BASIC TOPOLOGIES OF DIRECT PWM AC CHOPPERS
Annals of the University of Craiova, Electrical Engineering series, No. 30, 2006BASIC TOPOLOGIES OF DIRECT PWM AC CHOPPERSDan FLORICĂU1, Mariana DUMITRESCU2, Ioan POPA3, Sergiu IVANOV31Politehnica University of Bucharest, Faculty of Electrical Engineering313 Spl. Independentei, Sect.6, Bucharest, e-mail: floricau@stat.conv.pub.roTel.: (40) 021 4029720, Fax : (40) 021 31810162Dunarea de Jos University of Galati,3University of CraiovaAbstract In this paper some basic topologies of directPWM AC choppers are presented. Compared with thephase-controlled AC controllers using thyristors thePWM AC Choppers have important advantages:sinusoidal current waveforms, better power factor,faster dynamics and smaller input/output filters. Theypresent high robustness, offer safe commutation andhave high efficiency. The paper describes operationalprinciples and analysis of three basic direct topologiesusing a very simple DC snubber consisting of acapacitor only with no discharging resistors. This DCsnubber is directly attached to power commutation cellsto absorb energy stored in stray line inductances. Thereare also shown some modern line conditioners made upof basic PWM AC choppers and a transformer forseries voltage compensation.Keywords: direct AC-AC converter, direct PWM ACchopper, line conditioner1. INTRODUCTIONThe recent increase in the use of nonlinear loads hascaused serious concern for power quality andconsequently on the disturbances tolerated bysensitive electronics loads. Currently, most of the lineconditioners still rely on thyristor technology (linecommutated ac controllers). The absorbed harmonicscurrent present high amplitudes and low frequencies.Due to the emplacement of these harmonics near thefundamental harmonic at the power supply system itis not recommended to use passive filters. Also thesize, weight and price of the passive elements couldbe high. Therefore, direct AC-AC converters withthyristors are placed outside of new electromagneticcompatibility (CEM) standards that limit the admittedperturbations in the power supply system [1]. More,the reactive power absorbed by these converterschanges at the same time with the delay controlangle. Such conditioners, however, have slowresponse and need large input-output filters to ers can be replaced by pulse widthmodulation (PWM) AC choppers, which have betteroverall performance and the above problems can beimproved if these controllers are designed to operatein the chopping mode. In this case, the input voltageis chopped into segments and the output voltage levelis decided by controlling the duty cycle of thechopper switching function.Before the 90s, PWM AC choppers were made by fourquadrants switches [2]-[4]. Their application wasdelayed due to the commutation problems. To assuresafe commutation without high voltage spikes, somePWM control principles were developed [3]-[4].However, converters robustness depended on thecontrol accuracy and kept high the risk of overvoltagesand overcurrents appearance.After 1990, different AC choppers topologies using areduce number of switches [5]-[6] or standardcommutation cells in two quadrants [7] weredeveloped. To increase the converters robustnesssimple DC snubbers consisting of a capacitor onlyattached directly to commutation cells were used [8][11].In this paper some basic topologies of direct PWM ACchoppers are comparatively studied (fig.1). These havenumerous advantages, such as: quasi-sinusoidalcurrent waveforms, better power factor, fasterdynamics and smaller input/output filters. PWM ACchoppers present high robustness, offer safecommutation and have high efficiency. The two basicAC topologies work similar with DC choppers, at aconstant duty cycle, having a better dynamic and alarge industrial use.C1uaS1S1c C2S2cS2iLua CiLS1cS2cS2aS1bor any other switching deviceFig.1 Basic single-phase AC PWM choppers:a differential topology; b non-differential topology.There are also shown some modern line conditionersmade up of basic PWM AC choppers and atransformer for series voltage compensation.141
