（原文：）Introduction to D.C. Machines
D.C. machines are characterized by their versatility. By means of various combinations of shunt-, series-, and separately excited field windings they can be designed to display a wide variety of volt-ampere or speed-torque characteristics for both dynamic and steady state operation. Because of the ease with which they can be controlled, systems of D.C. machines are often used in applications requiring a wide range of motor speeds or precise control of motor output.
The essential features of a D.C. machine 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 is called the field axis or direct axis.
As we know, the A.C. voltage generated in each rotating armature coil is converted to D.C. 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 D.C. armature voltage as well as an armature m.m.f. Wave then is 90 electrical degrees from the axis of the field poles, i.e. in the quadrature axis. In the schematic representation the brushes are shown in quadrature 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 90 electrical degrees from their position in the schematic diagram because of the shape of the end
connections to the commutator.)
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 continue to assume a sinusoidal flux-density wave in the air gap. The torque can then be found from the magnetic field viewpoint.
The torque can be expressed in terms of the interaction of the direct-axis air-gap flux per pole
dFa
and space-fundamental component 1of the armature m.m.f.wave.
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
TP2dFa122 （1-1）
In which the minus sign gas been dropped because the positive direction of the torque can be determined from physical reasoning. The space fundamental Fa1 of the
2sawtooth armature m.m.f.wave is times its peak. Substitution in above equation
8then gives
TPCaaia(N•m)2m （1-2）
Where, Ia=current in external armature circuit;
Ca=total number of conductors in armature winding;
m=number of parallel paths through winding.
And
KaPCa2m （1-3）
is a constant fixed by the design of the winding.
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. In which each of the rectified sine wave 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 and is the sum of the rectified voltages of all the coils in series between brushes and is shown by the rippling line labeled ea 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 ea between brushes, Known also as the speed voltage, is
eaPCadmKadm2m （1-4）
where Ka 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.
From the above equations, with all variable expressed in SI units,
eaiaTm （1-5）
This equation simply says that the instantaneous power associated with the speed voltage equals the instantaneous mechanical power with the magnetic torque. The direction of power flow being determined by whether the machine is acting as a motor or generator.
The direct-axis air-gap flux is produced by the combined m.m.f.
Nfif 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 –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 magnetization curve in terms of the armature e.m.f.
ea0 at a constant speed
m0. The voltage ea for a given flux at
any other speed m is proportional to the speed, i.e.
eamea0m0 （1-6）
There is the magnetization curve with only one field winding excited. This curve can easily be obtained by test methods, no knowledge of any design details being required.
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 being the direct-axis air-gap permeance.
The outstanding advantages of D.C. machines arise from the wide variety of operating characteristics that 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.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.
The connection diagram of a separately excited 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 often 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 series generator. The field may be connected in shunt 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 compound generator. With self-excited generators residual magnetism must be present in the machine iron to get the self-excitation process started.
In the typical steady-state volt-ampere characteristics, constant-speed prime movers being assumed. The relation between the steady state generated e.m.f. Ea and the terminal voltage
Vt is
VtEaIaRa （1-7）
where Ia is the armature current output and Ra is the armature circuit resistance. In a generator,Ea is larger than Vt and the electromagnetic 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 air-gap flux and hence the voltage vary widely with load. As a consequence, series generators are normally connected so that the m.m.f. 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 that is nearly usually contains many turns of relatively small wire. The series winding, wound on the outside, consists of a few turns of comparatively heavy conductor because it must carry the full armature current of the machine. The voltage of both shunt and
compound 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 steady-state speed-torque characteristics, it is assumed that motor terminals are supplied from a constant-voltage source. In a motor the relation between the e.m.f. Ea generated in the armature and terminal voltage Vt is
VtEaIaRa （1-8）
where Ia is now the armature current input. The generated e.m.f. Ea is now smaller than the terminal voltage Vt, the armature current is in the opposite direction to that in a generator, and the electron magnetic 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 accompanied by a very nearly proportional increase in armature current and hence by a small decrease in counter e.m.f. to allow this increased current through the small armature resistance. Since counter e.m.f. is determined by flux and speed, the speed must drop slightly. Like the squirrel-cage induction motor, the shunt motor is substantially a constant-speed motor having about 5% drop in speed from no load to full load. Starting torque and maximum torque are limited by the armature current that can be commutated successfully.
An outstanding advantage of the shunt motor is case of speed control. With a
rheostat in the shunt-field 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 e.m.f. approximately equal to the impressed terminal voltage. A maximum speed range of about 4 or 5 to I can be obtained by this method. The limitation again being commutating conditions. By variation of the impressed armature voltage, very speed ranges can be obtained.
