Drawing of hand fairing a motor field.


In the last chapter, the generator was described as a device used to change MECHANICAL ENERGY into electricity. In this chapter, the motor is described as a mechanism that changes the ELECTRICITY back into MECHANICAL ENERGY.

Radiomen do not have many contacts with motors, other than by pressing a button to start or stop them. But every man in the radio rates should know and understand the principles of electric motors.

The MOTOR-GENERATOR sets that power the large transmitters use an ELECTRIC MOTOR to drive one or two GENERATORS, depending upon the model of transmitter. The motors take their power from the 110-, 220-, or 440- volt ship's supply, and the GENERATOR delivers several voltages-both a.c. and d.c.-to the transmitter.

Your ship's real source of power is the oil in the tanks.


In the boilers, burning oil changes water to steam. The steam drives a turbine, and the turbine turns the ship's generators. The emf from the generators runs the motor of the transmitter's MOTOR-GENERATOR set-and the generator changes the motor's MECHANICAL energy back into the ELECTRICAL energy to operate the transmitter.
Action of a conductor in a magnetic field.
Figure 68.-Action of a conductor in a magnetic field.
The ACTION of a motor is based upon the old, familiar law-UNLIKE POLES ATTRACT, and LIKE POLES REPEL.

To review the laws, look at figure 68. A conductor is hung in a position that will permit it to swing freely either in or out of the horse shoe magnet. Two dry cells are connected to the wire through a double-pole, double-throw switch. The switch is so connected that by throwing the switch from one set of contacts to the other the current through the conductor is reversed.

Closing the switch in one direction causes the CONDUCTOR to move INTO the magnet. And throwing the switch


in the opposite direction causes the conductor to move OUT.

The conductor's movement is caused by the COMBINED ACTION of TWO MAGNETIC FIELDS-the field around the conductor and the field of the horse shoe magnet.

Motor action.
Figure 69.-Motor action.
In the bottom drawing of figure 69A, the conductor's field and the flux of the field coil combine to CANCEL each other at the BOTTOM and ADD to each other at the TOP. This leaves a GREATER FORCE tending to move the conductor DOWN than up-and the conductor will move DOWN.

In figure 69B, the current is flowing in the opposite direction, and the effect of the field is reversed. The two fields CANCEL ON TOP and ADD on the bottom, so the conductor moves UP.


The action of a conductor in a magnetic field is known by many different names, but the term "motor action" is as good as any.


The essential parts of a d.c. motor are similar to those of a generator. Look at figure 70. The four main parts are-STATOR, ARMATURE, COMMUTATOR, and BRUSHES. A battery attached to the brushes provides the energy to drive the motor.

The differences between a d.c. motor and a generator are usually only in the manner of mounting the brushes and connecting the windings. Actually, some d.c. motors may be used as d.c. generators without any change at all.

Parts of an electric motor.
Figure 70.-Parts of an electric motor.
If you apply the MOTOR ACTION principle to the coil, the WHITE leg in figure 70 will go UP, and the BLACK leg will go DOWN.

When the loop has rotated 90° from its position in figure 70, the brushes will "slip" from one commutator


segment to the other, and the direction of the current in the loop will be reversed. The black leg will now move UP and the white leg DOWN.


While the St. Louis motor does not have any commercial uses, it does demonstrate the operation of a d.c. motor very well.

In figure 71 the STATOR (field yoke) is an electromagnet. The armature, figure 71D, is formed by winding the coil on a soft iron core.

The COMMUTATOR is a two-segment, copper ring mounted on the same shaft as the armature. Each segment

St. Louis motor.
Figure 71.-St. Louis motor.

is insulated from the shaft so that no electrical contact is made between armature core and commutator. The BRUSHES are strips of copper.

To start a cycle of rotation, look at figure 71A. The commutator is in a position to give the armature the indicated polarities. Since unlikes attract, and likes repel, the armature will rotate in a CLOCKWISE direction.

When the armature reaches the position indicated in figure 71B, N is opposite to S. This would cause the armature to stop if it were not for the commutator. The INERTIA of the armature carries the commutator far enough for the black brush to move onto the white segment, and for the white brush to move onto the black segment.

The "trading" of segments reverses the direction of the current through the coil, and this in turn reverses the polarity of the core. Now look at figure 71C-N is opposite N, and S opposite S. The REPULSION between the coil and armature fields causes the armature to continue turning.

When the armature assumes the vertical position of figure 71, the repulsion is traded for attraction of the opposite poles, and the cycle starts all over again.

While the d.c. motors used by the Navy are more elaborate in their windings and construction, the basic principle outlined here applies to the more complex types.


The a.c. motor is used more commonly than the d.c. types. The reason for this is not in the motor, but in the greater efficiency of using alternating current.

Some a.c. motors have WINDINGS, COMMUTATORS, and BRUSHES similar to those of d.c. motors. In addition a.c. motors have many variations. A few have armatures with no windings at all, just heavy bars of copper embedded in soft iron cores. Other armatures have windings but NO DIRECT electrical connection to the external source of power.


In many a.c. motors, it is INDUCTION that causes the armature to turn. A current flowing in the field coil causes a CURRENT to flow IN THE ARMATURE. The magnetic fields of these two currents oppose each other, causing the armature to turn.
Series a.c. motor.
Figure 72.-Series a.c. motor.
One feature about the operation of an a.c. motor that differs radically from d.c. types is due to PERIODIC reversal of the current in a.c. circuits.

In figure 72A, when the TOP lead is NEGATIVE and the BOTTOM POSITIVE, the left pole is North and right South. In the armature, the current is flowing in at the left, and OUT the right side.

In drawing 72B, the current has reversed itself (other half cycle), making the top lead positive and the bottom negative. The polarity of the field is reversed, and current in the armature is flowing in the opposite direction.


The REVERSING of the CURRENT in a.c. motors has the same effect on the TURNING of the armature as the "trading" of segments in d.c. motors. It REVERSES the FIELDS so that ATTRACTION and REPULSION will cause the armature to rotate.

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