International projectionist (Jan-Dec 1945)

Projectionists' Course on Basic Radio and Television By M. BERINSKY, E. E. MEMBER OF INSTITUTE OF RADIO ENGINEERS X DIRECT CURRENT MOTORS KNOWLEDGE of electric motors is of great importance to the projectionist and to the radio serviceman. It is the purpose of this article to present some of the principles on the theory and applications of direct current motors. The action of a direct current motor is similar to that of a direct current movable coil instrument. In the movable coil instrument the coil is prevented from making a complete revolution, but in the motor a continuous motion of the movable coil is necessary. Special means are provided so that the motion of the coil will be continuous, this being accomplished by the use of a commutator. Principle of the Motor A motor is a machine for converting electrical energy into mechanical energy. This action is opposite from that of the generator which is a device for converting mechanical energy into electrical energy. A direct current motor may also be used as a direct current generator. A motor depends upon electromagnetism for its operation. As a matter of fact, the most important application of the electromagnet is found in the electric motor. Figure 1 (a) shows a magnetic field having uniform strength or intensity in which is placed a conductor that carries no current. The field is not affected by the conductor and simply goes through the copper wire in the direction indicated by the arrows. In Figure 1 (b) the conductor is shown as carrying current toward the reader, but the field due to the north and south poles has been removed. A cylindrical magnetic field now exists about the conductor due to the current in it. The direction of this field is shown to be clockwise. This field is not distorted but exists in concentric circles because the main field has been removed. The resultant field obtained by combining the main field and that due to the current is shown in Figure 1 (c). The field attributed to the current in the wire acts in conjunction with the main field above the conductor, but is in opposition to the main field below the conductor. The effect is to crowd the flux into the space directly above the conductor and to reduce the flux density in the region directly below the conductor. Owing to the concentration of flux above the conductor, a force acts on the conductor trying to push it down, as indicated by the arrow. In Figure 1 (d) the current in the conductor is indicated by an arrow head. This means that it is flowing in the direction away from the reader. Under these conditions the field due to the current in the wire acts in opposition to the main field above the conductor while the same field acts in conjunction with the main field below the conductor. As shown in the illustration, the crowding N S (B) FIGURE 1. Force acting on a conductor carrying current in a magnetic field. of the lines of force below the conductor will tend to move it upward. To summarize, the operation of the electric motor depends upon the principles illustrated in Figure 1. A conductor that carries a current will have a thrust exerted upon it when placed in a magnetic field. The direction of the thrust will be at right angles to the magnetic field. A definite relation exists between the direction of the magnetic field, the direction of the current through a conductor in that field, and the resulting motion of the conductor. This relation is known as "Fleming's left-hand motor rule." Fleming's left-hand rule reads as follows: Place the thumb, forefinger, and middlefinger of the left hand so that they are at right angles to each other. