International Journal of Micro and Nano Electronics, Circuits and Systems, 3(1), 2011, pp. 61-66

ANALYSIS AND DESIGN OF EMBEDDED Z-SOURCEINVERTER FOR INDUCTION MOTOR DRIVES

P. Kannan1 & N. C. Lenin2

1EEE Department, St. Joseph’s College of Engineering, (e-mail: kannamuthu1987@gmail.com)2EEE Department, St. Joseph’s College of Engineering, (e-mail: nclenin@sify.com)

Abstract: This Project deals with an Embedded Z-source inverter to control the three phase induction motor.The Z-source inverters are recent topological options for buck–boost energy conversion with a number ofpossible voltage and current-type circuitries. The common advantage of Z-source inverter and EmbeddedZ-source inverter is the inclusion of an LC impedance network, placed between the dc input source andinverter bridge. The drawbacks of the conventional Z-source inverters are more harmonic, less reliable,more diode blocking voltage and capacitor size also high. By introducing Embedded Z-source inverters,the drawbacks of the conventional Z-source inverter can be eliminated and also gain produced is same asthat of Z-source inverter. It can produce smoother and smaller current or voltage maintained across the dcinput source without using any additional passive filter, less cost and low harmonic distortion. The projectis further carried to control the induction motor drive which is used in paper mills, textile mills and cementindustries etc.Keywords: Embedded Z-source inverters, motor drives, buck-boost, Z-source inverters.

1. INTRODUCTIONThe Inverter is the circuit, used to convert the directcurrent input signal into alternating current outputsignal. Traditionally there are three types ofinverters namely Voltage Source Inverter, CurrentSource Inverter and Z-source Inverter. In this thesis,the Z-Source Inverter is considered in place ofVoltage Source Inverter and Current SourceInverter, because the drawback of conventionalvoltage source and current source inverter can beeliminated by using Z-source inverter. The mainadvantages of the Z-source inverter are to improvethe voltage level of the inverter input voltage,reduce the voltage spikes and current spikes in theoutput. The efficiency of the inverter is highlyimproved through our project by means of theproposed Embedded Z-source inverter technique.The Z-source inverter, first proposed in [1] as shownin Figure 1, as the single-stage inverter topology todemonstrate both buck and boost power conversionability. Various Z-source topological options havebeen developed with either voltage- or current-typeconversion ability [1], [2]. Among them, the voltage-type inverters are more popular which are testedfor applications in motor drives [3]–[6] and fuel cell-[6]–[9] and photovoltaic (PV)- [9]–[11] powered

systems. The operation mode characteristics ofZ-source inverter have been developed [14] withsmall inductance and low power factor.Forcontrolling the Z-source inverters, many pulsewidth modulation schemes [12], [13] have also beenreported with some achieving a lower switching lossand others realizing an optimized harmonicperformance. The mathematical analysis of the Z-source inverter [17] is same as that of mathematicalmodel of the Embedded Z-source inverter.

2. VOLTAGE-TYPE Z-SOURCE INVERTERS

2.1. Z-source inverterThe voltage-type Z-source inverter is shown inFigure 1 where an X-shaped LC impedance networkis connected between the input dc source and three-phase inverter bridge. With the impedance networkadded, any two switches from the same phase-legcan now be turned on safely to introduce a shoot-through or short-circuit state with no surge incurrent observed, since all current paths in the dcfront-end are effectively limited by at least aninductive element (L1, L2, or both). In response tothe inserted shoot-through state, the Z-source invertercan then be proven to exhibit voltage-boosting

62 International Journal of Micro and Nano Electronics, Circuits and Systems

capability, whose corresponding gain expression isderived by considering the inverter-state equationsduring shoot-through and non shoot-through states,expressed by (1)–(4) with a balanced networkassumed (L1 = L2 = L and C1 = C2 = C). Note that forthe case of non-shoot-through state, it can representany of the six traditional active states (ii ≠ 0) or theremaining two null states (ii = 0), solely determinedby the modulation process.

2.2. Design of Z-source InverterThe conventional circuit consists of various modesof operation which include ordinary inverter modeoperation and also in this circuit can becharacterized mainly into two modes; there areshoot-through mode and non-shoot through mode.There is an inductive element placed along allcurrent paths in the dc front-end, the switches fromthe same phase-leg can, as usual, be turned onsimultaneously to introduce a shoot-through statewithout damaging semiconductor devices. In thismode capacitor can act as source of supply, theenergy stored in the capacitor can be dischargedthough inductor, so it charges more. The equivalentcircuit for this mode can be shown in the Figure 2.This circuit act as the two parallel LC circuitconnected in series, the resultant frequency isexpressed in the following manner.

