外文翻譯----感應電機

1、<p><b>　　附錄A：外文資料</b></p><p>　　Induction Motor</p><p>　　1 Starting an induction motor</p><p>　　High-inertia loads put strain induction motors because they prolong t

2、he starting period. The starting current in both the stator and rotor is high during this interval so that overheating becomes a major problem. For motors of several thousand horsepower, a prolonged starting period may e

3、ven overload the transmission line feeding the plant where the motor is installed. The line voltage may fall below normal for many seconds, thus affecting other connected loads. To relieve the problem, induction</p>

4、;<p>　　Rule I - The heat dissipated in the rotor during the starting period (from zero speed to final rated speed) is equal to the final kinetic energy stored in all the revolving parts.</p><p>　　This

5、 rule holds true, irrespective of the stator voltage or the torque-speed curve of the motor. Thus, if a motor brings a massive flywheel up to speed, and if the energy stored in the flywheel is then 5000 joules. the rotor

6、 will have dissipated 5000 joules in the form of heat. Depending upon the size of the rotor and its cooling system, this energy could easily produce overheating.</p><p>　　2 Plugging an induction motor</p&

7、gt;<p>　　In some industrial applications, the induction motor and its load have to be brought to a quick stop. This can be done by interchanging two stator leads, so that the revolving field suddenly turns in the

8、opposite direction to the rotor. During this plugging period, the motor acts as a brake.</p><p>　　It absorbs kinetic energy from the still-revolving load, causing its speed to hill. The associated mechanical

9、 power P, is entirely dissipated as heat in the rotor. Unfortunately, the rotor also continues to receive electromagnetic power P from the stator, which is also dissipated as heat (Fig. 14.10). Consequently, plugging pro

10、duces 12R losses in the rotor that even exceed those when the rotor is locked. Motors should not he plugged too frequently because high rotor temperatures may melt the rot</p><p>　　Rule 2- The heat dissipate

11、d in the rotor during the plugging period (initial rated speed to zero speed) is three times the original kinetic energy of all the revolving parts.</p><p>　　Fig. 14.10</p><p>　　When a 3-phase i

12、nduction motor is plugged, the rotor losses are very high.3 Braking with direct current An induction motor and its high-inertia load can also be brought to a quick stop by circulating dc current in the stator winding. An

13、y two stator terminals can be connected to the dc source.</p><p>　　The direct current produces stationary N, S poles in the stator. The number of poles created is equal to the number of poles which the motor

14、 develops normally. Thus, a 3-phase, 4-pole induction motor produces 4 dc poles, no matter how the motor terminals are connected to the dc source.</p><p>　　When the rotor sweeps past the stationary field, an

15、 ac voltage is induced iii the rotor bars. The voltage produces an ac current and the resulting rotor losses are dissipated at the expense of the kinetic energy stored in the revolving parts. The motor finally comes to r

16、est when all the kinetic energy has been dissipated as heat in the rotor.</p><p>　　The advantage of dc braking is that it produces far less heat than does plugging. In effect, the energy dissipated in the ro

17、tor is only equal to the original kinetic energy stored in the revolving masses, and not three times that energy. The energy dissipated in the rotor is independent of the magnitude of the dc current. However, a smaller d

18、c current increases the braking time. The dc current can be two or three times the rated current of the motor. Even larger values can be used, provided that</p><p>　　4 Abnormal conditions</p><p>

19、;　　Abnormal motor operation may due to internal problems (short-circuit in the stator, overheating of the bearing, etc. ) or to external conditions. External problems may be caused by any of the following:</p><

20、;p>　　1. Mechanical overload</p><p>　　2. Supply voltage changes </p><p>　　3. Single phasing</p><p>　　4. Frequency changes</p><p>　　We will examine the nature of these

21、 problems in the sections that follow.</p><p>　　According to national standards, a motor shall operate satisfactory on any voltage within ± 10% of the nominal voltage, and for any frequency within ±

22、;5% of the normal frequency. If the voltage and frequency both vary, the sum of the two percentage changes must not exceed 10 percent. Finally, all motors are designed to operate satisfactorily at altitudes up to 1000 m

23、above sea level. At higher altitudes the temperature may exceed the permissible limits due to the poor cooling afforded by the thin</p><p>　　5 Mechanical overload</p><p>　　Although standard indu

24、ction motors can develop as much as twice their rated power for short periods, they should not be allowed to run continuously beyond their rated capacity. Overloads cause overheating, which deteriorates the insulation an

25、d reduces the service life of the motor. In practice the overload causes the thermal overload relays in the starter box to trip. bringing the motor to a stop before its temperature gets too high.</p><p>　　So

