An Observer-based DTC of Induction Motors Driven by 3-Level Inverter for Improving Low Speed Operation

7/27/2019 An Observer-based DTC of Induction Motors Driven by 3-Level Inverter for Improving Low Speed Operation

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AN OBSERVER-BASED DTC OF INDUCTION MOTORS DRIVEN BY 3-LEVEL INVERTER FO RIMPROVING LOW SPEED OPERATION

K. B. Lee*, J. H. Song**, I. Choy**, and J. Y. Choi**

* Korea Univ., KOREA** ISCRC, KIST, KOREAABSTRACT Direct torque control algorithm for 3-levelinverter-fed induction motors is proposed. Conve ntionalselection method of the stator voltage vector showsproblems of stator flux drooping phenomenon andundesirable torque control appeared especially at the lowspeed operation. To overcome these problems, aproposed method uses intermediate voltage vectors,which are inherently generated in 3-level inverters. Anadaptive observer is also employed to estimate somestate-variables and motor parame ters, which takes a deepeffect on the performance of the low speed operation.Simulation and experiment results verify effectiveness ofthe proposed algorithm.INTRODUTION

Direct torque controlled (DTC) induction motors havebeen widely used in many industrial application areas.The DTC method gives attractive performances in termsof fast torque response, simple control scheme, androbustness against the motor parameter variation. TheDTC can be implemented with no speed sensor equippedto induction motors. The DTC algorithm has beenextended into the high power motor drives from smallpower ratings [1-21.

As for 2-level inverter to drive induction motors,several types of respective DTC methods can be found innumerous literatures. On the contrary, research results onDTC schemes for 3-level inverter are hardly reported.One of distinct features, which are especially shown inhigh-power 3-level inverter applications, is that theinverter switching frequency is generally limited belowat most IkHz, considering that higher switchingfrequency causes heavier cooling apparatus for sw itchingsemiconductors.This paper pays major attention on the low speedoperation characteristics of induction motors driven by 3-level inverters. A simple hysteresis control scheme for 3-level DTC inverters is studied and a new m ethod to copewith the stator flux-drooping problem appeared in thelow speed operation region is proposed. In order toresolve this flux-drooping problem in case of 2-levelinverter system, several methods such as a variableswitching sector and an improved look-up table are used[3,4]. Also, an adaptive full-order observer [5] isemployed to obtain good DTC performance based on theestimation of the stator flux, the rotor speed, and thestator resistance over the wide op eration region.

DTC FOR THR EE LEVEL INVERTER

In this section, the principle of DTC is brieflydiscussed and a torque co ntrol for 3-level inverter systemis studied. Figure 1shows sp ace vector representation of3-level inverter output voltages, in which subscripts z, h,i, f denote zero, half, intermediate, and full voltages,respectively. Since 27 possible choices for switchingvoltage vector selection exist in the 3-level inverters,appropriate selection of the inverter switching voltagevector is more complex than in the 2-level inverters.

Figure 1 Output voltage vectors of 3-level inverterFrom induction motor equations, neglecting thevoltage drop across the stator resistance, relationshipbetween the inverter output voltage vector and thevariation of the stator flux ca n be expressed as

-A A , = (V, - Cs R , ) sp = v , ~sp,where, Y, : he inverter output voltage vector,

t,s,,: ampling period.Equation (1) shows that an applied stator voltagevector produces a stator flux change. The amplitude ofthe stator flux change is proportional to the product ofthe applied voltage vector and the sampling period. Thevectorial direction of the stator flux change keeps thesame as hat of the selected voltage vector.The stator and rotor fluxes are written as the followingrelation

Power Electronics and Variable Speed Drives,18-19 September2000, ConferencePublication No. 475 Q IEE 2000170


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L 'L, 'L,where, (T= -

The electrom agnetic torque can be expresse d as

Equation. (3) show s that torque is determined by thestator flux m agnitude and the stator flux phase an gle withrespect to the rotor flux.Assumed that the stator flux vector is in the k-th sector,selection of the respective stator voltage vector isdescribed in Figure 1. The selection of yk+I, f ,h an dvk+ , , f is able to increase the angle between the statorflux and the rotor flux. As a result, developed torque canbe increased by the a pplication of ?k+?,/h or ? k + ] , f h . Itcan be seen in Figure 1that the stator flux is increased bythe selection of yk+] , f ,h , and decreased by c k + r , f , h . Ifhalf voltage vector is selected, the lower slope of torquecan be obtained.

Upper TorqueBand

LonerTorqueBand

Rd c r c ~ c v d u c

-LonerTorque Band

-Upper Torque Band - - L ew I TorqueFlippk- Lod Torque RipplcFigure 2: Torque slope pattern of 3-level inverterThe double torque hysteresis band method as shown inFigure 2 is applied for 3-level inverters. When torquecomes down to the negative ,upper hystersis band,appropriate full voltage vector is chosen to increase thetorque developed. When the controlled torque reachesthe positive lower hysteresis band, the full voltage vectoris changed into the respective half voltage. If torqueincreases over the positive upper torque band, zerovoltage vector is applied to decrease torque value. Thesame rules for voltage vector selection can be applied forthe reverse direction operation. The resulted switchinglook-up table is shown in Table I .Assumed that ds is located in the k-th sector, possiblevoltage vectors to increase torque are . ; (+I, , , i i k + ] , h ,c k + l J , and ? k + I , h . Among these voltage vectors, acertain voltage vector should be se lected, considering thedouble torque hysteresis band and the stator fluxcondition. Meanwhile, only one voltage vector, ?,, isselected to reduce the developed torque. The simulationresults of the forw ard and reverse op eration are shown infigure 3.

TABLE 1- Switching look-up tableTorque

in sector kFlux L

Torquein sector k

' k + l . h ' k + I , J v:Flux - - -L V k + . ? , h v k + 2 .f VI70

50

9.985 0.986 0.987 0.988 0.989 0.99

. 60i- IO

E-12.51 , ,%98 0.988 0.996 1.004 1.012 1.02

TIne

(a) Forward direction operation-30

," -50,O -60.? -10

-7&! 940.? 12.5y -152-42.58.98 0.388 0.396 1.001 1.012 1.02-7 rlne

(b) Reverse direction operationFigure 3:Torque control

To show effectiveness of 3-level DTC algorithmdescribed above, some simulations have been performed.The parameters of the induction motor used are listed inTable 3. In this case, speed estimation is carried out byan equation-based estimator, which is described in thefollowing equation,

It can be seen in Figure 4 that the DTC switchingmethod above-mentioned causes the stator fluxdemag netization phenom enon at the low speed operation.As seen in Figure 1 , around the boundary between twosectors, there is no effective voltage vector that canassure an increased stator flux, in which the rotatingstator flux vector moves its position from one sector toanother sector. Another problem that deterioratesperformance of the low speed operation is selection ofthe zero-voltage vector. At low speed region, the zero-voltage can not effectively assure control of torquereduction because the resultant phase angle reduction isnot easily obtained due to the low value of the stator fluxfrequency. The basic switching method of 3-level DTChas to be modified to overcome these problems at thelow speed operation.

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7 5 0 7 - I

Figure 4: Low speed performance o f the DTC; (from topto bottom)speed, flux magnitide, d-axis flux, a nd phasecurrent