Annals of the University of Craiova, Electrical Engineering series, No. 30, 20062. BASIC PWM AC CHOPPERS2.1. Single-phase topologyBy the supply mode, PWM AC Choppers areclassified in differential and non-differentialtopologies. Both structures are made by two invertercommutation cells with IGBTs bidirectional in currentand unidirectional in voltage. DC snubbers areattached directly to commutation cells to absorb theenergy stored in line stray inductance. These snubbershave a very simple structure, consisting of a capacitoronly with no need for discharging resistors. Indifferential topology (fig.2a) the S2 and S1 switches,the voltage source and the load are serial connected.By moving S2 switch between the voltage source andS1 switch non-differential topology is obtained (fig.2b)[12]-[13]. This second structure presents the neutralwire continuity advantage.us α ua(1)S1cωtDuring one switching cycle, basic PWM AC chopperspresent three possible operating modes: active mode,freewheeling mode and bypass mode.Active modeDuring this mode, the inductor current iL conductsthrough input and output side and provides outputenergy. The S1 and S2 switches are turned on and theinductor current iL conducts through S1 and the diodeacross S2 for iL 0 or S2 and the diode across S1 foriL 0, as shown in fig.4a.Freewheeling modeThis mode is complementary to the active mode.During this mode the switches S1c and S2c are turnedon so that the inductor current freewheels through theoutput side, as shown in fig.4b.Bypass modeThe bypass mode is imposed by non-linear regime ofpower devices. To avoid commutation problemsduring dead time, a specific control strategy for thistype of conversion was proposed [7]-[8]. Twoadditional switches are turned on for safe commutationin this mode. When the input voltage ua is positive, theswitches S2 and S2c are turned on for safecommutation. During the dead time the inductorcurrent iL conducts in the positive direction through theload, the S2c switch and the diode across S1c. Thenegative inductor current iL conducts through thevoltage source, the S2 switch and the diode across S1.Thus, a current path for the inductor current alwaysexists in every current direction during the bypassmode. The current paths in this mode, for ua 0, areshown in fig.4c. Table 1 presents the output voltagedepending on the switches control. The mainrestrictions for this control are the accuracy and speedfor detecting the voltage source sign.S2ωt2.2. Three-phase topologyC1uaWhen the sign of the voltage source is changed, theswitching pattern is reversed, S2 and S2c beingcomplementary PWM controlled with a constant dutyratio and S1 and S1c are fully turned on. In theseswitching patterns the current path always exitswhatever the inductor current direction. Since twoswitches are always turned on during the half period ofthe voltage source the switching loss is significantlyreduced. In the buck conversion topology the outputvoltage is proportional with the duty ratio:S1S2iLuaS1c C2S2cCusS1iLS1cS2cS2usbaFig.2 Basic single-phase PWM AC choppers, bucktype: a differential topology; b non-differentialtopology.Both converters have the same control, depending onthe voltage source ua sign. In this way, if ua is positive,S1 and S1c switches are PWM controlled with aconstant duty (α) ratio, while S2 and S2c switches arefully turned on (fig.3).uaπ2πωtS1ωtS2cωtusωtFig.3 Control strategy.Single-phase topologies structures can be extended forthree-phase structures. We are interested only in thecase of differential topology because of the number ofcommutation cells is two times smaller than the threephase non-differential topology [7], [14]. By adding athird commutation cell to the single-phase topology, adifferential three-phase PWM AC chopper is obtained(fig.5). A snubber made by only one capacitor isattached directly to each commutation cell. As in the142
