Figs. 13, 14.—Diagrams illustrating the Difference between an Electric Motor and a Generator.
An electric motor can be turned into an electric generator by simply reversing the direction in which the armature rotates—that is, any electric machine is either a generator or a motor. This fact can be illustrated by means of [Figs. 13 and 14], both of which show the armature and the poles of the field magnet. The first figure represents an electric motor, and, as can be seen, the pull between the N pole of the armature and the P pole of the field is in the direction of arrow b, hence the armature will rotate in the same direction, as indicated by arrow a. To obtain the polarity of the armature and field it is necessary to pass an electric current through both—that is to say, we must expend electrical energy to obtain power from the machine. As soon as the current ceases to flow, the polarity of the armature and field dies out, and the rotation of the former comes to an end. The magnetism, however, does not die out entirely; a small residue is always left, although it is never sufficient to produce rotation, and even if it were it could only cause the armature to revolve through one quarter of a turn. If, after the current has been shut off, the armature shaft is rotated in the reverse direction, as indicated by arrow a in [Fig. 14], the motion will be against the pull of the magnetism; therefore, although the poles may be very weak, an amount of power sufficient to overcome their attraction must be applied to the pulley, otherwise rotation can not be accomplished. In consequence of the backward rotation a current is generated in the armature coils, and this current, as it traverses the field coils as well as those of the armature, causes the polarity of both parts to increase. As a result of the increased polarity the resistance to rotation is increased, and more power has to be applied to the pulley. The increase in the strength of the poles results in increasing the current generated, and this in turn further increases the pole strength, so that one effect helps the other, the result being that the current, which starts with an infinitesimal strength, soon rises to the maximum capacity of the machine.
The motor shown in [Fig. 10] does not in any way resemble an electric railway motor, nevertheless the principle of action is precisely the same in both. The design of a machine of any kind has to conform to the practical requirements, and this is true of railway motors, just as it is true of printing presses, sawmills, or any other mechanism. A railway motor must be designed to run at a comparatively slow speed and to develop a strong rotative force, or torque, as it is technically called. It must also be so constructed that it will not be injured if covered with mud and water. It must be compact, strong, and light, and capable of withstanding a severe strain without giving out. To render the machine water- and mud-proof it is formed with an outer iron shell, which entirely incases the internal parts. The first railway motors were not inclosed, and the result was that they frequently came to grief from the effects of a shower of mud. When the modern inclosed type of motor, which is called the iron-clad type, first made its appearance it was frequently spoken of as the clam-shell type, and the name is not altogether inappropriate, for while the outside may be covered with mud to such an extent as to entirely obliterate the design, the interior will remain perfectly clean and dry, and therefore its effectiveness will not be impaired.
Fig. 15.—External View of Electric Railway Motor mounted upon Car-Wheel Axle.
To enable the motor to give a strong torque and run at a slow speed the number of poles in the field and armature is increased. The design of [Fig. 10] has two poles in the field and two in the armature, and is what is known as the bipolar type. Machines having more than two poles in each part are called multipolar machines. The number of poles can be increased by pairs, but not by a single pole—that is, we can have four, six, eight, or any other even number of poles, but not five, seven, or any odd number. This is owing to the fact that there must always be as many positive as negative poles, no more and no less. Railway motors at the present time are made with four poles. The external appearance can be understood from [Fig. 15], while [Fig. 16] and [Fig. 17] will serve to elucidate the internal construction. In [Fig. 15] the motor casing is marked M, and, as will be seen, it forms a complete shell. The motion of the armature shaft is transmitted to the car-wheel axle F through a pinion, which engages with a spur gear secured to the latter. In [Fig. 16] the pinion and gear are marked N and L respectively. As it is necessary that the armature shaft and the axle be kept in perfect alignment, the motor casing M is provided with suitable bearings for both, those for the armature shaft being marked P P in [Fig. 16], and one of those for the axle being marked T in [Fig. 15]. It will be understood from the foregoing that the motor is mounted so as to swing around the car-wheel axle as a center, but, as it is not desirable to have all this dead weight resting upon the wheels without any elasticity, the motor is carried by the crossbars B B, [Fig. 15], which rest upon springs s s at each end. The beam A and a similar one at the farther end of the B B bars extend out to the sides of the car truck and are suitably secured to the latter. The coils w w are the ends of the field coils and the armature connections, and to these the wires conveying the current from the trolley are connected. The cover C on top of the motor at one end closes an opening through which access to the commutator brushes is obtained. The armature is shown at H in [Fig. 16] and the commutator at K in the same figure. Directly under the armature may be seen one of the field magnet coils, it being marked R.
Fig. 16.—Railway Motor with Casing Open, showing Armature in Lower Half.
As will be noticed in [Fig. 16], the motor casing is made so as to open along the central line, and the lower half is secured to the top by means of hinges, g g, [Fig. 15], and also by a number of bolts, which are not so clearly shown. The gear wheels are also located within a casing, which ([Fig. 16]) is made so as to be readily opened whenever it becomes necessary. All the vital parts of the machine are entirely covered, and are not easily injured by mud or water.