electrical machines and drives systems電機(jī)和驅(qū)動器系統(tǒng)英文原版電氣工程教材教程

1、Electrical Machines And Drives Systems電機(jī)和驅(qū)動器系統(tǒng)英文原版電氣工程教材教程1Introduction and review ofbasic theory1.1 Aim of the bookOn entering the world of electrical machines, the student meets manyconceptual difficulties not experienced for example in the early studies ofdigital systems, with their simple and pr

2、ecise 2-state operation. Moreassistance is required to permit the new-comer to gain confidence indealing with non-linear, 3-dimensional, rotating electromagnetic devices.The purpose of this book is to provide this aid to understanding byshowing how, with a limited number of equations derived from ba

3、sicconsiderations of power flow and elementary circuit and electromagnetictheory, the electromechanical performance can be explained and pre-dicted with reasonable accuracy.Such an aim, which will permit the calculation of power-input/outputc haracteristics almost close enough in engineering terms t

4、o those of thedevice itself, can be achieved by representing the machine as a simpleelectrical circuit - the equivalent-circuit model. This concept is explainedin many books, for example in the authors companion volume ElectricalMachines and Their Applications. Though more detailed theoretical treat

5、-ment is given there, substantial portions of the present text may beregarded as suitable revision material. This expanded 3rd edition can, as awhole, be considered as a textbook with particular, but not exclusive,emphasis on Electrical Drives, taught through worked examples, for areader having some

6、 familiarity with basic machine theory.Perhaps it is appropriate to point out that complete and exact analysis ofmachine performance is so complex as to be virtually impossible. Theadditional accuracy achieved by attempts to approach such methods isprimarily of interest to the specialist designer wh

7、o must ensure that his2 Electrical Machines and Drive Systems(1.1)product will meet the users needs without breakdown and he must judgewhen the analytical complication is justified. For the user, and for theengineering student who is not yet a specialist, the simpler methods areadequate for general

8、understanding and provide a lead-in if necessary forlater specialisation.There are many features of all machine types which are common, theobvious example being the mechanical shaft equations. But apart fromthese and the fundamental electromagnetic laws, the input/outputrelationships and modes of op

9、eration have many similarities. These arebrought together where possible and also in this first chapter, someelementary mechanical, magnetic and circuit theory is discussed briefly, asa reminder of the basic knowledge required. Students should beware ofunderestimating the vital importance of this ma

10、terial, since experienceshows that it is these very points, improperly understood, which hold backprogress in coming to feel at ease with machines problems.However familiar one may become with theory, as a student, the true testof an engineer is his ability to make things work. First steps to this g

11、oal ofconfidence are reached when a student is prepared to commit himself toselecting equations and inserting values in the algebraic expressions,producing answers to a specific problem. Hence the importance ofpractice with numerical examples. Understanding grows in proportion toones ability to real

12、ise that the equations developed really can be used in asystematic fashion to solve such problems, since they describe the physicalbehaviour in mathematical terms. Appreciation of this last statement is thekey to successful problem-solving.The chapters are planned to sequence the examples at increas

13、ing levelsof difficulty. Much theoretical support is given, in that the equations arediscussed either at the beginning of each chapter, or as the need arises.Solution programmes indicate the kind of problems which can beformulated for the three basic types of rotating machine: d.c., induction,and sy

14、nchronous. Readers are encouraged to adopt an ordered approachto the solution; for example it is a good idea to incorporate the questiondata on a diagram. One of the difficulties of machines problems often liesin the amount of data given. By putting the values on a simple diagram,assimilation is eas

15、ier and it helps to avoid mistakes of interpretation,especially when working with 3-phase circuits. In following this recom-mended pattern, it is hoped that the text will help to remove the mysterywith which some students feel the machines area is shrouded.The emphasis is on machine terminal-charact

16、eristics, rather than on theinternal electromagnetic design. In other words, the electrical-drives aspectis uppermost since this is the area in which most engineering studentsneed to have some good knowledge. It is worth noting that about 60-70%of all electrical power is consumed by motors driving m

17、echanical shafts andvirtually all this power is produced by generators driven through6 Electrical Machines and Drive Systems(1.2)Figure 1.3 Motor conventions.are the assumed +ve ends. The directions of the arrows for theinstantaneous terminal voltage v and for e may be assigned arbitrarily butRi and

18、 L di/dt must oppose z, since the voltage arrowheads must be positivefor +ve i and +ve di/dt respectively. The direction of i may also be assignedarbitrarily but the decision has consequences when related to the v and/ore arrows. As shown, and with all quantities assumed to be +ve, then themachine i

19、s a power sink; i.e. in a MOTORING mode; the vi and ei products areboth positive. For GENERATING, when the machine becomes a power source,ei will then be negative; e or i reversed.The above is called the MOTORING convention and it is often convenientin electrical-drives studies to use this throughou

20、t and let a negative eiproduct indicate a generating condition. Alternatively, a GENERATINGconvention could be used, as sometimes preferred in power-systemsstudies. By reversing the i arrow say, ei would then be positive forgenerating and the circuit equation would have a sign reversed. It would bea

