Identifier: cyclopediaofappl04amer (find matches)
Title: Cyclopedia of applied electricity : a general reference work on direct-current generators and motors, storage batteries, electrochemistry, welding, electric wiring, meters, electric lighting, electric railways, power stations, switchboards, power transmission, alternating-current machinery, telegraphy, etc.
Year: 1916 (1910s)
Authors: American Technical Society
Subjects: Electric engineering
Publisher: Chicago : American Technical Society
Contributing Library: Northeastern University, Snell Library
Digitizing Sponsor: Northeastern University, Snell Library

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ther as are the primary andthe secondary of an alternating-current transformer. The result is,that when the field coils have many turns of wire, a dangerouslyhigh electromotive force may be induced in them. This production of high voltages in the field coils of a self-started polyphase synchronous motor may be, to a great extentavoided, by using few turns of large wire in the field winding, thusnecessitating the use of a low-voltage exciter. For this reason ex-citers giving an electromotive force as low as 50 volts are frequentlyused. Another method of obviating the danger referred to, is toprovide short-circuited metal rings around the field poles. Theserings limit the changes of magnetism in the pole-pieces, and therebyprevent the formation of excessively high induced voltages in thefield coils. In synchronous motors of the stationary field type, the fieldcircuit may be broken up into many separate parts so as to divideUP the induced electromotive force. Thus, Fig. 198 shows the 234
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ALTERNATING-CURRENT MACHINERY 223 stationary field of an alternator (synchronous motor) with the ter-minals of each field spool brought out to convenient switches onthe frame of the machine. During starting these switches are open,and when the machine has reached synchronous speed, they are allclosed, thus connecting all the field spools in series to the exciter. Hunting Action. When the load on a synchronous motor issuddenly increased, the motor slows down momentarily and fallsbehind the generator in phase. When the motor has fallen behindsufficiently to take in power enough to enable it to carry its load,it is still running sHghtly below synchronism; it, therefore, falls stillfurther behind, and takes an excess of power from the generator.

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