The direct-cur•ent machine used as a motor will now be considered. If a dynamo-electric machine which has been connected for working as a generator be supplied with a current from an exterior source instead, it will run as a motor. the direction of the rotation depending upon the manner of the field excitation. A series machine. since both the armature and field currents are then reversed, will run in the opposite direction from that in which it was driven as a generator. A shunt-wound machine will rotate in the same direction when run as a motor as it did when driven as a generator, for. while the armature current is reversed. the field current remains un changed in direction. A compound-wound machine will rotate in the same direction or in the opposite direction as a motor that it did as a generator, according as the shunt winding or the series winding is the more powerful; and while the field excitation as a generator was the sum of series and the shunt windings. the field excitation as a motor is their difference. Another peculiarity of t he dynamo-electric machine when operated as a motor which re quires to be noted is that in such a machine operating as a generator there is only one elec tro-motive force hut in the same machine operating as a motor there are two electro motive forces acting. The reason for this is that the armature of a motor revolving in a field uniter the influence of supplied electrical energy differs in no respect from the same armature revolving in a field under the influence of sup plied mechanical energy: .there is an electro motive force induced in it exactly as is done in a generator. The direction of this induced rice tro-motive force is, however. such as to tend to send a current in a direction opposite to the current flowing under the influence of the exter nal supply of electro-motive form. Therefore this pressure. which is induced in the armature of a nnotor. is enlled counter electro-motive force.
The eurrent in the armature is due and propor tional to the difference between the applied elec. tro-motive force and the counter eleetro-motive force. The counter electro-motive force is pro portional to the product of the speed of the conductor :Ind the field strength. The torque. or twisting of the armature is proportional to the product of armature current and field strength. It is evident that when the armature of a motor is at rest there is no counter electro motive force acting. and that the highest speed it may reach is that at which the counter electro motive force equals the applied electromotive force. In starting up a motor an external re sistance has therefore to be provided to take the place of the counter ele•tro-motive force in order to keep down the current of the IMAM' and prevent possible destructive action from it. hefore the inertia of the armature has been over come and rotation begun. This exterior re sistance is commonly provided by a starting box or rheostat, which is a device by which the current is gradually applied. Destructive cur rents may also arise during operation from vari ous causes, and to prevent damage from these, automatic attachments are often provided. 1\ Mehl open the circuit and stop the current. To regu late the speed of motors rheostats operated by hand may be employed. When. however. it be comes necessary to vary the speed of a motor to any eonsideralde degree, any of the rheostat methods introduce material losses of current, and to save these losses resort has to he made to other methods of regulation. It should be noted that a shunt motor when its armature is supplied with current at a constant potential runs at a practically constant speed, since the field strength constant, and any diminution of speed would cause a great increase of current and torque. The speed may he varied by varying the applied electro-motive force, as above described, or by other means. The series motor has no proper speed or torque. since the field strength varies with the armature current. that is, for a varying load at the constant applied electromotive force it has a varying speed and torque, the lighter the load the greater the speed and the less the cur rent and torque. Use is made above of the term
torque, and this perhaps requires some explana tion before proceeding to consider specific forms of motors. When a street-railway motor. for example, is first thrown into current thew is noth ing but the resistance to limit the amount of current flowing. the result being that at first a powerful current flows through the armature and field coils. This produces a very intense field, which in turn exerts great force upon the armature. The effort to turn the armature. or twisting moment, is teehnieally known as torque. The power of a motor is its torque nmItiplied by the number of revolutions.
Probably the form of motor which is used in the greatest numbers is the street-rail way motor. A series-wound motor operat ing on a constant•potential circuit is uni versally used for street railway work. These machines have to be unusually sturdy to with• stand the eonditions of service NvhicL require them to endure the shocks and ,jolt< due to rough track, as well as the dust. mud. slush. and rain of storms and dirty streets. and often the abuse of ignorant or careless motormen. Street-rail way motors. one of which is illustrated by Fig. 13. are usually constructed with four poles, hut use only t WO commutator brushes, placed in such a position as to he easily aecessible. The field frame which carries the poles is made of such shape as to inclose all the working parts and thus protect them from dint and moisture.
It is general practice to equip each trolley car with at least two motors and to regulate the speed of the car in the following Ina liner : First the two motors and a resistance arc connected in series. The resistance is then cut out step by step until the two motors are operating in series on 500 volts. This is called a running connec tion. To increase the speed further, the motors are placed in parallel with a resistance in series with both. This resistance is then cut out step by step until the motors are each operating on 500 volts. This again constitutes a running con nection. A further change is sometimes effected by placing a small resistance in shunt with the fields when all the series resistance is out. This reduces the field flux and causes a higher a•ma ture speed to maintain the eleetro-motive force. .1 car governed in this way has four running connections. on heavy ones, such as are used in elevated-railway or internrhan service, four motors are used on each car. In this ease the motors are governed in Iwo series-parallel combinations, as if there were two separate ears governed by one controller. The different conne0.ions are madc by a motor man, who operates a handle on tor of a con troller. Each different combination is called a point or notch. A pointer fixed to the con troller-handle indicates at what notch the car is running. A controller is almost invariably placed at each end of the ear, and is familiar to every one as an upright cylindrical device with a crank-like handle on its top. The in terior of an ordinary controller is shown in Fig. I t. The wires from the trolley, from the field. from the armature. and fr the different terminals of the series and shunt re sistance are brought up under the car to terminals on a connecting board in the bottom of the controller. On this connecting board there is also a switch, \Odell enables an injured motor to be cut out l\unbent interfering with the operation of the other. From the connecting board conductors are run to terminals called lingers or wipes. Mounted on sun insulating cylinder, which may be revolved by the con troller-handle, are insulated contact series. which at various angular positions of the cylinder make electrical connections between various wipes and give the proper connections for the various points or notches. A smaller cylinder connected to a reversing-lever is situated to the right of the main cylinder. This has con tact series which are arranged so as to enable the motorman to reverse the direction of rotation of both motors or to cut them out entirely.