Page:The New International Encyclopædia 1st ed. v. 06.djvu/652

DYNAMO-ELECTRIC MACHINERY. 568 DYNAMO-ELECTRIC MACHINERY. be supplied at a constant potential irrespective of the load. In separately excited or shunt- wound machines the regulation of the current to constant potential is usually performed by in- serting a rheostat or variable resistance in cir- cuit with the field winding. By increasing or decreasing this resistance, usually bj- hand, the fi^ld current and, therefore, the electro motive force, are decreased or increased. In compound- wound machines the regulation is accomplished as follows: When there is no load upon the gen- erator the excitation of llic magnets is produced entirely bj' the shunt winding, but as the load is increased the current flowing through the series coils aids that in the shunt coils, thus producing stronger fields and increasing the electro-motive force at the generator terminals. Small direct- current generators, whether operated at con- stant current or constant potential, arc usually driven by belting from the engine or other prime motor. b>it large machines, in modern jjractiee. are connected directly to the shaft of the prime motor. Typical direct-current constant-potential generators are shown on the adjoining pages. The direct-current 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 ctirrents 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 the series and the shunt windings, the fieW excitation as a motor is their ditTerence. Another peculiarity of the 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- tromotive force acting, but 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 under the influence of si])plied 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 indiiced elec- 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 force. Therefore this pressure, which is induced in the armature of a motor, is called counter electro-motive force. The current in the armatijre is due and propor- tional to the difTcrcnce between the applied elec- tro-motive force and the counter electromotive force. The counter electromotive force is pro- portional to the product of the speed of the conductor and the field strength. The torque, or twisting moment, of the armature is proportional to the product of armature current aiul 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 electro-motive force. In starting up a motor an external re- sistance has therefore to be provided to take the place of the counter electro-motive force in order to keep down the current of the motor and prevent possible destructive action from it before the inertia of the arnuiture has been over- come and rotation l)cgun. This exterior re- sistance is commonly provi<led by a starting box or rheostat, which is a device by which the current is graduallj- applied. Destructive cur- renvs may also arise during operation from vari- ous causes, and to prevent damage from these, automatic attachments are often provided, which open the circuit and stop the current. To regu- late the speed of motors rheostats operated by hand mav be employed. When, however, it be- comes necessary to vary the speed of a motor to any considerable degree, an.v of the rheostat methods introduce material losses of current, and to save these losses resort has to be 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 is constant, and any diminution of speed would cause a great increase of current and torque. The speed may be varied by varjMng the ajiplied 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 apidied electro-motive 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 nuide 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 there 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 technically knowni as torque. The power of a motor is its torque multiplied by the nuudier of revolutions. Probably the form of motor which is used in the greatest numbers is the streel-niil- uay motor. A series-wound motor operat- ing on a constant-potential circuit is imi- versally used for street-railway work. These machines have to be unusually sturdy to with- stand the conditions of service which require them to endure the shocks and jolts 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. 1.3. are usually constructed with four poles, but use only two commutator brushes, placed in such a position as to be easily accessible. The field frame which carries the poles is made of such shape as to inclose all the working parts and thus protect them from dirt and moisture.