Introduction-to-D.C.-Machines直流电机的介绍大学毕业论文外文文献翻译及原文

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　　翻译日期： 2017.02.14

　　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 ?d and space-fundamental component Faof the armature m.m.f.wave. With 1

　　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

　　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

　　ea?PCa?d?m?Ka?d?m （1-4） 2?m

　　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,

　　eaia?T?m （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.

　　ea??mea0 （1-6） ?m0

　　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

　　Vt?Ea?IaRa （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

　　Vt?Ea?IaR a （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