In the series motor, increase in load is accompanied by increase in the armature current and m.m.f. and the stator field flux (provided the iron is not completely saturated). Because flux increase with load, speed must drop in order to maintain the balance between impressed voltage and counter e.m.f. Moreover, the increased in armature current caused by increased torque is varying-speed motor with a markedly drooping speed-load characteristic. For applications requiring heavy torque overloads, this characteristic is particularly advantageous because the corresponding power overloads are held to more reasonable values by the associated speed drops. Very favorable starting characteristics also result from the increase flux with increased armature current.
In the compound motor the series field may be connected either cumulatively, so that its m.m.f. adds to that of the shunt field, or differentially, so that it opposes. The differential connection is very rarely used. A cumulatively compounded motor has speed-load characteristic intermediate between those of a shunt and a series motor, the drop of speed with load depending on the relative number of ampere-turns in the shunt and series fields. It does not have disadvantage of very high light-load speed associated with a series motor, but it retains to a considerable degree the advantages
of series excitation.
The application advantages of D.C. machines lie in the variety of performance characteristics offered by the possibilities of shunt, series and compound excitation. Some of these characteristics have been touched upon briefly in this article. Still greater possibilities exist if additional sets of brushes are added so that other voltages can be obtained from the commutator. Thus the versatility of D.C. machine system and their adaptability to control, both manual and automatic, are their outstanding features.
A D.C machines is made up of two basic components:
－The stator which is the stationary part of the machine. It consists of the following elements: a yoke inside a frame; excitation poles and winding; commutating poles (composes) and winding; end shield with ball or sliding bearings; brushes and brush holders; the terminal box.
－The rotor which is the moving part of the machine. It is made up of a core mounted on the machine shaft. This core has uniformly spaced slots into which the armature winding is fitted. A commutator, and often a fan, is also located on the machine shaft.
The frame is fixed to the floor by means of a bedplate and bolts. On low power machines the frame and yoke are one and the same components, through which the magnetic flux produced by the excitation poles closes. The frame and yoke are built of
cast iron or cast steel or sometimes from welded steel plates.
In low-power and controlled rectifier-supplied machines the yoke is built up of thin (0.5～1mm) laminated iron sheets. The yoke is usually mounted inside a non-ferromagnetic frame (usually made of aluminum alloys, to keep down the weight). To either side of the frame there are bolted two end shields, which contain the ball or sliding bearings.
The (main)excitation poles are built from 0.5～1mm iron sheets held together by riveted bolts. The poles are fixed into the frame by means of bolts. They support the windings carrying the excitation current.
On the rotor side, at the end of the pole core is the so-called pole-shoe that is meant to facilitate a given distribution of the magnetic flux through the air gap. The winding is placed inside an insulated frame mounted on the core, and secured by the pole-shoe.
The excitation windings are made of insulated round or rectangular conductors, and are connected either in series or in parallel. The windings are liked in such a way that the magnetic flux of one pole crossing the air gap is directed from the pole-shoe towards the armature (North Pole), which the flux of the next pole is directed from the armature to the pole-shoe (South Pole).
The commutating poles, like the main poles, consist of a core ending in the pole-shoe and a winding wound round the core. They are located on the symmetry
(neutral) axis between two main poles, and bolted on the yoke. Commutating poles are built either of cast-iron or iron sheets.
The windings of the commutating poles are also made from insulated round or rectangular conductors. They are connected either in series or in parallel and carry the machine's main current.
The rotor core is built of 0.5～1mm silicon-alloy sheets. The sheets are insulated from one another by a thin film of varnish or by an oxide coating. Both some 0.03～0.05mm thick. The purpose is to ensure a reduction of the eddy currents that arise in the core when it rotates inside the magnetic field. These currents cause energy losses that turn into heat. In solid cores, these losses could become very high, reducing machine efficiency and producing intense heating.
The rotor core consists of a few packets of metal sheet. Redial or axial cooling ducts (8～10mm inside) are inserted between the packets to give better cooling. Pressure is exerted to both side of the core by pressing devices foxed on to the shaft. The length of the rotor usually exceeds that of the poles by 2～5mm on either side－the effect being to minimize the variations in magnetic permeability caused by axial armature displacement. The periphery of the rotor is provided with teeth and slots into which the armature winding is inserted.
The rotor winding consists either of coils wound directly in the rotor slots by means of specially designed machines or coils already formed. The winding is carefully insulated, and it secured within the slots by means of wedges made of wood or other
insulating material.