21、 good check to complete the following short exercise to see if the abovestatements are properly understood.Write down the MOTORequation;Write down the GENERATORequation;Write down the GENERATORequation;Write down the MOTORequation;with MOTORconventions:with MOTORconventions:With GENERATORconventions

22、:With GENERATORconventions:V = E RIV = E RIV = E RIV= E RI10 Electrical Machines and Drive Systems(1.2)Meaning ofVxl componentsMultiplying the /, /cos (f> and /sin <p current phasors by Vgives:VI (Svoltamperes, VA)VI cos <p (P watts) andV7sinest single-unit steam-turbine generators for powe

23、r stations are now over1000MW= 1GW, (109W).3-phase circuit theoryFor many reasons, including efficiency of generation and transmission,quite apart from the ease of producing a rotating field as in any polyphasesystem, the 3-phase system has become virtually universal though there areoccasions when o

24、ther m-phase systems are used. For low powers of course,as in the domestic situation for example, single-phase supplies aresatisfactory. For present purposwer flow.There are two symmetrical ways of connecting the three phases as shownon Figure 1.6:in STAR (or wye);for which it is obvious that the cu

25、rrent through the line terminals isthe same as the current in the phase itself, or:in DELTA (or mesh);for which it is obvious that the voltage across the line terminals is thesame as the voltage across the phase itself.(1.2)Introduction and review of basic theory 11Figure 1.6 3-phase circuits.For th

26、e delta case shown:?ne = IA IG and IB - IA and Ic - IB.For the star case shown:V,ine=VA-VB and VB-VC and Vc - VA;assumed positive senses of phase currents and voltages being indicated.The 120 displacement means that the magnitude of the line quantitiesin these two cases is equal to V3 times the magn

27、itude of the phasequantities and there is a 30 displacement between line and phasephasors; depending on which phasors are differenced. Only one linevalue involves a x3 factor, hence for both star and delta circuits:which can be solved for any one unknown. Frequently this is the current,from the know

28、n power and voltage ratings. Sometimes, for parametermeasurements, <p is required for dividing currents, voltages or impedancesini o resistive and reactive components. The total voltamperes for a 3-phasesystem are thus given by V3V7 where V and / are here, line values, oralternatively, three time

29、s the phase VI product. A.C. devices are rated on aVA. (kVA or MVA) basis, since they must be big enough magnetically to dealwi1;h full voltage, whatever the current, and big enough in terms of theelectrically-sensitive parameters to deal with the current-carrying capacityspecified in the rating, wh

30、atever the voltage. This means for example, thatat zero power-factor, at full voltage and current, the temperature rise will16 Electrical Machines and Drive Systems(1.3)Figure 1.9 Machine equivalent circuits.coupling3-ph synchronousmotorv3VL/Lcosp= SW.cosp3/a2fla3 ,/a cos (<p -6)= u)aT9 3Wa cosy)

31、/,2flFV,/f = /f2flF ouplingNote. The Fe toss mostly manifests itself as a torque loss, part of 7oss, though it is usual on inductionmachines to show it as part of the electrical loss.(1.4)Introduction and review of basic theory 19always (speed X electromagnetic torque) = a>m Te = -Pgap? where om

32、= wsfor the synchronous machine. For the induction machine the speed is o>m= ws (1 - 5) so that there is a power sPSSip converted not to mechanicalpower, but to electrical power in the secondary circuits. Continuing alongthe motoring power-flow path, the remaining elements are the same for allmac

33、hine types. comTe is reduced by the mechanical loss wmTloss towm Coupling which is the output motoring power at .onents of powercancel, giving steady power flow.There are two symmetrical ways of connecting the three phases as shownon Figure 1.6:in STAR (or wye);for which it is obvious that the curre

34、nt through the line terminals isthe same as the current in the phase itself, or:in DELTA (or mesh);for which it is obvious that the voltage across the line terminals is thesame as the voltage across the phase itself.(1.2)Introduction and review of basic theory 11Figure 1.6 3-phase circuits.For the d

35、elta case shown:?ne = IA IG and IB - IA and Ic - IB.For the star case shown:V,ine=VA-VB and VB-VC and Vc - VA;assumed positive senses of phase currents and voltages being indicated.The 120 displacement means that the magnitude of the line quantitiesin these two cases is equal to V3 times the magnitu

36、de of the phasequantities and there is a 30 displacement between line and phasephasors; depending on which phasors are differenced. Only one linevalue involves a x3 factor, hence for both star and delta circuits:which can be solved for any one unknown. Frequently this is the current,from the known p

37、ower and voltage ratings. Sometimes, for parametermeasurements, <p is required for dividing currents, voltages or impedancesini o resistive and reactive components. The total voltamperes for a 3-phasesystem are thus given by V3V7 where V and / are here, line values, oralternatively, three times the phase VI product. A.C. devices are rated on aVA. (kVA or MVA) basis, since they must be big enough magnetically to dealwi1;h full voltage, whatever the current, and big enough in terms of theelectrically-sensitive parameters to deal with the current-carry