1 =2

πf Hz

LC (8)Figure 1: Z-source Inverter

Shoot-Through (Sx = Sx‘ON, x = A, B, or C; D=

OFF)

VL =VC Vi = 0 Vd = 2VC VD = Vdc — 2VC (1)IL = — IC Ii = IL — IC Idc = 0. (2)

Non-shoot-Through (Sx ≠ Sx‘, x = A, B, or C; D= ON)

VL =Vdc — VC Vi = 2VC — Vdc Vd = Vdc VD = 0 (3)Idc = IL + IC Ii = IL — IC Idc≠ 0 (4)

Performing state-space averaging on (1) and (3)then results in the following expressions derived forthe capacitive voltage VC, peak dc-link voltage VP,and peak ac output voltage Vac.

o

o

1- /=1-2 /c dc

T TV VT T (5)

=1-2 /

dcp

o

VVT T (6)

=2P

ac

VV M (7)

where T0/T refers to the shoot-through ratio (T0/T <0.5) per switching period, M represents themodulation index used for traditional invertercontrol, and B = 1/(1 — 2T0/T ) is the boost factor.Clearly, the term enclosed by the parentheses in theexpression for Vac represents the output amplitudeproduced by a traditional VSI, which can be boostedby raising B above unity and adjusting Maccordingly.

Figure 2: Equivalent Circuit of Shoot Through Mode

The shoot-through time period is 0.3ms.Using(8), choose any one parameter and find out anotherparameter. Using (8) C=1000µF, and L=2.3µH.

3. VOLTAGE-TYPE EMBEDDED Z-SOURCEINVERTERS

3.1. Embedded Z-source InverterComparing with Figure 1, the voltage-type EZ-source inverter shown in Figure 3 has its dc sourcesembedded within the X- shaped LC impedancenetwork with its inductive elements L1 and L2 now,respectively, used for filtering the currents drawnfrom the two dc sources without using any externalLC filter. The switches from the same phase-leg can,as usual, be turned on simultaneously to introducea shoot-through state without damagingsemiconductor devices. The resulting equivalentcircuit is shown in Figure 4(a), where it is shown

Analysis and Design of Embedded Z-Source Inverter for Induction Motor Drives 63

Where the subscripts in (16) represent thenumberings of the respective VC expressions. Notingthat T0/T is always smaller than 0.5; the ratio in (16)is calculated to span from 0.5 to 1 as T0/T rises from0 to 0.5, inferring that the second advantageintroduced by embedding the sources is a significantreduction of the capacitor sizing (voltage rating).The reduction is as much as 50% under nominalcondition during which M is set close to unity (or1.15 if triplen offset is injected) and T0/T is kept small.

that when the inverter bridge is shot-through, thefront-end diode D is reverse biased with itsblocking-voltage expression and other stateequations written as follows.

Figure 3: Embedded Z-source Inverter

Shoot-Through (Sx = Sx‘ON, x = A, B, or C; D=

OFF)

VL =VC +Vdc /2 Vi = 0 Vd = VD = — 2VC (9)IL = — IC Ii = IL — IC Idc = 0 (10)

Non-shoot-Through (Sx ≠ Sx‘, x = A, B, or C; D = ON)

VL =Vdc /2— VC Vi = 2VC Vd = VD = 0 (11)Idc = IL + IC Ii = IL — IC Idc ≠ 0 (12)

Performing state-space averaging on (1) and (3)then results in the following expressions derived forthe capacitive voltage VC, peak dc-link voltage VP,and peak ac output voltage Vac.

/2=1-2 /

dcc

o

VVT T

(13)

=1-2 /

dcp

o

VVT T (14)

=2

Pac

VV M (15)

where (18), when compared with (7), clearly showsthat both Z- and Embedded Z-source invertersproduce the same transfer gain even though theEmbedded Z-source inverter has its dc sourcesembedded within the impedance network forachieving inherent filtering. Observing carefully, asecond advantage is also noted in (16) whencomparing its capacitive voltage VC with thatexpressed in (5). To be specific, VC in (13) is only afraction of that in (5) with their ratio mathematicallyexpressed as

c(16)

c(5)

1=2(1- )o

VTVT

(16)

Figure 4: Equivalent Circuit of Embedded Z-source Inverter(a) Shoot Through State (b) Non-shoot-throughState