The winding overcharge are bent over and tied to one another with steel wire in order to resist the deformation that could be caused by the centrifugal force.
The coil-junctions of the rotor winding are connected to the commutator mounted on the armature shaft. The commutator is cylinder made of small copper. Segments insulated from one another, and also from the clamping elements by a layer of minacity. The ends of the rotor coil are soldered to each segment.
On low-power machines, the commutator segments form a single unit, insulated from one another by means of a synthetic resin such as Bakelite.
To link the armature winding to fixed machine terminals, a set of carbon brushes slide on the commutator surface by means of brush holders. The brushes contact the commutator segments with a constant pressure ensured by a spring and lever. Clamps mounted on the end shields support the brush holders.
The brushes are connected electrically－with the odd-numbered brushes connected to one terminal of the machine and the even-numbered brushes to the other. The brushes are equally spaced round the periphery of the commutator－the number of rows of brushes being equal to the number of excitation poles.
附：中文译文
直流电机的介绍
直流电机的特点是他们的多功用性。依靠不同的并励、串励和他励励磁绕组的组合，他们可以被设计为动态的和静态的运转方式从而呈现出宽广范围变化的伏安、－特性或速度－转矩特性。因为它简单的可操纵性，直流系统经常被用于需要大范围发动机转速或精确控制发动机的输出量的场合。
直流电机的总貌如图所示。定子上有凸极，而且由一个或几个励磁线圈励磁。气隙磁通量以磁极中心线为轴线对称分布。这条轴线叫做磁场轴线或直轴。
我们都知道，在每个旋转电枢线圈中产生的交流电压，经由一与电枢联接的旋转的换向器和静止的电刷，在电枢线圈出线端转换成直流电压。换向器－电刷组合构成了一个机械整流器，它形成了一个直流电枢电压和一个被固定在空间中的电枢磁势波形。电刷的位置应使换向线圈也处于磁极中性区，即两磁极之间。这样，电枢磁势波的轴线与磁极轴线相差90度,也就是在交轴上。在示意图中，电刷位于交轴上，因为这是线圈和电刷相连的位置。这样，电枢磁势波的轴线也是沿着电刷轴线的（在实际电机中，电刷的几何位置大约偏移图例中所示位置90度，这是因为元件的末端形状构成图示结果与换向器相连。）。电刷上的电磁转矩和旋转电势与磁通分布的空间波形无关；为了方便我们可以假设在气隙中有一个正弦的磁通密度波形。转矩可以从磁场的观点分析得到。
转矩可以用每个磁极的直轴气隙磁通d和电枢磁势波的空间基波分量Fa1相互作用的结果来表示。在交轴上的电刷和这个磁场的夹角为90度，其正弦值等于1，对于一台P极电机
TP2dFa122 （1-1）
式中带负号被去掉因为转矩的正方向可以由物理的推论测定出来。锯齿电枢磁势波的空间基波是它最大值的
82。代替上面的等式可以给出：
