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1、Introduction to D.C. MachinesD.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 stat
2、e 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
3、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 extern
4、al 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
5、 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 i
6、n 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;
7、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 and space-fundamental component of the armature m
8、.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 pole machine （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 fu
9、ndamental of the sawtooth armature m.m.f.wave is times its peak. Substitution in above equation then gives （1-2）Where, =current in external armature circuit; =total number of conductors in armature winding; =number of parallel paths through winding.And （1-3）is a constant fixed by the design of the w
10、inding.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, commutat
11、ion 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 in figure. With a dozen or so commutator segmen
12、ts 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 between brushes, Known also as the speed voltage, is （1-4）where is the design constant. The rectified volt
13、age 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, （1-5）This equation simply says that the instantaneous power associated with the speed voltag
14、e 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. of the field windings. The flux-m.m.f. Characteristic being the
15、 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
16、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. at a constant speed . The voltage for a given flux at any other speed is proportional to the speed, i.e. （1-6）There is th
17、e 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 i
18、s 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 excitatio
19、n 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
20、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
21、.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 se
22、ries 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 s
23、elf-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. and the terminal voltage
24、 is （1-7）where is the armature current output and is the armature circuit resistance. In a generator, is larger than and the electromagnetic torque is a counter torque opposing rotation.The terminal voltage of a separately excited generator decreases slightly with increase in the load current, princ
25、ipally 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 wi
26、nding 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 fe
27、w 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 b
28、e 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. generated in the armature and terminal voltage is （1-8）where is now the armature current input. The
29、generated e.m.f. is now smaller than the terminal voltage , 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. Con
30、sequently 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 s
31、lightly. 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
32、 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
33、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
34、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 marke
35、dly 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
36、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 mo
37、tor 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
38、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 th
39、is 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
40、 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 term
41、inal 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 mea
42、ns 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 rec
43、tifier-supplied machines the yoke is built up of thin (0.51mm) 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 slidi
44、ng bearings.The (main)excitation poles are built from 0.51mm 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 m
45、eant 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
46、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 po
47、les, 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 i
48、nsulated round or rectangular conductors. They are connected either in series or in parallel and carry the machines main current.The rotor core is built of 0.51mm silicon-alloy sheets. The sheets are insulated from one another by a thin film of varnish or by an oxide coating. Both some 0.030.05mm th
49、ick. 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 r
