Physics Part 68

[Ill.u.s.tration: FIG. 445.--The "stator" of an induction motor.]

=444. The Induction Motor.=--Another common type of a.-c. motor is the _induction motor_. Its advantage lies in its simplicity. It has neither commutator nor brushes, the armature having no connection with an external circuit. If the wires of a three-phase line be connected to a coil wound in the form of a _gramme ring_, the connections being 120 degrees apart as in Fig. 444, the magnetic field within this coil will change in the same manner as if a magnet were spinning upon a pivot at the center of the coil. Suppose the _N_ pole at one instant is at _A_, in one-third of a cycle it moves to _B_, in another third to _C_, and in one cycle it makes a complete revolution. Thus we have a _rotating magnetic field_. If a cup of some non-magnetic metal such as aluminium or copper be placed on a pivot in the center of this coil, the cup is cut by the moving lines of force and currents are induced in it.

Because of these currents, the cup has a magnetic field of its own, and the action of the two magnetic fields is such as to pull the cup around and cause it to rotate in the same direction as that in which the field of the coil rotates. The coil represents the stationary part, the _stator_ (Fig. 445) and the cup the rotating part, the _rotor_, of an induction motor. While the cup rotates in the same direction, it does not rotate so rapidly as the magnetic field. If it should it is plain that it would not cut the lines of force. The difference between the rate of rotation of the rotor and that of the magnetic field is called the "slip." The rotating part in small induction motors is frequently made in a single casting. In large motors, it is built up of heavy copper bars. Thus, from its appearance the common form of rotor is known as the "squirrel cage" rotor. (See Fig. 446.)

[Ill.u.s.tration: FIG. 446.--The "rotor" of an induction motor.]

[Ill.u.s.tration: FIG. 447.--Diagram ill.u.s.trating the principle of the synchronous motor. The armature coil pa.s.ses 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 ill.u.s.trated 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 princ.i.p.al 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 c.o.ke coils, uses, applications.

Impedance, reactance, and resistance; relation and effects.

Condensers, uses and applications with a-c. circuits.

Alternating current power transmission; uses, advantages.

Power factor, lag, lead, volt-amperes, true watts.

Single- and three-phase currents; uses and nature of each.

Three-wire transmission systems, alternators, construction, and action.

A-c. motors, series, induction, synchronous.