Industrial Electronics Laboratory Technical Note

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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



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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Industrial Electronics Laboratory Technical Note