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.
Fig. 300.—The frame and electromagnet (at left), front bracket and brush holder (at right) of the motor shown in Fig. 299.
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?