26、me drip-proof motors are designed to carry a continuous overload of I 5 percent. This overload capacity is shown on the nameplate by the service factor 1.15. The allowable temperature rise is then 10°C higher than t

27、hat permitted for drip-proof motors operating at normal load.</p><p>　　During emergencies a drip-proof motor can be made to carry overloads as much as 125 percent, as long as supplementary external ventilati

28、on is provided. This is not recommended for long periods because even if the external frame is cool, the temperature of the windings may be excessive.</p><p>　　6 Line voltage changes</p><p>　　Th

29、e most important consequence of a line voltage change is its effect upon the torque-speed curve of the motor. In effect, the torque at any speed is proportional to the square of the applied voltage. Thus, if the stator v

30、oltage decreases by 10%, the torque at every speed will drop by approximately 20%. A line voltage drop is often produced during start-up, due to the heavy starting current drawn from the line. As a result of the lower vo

31、ltage, the starting torque may he much less than its rated</p><p>　　On the other hand, if the line voltage is too high when the motor is running, the flux per pole will be above normal. For a motor running a

32、t full-load, this increases both the ion losses and the magnetizing current, with the result that the temperature increases slightly and the power factor is slightly reduced.</p><p>　　If the 3-phase voltages

33、 are unbalanced, they can produce a serious unbalance of the three line currents. This condition increases the stator and rotor losses, yielding a higher temperature. A voltage Unbalance of as little as 3.5% can cause th

34、e temperature to increase by 15°C. The utility company should be notified whenever the phase-to-phase line voltages differ by more than 2 percent.</p><p>　　7 Single-phasing</p><p>　　If one

35、line of a 3-phase line is accidentally opened or if a fuse blows while the 3-phase motor is running, the machine wilt continue to run as a single-phase motor. The current drawn from the remaining two tines will almost do

36、uble, and the motor will begin to overheat. The thermal relays protecting the motor will eventually trip the circuit-hrC1’ hereby disconnecting the motor from the line.</p><p>　　The torque-speed curve is ser

37、iously affected when a 3-phase motor operates on single phase. The break- down torque decreases to about 40% of its original value, and the motor develops no starting torque at all. Consequently, a fully loaded 3-phase m

38、otor may simply stop if one of its lines is suddenly opened. The resulting locked-rotor current is about 90% of the normal 3-phase LR current. It is therefore large enough to trip the circuit breaker or to blow the fuses

39、.</p><p>　　Figure 14.11 Typical torque-speed curves when a 3-phase squirrel-cage motor operates normally and when it operates on single-phase.</p><p>　　Fig.14.11 shows the typical torque-speed c

40、urves of a 3-phase motor when it runs normally and when it is single-phasing. Note that the curves follow each other closely until the torque approaches the single-phase breakdown torque.</p><p>　　8 Frequenc

41、y variation</p><p>　　Important frequency changes never take place on a large distribution system, except during a major disturbance. However, the frequency may vary significantly on isolated systems where el

42、ectrical energy is generated by diesel engines or gas turbines. The emergency power supply in a hospital, the electrical system on a ship, and the generators in a lumber camp, are examples of this type of supply.</p&g

43、t;<p>　　The most important consequence of a frequency change is the resulting change in motor speed: if the frequency drops by 5%, the motor speed drops by 5%.</p><p>　　Machine tools and other motor-d

44、riven equipment imported from countries where the frequency is 50 Hz may cause problems when they are connected to a 60 Hz system. Everything runs 20% faster than normal, and this may not be acceptable in some applicatio

45、ns. In such cases we either have to gear down the motor speed or supply an expensive auxiliary 50 Hz source.</p><p>　　A 50 Hz motor operates well on a 60 Hz line, but its terminal voltage should be raised to

46、 6/5 (or I 20%) of the nameplate rating. The new breakdown torque is then equal to the original breakdown torque and the starting torque is only slightly reduced. Power factor, efficiency, and temperature rise remain sat

47、isfactory.</p><p>　　A 60 Hz motor can also operate on a 50 Hz line, but its terminal voltage should be reduced to 5/6 (or 83%) of its nameplate value. The breakdown torque and starting torque are then about

48、the same as be- fore, and the power factor, efficiency, and temperature rise remain satisfactory.</p><p><b>　　附錄B：中文翻譯</b></p><p><b>　　感應電機</b></p><p><b&