TPCaaia(N•m)2m （1-2）
其中：Ia=电枢外部点路中的电流；
Ca=电枢绕组中总导体数；
m=通过绕组的并联支路数；
及
KaPCa2m （1-3）
其为一个由绕组设计而确定的常数。
简单的单个线圈的电枢中的整流电压前在面已被讨论过。将绕组分散在几个槽中的效果可用图形表示，在图示中每一个整流的正弦波是在线圈中产生的电压,换向线圈边处于磁中性区。从电刷观察到的电压是电刷间所有串联线圈中整流电压的总和，在图中标以ea的文波表示。每个磁极用12个或更多换向片,可以使波动变得很小。从电刷中观测到平均产生的电压等于整流线圈电压的平均值的总和。电刷之间整流电压ea，即旋转电势为
eaPCadmKadm2m （1-4）
Ka为常数。分布绕组的整流电压与集中绕组有相同的平均值，不同的是波动大大减低了。
在上面的等式中，所有的变量都是标准国际单位制。
eaiaTm （1-5）
这个等式清楚地说明，与旋转电势相关的瞬间功率等于与磁场转矩有关的瞬时机械功率，能量的流向是由设备的确定，是发动机还是发电机。
直轴气隙磁量由励磁绕组的合成磁势
Nfif产生，其磁通—磁势曲线就是电机的具体铁磁
材料的几何尺寸决定的磁化曲线。在磁化曲线中, 假设电枢磁势波的轴线与磁场轴垂直，因此假定电枢磁势对直轴磁通不产生作用。在本文的后面有必要重新检验这一假设，饱和效应会深入研究。因为电枢电势是与磁通、时间、速度成比例，所以通常用恒定转速m0下的电枢电势ea0来表示磁化曲线更为方便。任意转速电压m时，任一给定磁通下的电压ea与转速成正比，也就是说
mema0ea0 （1-6）
图中磁化曲线只有一个励磁绕组励磁的，这种曲线可以通过测试的方法轻松获得，不需要任何设计步骤的知识。
大范围励磁下的铁磁阻与空气气隙相比可以忽略不计，在这种情况下磁通与励磁绕组的总磁势成线性比例关系，比例常数就是直轴的气隙导磁性。
直流电机的显著优势源自于通过选择励磁绕组的励磁方式而获得不同的运转方式。励磁绕组可以从外部直流电源以他励的方式励磁，也可以以自励的方式励磁。换句话，直流电机可以提供自身励磁。励磁方式不仅极大地影响它的静态特性，而且极大地影响在控制系统中电机的动态性能。
他励发电机的联接图解已经给出的。所需的励磁电流只是电枢电流中的一小部分。在励磁电路中少量的功率可以控制相对一大部分电枢电路的功率。换句话说，发电机是一个功率放大器，当需要在大范围控制电枢电压时，他励发电机通常在反馈控制系统中使用。自励发电机的励磁绕组可以有三种不同的供电方式。励磁线圈可以与电枢串联起来，这便是串励发电机；励磁绕组可以与电枢并联在一起，这便是并励发电机。也可以同时以两种方式相连接组成一个复励发电机。为了引起自励过程，在自励发电机中必须存在剩磁。
在典型的静态伏-安特性中，假定原动机速度恒定，稳态电动势与端电压之间的关系为
VtEaIaRa （1-7）
其中Ia是电枢输出电流，Ra是电枢回路电阻。在发动机中，Ea大于Vt。电磁转矩T是一个反转矩。
他励发电机的端电压随着负载电流的增大而轻微的减小，主要是因为电压在电枢电阻上的压降。串励发电机中的励磁电流与负载电流相同，所以气隙磁通和电压随负载变化很大，因此很少采用串励发电机。并励发电机电压随负载增加会有所下降，但在许多应用场合，这并不妨碍使用。复励发电机的连接通常使串励绕组的磁势与并励绕组磁势相加，其优点是通过串励绕组作用，每极磁通随着负载增加，从而产生一个随负载增加近似为常数的输出电压。通常，并励绕组匝数多，导线细；而绕在外部的串励绕组由于它必须承载电机的整个电枢电流，所以其
构成的导线相对较粗。不论是并励还是复励发电机的电压都可借助并励磁场中的变阻器在适度的范围内得到调节。
所有励磁的方法在电动机上同样适用。在电动机典型的静态转速—转矩特性中，电机端电压假设由恒压源供电，在电动机中感应的电势Ea与路端电压Vt间关系是
VtEaIaRa （1-8）
Ia是电枢输入电流。电势Ea小于端电压Vt。电枢电流与发电机中的方向相反，且电磁转矩
与电枢旋转方向相同。
对于并励与他励电动机来说，磁场磁通基本近似为常数，因此转矩的增加必须要求电枢电流近似成比例增大，同时为允许增大的电流通过小的电枢电阻，要求反电势稍有减少。由于反电势决定于磁通和转速，因此，转速必须稍稍降低。与鼠笼式感应电动机类似，并励电动机实际是一种从空载到满负荷的速度基本上只有5%的下降的恒速电动机。从起动转矩到达到最大转矩之间一直是被电枢电流所控制可以正常交替进行。
