【正文】 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 terminals are supplied from a constantvoltage source. In a motor the relation between the . generated in the armature and the terminal voltage is Where is now the armature current input. The generated . is now smaller than the terminal voltage , the armature current is in the opposite direction to that in a motor, and the electromagnetic 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 saturated). Because flux increases with load, speed must drop in order to maintain the balance between impressed voltage and counter .。 similarly and .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 before this ideal transformation is applied. The behavior of a transformer as detected at both sets of terminals is the same as the behavior detected at the corresponding terminals of this circuit when the appropriate parameters are inserted. The slightly different representation showing the coils and 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.Very little error is introduced if the magnetizing branch is transferred to the primary terminals, but a few anomalies will arise. For example, 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 error. With this simplified circuit, the primary and referred secondary impedances can be added to give: And It should be pointed out that the equivalent circuit as derived here is only valid for normal operation at power frequencies。 . ,。 ., the generator is a power amplifier. Separately excited generators are often used in feedback control systems when control of the armature voltage over a wide range is required. The field windings of selfexcited 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 two sections, one of which is connected in series and the other in shunt with the armature, resulting in a pound generator. With selfexcited generators residual magnetism must be present in the machine iron to get the selfexcitation process started.In the typical steadystate voltampere characteristics, constantspeed prime movers being assumed. The relation between the steadystate generated . and the terminal voltage is Where the armature is current output and is the armature circuit resistance. In a generator, is large than 。這樣的話，互感磁通只經(jīng)歷很小的變化，并且實(shí)際上只需要與空載時(shí)相同的凈磁勢。盡管變化很小，但是如果沒有互感磁通和電動(dòng)勢的變化來允許一次側(cè)電流變化，那么二次側(cè)的需求就無法滿足。在空載情形下，如果忽略電阻，幅值大小不變，因?yàn)樗蕴峁┮粋€(gè)等于的反總電動(dòng)勢。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。如果將一次側(cè)匝數(shù)作為參考匝數(shù)，那么這種過程稱為往一次側(cè)的折算。在選擇折算基準(zhǔn)時(shí)，無非是將一次側(cè)與折算后的二次側(cè)匝數(shù)設(shè)為相等，除此之外再?zèng)]有什么更要緊的了。如果將激磁支路移至一次繞組端口，引起的誤差很小，但一些不合理的現(xiàn)象又會(huì)發(fā)生。以此為分析基礎(chǔ)的電路會(huì)擁有一個(gè)既產(chǎn)生電壓大小的變化，也產(chǎn)生相位移的匝比，其阻抗也會(huì)與繞組的阻抗不同。由于直流電機(jī)易于控制，因此該系統(tǒng)用于要求電動(dòng)機(jī)轉(zhuǎn)速變化范圍寬或能精確控制電機(jī)輸出的場合。在示意圖中，電刷位于交軸上，因?yàn)榇颂幷桥c其相連的線圈的位置。電樞磁勢鋸齒波的空間基波是峰值的8/。將上述幾式中的所有變量用SI單位制表達(dá)，有 這個(gè)等式簡單地說明與速度電壓有關(guān)的瞬時(shí)功率等于與磁場轉(zhuǎn)矩有關(guān)的瞬時(shí)機(jī)械功率，能量的流向取決于這臺電機(jī)是電動(dòng)機(jī)還是發(fā)電機(jī)。勵(lì)磁繞組可以由外部直流電源單獨(dú)激磁，或者也可自勵(lì)，即電機(jī)提供自身的勵(lì)磁。 在典型的靜態(tài)伏安特性中，假定原動(dòng)機(jī)恒速運(yùn)行，穩(wěn)態(tài)電勢和端電壓關(guān)系為： 式中為電樞輸出電流，為電樞回路電阻。任何用于發(fā)電機(jī)的勵(lì)磁方法都可用于電動(dòng)機(jī)。并勵(lì)電動(dòng)機(jī)的突出優(yōu)點(diǎn)是易于調(diào)速。磁通隨著電樞電流的增大而增大，同時(shí)還帶來非常有用的起動(dòng)特性。23