Annals of the University of Craiova, Electrical Engineering series, No. 30, 2006single-phase topology, this structure presents theadvantage of using standard commutation cells in twoquadrants. The control can be realized by taking intoaccount the current or the voltage source sign.CuaS1S2iLuaS1c CCS1S2cS1cS2S2ciLdisadvantage is to determine the current’s sign near thepassing through zero.In the single-phase topology, the PWM AC chopperoperates like a unidirectional voltage chopper. In thesecond control (depending on the voltage source sign)the three-phase structure operates like two independentunidirectional voltage choppers. It means that the dutycycles can be different, which is an importantadvantage in comparison with the previous control.uaaC1S1S1cCuaS1ua CS1S2cS1cS2S2cuaiLS1uaS1c CS1S2cS1cS2S2ciLFig.4 Current paths for operating modes: a activemode; b freewheeling mode; c bypass mode.ua 0ua 0100111S1c001111S3S3cIn the buck PWM AC chopper for three-phasesystems, the switches with the smallest voltage sourceamong three voltage sources are fully turned on. Otherswitches are modulated with a constant duty ratio. Forexample, if the voltage of phase a is the smallest,switches S1 and S1c are fully turned on, while S2, S2cand S3, S3c are complementary switched with aconstant duty ratio. The operational modes are thesame with the single-phase buck AC chopper: activemode (fig.6a), freewheeling mode (fig.6b) and bypassmode.The imposed voltages across commutation cells arenot sinusoidal, as shown in fig.7. For this reason, thecurrents through the snubber capacitors presentimportant discontinuities. The capacitors size has to besmall enough to limit their currents, so that theharmonic content does not influence the input filter.cS1S2cC3Fig.5 Three-phase differential AC chopper.S2 CS2 iLbCC2uc S2iLS1cCub S2111100S2c111001usiL 0ua00uaua03. MODERN LINE CONDITIONERSiL 0uaua0ua00Tab.1 Output voltage depending on control.The first control, depending on the current’s sing, isrealized by having identical duty cycles. In this caseonly active and freewheeling operation modes areallowed. As a result, there is no indirect energy changesimilar to a bypass operating mode. AnotherMany factors, such as energy sources, supply lines andloads, disturb the distribution network voltage quality.The energy producer has to maintain at standard valuesthe voltage amplitude and frequency. However,switching in voltage sources, natural phenomena(thunderbolts) or accidental phenomena (broken line)lead to modify the voltage quality, by generatingimportant and unforeseeable disturbances. Also, highloads variations produce more or less local voltagesource changes. The use of power electronic inindustrial and household applications generatesharmonic pollution to the power supply system:harmonic currents absorbed by the nonlinear loadsmodify the voltage through the input impedance.143
Annals of the University of Craiova, Electrical Engineering series, No. 30, 2006uaS1C1ub C2S1cuc S2C3S2cLine conditioners have been used in industry toprovide and to protect sensitive loads. There is a largevariety of line conditioners which absorb one or moretypes of disturbances, like line conditioners with orwithout energy storage. Line conditioners with energystorage, for example uninterruptible power supply(UPS), protect the equipments against a high numberof disturbances, including short disconnections whichare the most expensive. The line conditioner’s withoutenergy storage category includes compensators, whichstabilize the voltage source by reactive power controland active filters. In the same category are includedline conditioners made by basic AC choppers. Theywork at high commutation frequency and allow a fastand continue output voltage control. There are directand indirect types of line conditioners without energystorage. In the direct type case one or more ACchoppers are connected between the voltage sourceand the load (fig.8a). The indirect type is made up of aPWM AC chopper and a transformer for series voltagecompensation (fig.8b). S3S3cauaS1C1S1cub C2S2S2cuc C3 S3S3cSensitiveelectronicloadBasic PWMAC chopperPowersupplyabFig.6 Operational modes for three-phase differentialAC chopper, buck type: a active mode; b nicloadBasic PWMAC chopperbFig.8 Modern line conditioners: a direct topology;b indirect topology.abFig.7 Cells 1 and 2 voltages: a single-phase PWM ACchopper; b three-phase PWM AC Chopper.In the case of direct line conditioners differential ACchoppers topologies are preferred, while indirect lineconditioners use differential or non-differential ACchoppers topologies. The serial voltage (uc) given bythe transformer is controlled by AC chopper in orderto compensate the voltage source fluctuations. As aresult, a constant load voltage with the indirect lineconditioner is obtained. The serial voltage (uc) is onlya small part of the voltage source, which allows theline conditioner to absorb a reduced power. The ACchopper output voltage is in phase with the voltagesource. The relative sign between the primary and thesecondary of the series transformer (indicated byblackened points) allows to increase or to decrease thevoltage delivered to the load ( u s u r u c ), dependingon the connection mode (fig.9). To reduce the PWMAC chopper absorbed power the use of bucktopologies is recommended.144
