Fig. 289.—A drum armature.
The field coils vary in number and position. The purpose of their construction is always to send the largest possible number of lines of force through the armature. Some dynamos are bipolar, or have two poles, others are multipolar or have more than two. In Fig. 288 No. 4 has four poles. The armature of a dynamo differs from a magneto armature in that it consists of a series of coils of insulated copper wire wound in numerous slots cut in the surface of a cylindrical piece of iron. Fig. 289 shows a side view of the iron core of such an armature. Iron is used to form the body of the armature since the magnetic lines of force flow easily through the iron. The iron by its permeability also concentrates and increases the magnetic flux. The best armatures are made of many thin sheets of soft iron. These are called laminated armatures. An armature made of a solid piece of iron becomes hot when revolving in a magnetic field. This is due to electric currents induced in the iron itself. This heating is largely reduced by laminating the armature. Why?
Fig. 290.—Armature connected to slip rings producing an alternating current.
303. Methods of Collecting Current from the Armature.—The electric currents produced in the armature are conducted away by special sliding contacts. The stationary part of the sliding contact is called a brush. The moving part is a slip ring or a commutator. Fig. 290 shows an armature coil connected to slip rings. As the armature revolves, the coils and slip rings revolve with it. The two ends of the armature coils are connected to the two rings respectively. Now as the armature revolves it cuts the lines of force first in one direction and then in the other. This produces in the coils an E.M.F. first one way and then the other. This E.M.F. sets up a current which is conducted to the outside circuits through the slip rings and brushes. Such a current which repeatedly reverses its direction is called an alternating current. Fig. 291 (1) indicates graphically how the current moves alternately one way and then the other. Alternating currents are extensively used for electric light, heat, and power. Direct currents or those going continuously in one direction are however in much demand especially for street car service, for electrolysis, and for charging storage batteries.
304. The Commutator.—For a dynamo to deliver a direct current it must carry upon the shaft of the armature a commutator. The commutator is used to reverse the connections of the ends of the armature coils at the instant that the current changes its direction in the armature. This reversal of connection when the direction of current changes, keeps the current in the outside circuit flowing in the same direction. Fig. 291 is a diagram of an armature with a commutator. The commutator is a split ring, having as many parts or segments as there are coils upon the armature. The brushes touch opposite points upon the commutator as they slide over the surface of the latter. Suppose that the armature viewed from the commutator end rotates in a counter-clockwise direction, also that the currents from the upper part move toward the commutator and out the top brush.
Fig. 291.—The armature coils are connected to a commutator producing a direct current.
As the armature revolves, its coils soon begin to cut the force lines in the opposite direction. This change in the direction of cutting the lines of force causes the current to reverse in the coils of the armature. At the instant the current changes in direction, what was the upper segment of the commutator slips over into contact with the lower brush, and the other segment swings over to touch the upper brush. Since the current has reversed in the coils it continues to flow out of the upper brush. This change in connection at the brushes takes place at each half turn of the armature, just as the current changes in direction in the coils. This is the manner in which the commutator of a dynamo changes the alternating current produced in the armature coils, into a direct current in the external circuit. Fig. 292 (1) represents graphically an alternating current, (2) of the same figure shows current taken from the brushes of the commutator of a dynamo with one coil on the armature.
