Controlled Multi-motor Drives85

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SPEEDAM 2006International Symposium on Power Electronics,Electrical Drives, Automation and Motion

1-4244-0194-1/06/$20.00 ©2006 IEEE

Abstract: Controlled multi-motor drives are used in a great number of industrial plants. The functioning of large and complex systems such as ironworks, paper mills, excavators, etc. depends on the performance of thecontrolled multi-motor drives. A systematic survey of multi-motor drives based on the method of coupling and mutual influence is presented in the paper. Following that, an overview of the appropriate control algorithms, sorted by the principle of operation and the means of realization, is given. The paper is illustrated with selected applications of controlled multi-motor drives from theauthor’s practice.

Key words: Controlled drive, speed synchronization,load distribution, load sharing

1. INTRODUCTION

The term multi-motor drive is used to describe all thedrives in a technological process. If the controlledoperation of the drives is required by the process, based

on the controlled speed of the individual drives, the

expression controlled multi-motor drives is adequate. For the great deal of such drives, the mechanical coupling onthe load side is typical. The load torque component, that isthe consequence of the coupling between the drives withindexes i and i+1, is, in general, represented by thefollowing:

)()( 111,, +++ −+−= iieii pii s K K m θ θ ω ω (1)

K p is the viscous damping coefficient, K e is the saliency

coefficient of the coupling material, and ω and θ are thecorresponding angular speed and angle of the motor shaft.The practical values of the coefficients vary in a very broad range, starting from zero; therefore, diverse cases of coupling exist. In general, the practical cases may be

characterized as follows:

a) Drives with rigid coupling K p≈0 K e→∞;

b) Drives with resilient coupling, K p≈0 K e≠0;

c) Drives with viscous damping coupling, K p≠0 K e≈0;

d) Mech. uncoupled drives. K p≈0 i K e≈0.The coupling of the multi-motor drives in the process,

dictates the coordinated control. The necessity for suchcontrol is imposed for two reasons; the first comes fromthe fact that the drives are mechanically coupled, and thesecond is the consequence of the process / technological

requirements for the multi-motor drive.

The structure of the control algorithm for the multi-motor drive is determined by the above reasons, e.g. bythe dominant component of the coupling load torque, or

by the reasons given by the process. Sometimes, thefeatures of the selected equipment can significantlyinfluence the implementation of the control algorithm.

2. MULTIMOTOR DRIVES STRUCTURES

According to the former classification of the multi-motor drives, this section presents a survey of the typical

drive configurations, accompanied with the requirementsthat need to be fulfilled by the control subsystem, tosuccessfully realize the multi-motor drive.

2.1. Drives with rigid coupling

With this type of drives, the coupling of the individualmotors is by the mechanical transmission devices, and isusually unbreakable. The coupled motors have the samespeed, or the speeds may be different, but in a fixed ratio,

predetermined by the mechanical gear-box. Theconfiguration may be found in the very high power drives,where, given the technical or economic circumstances, itwas not possible to use the single motor drive. The

examples are the big press rolling mills, where the limitedspace for the motor rules out the single big motor,therefore, a pair of two smaller motors is used. Another

reason for selection of this configuration is the plannedincrease in the capacity of the ironworks. In the initial phase of construction a single motor is placed, after that,in the second phase, the second drive motor is added [1].

The second distinctive example for the drives withrigid coupling is the slewing drive of the excavator

superstructure, used in open cast mines. Thesuperstructure is supported by the base of the excavator,over the horizontal axial ball bearing, with the radius of 10 to 20m, depending on the size of the excavator. The

slewing of the superstructure about the vertical axis is provided over the system of gears. The big gear-wheel(with the diameter similar to the diameter of the bearing)is located on the platform and the small gear-wheels arefitted on the shaft of the drive motors, or the shaft of theappropriate gear-boxes. The concept of the slewing driveis illustrated in Fig. 1.

In order to keep the superstructure of the excavator

vertical, the drive must be realized with two or three drivemotors, positioned evenly on the circumference of the biggear wheel [2].