SWITCHING METHOD FO R LOW SPEEDOPERATION

As mentioned in previous section, problems of thebasic DTC algorithm are stator flux drooping and theapplication of the zero-voltage vector, especially at thelow speed region. The demagnetization effect is chieflycaused by the non-linearity of the induction machine.This problem is appea red just after the position of thestator flux vector changes from one sector to anothersector. The application of the zero-voltage vector alsodeteriorates performance at the low speed operation. Thisphenomenon is easily comprehended from equations(2),(3). To overcome this problem, the reverse voltagevector is applied to decrease torque instead of zero-voltage vector. The rapid reduction of torque at the lowspeed region can be obtained by this method, but it maycause the increase of the switching frequency and th etorque ripple.The basic sw itching look-up table can not solve theseproblems that are occurred in the low speed operation.Even if the demagnetization problems can be w orked outby a rotation of reference frame or the switching sectorthat are similar to methods for 2-level inverters, thephenomenon caused by the application of the zero-voltage vector can not be easily answered. Therefore, tosolve these problems, a newly modified switching look-up table has to be a pplied in the low speed operation. In3-level inverters, intermediate voltage vectors can givepossibility to resolve the demagnetization problem,without considering the reference frame rotation, or theswitching sector rotation adopted in the 2-level. As seenin Figure 5 , the switching sectors are divided into 12sectors to choose the stator voltage vector precisely. Thek-th sector is subdivided into the lower sub-sector andthe upper sub-sector, each of which has the width of 30".In the lowe r sub-sector, the stator vo ltage vector, i$(+, f ,which is determined according to Table I , becomes to beineffective to boost the stator flux. If the intermediate

voltage vector, vk,;, is selected instead of Vk+,,, , thedemagnetization problem is resolved. To obtaineffectively magnetizing effect around the boundarybetween two sectors, especially at the low speed region,modified look-up table as shown in Table 2 is used. Thetransition from conventional look-up table to modifiedone is taken by detecting a certain level of the droopingmagnitude of the stator flux. When the magnitude of thestator flux is drooped below minimum fixed level, thebasic look-up table, Table 1, is replaced with themodified look-up table, Table 2.

k-th sector

k-th ower subsectorIFigure 5 : Switching method for low speed operation

TABLE 2 - Modified Look-up TableI I Toraue I

$ 12.50 6.25:: 00.5 0.6 1.1 1.4 1.79 0.457 0.32 0.15

q-~ , , 18.5 0.8 1.1 l .+ 1.7 2T2 n d . l

Figure 6: Improved performance in low speed ;(from opto bottom)speed, flux magnitude, torque, and sw itchingfrequency

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Advanced control scheme

.. .r -y r i 1." > ........................................... I. . . j i : I , I I I ; 1 -r h l l CO I I I p8m ID I :..................... V4e v

Double torque hysteresis eaku la t ia icomparator and nuxcomparator

..............................Adaptive observer

Figure 7: Schem atic diagram of the proposed control strategy

If the stator flux moves into the upper sub-sector, theappropriate voltage vector is determined according toTable 2. This selection is basically same as Table 1,except the reve rse voltage vector is chosen instead of thezero voltage vector for torque control.Figure 6 shows the low speed operation of the DTCusing the advanced look-up table in the speed regionabout 1 -%. rated speed. It is shown in this figure thatproblems of the demagnetization and the zero-voltagevector work out effectively even without the increase ofthe switching frequency. The performance of theproposed control scheme indicates that it is feasible forthe low speed operation.

6.25 1 h8 0. 2 1.2 2.2 3.2 4.2 5.2

; 0 . 1 5 / , , , , j0. 2 1.2 2.2 3.2 4.2 5.2rI $1

Lb.2 1.2 2.2 3.2 1.2 5.21,nd SIFigure 8: Equation-based DTC ag ainst stator resistancechange( 150[%]); (from top to bottom)speed, fluxmagnitude, torque, and stator resistance

---l---1l

ADAPTIVE OBSERVER-BASED CONTR OLThe overall control scheme is shown in Figure 7,which consists of speed controller, torque and fluxcomparators, advanced look-up table, adaptive observer,and 3-level inverter.Figure 8 shows the DTC operation with the modifiedlook-up table applied in the low speed region when thevariation of the stator resistance(150%) is considered.Estimation performance makes worse owing to the statorresistance variations, which may deteriorate the driveperformance by consequently introducing errors inestimating the stator flux, speed, and electromagnetictorque. This is critical at the low speed region in

particular, because the voltage drop across the statorresistance constitutes significant proportion of theapplied voltages. To get robust performance against thestator resistance variation, especially in the low-speedoperation, adaptive observer-based DTC is used asshown in Figure 7.

SIMULATION AND EXPERIMENT RESULTSTo confirm validity of the proposed co ntrol algorithm,simulation and experiment have been carried out. Theperformance of the low spe ed operation is focused on thesimulation and experiment.The following simulation results have obtained byusing Boland C". All simulation results are executedwith no load condition and same sampling frequency(12O[p]) . Figure 9 shows several responses ofobserver-based DTC in the low speed region using theadvanced look-up table even though the stator resistanceincreases up to 150ph] rated value. The proposed DTCscheme shows good responses against the parameter

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variation, whereas the eq uation-based estimator is easilydeteriorated by stator resistance variation.The experimental set-up of the proposed controlsystem is shown in Figure 10. It consists of 7.4[kW]-induction motor, power circuit, main controller boardDS 1003 containing TMS320C40, I/O board DS400 1,and A/D board DS2001. Experiment is executed with noload condition and same sampling frequency (120[ps 3).Figure 11 shows speed estimation and phase currentresponses of the proposed DTC scheme in the forwardand reverse operation. Figure 12 shows speed and torqueestimation in the low speed operation. Good estimationof the speed is achieved down to about 1% rated speed.Figure I3 shows the stator resistance estimation value inusing adaptive scheme.

2;q i:: 8. 2 1.2 2.2 3.2 4.2 5.2; 1 5 1 , , , I 1

8. 2 1.2 2.2 1l"PISl 3.2 4.2 5.2-2 50 I I

I3.2 4.2 5.2r ~ r s i2.2

z o ; ~ ~ ~20.075

8 . 2 1.2 2.2 3.2 4.2 5.21 I " d I lFigure 9: Observer-base d DTC a gainst stator resistancechange (150[%]); (from top to bottom) speed, fluxmagnitude, torque estimation, stator resistance variation

Figure 10: Experime ntal set-up

TABLE 3 -Induction motor parame tersI Rated Dower I 7.5 [kWl

Rated voltage 220 [VI

Rated velocity 1740 [rpm]I Rated torque 1 40 " m l

I Mutual inductance I 0.02456 [HIRotor inductance I 0.02277 [HI

Figure 11 Observer-based DTC, forward and reverseoperation, experimental results; (from top to bottem)speed, phase current

1 1 5s 2

Figure 12: Observer-based DTC, e xperimental results ;(from top to bottom) estimated speed, estimated torque

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Figure 13: Stator resistance estimation

CONCLUSIONAdvanced DTC algorithms for high power 3-levelinverters are presented. The torque and flux controlmethod for 3- level inverter is studied. The

dem agnetiza tion effect and the applica tion of the zero-voltage vector deteriora te the performance at the lowspeed operation. To overcome these problems, theadva nced look-up table, in which interm ediate voltagevectors are used, is proposed. Adaptive observer isemployed to obtain good performa nce even against thema chine parameter variation.

REFERENCE1. G. Buja, D. Casadei, and G. Serra, 1998, IECON98,2. Jam es N. Nash, 1997, IEEE T rans. Ind. Appli., Vol. 33 ,3. CG Mei, SK Panda, JX Xu, an d KW Lim, 1999,4. D. Alfonso, G. Gianluca, M. Ignazio, and P. Aldo,5.H. uboda, K. Matsuse, and T. Nagano, 1993, I EEE

T50-T64NO.2, pp. 333-341PEDS99, pp. 80-851999, EPE99ms. nd. Appli., Vol. 29 , No . 2, pp. 344-348

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An Observer-based DTC of Induction Motors Driven by 3-Level Inverter for Improving Low Speed Operation