負(fù)載運(yùn)行的變壓器及直流電機(jī)導(dǎo)論

1、苷澆駿裴竟糠浩懼掇擴(kuò)餓The Transformer on loadIntroduction to DC Machines氫鷦腿墾妨圖符秈戇修矬The Transformer on load虱天漆忸創(chuàng)皤攢鱷卵碇泰It has been shown that a primary input voltage can be transformed to any desired open-circuit secondary voltage by a suitable choice of turns ratio. is available for circulating a load current i

2、mpedance. For the moment, a lagging power factor will be considered. The secondary current and the resulting ampere-turns will change the flux, tending to demagnetize the core, reduce and with it . Because the primary leakage impedance drop is so low, a small alteration to will cause an appreciable

3、increase of primary current from to a new value of equal to . The extra primary current and ampere-turns nearly cancel the whole of the secondary ampere-turns. This being so , the mutual flux suffers only a slight modification and requires practically the same net ampere-turns as on no load. The tot

4、al primary ampere-turns are increased by an amount necessary to neutralize the same amount of secondary ampere-turns. In the vector equation , ; alternatively, . At full load, the current is only about 5% of the full-load current and so is nearly equal to. Because in mind that , the input kVA which

5、is approximately is also approximately equal to the output kVA, .宓螢段松桐鄶商潰拱穸擘The physical current has increased, and with in the primary leakage flux to which it is proportional. The total flux linking the primary , is shown unchanged because the total back e.m.f.,（）is still equal and opposite to . H

6、owever, there has been a redistribution of flux and the mutual component has fallen due to the increase of with . Although the change is small, the secondary demand could not be met without a mutual flux and e.m.f. alteration to permit primary current to change. The net flux linking the secondary wi

7、nding has been further reduced by the establishment of secondary leakage flux due to , and this opposes . Although and are indicated separately , they combine to one resultant in the core which will be downwards at the instant shown. Thus the secondary terminal voltage is reduced to which can be con

8、sidered in two components, i.e. or vectorially . As for the primary, is responsible for a substantially constant secondary leakage inductance . It will be noticed that the primary leakage flux is responsible for part of the change in the secondary terminal voltage due to its effects on the mutual fl

9、ux. The two leakage fluxes are closely related; , for example, by its demagnetizing action on has caused the changes on the primary side which led to the establishment of primary leakage flux.噌笆愆蠣?wèi)яZ扶僻補(bǔ)鞒If a low enough leading power factor is considered, the total secondary flux and the mutual flux

10、are increased causing the secondary terminal voltage to rise with load. is unchanged in magnitude from the no load condition since, neglecting resistance, it still has to provide a total back e.m.f. equal to . It is virtually the same as , though now produced by the combined effect of primary and se

11、condary ampere-turns. The mutual flux must still change with load to give a change of and permit more primary current to flow. has increased this time but due to the vector combination with there is still an increase of primary current.胯譙嗽躚縈堰錕和刃涂高Two more points should be made about the figures. Fir

12、stly, a unity turns ratio has been assumed for convenience so that . Secondly, the physical picture is drawn for a different instant of time from the vector diagrams which show , if the horizontal axis is taken as usual, to be the zero time reference. There are instants in the cycle when primary lea

13、kage flux is zero, when the secondary leakage flux is zero, and when primary and secondary leakage flux is zero, and when primary and secondary leakage fluxes are in the same sense.觥柑鄙樾膚火將鱧囔醉槌The equivalent circuit already derived for the transformer with the secondary terminals open, can easily be

14、extended to cover the loaded secondary by the addition of the secondary resistance and leakage reactance.皮笫伯竟未莠塊庾吱繃訃Practically all transformers have a turns ratio different from unity although such an arrangement is sometimes employed for the purposes of electrically isolating one circuit from anot

15、her operating at the same voltage. To explain the case where the reaction of the secondary will be viewed from the primary winding. The reaction is experienced only in terms of the magnetizing force due to the secondary ampere-turns. There is no way of detecting from the primary side whether is larg

16、e and small or vice versa, it is the product of current and turns which causes the reaction. Consequently, a secondary winding can be replaced by any number of different equivalent windings and load circuits which will give rise to an identical reaction on the primary .It is clearly convenient to ch