The belt drives, web and felt drives in the paper machines, also belong to this group, if the length demands

the use of more then one drive motors.In the above or similar drives, only one speed regulator

is sufficient, and only one speed sensor is needed to fulfill

CONTROLLED MULTI-MOTOR DRIVES

Prof. Borislav Jefteniü, PhD, Milan Bebiü, MScEE, Saša Štatkiü* MScEEUniversity of Belgrade, Faculty of Electrical EngineeringBulevar Kralja Aleksandra 73 11000 Beograd, Serbia and Montenegro

*University of Kosovska Mitrovica, Faculty of Technical Sciences

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the speed control. However, special care must be taken tocoordinate the load distribution between the coupleddrives. The technique used is determined by the type of the motor and the converter used.

Fig. 1. The excavator superstructure slewing drive

2.2. Drives with resilient coupling

The drives in resilient connection are the drivescoupled by extremely long shafts, chains or belts, wheretwisting and elongation becomes significant. On the other

hand, from the practical point of view, much moreinteresting are the drives where the mechanical couplingis formed over the material being processed, tapes, pipes,tubes, stripes elongated in hot rolling mills, slabs inironworks, steel plates in cold rolling mills, paper or board in paper machines. All this drives require the

accurate speed control. The coupling in the former groupis unbreakable due to the technology. In the later group,while in normal operation, the drives are coupled, if thematerial being processed is “loaded”. The coupling doesnot exist if the material is “not loaded”. In any case, thedrives must keep the set speed ratio. In the first case, withthe material loaded, the processing of the material

determines the ratio, and in the second case, the drivemust be ready for the loading of the material, duringwhich the deformation of the material is unwanted.

Fig.2. The foundry drive

The basic layout of the drives in the vertical foundryfor outpouring of slabs in the steel works is presented inFig. 2 [4].

In the finishing sections of the paper machine, with the paper already dry and firm, the drives are in resilientcoupling by the sheet. The principal layout of the drivesin this phase of the paper making is displayed in Fig. 3 [4,5, and 7].

Fig. 3. The drives in the dry section of the paper machine

All the drives with resilient coupling have individualspeed control loop, controller and a speed sensor. Thedrives coupled by the material, have an additional reasonfor the separate speed regulator, namely the synchronized

operation before and after the “loading” of the material. Itshould be emphasized that these drives, due to theelasticity of the coupling material, have a cross-coupling

forces, torques in the material, given by Equation (1). The presence of this cross-coupling gives rise to the problemsof load distribution [4, 7], hence one drive usually takes

over the load of the other, sending it to breaking region,making the load distribution drastically violated. Theforce F in the material is given by (2):

[ ])()( 2211 meme mmmm K F −−−= (2)

In Equation (2), me is the motor torque, mm is the loadtorque of the drive in uncoupled state, and K is thecorrelation coefficient.

This mode of operation is especially significant in thedrives coupled by the material being processed, namely, if

the force F exceeds the required, permitted values, a plastic deformation, or in extreme cases, rupture of thematerial occurs. To overcome this problem, an additionalregulation of the force is necessary, either with themeasurement of the force magnitude, or indirectly, by the

regulator of the load distribution [4, 7].

2.3. Drives with viscous damping coupling

The drives with viscous damping coupling areconnected by the material being processed, which is plastically deformed during processing. The examples arerolling drives in hot rolling mills, where the processing isdone by pressing, as opposed to stretching, or the drives

in the paper machine in the early section where the sheetis still wet. To avoid the longitudinal strain of thematerial, with drives in the mentioned technological processes, the speeds of the drives should be in the exact predetermined ratios, that is, the line speed of the materialshould be equal, or slightly different, to compensate for the stretching of the material due to the pressing. If the

configuration permits, to totally avoid longitudinal strain,the material may form the loop between the drives, a wellknown solution in engineering practice. An early, formingsection of the paper machine, where the drying isaccomplished by pressing is shown in Fig. 5. In the early phase the sheet is very wet and soft, having no tolerance

for longitudinal strain, therefore, the synchronization of the drives deserves great attention. Motors M1 and M2 are

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in rigid connection over the colander that forms the web, but, this two-motor drive is coupled with the next and allthe others between each other are in viscous dampingcoupling [5, 6].

