The Practical Motor
Magnets and Currents
A motor constructed in accordance with earlier Section would be of little value in practical everyday affairs; its armature rotates too slowly and with too little force. If a motor is to be of real service, its armature must rotate with sufficient strength to impart motion to the wheels of trolley cars and mills, to drive electric fans, and to set into activity many other forms of machinery.
The strength of a motor may be increased by replacing the singly coiled armature by one closely wound on an iron core; in some armatures there are thousands of turns of wire. The presence of soft iron within the armature causes greater attraction between the armature and the outside magnet, and hence greater force of motion. The magnetic strength of the field magnet influences greatly the speed of the armature; the stronger the field magnet the greater the motion, so electricians make every effort to strengthen their field magnets. The strongest known magnets are electromagnets, which, as we have seen, are merely coils of wire wound on an iron core. For this reason, the field magnet is usually an electromagnet.
When very powerful motors are necessary, the field magnet is so arranged that it has four or more poles instead of two; the armature likewise consists of several portions, and even the commutator may be very complex. But no matter how complex these various parts may seem to be, the principle is always that stated in Section 309, and the parts are limited to field magnet, commutator, and armature.
The motor is of value because by means of it motion, or mechanical energy, is obtained from an electric current. Nearly all electric street cars, are set in motion by powerful motors placed under the cars. As the armature rotates, its motion is communicated by gears to the wheels, the necessary current reaching the motor through the overhead wires. Small motors may be used to great advantage in the home, where they serve to turn the wheels of sewing machines, and to operate washing machines. Vacuum cleaners are frequently run by motors.
FIG. - A modern power plant.
FIG. - The electric street car.