Part 52 (1/2)

[Ill.u.s.tration: FIG. 293.--DeLaval multi-stage turbine and gear driving 750-kw., 750-r.p.m., 600-volt direct-current generator.]

A practical dynamo, however, has many coils upon its armature with a corresponding number of segments upon the commutator. (See Figs. 289 and 293.) As each coil and commutator segment pa.s.ses a brush, it contributes an impulse to the current with the result that armatures with many coils produce currents that flow quite evenly. (See Fig. 292, 3.)

The current represented in Fig. 292 (2) is called a _pulsating_ current.

[Ill.u.s.tration: FIG. 294.--A wire carrying a current across a magnetic field is pushed sideways by the field.]

=305. The electric motor= is a machine which transforms the energy of an electric current into mechanical energy or motion. The _direct current motor_ consists of the same essential parts as a direct current dynamo, viz., the field magnet, armature, commutator and brushes. Its operation is readily comprehended after one understands the following experiment:

Set up two bar electromagnets with unlike poles facing each other about an inch apart. A wire connected to a source of current is hung loosely between the poles as in Fig. 294. The circuit through the wire should contain a key or switch. If a current is sent through the electromagnets and then another is sent through the wire, the latter will be found to be pushed either up or down, while if the current is reversed through the wire it is pushed in the opposite direction. These results may be explained as follows:

Consider the magnetic field about a wire carrying a current (See Fig.

295.) If such a wire is placed in the magnetic field between two opposite poles of an electromagnet (Fig. 296), the wire will be moved either up or down. The reason for this is shown by the diagram in Fig.

297. Here a wire carrying a current and therefore surrounded by a magnetic field pa.s.ses across another magnetic field. The two fields affect each other causing a crowding of the force lines either above or below the wire. The wire at once tends to move sideways across the field away from the crowded side. In the figure, the wire tends to move downward.

[Ill.u.s.tration: FIG. 295.--The magnetic field about a wire carrying a current.]

[Ill.u.s.tration: FIG. 296.--The magnetic field between two unlike poles.]

[Ill.u.s.tration: FIG. 297.--The crowding of the lines of force above the wire, pushes it downward.]

In a practical motor, the wires upon the armature are so connected that those upon one side (see Fig. 298), carry currents that pa.s.s in, while on the other side they pa.s.s out. To represent the direction of the current in the wires, the following device is employed; a circle with a cross (to represent the feather in the tail of an arrow) indicates a current going away from the observer, while a circle with a dot at its center (to represent the tip of an arrow) indicates a current coming toward the observer.

[Ill.u.s.tration: FIG. 298.--The crowding of the lines of force causes the armature to revolve in a clockwise direction.]

In Fig. 298 the north pole is at the left and the south pole at the right. The field of the magnets therefore pa.s.ses from left to right as indicated in the figure. Now in the armature the currents in the wires on the left half of the armature are coming toward the observer while those on the right move away. Applying the right-hand rule, the magnetic lines will crowd _under_ the wires on the left side of the armature while they will crowd _over_ the wires on the right side. This will cause a rotation up on the left side and down on the right, or in a _clockwise_ direction.

[Ill.u.s.tration: FIG. 299.--View of a one-half horse-power motor.]

If the current in the armature is reversed (in on the left and out on the right), the lines of force will crowd the armature around in the opposite direction or _counter clockwise_. The rotation of the armature will also be reversed if, while the current in the armature is unchanged in direction, the poles of the magnet are changed thus reversing the magnetic field.

The motorman of a street car reverses the motion of his car by reversing the direction of the current in the _armature_ of the motor.

[Ill.u.s.tration: FIG. 300.--The frame and electromagnet (at left), front bracket and brush holder (at right) of the motor shown in Fig. 299.]

[Ill.u.s.tration: FIG. 301.--The armature of a motor.]

=306. Practical motors= have many coils upon the armature with a corresponding number of segments upon the commutator. A large number of coils and commutator segments enables some one of the coils to exert its greatest efficiency at each instant, hence a steady force is provided for turning the armature which causes it to run smoothly. Fig. 299 represents a 1/2 horse-power motor ready for use while Fig. 300 shows the frame and poles and the front bracket and brush holder, and Fig. 301 represents the armature.

Important Topics

1. The dynamo, four essential parts, action (a) for alternating currents, (b) for direct currents.

2. The electric motor: (a) essential parts, (b) action.

Exercises

1. _Why_ is an alternating current produced in the armature of a dynamo?

2. _How_ is this current produced? Give careful explanations.