并励电动机的一个显著优点是速度控制，通过在并励绕组回路装上内部变阻器，励磁电流和每极磁通都可任意改变。而磁通的变化导致转速相反的变化以维持反电势大致等于外施加端电压。用这种方法我们可以获得最大调速范围为4或5比1，最高转速同样受到换向条件的限制。通过改变外施加电枢电压，可以获得很宽的调速范围。
对于串励电动机来说，电枢电流、电枢磁势波以及定子磁场磁通随负载增长而增长。因为由于负载增大而造成的磁通增大，速度必须降低，这样才可以维持反电势与外加电压之间的平衡。此外，由于磁通增加，所以转矩增大所引起电枢电流的增大比并励电动机中的要小。因此
串励电动机是一种具有明显下降的转速-负载特性的变速发电机。对于要求转矩过载很多的应用场合，由于对应的过载功率随相应的转速下降而维持在一个合理的范围内。因此，这种特性具有特别的优越性。磁通随着电枢电流的增大而增大，同时还带来非常有用的起动特性。
在复励电动机中，串励磁场可以连接成积复励式，使其磁势与并励磁场相加；也可以连接成差复励式，两磁场方向相反,差复励很少使用。积复励电动机具有界于串励和并励之间的速度—负载特性，转速随负载的降低取决于并励磁场和串励磁场的相对安匝数。这种电动机没有像串励电动机那样轻载高转速的缺点，但它在相当的程度上保持着串励方式的优点。
直流电机的应用优势是可以连接成串励、并励及复励式等各种励磁方式。其中的一些特性我们已在本文中的提及到了。如果增加电刷可以通过换向器获得更多的电压，那么还会存在更多的应用场合，不论对人工的还是自动控制的适应性是它们的显著特性。
一个直流电机是由两个基本元素组成：
－定子是电机固定的部分。它由以下基础组成；在结构中有一个磁轭；励磁磁极和绕组；换向极和绕组；有滑动轴承的端罩；电刷和电刷固定器；出线盒。
－转子是电机旋转的部分。它构成了一个中心，这个中心是安放在设备轴上并且已经平均地隔开，把电枢绕组放入槽中。还有一个换向器和一个风扇组成，被放在设备的轴上。
它用螺栓和底座固定在地板上。低压电机的磁轭和本身的结构是一体的，穿过励磁磁极闭合而产生的磁通量。它的结构和磁轭是用生铁和铸钢制造成的，有时候也用焊接的钢板。在低压和可控补偿整流器电机中，磁轭是由0.5～1毫米的薄铁板制成的。磁轭经常被安放在一个非铁磁性的结构内（通常是由铝合金制成，为了缩减重量）。在内部有两个端盖并且都包含球
体和滑动轴承。
励磁磁极由用0.5～1mm的铁片通过用螺栓钉牢互相支撑。磁极被放入结构内的依靠螺栓固定。它们支撑绕组，让它运送励磁流动。在电枢上，磁极铁心的末端是极靴，它通过气隙有助于磁通量的分布。绕组被放置在一个绝缘结构内的中心处，被极靴保护。
励磁绕组是由绝热的圆形物或矩形的导体制成，并且和另一个连续或平行的相连接。绕组是以一个磁极的磁通量穿过气隙，然后被指引由极靴向电枢（北极），下一个磁极的磁通量由电枢到极靴（南极）。换向极的磁极就像主磁极，它组成了一个中心，末端在极靴中并且一个绕组绕在中心周围。它们被放在两个主磁极中间的对称轴，拴在磁轭上。换向极的磁极是由生铁或铸铁制成。换向极的绕组是由周围绝缘的或垂直的导线制成。它们相互平行或首尾连接，带动设备的主电流。
转子的中心是由0.5～1毫米硅合金薄板制成。薄板是通过清漆薄膜或氧化物涂层和其他物质绝缘。绝缘物质厚度为0.03～0.05毫米。目的是当它在磁场中旋转时涡流升高时，减少涡流。它变得很热将导致能量损失。在实心物的中心，它损失得很高，减少电机的效率和产生剧烈的热量。转子的中心包含了一些金属薄片。轴向的冷却管（8～10毫米）被嵌在金属薄片中给它更好的冷却。压力施加在中心的两端。转子的长度超过磁极2～5毫米，作用是减少磁力渗透性导致的轴向位移。转子的外围提供了槽放入电枢绕组。每个转子绕组包含了一个线圈直接绕在转子槽中依靠特殊设计机械或成行的线圈。绕组是绝缘的，依靠木制的或绝缘物质制成的槽楔保护它。绕组过载是其弯曲，用钢丝相互连接为了抵抗由地心引力产生的变形。转子绕组的线圈交叉点连接到放在电枢轴的换向器上。换向器是圆柱体含有少量的铜。换向片是绝缘的。转子线圈被焊接在换向片上。低压电机的换向器片被分割成一个独立的单元，依靠合成树脂互相绝缘，例如人造树脂。为了连接电枢绕组固定接线端，一组电刷在换向器的表面上依靠支架滑动。电刷通过弹簧给予不变的压力连接换向片。卡钉安放在端盖上并且支撑碳刷支架。
奇数的炭刷连接到电机的一个接线端上，偶数的炭刷连接到另一个接线端上。炭刷围绕换向器的外围等距隔开，有多少排炭刷就有等量的励磁磁极。
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