Fig.5. The drives of a paper machine in the early phase of web forming

The drives with viscous damping coupling must haveindividual speed regulation, consequently, the converter and the controller must provide it. The longitudinal strainis eliminated by cascading the reference and by forming

the loop between the drives.

2.4. Uncoupled drives

This group forms the drives without the mechanicalconnection, but with the technological coupling, i.e. thetechnological process is possible by the coordination of the drives. The typical example is the plant for continuoustransversal sheet cutting, known as the flying shear. The

principle of operation is shown in Fig 6. [7, 8].The first in the line of the flying shear is the press

drive. It unwinds the sheet from the roll, and feeds it between the cylinders that carry the blades of the shear.Strictly speaking, the two drives are mechanically coupledonly during the short cutting period, while the blades cutthe material, however this may be neglected from the practical point of view, so the drives are mechanicallyuncoupled. The drive for the knives must be preciselycoordinated with the press drive, since this determines theaccuracy of the length being cut. To get the high qualityof the cut, with no rupture or crumpling, the peripheralvelocity of the sheet and the knives during the cut must be

equal. During one round, the blades travel the distance of Lk =π Dk, which is, in general, different from the selectedlength to be cut ( L). This is why the blades must have thevariable speed between the two consecutive cuts. Thereare two possible solutions for this problem. In one case,the motor itself provides the variable speed, but, due tothe low bandwidth of the mechanical system, this methodcan be used only where the sheet speed is low, with therelatively long lengths to be cut. With the higher sheetspeed, the mechanical variator is used to provide thevariable speed of the knives during the single revolution.The variation of the speed is provided by the mechanical

mechanism, i.e. the flying shear mechanism, with the

drive motor kept at the constant speed. The ratio of thespeed of the press motor, and of the shear drive is proportional to the ratio L/ Lk . The accuracy of the cut

length depends on the possibility to keep the ratioconstant; the usual required accuracy is at 0.1%, for example, in the paper industry, the tolerated error is at themost 1mm, for the cut of the length 1000mm. It isobvious that the two drives need independent, extremelyaccurate speed regulation.

Fig. 6. The flying shear

The remaining belt drives shown in Fig. 6 should also

have synchronized speed, to properly align the parts,however the required accuracy is not very high, thereforethe slip compensation as a function of the load, isadequate.

2.5. Drives with stochastic coupling structure

This group is represented by the caterpillar drive onlarge mining machines such as bucket wheel excavatorsused on open cast mines, shown in Fig. 7.

Fig. 7. SRs 2000 bucket wheel excavator on an open cast mine

The travel drive of an excavator has a stochasticallychanging structure due to the changes in the structure andthe geometry of the supporting ground, as well as thetravel along the curved path. The stochastic changesinfluence the values of the parameters, and therefore thestructure of the coupling. The travel drive has the mainassignment to keep the desired path and the average speedof the movement [3]. Similar to the rigidly coupleddrives, the load of individual motors should be evenlydistributed, but the speed of individual drives may

decrease, in the event of overcoming the obstacle in the path of the crawler. This type of drive, with ideally flat

surface becomes rigidly coupled.

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3. THE CONVERTER AND THE CONTROLLER FOR THE CONTROLLED MULTI-MOTOR DRIVES

Controlled drives are usually fed from the power converter, which is also true for controlled multi-motor drives. The kind, the type and the number of converters

used depend on the type of motors, their power ratings,and of the kind of the multi-motor drive. The control andregulation also depend on the type of the multi-motor drive, but on the type of the converter selected too,therefore the selection of the converter and the controller for these drives must be analyzed together.

3.1. Drives with rigid coupling

The drives with rigid coupling require only one speedcontroller, therefore for the drives with lower power

ratings, only one power converter of the adequate size,can feed all the motors in the drive. The method for proper distribution of the load among the motors depends

on the type of the motors used. With DC drives withseparate excitation, with the armatures connected in parallel, the load distribution may be adjusted through the

field. For the motors with the same power ratings, the proper load distribution is achieved with the seriesconnection of the armature windings.

With induction motors connected in parallel, the loaddistribution is influenced only by the correct selection of the torque-speed mechanical characteristic. For thesquirrel-cage induction motors there exist no economical

method for adjustment of the mechanical characteristic of the ready-made motors, it has to be done during theselection. For the slip-ring induction motor, the

mechanical characteristic can be adjusted afterwards, withthe inclusion of the rotor resistors.

