Fig. 443.—Diagram of a "Series Motor."
443. The A.-C. Series Motors.—The only type of motor that will run on either alternating or direct current is the series motor. The "universal" motor used in household appliances such as electric fans, vacuum cleaners, etc., is a series motor. The reason a series motor will run on either direct or alternating current is because the direction of rotation of the armature of a motor depends on (a) the direction of the current in the armature, and (b) the polarity of the field. Reversing either of these alone, reverses the direction of rotation of the armature, while reversing both at the same instant leaves the direction of rotation unchanged. Fig. 443 is a diagram of a series motor since the field coils and armature are connected in series. On an a.-c. line, both field and armature current must therefore reverse at the same instant. In a shunt motor (similar to Fig. 286) we have a divided circuit, and the greater self-induction of the field coils causes an a.-c. current through these coils to lag behind that flowing in the armature so that the two currents do not reverse at the same instant.
Fig. 444.—Diagram of a gramme ring. It is shown connected to a single-phase current so as to produce a rotating magnetic field, similar to that obtained with a three-phase current. (Ahrens, Harley and Burns.)
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.)
Fig. 446.—The "rotor" of an induction motor.