Annals of the University of Craiova, Electrical Engineering series, No. 30, 2006injected through a transformer with the voltage source.The voltage delivered to the load can be obtained as afunction of the voltage source ur and the AC choppersinput voltages (ua1, ua2) by taking into account theirduty cycles (α1, α2):uciLur BasicPWM ACchopperuausu s u r m (α 1 u a1 α 2 u a 2 )ucamucur iLur BasicPWM ACchopperuaBasicPWM ACchopperusuacuciL us1 us2us4. CONCLUSIONSiLuriLPWM ACchopper non- ua2differential 2Fig.10 Line conditioner with voltage up/downcapability.uc ua1PWM ACchopper nondifferential 1usbur(2)BasicPWM ACchopperuausdFig.9 Line conditioners without energy storage:a boost circuit with power supply from input side;b buck circuit with power supply from input side;c boost circuit with power supply from output side;d buck circuit with power supply from output side.To allow the compensation of voltage variations inboth directions a serial boost line conditioner with abuck one are associated. There are many possibilitiesto associate basic AC choppers with serialtransformers. Fig. 10 presents a line conditioner withvoltage up/down capability made by two nondifferential AC choppers [14]. The first structuredelivers the voltage us1, which have an increasevoltage effect, while the second one delivers thevoltage us2 with a decrease voltage effect. As adifference between the two previous voltages (us1, us2)a compensation voltage is obtained, which is serialIn this paper basic PWM AC choppers made by twoquadrants commutation cells were presented. Themost simple control principle to implement is the onebased on the voltage source sign. More, it no needsdevices to limit the current, which reduce losses.Structures based on two quadrants commutation cellsare more robust than other based on four segmentsswitches. Their devices components have notcommutation voltage spikes due to the simplesnubbers attached directly to each commutation cell.The non-differential AC chopper has the samerobstness like the differential one and more, itpresents the advantage of neutral wire continuity. Thethree phase differential topology is made only bythree commutation cells representing an importantadvantage.In accordance with standard CEM, the phase controlsmust be replaced by modern structures like the onespresented in this paper. PWM AC choppers deliverquasi-sinusoidal voltages. As a result, these modernconverters can be used in a bigger number ofapplications. At an equal apparent power a PWM ACchopper is more expensive than a transformer. Inorder to reduce the converter’s apparent power and toincrease the voltage supply quality the paperpresented some modern line conditioners made up ofbasic PWM AC choppers.References[1] IEEE Recommended Practice for Emergency andStandby Power Systems for Industrial andCommercial Applications, IEEE Standard 4461987 (Orange Book), 1987.[2] M.Venturini,A.Alesina,Thegeneralizedtransformer: a new bi-directional sinusoïdal145
Annals of the University of Craiova, Electrical Engineering series, No. 30, 2006waveform frequency converter with continuouslyadjustable input power factor, Proceedings IEEEPower Electronics Specialists Conference, 1980,pp.242-252.[3] N.Burany, Safe Control of Four-QuadrantSwitches, Conference Record IEEE IAS Annualmeeting, Vol.1, 1989, pp.1190-1194.[4] L.Zhang, C.Watthanasarn, W.Shepherd, Analysisand comparison of control techniques for AC-ACmatrix converters, IEE Proc.-Electr. Power Appl.,Vol. 145, No.4, July 1998, pp.85-91.[5] D.Vincenti, J.Hua, P.Ziogas, Design andimplementation of a 25-kVA three-phase PWMac-ac line conditioner, IEEE Trans.PowerElectron., vol.9, July 1994, pp.384-389.