3.2. Input-Source RequirementAlthough the Embedded Z-source inverter shownin Figure 3 uses two independent dc sources forproducing a balanced front-end impedancenetwork, in practice, it is not necessary for bothsources to be balanced. For the extreme case, one ofthe sources can in fact be omitted. The omission ofone source is in principle favourable to the industry,where locating a single source is definitely mucheasier. Relevant mathematical analysis andexperimental testing for the case of only a singlesource powering the Embedded Z-source inverterhave already been presented by the authors in [17],where it is generally concluded that a single sourceis sufficient, if unbalanced voltage drops across the

64 International Journal of Micro and Nano Electronics, Circuits and Systems

front-end passive LC elements are acceptable. Inaddition to [17], the same analysis can also be foundin [18] and [19], which in principle are independentresearch papers reporting on the same topic andprinted at about the same time in the sameconferences.

3.3. Experimental ResultsThe embedded inverters proposed in this paperwere verified experimentally using a hardwareplatform that could flexibly be configured to anydesired topology for testing. Upon completing thetests, most of the captured results were observed tobe the same, as proven conceptually in earliersections.

Figure 5(a), 5(b) clearly shows the boosting ofline-voltage in both Z-source inverter andEmbedded Z-source inverter. Figure 6(a), 6(b)clearly shows the boosting of output current in bothZ-source inverter and Embedded Z-source inverter.Figure 7(a),7(b) shows that line current harmonicdistortion of both Z-source inverter and EmbeddedZ-source inverter, which clearly implies the relationbetween percentage magnitude of fundamentalharmonics and harmonic order. These Z-source,Embedded Z-source inverter can be used for currenttype inverter also.

Figure 5(a): Experimental Line Voltage of Z-source Inverterwith T0/T = 0.3

Figure 5(b): Experimental Line Voltage of Embedded Z-sourceInverter with T0/T = 0.3

Therefore, to avoid excessive duplication, onlyresults for the EZ-source inverters are presentedhere for illustration purposes. With an EZ-sourcenetwork constructed using L =2.3µH, C = 1000 µF,and Vdc ≈ 20 V and connected to a two-level voltagetype inverter controlled by a micro controller, Figure5 shows the relevant waveforms obtained by settingthe relevant control parameters to M = 1.15 × 0.7and a shoot-through duration of T0/T = 0.3 addedto the inverter-state sequence.

Figure 6(a): Experimental Line Current of Z-source Inverterwith M = 1.15 × 0.7 and T0/T = 0.3

Figure 6(b): Experimental Line Voltage of Embedded Z-sourceInverter withM = 1.15 × 0.7 and T0/T = 0.3

Figure 7(a): Total Harmonic Distortion of Z-source Inverter

Analysis and Design of Embedded Z-Source Inverter for Induction Motor Drives 65

3.4. Hardware SetupFigure 8 clearly shows the hardware setup of theembedded Z-source inverter fed induction motordrives control. It consist of many sub modules

namely power supply circuit, Impedance network,Driver circuit, and Inverter. This impedancenetwork includes the direct current source, so thiscircuit named as Embedded Z Source inverter.

Figure 9(a), (b) shows that line and phase voltage waveform for these experimental setuprespectively.

Table 1Comparative Summary between z-source Inverter and

Embedded z-source InverterParameter Z-source Embedded

Inverter Z-SourceInverter

Line voltage 80 Volts 85 VoltsLine current 4 Amps 4 AmpsTotal harmonic distortion 7.43% 4.65%Diode blocking voltage 5 Volts 1.5 VoltsCapacitor voltage 30 Volts 15 Volts

The tabular summarization of comparativeevaluation between Z-source inverter and theEmbedded Z-source inverter is presented inTable 1.

4. CONCLUSIONThis paper has proposed a new family of EmbeddedZ-source inverters implemented using animpedance network with the relevant dc sourcesembedded within. Comparing with the Z-sourceinverters, the Embedded Z-source inverters have theadvantages of drawing a smoother current from thedc input sources without using external second-order filters and a lower required capacitive voltage.The testing of the inverters has been performedexperimentally with favourable results obtained,hence confirming the practicality of the newEmbedded Z-source inverters. Needless to say, theembedded concepts can also be applied to thecurrent type inverter also.

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Figure 7(b): Total Harmonic Distortion of Embedded Z-sourceInverter

Figure 8: Hardware Setup of Embedded Z-source Inverter

Figure 9(a): Experimental Line Voltage Waveform

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66 International Journal of Micro and Nano Electronics, Circuits and Systems

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