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GENERATORS AXD MOTOl
One type of synchronous phonograph motor has no
brushes. Its armature consists only of a metal disc. Alternating current in the field windii . <1<1\
currents (Page 86 I in this disc; the direction of the eddy currents is such that the portion of the disc nearest a field piece is repelled and a more remote portion attracted : hence, the disc revolves. Since the eddy currents in the discs are generated by a.c. excitation of the fields, the rate of rotation is strictly controlled by the frequency of the applied current and cannot vary, [f excessive load is applied the motor stops; it cannot slow down.
(21) Synchronous motors used to drive the projector mechanism may be regarded essentially as being trans formers which have secondary windings so mounted that they are free to rotate. Current is induced in the anna ture windings by virtue of a.c. flowing through the field coils; the magnetic reaction between armature and field currents causes the armature to rotate. The rotation is controlled by the frequency of the applied current. A kind of "squirrel cage" construction, conlisting of copper bars embedded in the pole faces and connected at their ends, helps steady the rotation. For this reason such motors are sometimes known a "squirrel cage" motors. Polyphase motors of this type are selfstarting, but the single-phase motors used for projector operation would not be, except for the addition of a brush connection to the armature. The alternating current supplied to the armature through the brushes is nol in phase with the a.c. generated in the armature by induction, hence a motor of this kind is further distinguished by the title of split-phase motor.
(22) Synchronous motors are used for projector operation (also frequently for phonographs) becaus the need for constant speed to assure good reprodu
of sound. The frequency of commercial power lin< very accurately regulated', and hence can be trusted t<> assure constant speed operation of all motors which depend upon the frequency rather than applied voltage for speed control. Voltage is still rather poorly regulated in commercial supply lines.