49、gt;　　1 感應電機的啟動</b></p><p>　　高慣性負載由于其延長了啟動時間，所以會拖累感應電機。啟動時定子和轉子中的啟動電流都很高，故過熱成為了感應電機啟動的主要問題。比如一個幾千馬力的電機，因啟動時間的延長甚至會使裝載電機設備的饋電回路過載。線電壓會低于正常值好幾秒鐘時間，因此影響到其他相連的負載。為了解決這個問題，感應電機經常在低電壓下進行啟動。這樣就減少了電機的電力牽引，并以此降低

50、了線電壓的壓降和繞組上產生熱量的速度。雖然低電壓加長了啟動時間，但這一般來說并不重要。對于一個無機械性加載電機來說，無論啟動時間是長是短，記住下列規(guī)則是很有用的：</p><p>　　規(guī)則1：在啟動過程中（從零速度到額定速度）在轉子上消耗的熱量相當于最終所有旋轉部分所儲存的電能。</p><p>　　不管電機的定子電壓或者轉矩的大小，這個規(guī)則都是成立的。因此，如果一個電機要帶動一個大型轉速

51、輪達到預定速度并且使該轉速輪所帶的動能達到5000焦耳，那么轉子將以熱能的形勢消耗5000焦耳。取決于轉子的大小和冷卻系統(tǒng)的好壞，這些能量會很容易導致過熱。</p><p><b>　　2 感應電機的制動</b></p><p>　　在一些工業(yè)上的應用中，感應電機和它的負載需要在瞬間停止。這可以通過相互交換定子的兩個引線來實現(xiàn)，一旦交換，旋轉場馬上轉變?yōu)橄鄬D子旋轉的

52、反方向。在這個制動過程當中，電機充當了剎車閘的作用。</p><p>　　電機吸收了仍在旋轉負載的動能，使它自己的速度降了下來。相關的機械能Pm在轉子當中完全地耗盡。遺憾的是，轉子仍然從定子接收電磁能量Pr，這些能量也以熱能的形式發(fā)散。因此，制動在轉子上產生了的消耗，在轉子被鎖住的時候甚至超過了這個值。電機不能被頻繁地制動，因為轉子的高溫會熔化轉子鐵片或者使定子線圈過熱?？紤]到這些，有必要記住一下無機械性加載電機

53、的制動操作規(guī)則：</p><p>　　規(guī)則2：轉子在制動過程當中（從額定轉速到零轉速）散發(fā)的熱量是原來所有旋轉部分所含動能的三倍。</p><p><b>　　3 感應電機的剎車</b></p><p>　　一個感應電機和它的高慣性負載同樣可以由定子繞組回路中的電流來進行快速制動。任何的兩個定子端口都可以接在直流電源上。</p>

54、<p>　　直流電流會在定子上產生恒定的N極和S極，產生的磁極的數(shù)量跟電機在正常情況下產生的磁極數(shù)相等。因此，一個三相四極的感應電機不管其端口是如何接在直流電源上的，它都能產生4個直流磁極。</p><p>　　當轉子經過這個恒定磁場時，在轉子鐵片的上會產生一個交流電壓。這個電壓將產生一個交流電流，這個電流會在轉子上產生的能量損耗而轉移到旋轉部分的動能中。當所有的動能都以熱能的形式的耗盡的時候，電機就停

55、下來了。</p><p>　　直流剎車燈的優(yōu)點是它產生的熱量遠遠低于上一節(jié)的制動方法。這種方法非常有效，因為它在轉子中所消耗的能量只相等于一開始儲存在旋轉部分的動能，而不是像上節(jié)的制動方法那樣是這個數(shù)字的三倍。這個能量損耗跟強直流電流是沒有關系。然而，用較弱的直流電流來剎車會增加剎車時間。故直流電流可以是額定電流的二到三倍，如果假設定子尚未變得過熱，這個電流甚至可以更高。剎車轉矩同比于直流剎車電流的平方。<

56、/p><p><b>　　4 非正常情況</b></p><p>　　電機的非正常運轉可能是由于內部的原因（定子的短路、軸承過熱等）或者外部環(huán)境的問題引起的。外部問題可能由以下原因引起：</p><p><b>　　機械性過載</b></p><p><b>　　供電電壓的改變</b&g

57、t;</p><p><b>　　單相失相運行</b></p><p><b>　　頻率的變動</b></p><p>　　我們將在下一節(jié)探討一下這些問題的本質。</p><p>　　由國家標準可知，電機在電壓浮動不超過正常電壓的＋10％，頻率浮動不超過正常頻率的＋5％是才能正常運行。如果電壓頻率同