17、ange the secondary winding to an equivalent winding having the same number of turns as the primary. 貪首蝎瓏蟑檔仙葸弟嵊駐With changes to , since the e.m.f.s are proportional to turns, which is the same as .涪枝鈁逝淚蘺磯緹粢叵吻For current, since the reaction ampere turns must be unchanged must be equal to .i.e. .鍵锿謙非瓜沂

18、柩竄牒析野For impedance , since any secondary voltage becomes , and secondary current becomes , then any secondary impedance, including load impedance, must become . Consequently, and .卞抗瑣戽趵閻噦仝交據(jù)囗If the primary turns are taken as reference turns, the process is called referring to the primary side.壓宋錢鶇崽繼

19、堊鉉邏燉遑There are a few checks which can be made to see if the procedure outlined is valid.矬騅珞圪瀕芤柒肱犧柔升For example, the copper loss in the referred secondary winding must be the same as in the original secondary otherwise the primary would have to supply a different loss power. must be equal to . does i

20、n fact reduce to .澮拶味牛配璩墜趁咴始砘Similarly the stored magnetic energy in the leakage field which is proportional to will be found to check as . The referred secondary .酢塢鱉襁謄畫翅啊驥邊默The argument is sound, though at first it may have seemed suspect. In fact, if the actual secondary winding was removed physi

21、cally from the core and replaced by the equivalent winding and load circuit designed to give the parameters ,and , measurements from the primary terminals would be unable to detect any difference in secondary ampere-turns, demand or copper loss, under normal power frequency operation.焊鰾枸搡匍殲絳飯魂賴剮Ther

22、e is no point in choosing any basis other than equal turns on primary and referred secondary, but it is sometimes convenient to refer the primary to the secondary winding. In this case, if all the subscript 1s are interchanged for the subscript 2s, the necessary referring constants are easily found;

23、 e.g. ,; similarly and .宮旭庫(kù)搪賾槽怯磕猛逶裁The equivalent circuit for the general case where except that has been added to allow for iron loss and an ideal lossless transformation has been included before the secondary terminals to return to .All calculations of internal voltage and power losses are made be

24、fore this ideal transformation is applied. The behaviour of a transformer as detected at both sets of terminals is the same as the behaviour detected at the corresponding terminals of this circuit when the appropriate parameters are inserted. The slightly different representation showing the coils a

25、nd side by side with a core in between is only used for convenience. On the transformer itself, the coils are , of course , wound round the same core.技淌庖歡閏鐮菸颯韭恫幬Very little error is introduced if the magnetising branch is transferred to the primary terminals, but a few anomalies will arise. For exam

26、ple ,the current shown flowing through the primary impedance is no longer the whole of the primary current. The error is quite small since is usually such a small fraction of . Slightly different answers may be obtained to a particular problem depending on whether or not allowance is made for this e

27、rror. With this simplified circuit, the primary and referred secondary impedances can be added to give:颶滄稱瓦活衍菜螵悚銩賀 and 韓鳩拭挖舾蹩狺捷速伏攤It should be pointed out that the equivalent circuit as derived here is only valid for normal operation at power frequencies; capacitance effects must be taken into accou

28、nt whenever the rate of change of voltage would give rise to appreciable capacitance currents, . They are important at high voltages and at frequencies much beyond 100 cycles/sec. A further point is not the only possible equivalent circuit even for power frequencies .An alternative , treating the tr

29、ansformer as a three-or four-terminal network, gives rise to a representation which is just as accurate and has some advantages for the circuit engineer who treats all devices as circuit elements with certain transfer properties. The circuit on this basis would have a turns ratio having a phase shif

30、t as well as a magnitude change, and the impedances would not be the same as those of the windings. The circuit would not explain the phenomena within the device like the effects of saturation, so for an understanding of internal behaviour .徂擠廾酌鮭旋毿柚袈氯癡There are two ways of looking at the equivalent

31、circuit:蕪澎薹瘋宏羚螋巢瘠湊矗(a) viewed from the primary as a sink but the referred load impedance connected across ,or苣瑁驟塘幌捃床鼙蕩糈厥(b) viewed from the secondary as a source of constant voltage with internal drops due to and . The magnetizing branch is sometimes omitted in this representation and so the circuit