Drives with medium and high power ratings require the

use of separate converter for each motor; therefore, theload distribution is accomplished at the control level, suchthat every motor takes a part of the total load, proportionate to its ratings. The common speed controller determines the needed value of the total torque reference.The block diagram of such a drive is given in Fig. 8.

Fig.8. The converter and the controller for the two rigidly

coupled drives, with separate power converters

3.2. The drives with resilient coupling

Each of the drives in the resilient coupling must be inthe speed closed loop, that is, it must have its owncontroller, speed sensor, and the separate converter .Each

speed controller, should receive its own reference value.There exist two basic methods for the speed referencedistribution, the parallel and the series (cascade) method.The reference distribution in parallel is used in drives thathave the predetermined speed ratios, Fig. 9. The constants K 1 to K 3, in Fig. 9 depend on the individual drive’srequired speed ratio to the main reference, and are thefunction of the gear-ratio of the mechanical transmission,and the other mechanical parameters of the system. Thecascade distribution, shown in Fig. 10 is used with drivesthat need to transmit the change of the speed, called thedraw, of each drive to the other drives later in thetechnological line. The examples are rolling mills and paper industries. Constants K 1 to K 3 in Fig. 10 are

determined as in the above, while z 1, z 2 ... are the selecteddraw settings.

The problem of load distribution that is possible inthese drives is solved with the additional correction of thereference speed, based on the measured strain in thematerial that couples the drives, Fig. 11. The strain sensor

or the force sensor can be measurement tapes, or thesensor in the bearing, or the dancer roll. Forcemeasurement sensors are susceptible and expensive;however, they are required only between every other drive in the line.

Fig.9. Parallel method of reference distribution

Fig.10. Series (cascaded) speed reference distribution

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Fig.11. Control of the strain in the material.

In many cases, an adequate solution, that is morerobust and is cost effective, is the control of the loaddistribution, among subsequent drives [4, 6], shown in Fig12. The use of the load distribution regulator gives thesystem the required stability, and provides the satisfactorycontrol over the strain in the material, for most

applications encountered in practice. The desired loadratio for the drives m1 and m2 can be adjusted through the

reference value of the load differenceΔm*.

Fig.12. The load distribution regulator.

3.3. Drives with viscous damping coupling

The requirements for the controller with this type of drives are similar to the previous case; the separate speed

closed loop control is necessary, which requires theseparate power converters. The viscous damping of thematerial establishes the increase of the speed along thetechnological line, but the increase is higher than with the

drives with the resilient coupling. Instead of the draw, inthis case, the correction is for the compensation of theelongation of the material. Since the correction should betransferred to the subsequent drives, the selection of cascaded reference distribution is mandatory, shown onFig 10. The problems of the load distribution do not affect

these drives, and even if it does, it gives the unwantedstrain in the material, and must be eliminated. The problem of the unwanted strain in the material can be

effectively overcome by the use of the “loop” between thedrives. To ensure the function of the loop, an additionalcontrol of the loop depth is used. The measured depth of the loop d is compared to the reference depth d

*and the

difference is added to the reference speed of the drivesubsequent to the loop, Fig. 13.

Fig.13. The control of the loop depth.

3.4. Uncoupled drives

All the uncoupled drives require separate speedfeedback control and a separate power converter. Thecoordinated work is obtained over the reference. The

drive type determines the method of referencedistribution; parallel as well as cascaded distribution is possible. The principle of the reference distribution in theform of the master-slave drive is also possible, and has been applied in the aforementioned case of the flyingshear. The measured speed of the press motor is used as areference speed for the shear drive. Given the continuousdemand for the precise control of the speed ratio of thetwo drives, the great attention is given to the selection andthe tuning of the shear drive speed controller. PIDcontroller, with the speed and acceleration feed-forwardaction included. The speed of the press drive may be used

as a reference for the conveyor belt drives with the speedsensor-less control. The block diagram showing thereference distribution is shown in Fig 14.

Fig.14. The reference distribution in the flying shear

drive.

