Figs. 1,585 to 1,588.—Synchronous motor principles: I. A single phase synchronous motor is not self-starting. The figures show an elementary alternator and an elementary synchronous motor, the construction of each being identical as shown. If the alternator be started, during the first half of a revolution, beginning at the initial position ABCD, fig. 1,585, current will flow in the direction indicated by the arrows, passing through the external circuit and armature of the motor, fig. 1,586, inducing magnetic poles in the latter as shown by the vertical arrows. These poles are attracted by unlike poles of the field magnets, which tend to turn the motor armature in a counter-clockwise direction. Now, before the torque thus set up has time to overcome the inertia of the motor armature and cause it to rotate, the alternator armature has completed the half revolution, and beginning the second half of the revolution, as in fig. 1,587, the current is reversed and consequently the induced magnetic poles in the motor armature are reversed also. This tends to rotate the armature in the reverse direction, as in fig. 1,588. These reversals of current occur with such frequency that the force does not act long enough in either direction to overcome the inertia of the armature; consequently it remains at rest, or to be exact, it vibrates. Hence, a single phase synchronous motor must be started by some external force and brought up to a speed that gives the same frequency as the alternator before it will operate. A single phase synchronous motor, then, is not self-starting, which is one of its disadvantages; the reason it will operate after being speeded up to synchronism with the alternator and then connected in the circuit is explained in figs. 1,589 to 1,592.

3. With respect to speed, as
a. Constant speed;
b. Variable speed.
4. With respect to structural features, as
a. Enclosed;
b. Semi-enclosed;
c. Open;
d. Pipe ventilated;
e. Back geared;
f. Skeleton frame;
g. Riveted frame;
h. Ventilated; etc.

Of the above divisions and sub-divisions some are self-defining and need little or no explanation; the others, however, will be considered in detail, with explanations of the principles of operation and construction.

Synchronous Motors.—The term "synchronous" means in unison, that is, in step. A so called synchronous motor, then, as generally defined, is one which rotates in unison or in step with the phase of the alternating current which operates it.

Strictly speaking, however, it should be noted that this condition of operation is only approximately realized as will be later shown.

Any single or polyphase alternator will operate as a synchronous motor when supplied with current at the same pressure and frequency as it produces as a generator, the essential condition, in the case of a single phase machine, being that it be speeded up to so called synchronism before being put in the circuit.

In construction, synchronous motors are almost identical with the corresponding alternator, and consist essentially of two elements: