For power distribution in factories both series and shunt motors are used. The series motor is employed where a powerful starting effort is re quired, as on cranes and hoists, but where close speed regulation is unimportant. The speed of the shunt motor may he controlled by controlling the applied electromotive 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.
Cox \ EirrEns. DTNAMOTORS, BOOSTEDS, ETC. Be fore passing to the subject of alternating ma chines proper. some types of machines possess ing special properties which make them extremely useful devices under certain circum stances will he briefly considered. if the arma ture winding of any direct-current dynamo be tapped at two points which are in phase. that is, represented by a distance equal to that het \veto' two consecutive lield-poles. 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 it current de livered to the direct-eurrent brushes, then alternating currents may be drawn from the slip rings. If. after the machine is brought up to full speed. alternating currents of the same periodieity, or the same number of alternations per minute, as the machine will produce when run as a generator, he supplied to the slip rings, it will rim as a synchronous motor. and direct current en II be (Ira W11 from the reg,ular direct ern-rent brushes. To summarize, then, by simply tapping the armature winding of a direet-current dynamo in the manner described, a machine is prodneed which will act as an alternatMg-cur rent .enerator as well: which will transform
direct current into alternating. and which will also transform alternating current into direct current. According as the machine is used for tir.t, second, or third of these purposes it is called a double-current generator, an inverted converter. or a rotary converter. As these ma chines are most commonly employed to trans form alternating current into direct current, they arc commonly termed rotary concerters. (Mc of the principal uses of rotary converters is ill electric-railway operation, where the distance of transmission is great. For long-distance trans mission alternating current is desirable. since it may he raised easily to a higher potential, as will be explained below. The current fur operat ing the car-motors is usually, however. direct, current, so that when energy is transmitted in the torn of alternating currents, rotary converter substations are placed at intervals along the line to the alternating cur rent to direct current for distribution to the trolley-line. A dynamotor is a transforming device combining both motor and generator ac tion in one magnetic field by employing two armatures or one armature having two separate windings. These machines enable one to take direct current from a system of supply at one voltage and deliver it at another voltage to a circuit when it is to be utilized. They are used in electroplating works aml for various minor purposes. acnerators are transforming de vices of two machines, a motor and generator, mechanically connected together. A booster is a machine inserted in series in a cir cuit to change its voltage, and may be driven by an electric motor or otherwise. Boosters are used extensively in street-railway systems.