Introduction to D.C. Machines
D.C. machines are characterized by their versatility. By means of various combinations of shunt-, ries-, and parately 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. Becau of the ea with which they can be controlled, systems of D.C. machines are often ud in applications requiring a wide range of motor speeds or preci control of motor output.
lowbatteryThe esntial 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 for
怪兽婆婆ms a mechanical rectifier, resulting in a D.C. armature voltage as well as an f. Wave then is 90 electrical degrees from the axis of the field poles, i.e. in the quadrature axis. In the schematic reprentation the brushes are shown in quadrature axis becau this is the position of the coils to which they are connected. The 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 becau of the shape of the end connections to the commutator.)
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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 expresd in terms of the interaction of the direct-axis air-gap flux per pole and space-fundamental component of the f.wave. With the brushes in the quadrature axis the angle between the fields is 90 electrical degrees, and its sine equals unity. For a pole machine
（1-1）
In which the minus sign gas been dropped becau the positive direction of the torque can be determined from physical reasoning. The space fundamental 英语六级分值分布明细 of the sawtooth f.wave is times its peak. Substitution in above equation then gives
（1-2）
Where, =current in external armature circuit;
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=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 winding.
promptnessThe rectified voltage generated in the armature has already been discusd before for an elementary single-coil armature. The effect of distributing the winding in veral 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 obrved from the brushes and is the sum of the rectified voltages of all the coils in ries between brushes and is shown by the rippling line labeled in figure. With a dozen or so commutator gments per pole, the ripple becomes very small and the average generated voltage obrved 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
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（1-4）
少年中国梦where 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 expresd in SI units,
（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 f. of the field windings. 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. Becau the f. is proportional to flux times speed, it is usually more convenient to express the magnetization curve in terms of the f. at a constant speed . The voltage for a given flux at any other speed 运动虚脱 is proportional to the speed, i.e.
（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 wit
h that of the air gap. In this region the flux is linearly proportional to the f. of the field windings, the constant of proportionality being the direct-axis air-gap permeance.
The outstanding advantages of D.C. machines ari from the wide variety of operating characteristics that can be obtained by lection of the method of excitation of the field windings. The field windings may be parately excited from an external D.C. source, or they may be lf-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. hope to e you soon