[6] F.Z.Peng, L.Chen, F.Ahang, Simple Topologiesof PWM AC-AC Converters, IEEE Trans.PowerElectron. Letters, Vol.1, No.1, March 2003,pp.10-13.[7] B.H.Kwon, B.D.Min, J.H.Kim, Novel topologiesof AC choppers, IEE Proc.-Electr. Power Appl.Vol.143, No.4 July 1996, pp.323-329.[8] B.H.Kwon,B.D.Min,J.H.Kim,Novelcommutation technique of AC-AC converters, IEEProc.-Electr. Power Appl. Vol.145, No.4, July1998, pp.295-300.[9] J.-H.Youm, B.-H. Kwon, Switching Technique forCurrent-Controlled AC-to-AC converters, IEEETransaction on Industrial Electronics, Vol.46,No.2, pp.309-317, April 1999, pp.85-91.[10]B.H.Kwon, J.H.Youm, J.H.Choi, Automaticvoltage regulator with fast dynamic speed, IEEProc.-Electr. Power Appl. Vol.146, No.2, March1999, pp.201-207.[11]D.Floricau, B.Dagues, M.Fadel, J.C.Hapiot,M.Dumitrescu, PWM AC Choppers for highvoltage applications, ICATE, Baile Herculane,2004, pp.200-205.[12]E.Lefeuvre, T.Meynard, P.Viarouge, Robust twolevel and multilevel PWM AC Chopper, EPEGraz, Proc.CD, 2001.[13]D.Floricau, J.C.Hapiot, Novel Topologies ofDPWM multi-cells AC Choppers, 11th NationalConference on Electrical Drives, Galati, 2002,pp.73-78.[14]E.Lefeuvre, Convertisseurs alternatif-alternatifdirects à base de cellules de commutation deuxquadrants, Thèse de docteur ingénieur, INPToulouse, 2001.146
BASIC TOPOLOGIES OF DIRECT PWM AC CHOPPERS Dan FLORICĂU1, Mariana DUMITRESCU2, Ioan POPA3, Sergiu IVANOV3 1Politehnica University of Bucharest, Faculty of Electrical Engineering 313 Spl. Independentei, Sect.6, Bucharest, e-mail: floricau@stat.conv.pub.ro Tel.: (40) 021 4029720, Fax : (40) 021 3181016
Fan speed response to PWM control signal The fan‘s speed scales broadly linear with the duty-cycle of the PWM signal between maximum speed at 100% PWM and the specified minimum speed at 20% PWM (see www.noctua.at for individual fan specifications): For example, the NF-A12x25 PWM has a maximum speed of 2000r
,minimum ripple current PWM, Space vector PWM (SVM),Random PWM, hysteresis band current control PWM, Sinusoidal PWM with instantaneous current control etc.PWM feed forward or feedback methods, carrier or non carrier based control methods etc[1-3]. In [8] digital simulation of sinusoidal pulse width modulation presented, at rated
The PWM input wire (orange) and the feedback wire (white) for each servo will be attached to the Arduino pins as follows: Servo One - PWM - Digital 8, Feedback - Analog 1 Servo Two - PWM - Digital 9, Feedback - Analog 2 Servo Three - PWM - Digital 10, Feedback - Analog 3 Servo Four - PWM - Digital 11,
sinusoidal PWM and space Vector PWM. With the development of DSPs, space-vector modulation (SVM) has become one of the most important PWM methods for three-phase voltage source inverters. In this technique, Space-vector concept is used to compute the duty cycle of the switches. It is simply the digital implementation of PWM modulators.
compensation topology. The classical compensation topologies include series-series, series-parallel, parallel-series and parallel-parallel connected capacitors and inductors. There are also other topologies such as LLC. The objective of this thesis is to compare the four classical topologies in terms of their size and performance.
For the state of the art topologies, it almost reached the limit along this path with current technology. Switching loss and wide input range put lot of burden on these topologies, which prevented these topologies from increasing switching frequency and reaching higher efficiency. Several techniques will be developed to improve the state of the art
Figure 2: Schematic of the digital current to PWM converter B. Analog Current to PWM Converter: The analog current to PWM converter in Fig. 3 consists of a capacitive TIA, a comparator, and digital logics. It is also operated by a clock. When the clock is low, the conversion operation is performed and when th
Agile software development refers to a group of software development methodologies based on iterative development, where requirements and solutions evolve through collaboration between self-organizing cross-functional teams. The term was coined in 2001 when the Agile Manifesto was formulated. Different types of agile management methodologies can be employed such as Extreme Programming, Feature .