3.5. Drives with stochastic coupling structure

The drives with stochastic coupling requireinformation about speed of individual motors, but do notneed individual speed controllers. A single speed PIDcontroller controls the average speed of all motors.

Individual motors may have different values of speed.The principal block diagram of the control for thetransport drive of the bucket wheel excavator is shown inFig. 15 [3].

Stochastic coupling is indicated by the blue bar connecting the motors. Fig. 15 shows the speed sensors,

although in practical realizations the sensor-less controlalgorithm proved excellent performance, with increasedsystem reliability. The window control presented in Fig.15 is activated only in the situation where the speeddeviation is higher than the preset limits. In case of roughterrain, the individual motors may slow down to very lowspeed, due to the obstacles found in the path of the

caterpillar. Stalling of motors must be detected andrestricted by the controller.

4. LOAD SHARING

The consideration of rational energy consumption isimportant with multi-motor drives, especially if the high power ratings are used. The right choice of drive’scomponents, undoubtedly contributes to the rational

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energy consumption. However, only a few multi-motor drives are able to save energy, based on the converter sideload sharing. To be able to use load sharing, it isnecessary that the drives have instantaneous electric power with the opposite signs. The total of the consumedenergy for the complete drive can then be minimized.

Fig.15. The Principal block diagram of the transport drive of bucket wheel excavator (n = 6 ).

All of the drives analyzed previously do not satisfy the

necessary condition, since the motors were always in thesame operating regime. Some of the drives used thecontrol algorithm to disable the potential operation in thedifferent regimes, e.g. the load distribution regulator.

The typical example of the drive with the ability to usethe breaking energy is the rewinder drive. One of such plants is the reversible cold rolling mill, where the rolling process is accomplished by the multiple reeling of the tin

foil during which the foil is wriggled through the pressrolls in both directions. At both ends of the plant, thewinder and unwinder drives are placed. The motor in theunwinder provides the constant tension force during theunwinding process, operating in the breaking regime.

Provided the breaking is regenerative, the producedenergy can be used by the other drives in the system. Inthe aforementioned flying shear drive, the existingunwinder drive could share the energy with the other drives. In the example cited [7], the tension force isobtained by the pneumatic mechanical breaks. Thecharacteristic example with the ability for considerable

energy savings, based on the load sharing principle is therewinder drive, found in the paper industry. During therewinding process of the completed rolls of paper, thelongitudinal cutting is performed along the sides and tothe desired width. The same tension force is neededthroughout the rewinding of the roll, over the entire

diameter, for the storage and transportation purposes.

The load sharing option, among the drives, depends onthe type and the concept of the drive. The first controlleddrives, in the present sense, have been realized with theVard-Leonard groups. The generators feeding the

individual drives were placed on the shaft of the maindrive motor. During the breaking of an individual motors,

the energy would be transferred to the other drivesthrough the main drive’s shaft. In DC drives withthyristor converters, internal load sharing is performedover the main bus bars, feeding all the drives, under theassumption that multi-quadrant converters were used. In both cases, there was no need for additional investment toenable the internal load sharing in the drive.

Today, the controlled drives are most often realizedwith the frequency converters (AC-DC-AC). Theconverters usually utilize the diode rectifier units; but thereversible rectifiers are not uncommon. The load sharing

between the frequency converters is accomplished over the DC load bus. Fig 16 shows the principal block

diagram of the rewinder drive, with the frequencyconverters and induction motors [9, 10].

Fig.16. The rewinder drive.

The converter for the unwinder drive is configured inthe torque control mode (with speed feedback) to controlthe tension force from the presductor sensor. Owing to theclosed loop control of the tension force, the drive

automatically adapts to the changes of the roll diameter keeping the constant tension force equal to the referencevalue ( F*) given by the operator. The winder driveconverter is configured in the speed closed loop, to follow

the speed reference ω∗ . The DC links of the convertersare connected to establish the load sharing. Besides that, areversible rectifier feeds the DC link, to enable bi-directional flow of energy, transferring even the energygenerated during the slow-down, when both drives are in

breaking regime, converting the mechanical energy toelectrical. The described concept of the drive assures the

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operation with minimum power requirements, equal to thelosses in the system only.

5. REALIZATION AND APPLICATIONEXAMPLES

The practical realizations of the controlled multi-motor

drive are very complex, considering the fact that thegeometric layout for the drives and the converters, power supply, controll algorithm, the user interface, distribution

of the reference, supervison and protection, must all bedesigned and planned ahead.

Power distribution, together with the power converterscan be placed a few tens of meters from the motors,requiring the placement of a few kilometers of power cables. Long cable lines can cause a number of difficulties, such as voltage drop, electromagnetic

interference, considerable capacitive currents may occur in the drives with high carrier frequencies, etc.

The power converters of newer generation, used today,

are very compact, enabling the packing of a great number of converters with high power ratings in a small space.Figure 17 shows the converters of a drive with 12 motors,

with the total installed power of 460kW.In modern controlled multi-motor drives, the control

and the protection system can be realized in many ways, but in all cases the use of a programmable controller ismandatory. With simple drives, the integrated resourcesof the power converter’s control system, with the modestsupport of a single computer can satisfy the requirements

for the control and the protection system [7, 8]. Bigger drives require the support of powerful programmablelogic controllers (PLC), with high computing power, and

support of high performance field buses.

Fig.17. The frequency converters in a multi-motor drive.

The core of the system features the PLC that

communicates with the power converters over thePROFIBUS protocol and with the control panels over theMPI protocol. One panel controls the operation of eachdrive, while the supervisory control panel incorporates allthe information about the system state. The performanceof the PLC enabled the load distribution controller to beimplemented in the controller, using the PROFIBUS.However, basic protective functions, safety-stop, and the

trip of all the drives, is realized in the classical way, withdedicated, special purpose relays.

6. CONCLUSION

Analyzing and writing about the broad and complexsubject, permits the authors to forget an issue, but it also permits the authors to deliberately omit something. Theauthors intended to consider the typical examples of thedrives with high power ratings, but also the drives they

are entitled to consider. The drives that are very wellknown to them, studied both theoretically and practically,during the design, realization and finally successfulcommissioning.

7. REFERENCES

[1] Z. Stojiljkoviü, B. Jefteniü, “The reconstruction andcommissioning of the drive for the first press roll2x4.5MW, during the reconstruction of the hot rollingmill”, in Serbian, SARTID, Smederevo, 1995.

[2] B. Jefteniü, M. Bebiü, “Superstructure slewing drivesfor two bucket wheel excavators: SRs2000 and

SRs1300”, Project for Kostolac Open Cast Mine, for EPS, Serbia. 2004.

[3] B. Jefteniü, M. Bebiü, N. Rašiü “Transport drives for two bucket wheel excavators: SRs2000 andSRs1300”, Project for Kostolac Open Cast Mine, for EPS, Serbia. 2004.

[4] B. Jefteniü, M. Gvozdenoviü, “Synchronized work of two controlled DC drives with resilient mechanicalconnection”, Publications of the Faculty of Electrical,Belgrade, 1989.

[5] M. Bebiü, B. Jefteniü, M. Belinþeviü, “Electric drives

for paper machines”, in Serbian, Ee'2001, Novi Sad, Nov. 2001.

[6] B. Jefteniü, M. Bebiü, D. Jevtiü, M. Belinþeviü, “Thereplacement of the line shaft drive in a paper machinewith the sectional induction motor drives”, (inSerbian), VII Yugoslav symposium of cellulose, paper, packaging and printing , Zlatibor, 20-22. June 2001.

[7] B. Jefteniü, M. Bebiü, M. Milojeviü, “Thereconstruction of the flying shear drive based on thefrequency converters”, in Serbian, VI Yugoslav symposium of cellulose, paper, packaging and printing , Zlatibor, 14-16. June 2000.

[8] M. Belinþeviü, M. Bebiü, B. Jefteniü, “The controlalgorithm for synchronization of the flying shear drive”, in Serbian, Ee'2001, Novi Sad, Nov. 2001.

[9] B. Jefteniü, “The Rewinder Drive - the project for the paper mill Komuna Skopje”, in Serbian, Beograd,2004.

[10] B. Jefteniü, M. Bebiü “The Rewinder Drive - the project for the paper mill Umka, near Belgrade”, inSerbian, Beograd, 2005

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Controlled Multi-motor Drives85