【正文】 在并勵(lì)繞組回路裝上變阻器，勵(lì)磁電流和每極磁通都可任意改變，而磁通的變化導(dǎo)致轉(zhuǎn)速相反的變化以維持反電勢(shì)大致等于外施端電壓。在電動(dòng)機(jī)典型的靜態(tài)轉(zhuǎn)速 轉(zhuǎn)矩特性中，假設(shè)電動(dòng)機(jī)兩端由一個(gè)恒壓源供電。在發(fā)電機(jī)中， aE 比 tV 大，電磁轉(zhuǎn)矩 T 是一種阻轉(zhuǎn)矩。勵(lì)磁防哪個(gè)法不僅極大地影響控制系統(tǒng)中電機(jī)的靜態(tài)特性，而且影響其動(dòng)態(tài)運(yùn)行。 直軸氣隙通由勵(lì)磁繞組的合成磁勢(shì) ffiN? 產(chǎn)生，其磁通 磁勢(shì)曲線就是電機(jī)的具體鐵磁材料的幾何尺寸決定的磁化曲線。上式變換后有 adaada iKimPCT ??? ?? 2 式中 ai =電樞外部電路中的 電流； aC =電樞繞組中的總導(dǎo)體數(shù)； m =通過(guò)繞組的并聯(lián)支路數(shù)； 且 mPCK aa ?2? 其為一個(gè)由繞組設(shè)計(jì)而確定的常數(shù)。這樣，如圖所示電樞磁勢(shì)波的軸線也是沿著電刷軸線的。 定子上有凸極，由一個(gè)或一個(gè)以上勵(lì)磁線圈勵(lì)磁。 等效電路有兩個(gè)入端口形式： （ a） 從一次側(cè)看為一個(gè) U 形電路，其折合后的負(fù)載阻抗的端電壓為 39。由于 0I 通常只是 1I 的很小一部分，所有誤差相當(dāng)小。 21 NN ? 的通常情形時(shí)的等效電路，它除了為了考慮鐵耗而引入了 mr ，且為了將 39。2I的等效繞組和負(fù)載電路替換，在正常電網(wǎng)頻率運(yùn)行時(shí)，從一次側(cè)兩端無(wú)法判斷二次側(cè)的磁勢(shì)、所需容量及銅耗與前有何差別。 22 XI 成正比。 如果將一次側(cè)匝數(shù)作為參考匝數(shù)，那么這種過(guò)程稱為往一次側(cè)的折算。39。 實(shí)際中所有的變壓器的匝數(shù)比都不等于 1，盡管有時(shí)使其為 1也是為了使一個(gè)電路與另一個(gè) 在相同電壓下運(yùn)行的電路實(shí)現(xiàn)電氣隔離。盡管現(xiàn)在 p? 是一次側(cè)和二次側(cè)磁勢(shì)的共同作用產(chǎn)生的，但它實(shí)際上與 11? 相同。交鏈二次繞組的凈磁通 s? 由于 2I 產(chǎn)生的二次側(cè)漏磁通（其與 m? 反相）的建立而被進(jìn)一步15 削弱。一次側(cè)總磁勢(shì)增加了 22NI ，它是平衡同量的二次側(cè)磁勢(shì)所必需的。 and the electromagic torque T is a counter torque opposing rotation. 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 airgap 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 . of the series winding aids that of the shunt winding. The advantage is that through the action of the series winding the flux per pole can increase with load, resulting in a voltage output which is nearly constant. Usually, shunt winding contains many turns of paratively heavy conductor because it must carry the full armature current of the machine. The voltage of both shunt and pound generators can be controlled over reasonable limits by means of rheostats in the shunt field. Any of the methods of excitation used for generators can also be used for motors. In the typical steadystate speedtorque characteristics, it is assumed that the motor 12 terminals are supplied from a constantvoltage source. In a motor the relation between the . aE generated in the armature and the terminal voltage tV is aaat RIEV ?? Where aI is now the armature current input. The generated . aE is now smaller than the terminal voltage tV , the armature current is in the opposite direction to that in a motor, and the electromagic torque is in the direction to sustain rotation of the armature. In shunt and separately excited motors the field flux is nearly constant. Consequently, increased torque must be acpanied by a very nearly proportional increase in armature current and hence by a small decrease in counter . to allow this increased current through the small armature resistance. Since counter . is determined by flux and speed, the speed must drop slightly. Like the squirrelcage induction motor, the shunt motor is substantially a constantspeed motor having about 5 percent drop in speed from no load to full load. Starting torque and maximum torque are limited by the armature current that can be mutated successfully. An outstanding advantage of the shunt motor is ease of speed control. With a rheostat in the shuntfield circuit, the field current and flux per pole can be varied at will, and variation of flux causes the inverse variation of speed to maintain counter . approximately equal to the impressed terminal voltage. A maximum speed range of about 4 or 5 to 1 can be obtained by this method, the limitation again being mutating conditions. By variation of the impressed armature voltage, very wide speed ranges can be obtained. In the series motor, increase in load is acpanied by increase in the armature current and . and the stator field flux (provided the iron is not pletely 13 saturated). Because flux increases with load, speed must drop in order to maintain the balance between impressed voltage and counter .。 capacitance effects must be taken into account whenever the rate of change of voltage would give rise to appreciable capacitance currents, dtCdVIc /? . 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 transformer as a threeor fourterminal work, 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 shift 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 behavior. There are two ways of looking at the equivalent circuit: (a) viewed from the primary as a sink but the referred load impedance connected across 39。2I , measurements from the primary terminals would be unable to detect any difference in secondary ampereturns, kVA demand or copper loss, under normal power frequency operation. There 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 1’s are interchanged for the subscript 2’s, the necessary referring constants are easily found。 222 RI Must be equal to 222RI . ）222122122 /()/( NNRNNI ?? does in fact reduce to 222RI . Similarly the stored magic energy in the leakage field )2/1( 2LI which is proportional to 2239。39。 2? , for example, by its demagizing action on m? has caused the changes on the prim