Fig. 447.—Diagram illustrating the principle of the synchronous motor. The armature coil passes the position shown in the figure at the instant the current in the line reverses. Thus the armature keeps with the line current, making one revolution with each "cycle."
445. A synchronous motor is one that keeps step with the alterations of an alternating current. The line current is fed into the armature by means of two slip rings and brushes. The principle of the synchronous motor is illustrated in Fig. 447. This shows a motor having a two-pole field. The armature current must be reversed twice in each revolution. The reversal must take place when the armature winding is perpendicular to the lines of force of the field. In a direct current motor this reversal is brought about by the commutator. In a synchronous motor the armature reaches the 90 degree position at the exact instant at which the current reverses in the line. Thus in the case of a two-pole motor the armature must make exactly one revolution for each cycle; it is, therefore, a constant speed motor. Such motors are frequently employed in converter stations where alternating current is converted into direct current by what are called rotary converters.
In practice the synchronous motor has a number of pairs of field poles. It is essentially an alternating current generator running as a motor. One of the principal uses of the synchronous motor is that of a converter, receiving alternating current and delivering direct current. Synchronous motors are also used in transmission lines to aid in maintaining constant voltage.
Important Topics
The wireless telephone, essential parts, action, arrangement.
Alternating currents, alternating fields.
Transformers, voltage relation of coils, power and core losses.
Self-induction, inductance, and coke coils, uses, applications.
Impedance, reactance, and resistance; relation and effects.
Condensers, uses and applications with a-c. circuits.