Page:The New International Encyclopædia 1st ed. v. 06.djvu/655

* DYNAMO-ELECTEIC MACHINERY. 569 DYNAMO-ELECTRIC MACHINERY. It is geueral practice to equip eacli trolley car with at least two motors ami to re>;ulate the speed of the car in the I'oUowing manner: First the two motors and a resistance are connected in Fig. 13. BAn.wAT motok (g. e. form). Ijower frame dropped showing armature in position. 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 runnius connec- tion. To increase the speed further, the motors are placed in parallel with a resistance in series with both. Tins 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 Fl«. 14. COSTKOLLEK. by placing a small resistance in shunt with the fields when all the series resistance is out. This reduces the field (lux and causes a higher arma- ture speed to maintain the necessary counter electromotive force. . oar governed in this way has four running connections. On heavy ones, siich as are used in elevated-railway or intcrurban ser-ice, four motors are used on each car. In this case the motors are governed in two series-parallel combinations, as if there were two separate cars governed by one controller. The diH'erent connections are made by a motor- man, wlio operates a handle on top of a con- troller. Kacli Qill'erent combination is called a point or notch. .V pointer fixed to the con- tr'iller-liandle 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 street-car controller is shown in Fig. 14. The wires from the trolley, from the field, from the armature, and from 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 (•(innecting board there is also a switch, which enables an injured motor to be cut out without interfering with the operation of the other. From the connecting board conductors are run to terminals called fingers or wipes. Mounted on an 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 scries which are arranged so as to enable the motorman to reverse the direction of rotation of both motors or to cut them out entirely. For power distribution in factories both series and sliunt motors are used. The series motor is employed w-here a powerful starting efl'ort is re- quired, as on cranes and hoists, but where close speed regulation is unimportant. The speed of the sliunt motor may he controlled by controlling the applied electro-motive force, as explained above, and the shunt motor is thus especially adapted for driving machines at constant speed. The characteristics of the compound-wound motor are intermediate between those of the shunt and series motors. CoxvEKTERS. DyN.MOTORS, Boo.STERS, ETC. Be- fore passing to the subject of alternating ma- chines proper, some types of machines possess- ing certain special properties which make them extremely usefvil devices under certain circum- stances will be briefly considered. If the arma- ture winding of any direct-current dynamo be tapped at two points which are opposite in phase, that is, represented by a distance equal to that between two consecutive field-pcdes, and these taps are led to two slip rings upon the shaft of the machine, alternating currents may be drawn therefrom. If the machine be run as a motor by supplying it with direct current de- livered to the direct-current brushes, then alternating currents maj' be drawn from the slip rings. If, after the machine is brought up to full speed, alternating currents of the same periodicity, or the same number of alternations per minute, as the machine will produce when run as a generator, be supplied to the slip rings, it will run as a s'nchronous motor, and direct current can be drawn from the regular direct- current bru.shcs. To summarize, then, by simply tapping the armature winding of a direct -current dynamo in the manner descril)€d, a machine is produced which will act as an alternating-cur-