32、 reduces to a generator producing a constant voltage (actually equal to ) and having an internal impedance (actually equal to ).烏擦擷瘴鏊巧彀了骯拎盤In either case, the parameters could be referred to the secondary winding and this may save calculation time .胎撩貿(mào)飩境兀攤紛錳尿艫The resistances and reactances can be ob

33、tained from two simple light load tests.廉媧鍰鋸菲芩京令遁陷跗Introduction to DC Machines騷墩剔枉侵煞嘶獻(xiàn)甾砭躅DC machines are characterized by their versatility. By means of various combination of shunt, series, and separately excited field windings they can be designed to display a wide variety of volt-ampere or speed-

34、torque characteristics for both dynamic and steadystate operation. Because of the ease with which they can be controlled , systems of DC machines are often used in applications requiring a wide range of motor speeds or precise control of motor output.看琚昆摞發(fā)萌祭巧賑亮沽The essential features of a DC machine

35、 are shown schematically. The stator has salient poles and is excited by one or more field coils. The air-gap flux distribution created by the field winding is symmetrical about the centerline of the field poles. This axis is called the field axis or direct axis.季鼢氖淇锘菝輾彬捐舀嶇As we know , the AC voltag

36、e generated in each rotating armature coil is converted to DC in the external armature terminals by means of a rotating commutator and stationary brushes to which the armature leads are connected. The commutator-brush combination forms a mechanical rectifier, resulting in a DC armature voltage as we

37、ll as an armature m.m.f. wave which is fixed in space. The brushes are located so that commutation occurs when the coil sides are in the neutral zone , midway between the field poles. The axis of the armature m.m.f. wave then in 90 electrical degrees from the axis of the field poles, i.e., in the qu

38、adrature axis. In the schematic representation the brushes are shown in quarature axis because this is the position of the coils to which they are connected. The armature m.m.f. wave then is along the brush axis as shown. (The geometrical position of the brushes in an actual machine is approximately

39、 90 electrical degrees from their position in the schematic diagram because of the shape of the end connections to the commutator.)羯蜻箐寅圓翻貉安襁坷糴The magnetic torque and the speed voltage appearing at the brushes are independent of the spatial waveform of the flux distribution; for convenience we shall

40、continue to assume a sinusoidal flux-density wave in the air gap. The torque can then be found from the magnetic field viewpoint. 療嵊鋟嗽庥浼郜妾蘊(yùn)趙扉The torque can be expressed in terms of the interaction of the direct-axis air-gap flux per pole and the space-fundamental component of the armature m.m.f. wav

41、e . With the brushes in the quadrature axis, the angle between these fields is 90 electrical degrees, and its sine equals unity. For a P pole machine開(kāi)遢俚胩閱西譽(yù)胎牡閏奢 潔绔茅悔魯攘懷廴事妥軌In which the minus sign has been dropped because the positive direction of the torque can be determined from physical reasoning.

42、 The space fundamental of the sawtooth armature m.m.f. wave is 8/ times its peak. Substitution in above equation then gives 拍睽硇船釘豇溽錠糧囈鍵 烯矗文鈷槳道躡喘崞岙喚W(wǎng)here =current in external armature circuit;攔米彈郊哚謗摻嫡蹤閎喂 =total number of conductors in armature winding;汽濮卒桐畹顯扌颼胥標(biāo)脫 =number of parallel paths through win

43、ding;榭頭西咎芄喚省塬季哪善And 晡聊濂苒曳呆掄捉瞬羥貉 崧斤拂霽锪弧枯抓弧嚴(yán)定Is a constant fixed by the design of the winding.核襖掙獨(dú)義酐退寤康笆忿The rectified voltage generated in the armature has already been discussed before for an elementary single-coil armature. The effect of distributing the winding in several slots is shown in figure

44、,in which each of the rectified sine waves is the voltage generated in one of the coils, commutation taking place at the moment when the coil sides are in the neutral zone. The generated voltage as observed from the brushes is the sum of the rectified voltages of all the coils in series between brus

45、hes and is shown by the rippling line labeled in figure. With a dozen or so commutator segments per pole, the ripple becomes very small and the average generated voltage observed from the brushes equals the sum of the average values of the rectified coil voltages. The rectified voltage between brush

46、es, known also as the speed voltage, is 佬瓢惰瞪戎鼉嫗箭沼第香 冕卣癩朦拙茚浙所片詹噤Where is the design constant. The rectified voltage of a distributed winding has the same average value as that of a concentrated coil. The difference is that the ripple is greatly reduced. 薅刷栽崛獺瞎驍哌鮞丶締From the above equations, with all v

47、ariable expressed in SI units:龕癇櫬鮚搏櫛膿化讠砂套 儲(chǔ)哭丐鷂褥柜傲等祖宋艤This equation simply says that the instantaneous electric power associated with the speed voltage equals the instantaneous mechanical power associated with the magnetic torque , the direction of power flow being determined by whether the machine i

48、s acting as a motor or generator.淌擒酆蚍滾脖梢美莼影激The direct-axis air-gap flux is produced by the combined m.m.f. of the field windings, the flux-m.m.f. characteristic being the magnetization curve for the particular iron geometry of the machine. In the magnetization curve, it is assumed that the armature

49、 m.m.f. wave is perpendicular to the field axis. It will be necessary to reexamine this assumption later in this chapter, where the effects of saturation are investigated more thoroughly. Because the armature e.m.f. is proportional to flux times speed, it is usually more convenient to express the ma

50、gnetization curve in terms of the armature e.m.f. at a constant speed . The voltage for a given flux at any other speed is proportional to the speed,i.e.蚜呋胄湃緝螗鉭坯氦簧干 紡略讒孔茲闋嘔虼獫漠是Figure shows the magnetization curve with only one field winding excited. This curve can easily be obtained by test methods,

51、 no knowledge of any design details being required.爿堍浠衣薌獅巒癱瘧親揩Over a fairly wide range of excitation the reluctance of the iron is negligible compared with that of the air gap. In this region the flux is linearly proportional to the total m.m.f. of the field windings, the constant of proportionality

52、 being the direct-axis air-gap permeance.強(qiáng)妣桅隔灄恕淋踅局塑悟The outstanding advantages of DC machines arise from the wide variety of operating characteristics which can be obtained by selection of the method of excitation of the field windings. The field windings may be separately excited from an external D

53、C source, or they may be self-excited; i.e., the machine may supply its own excitation. The method of excitation profoundly influences not only the steady-state characteristics, but also the dynamic behavior of the machine in control systems.吏睚憤埽岳欽洧扌飚苻戤The connection diagram of a separately excited

54、generator is given. The required field current is a very small fraction of the rated armature current. A small amount of power in the field circuit may control a relatively large amount of power in the armature circuit; i.e., the generator is a power amplifier. Separately excited generators are ofte

55、n used in feedback control systems when control of the armature voltage over a wide range is required. The field windings of self-excited generators may be supplied in three different ways. The field may be connected in series with the armature, resulting in a shunt generator, or the field may be in

56、 two sections, one of which is connected in series and the other in shunt with the armature, resulting in a compound generator. With self-excited generators residual magnetism must be present in the machine iron to get the self-excitation process started.幽吩屠計(jì)披拽蛔沃橐哪奈In the typical steady-state volt-a

57、mpere characteristics, constant-speed prime movers being assumed. The relation between the steady-state generated e.m.f. and the terminal voltage is 郅倒副葆涼醞陡婺亢丟虐 松疚蕁月覆逼郗庭獺崇眷Where is the armature current output and is the armature circuit resistance. In a generator, is large than ; and the electromagn

58、etic torque T is a countertorque opposing rotation.畀丸稚闞略韋餡逢腌她兢 The terminal voltage of a separately excited generator decreases slightly with increase in the load current, principally because of the voltage drop in the armature resistance. The field current of a series generator is the same as the load current, so that the air-gap flux and hence the voltage vary widely with load. As a consequence, series generators are not often used. The voltage of shunt generators drops off somewhat with load. Compound generators are normally connected so that the m.m.f. of t