58、時變動，它們的浮動百分比之和絕對不能超過10％。最后還要提到的是，所有的電機都是被設計成可在海拔1000米的地方正常工作，那么在高海拔地區(qū)因為空氣較為稀薄，冷卻力就較為薄弱，所以電機溫度可能會超過允許極限。</p><p><b>　　5 機械性過載</b></p><p>　　盡管標準感應電機可以在短時間內產生兩倍于額定的功率，但它們不能一直工作在超過額定能力的狀態(tài)

59、。過載會產生過熱，導致絕緣部分被破壞，減短了電機的工作壽命。實際上，過載會引起在電機箱中的繼電器過熱，并引起繼電器的斷開，最后使電機在溫度變得過高之前就停下來。</p><p>　　某些具有防水功能的電機被設計成可持續(xù)加載超過15％的過負載。這種過負載能力可以在銘牌上的保險系數(shù)上看到。這時就允許防水電機的溫度比加載正常電阻時升高。</p><p>　　在緊急狀況下，一個防水電機可以加載12

60、5％的過負載，只要在電機外部安裝了額外的通風設備。但是我們還是不建議長時間的過載，因為盡管電機外殼已經被冷卻，但是內部繞組的溫度可能已經超過了。</p><p><b>　　6 線電壓的改變</b></p><p>　　由線電壓的改變引起的最大的問題是它對轉矩速度特征曲線的影響。在任何速下的轉矩實質上總是和應用電壓的平方成正比的。因此，如果定子電壓降低了10％，那么任

61、何速度下的轉矩都將降低將近20％。由于啟動電流比較大，所以線性電壓壓降一般都形成于啟動過程當中。故由于低壓的原因，啟動轉矩一般都比額定值小了很多。</p><p>　　另一方面，如果電機在運行過程當中電壓過高，各個磁極上的磁通就會超過正常值。由于電機一直運行在全荷負載，將增加鐵損耗和磁化電流，隨之產生輕微的升溫和功率因素的降低。</p><p>　　如果三相電壓出現(xiàn)了不平衡情況，會在線電路

62、引起嚴重的線電流的不平衡。這種情況將增加在定子和轉子上的損耗，導致高溫產生。如果一個電壓不平衡情況盡管只有3.5％，還是會引起的升溫。當電壓之間的相位之差大于2％的時候，就必須要通知廠家了。</p><p><b>　　7 失相運行</b></p><p>　　如果三相線電路中有一路出現(xiàn)了開路，或者三相電機在運行時有一個熔斷器被斷開，電機會像單相電機一樣繼續(xù)運行。在剩

63、下的兩條電路中流過的電流將變成原來的兩倍，電機會由此過熱。保護電機的熱動繼電器會斷開電路中的斷路器，從而使電機從線電路中斷開。</p><p>　　當電機在失相運行時，轉矩速度特征曲線會收到嚴重的影響。制動轉矩將降低到原來大小的大約40％，電機也不會產生啟動轉矩。因此一個全荷負載的三相電機如果其中一條線路忽然斷開，那么它會很容易就停下來。被鎖轉子的最終電流是正常的三相LR電路電流的90％，因此它足夠大去關斷電路的

64、斷路器或者燒掉熔斷器。</p><p>　　圖14.11 三相鼠籠電機在正常運行時和在單相失相情況下的標準轉矩－速度曲線（圖略）</p><p>　　圖14.11表示了一個三相電機在正常運行和單相失相時的典型的轉矩－速度特征曲線。我們注意到這兩個特征曲線緊緊地互相跟隨，直到轉矩接近單相極限轉矩。</p><p><b>　　8 頻率的變動</b>

65、;</p><p>　　在大型的分布式系統(tǒng)中除非經歷在一個巨大的干擾，一般從不對重要的頻率進行變動，。然而，當獨立系統(tǒng)的電能是由柴油機或燃氣渦輪產生時，它的頻率或許會進行巨變。醫(yī)院的緊急供電設備，船上的電力系統(tǒng)，伐木場中的發(fā)電機等都上這種類型的供電設備。</p><p>　　頻率變動導致的最重要的結果是電機速度的改變：如果頻率降低了5％，那么電機的速度也將降低5%。</p>

66、<p>　　從標準頻率是50赫茲的國家進口的機床和其他電機驅動的裝置如果接到60赫茲的系統(tǒng)中將會出現(xiàn)問題。所有部件都比正常時運行快了20％，這在一些電器中是不允許的。在這種情況下我們必須給電機速度降擋，或者提供一個昂貴的額外的50赫茲的電源。</p><p>　　一個50赫茲的電機在60赫茲的線電路中正常運行，但是它的端口電壓必須提高到銘牌標志的6/5（或者是120％）。新的極限轉矩跟原來的極限轉矩是相