:
This equation isderived from thepreviousequation (4) inthe singlethyristorexperiment.
Hence:
This makessense, as thepeak amplitudeof a sine wave isequal to the
square root timesthe RMS voltage,and the period of integration is .
In order to verifythis expression iscorrect, a range
of values for theaverage voltagewere collectedand plottedagainst the firingangle. This is
compared withthe theoreticalresults in Figure15 . As can beseen, the lines donot match veryclosely, with theexperimentalresults tendingtowards the peakvoltage of thesupply whereasthe theoreticalresults aretending towardsthe RMS value.
This discrepancycould be due to arelativelyunstable supplywith large
variations inoutput voltage,lowering theaccuracy of theRMSapproximation.
The two linesshow the sameshape andsimilar trends,
therefore it isbelieved theerrors are in theaccuracy of results andequipment ratherthan errors incalculations.
V AVE = 2 × V RMSSupply
π 1 + cos α ( )
2 × V RMSSupply
π = A
π
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Figure 16: AverageVoltage of a Half Bridge Rectifier
G2: Fully Controlled BridgeRectifier Circuit
Figure 17: FullyControlled Rectifier
Setup
The purpose of afully controlledrectifier is toprovide avariable DCvoltage from anAC source. Thecircuit is shownin Figure 17 . Itconsists of twopairs of thyristors, whichalternate toprovide outputrectification onboth half cycles.When the supplyvoltage ispositive, T1 and
T3 can betriggered and
current wouldflow through T1,to the load andback in through
T3. In the nexthalf cycle, the
other pair of thyristorsconducts. Thiseffect can beseen in Figure18 . It isimportant to notethat although thesupply voltagealternatespolarity, the loadvoltage remainsunidirectional.
The variance inoutput voltage isachieved byvarying the firingangle.
Anotherimportant factor
to consider is thecontinuity of loadcurrent. At thepoint where thesupply switchesfrom a positivehalf cycle to anegative half cycle, the loaddoes not fall to 0.
The inductor hasstored energy,acting as atemporarysource to keepthe thyristor pairconducting whilethe other pair is
triggered. If thefiring angle isless than theload angle, theenergy stored inthe inductor is
sufficient to keepconducting. If theopposite is true,the energystored in theinductor will runout before thesecond thyristorpair starts toconduct,therefore causinga discontinuity inthe currentacross the load.
The choice of inductor andfiring angle aretherefore keyparameters whendesigning such acircuit to avoid
voltage spikes,ripples and othersuch unwantedbehaviour.
Figure 18: OutputVoltage of a Fully
Controlled Rectifier
CONCLUSION
A variety of experimentswere carried outto analyse hebehaviour of thyristors indifferentconfigurations.
The singlethyristor exerciseexplored thebasic propertiesof thyristors in acircuit with a RLload. It wasfound that theoutput voltagelags behind the
input current,reaching zeroand becomingnegative beforethe input currentreaches zero. Adiode was
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introduced intothe circuit to stopthe outputvoltage goingnegative.
The triacexperimentresulted in a pairof thyristorsbeing used toconduct on bothhalf cycles of thepower supplyoutput. Aproblem wasencounteredwhere for smallfiring angles, thesecond thyristorwould attempt totrigger while thefirst was still on.
This was solvedusing multiplefiring pulses.
The last twoexperimentsanalysed thebridge rectifiercircuit, firstly thehalf controlled
and then thefully controlledvariations. Thehalf controlledrectifier providescontrol over thepositive voltageand clamps thenegative voltageat 0. The fullycontrolledrectifier providescontrol over boththe positive andnegative half cycles, allowingfor a variable DCoutput.
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