【正文】 ng and lever. Clamps mounted on the end shields support the brush holders. The brushes are connected electrically－ with the oddnumbered brushes connected to one terminal of the machine and the evennumbered brushes to the other. The brushes are equally spaced round the periphery of the mutator－ the number of rows of brushes being equal to the number of excitation poles. 翻譯全文 直流電機(jī)的介紹 直流電機(jī)的特點(diǎn)是他們的多 功用性。 brushes and brush holders。 mutating poles (poses) and winding。 . 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 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 pound generator. With selfexcited generators residual magism 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 steady state generated . aE and the terminal voltage tV is t a a aV E I R?? （ 17） where aI is the armature current output and aR is the armature circuit resistance. In a generator, aE is larger than tV and the electromagic torque T is a counter torque opposing rotation. 7 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 airgap flux and hence the voltage vary widely with load. As a consequence, series generators are normally connected so that the . 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 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 motor terminals are supplied from a constantvoltage source. In a motor the relation between the . aE generated in the armature and terminal voltage tV is t a a aV E I R?? （ 18） where aI is now the armature current input. The generated . aE is now smaller than the terminal voltage tV , the armature current is in the opposite direction to that in a generator, and the electron magic 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% drop in speed from no load to full load. Starting torque and maximum torque are 8 limited by the armature current that can be mutated successfully. An outstanding advantage of the shunt motor is case 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 I can be obtained by this method. The limitation again being mutating conditions. By variation of the impressed armature voltage, very 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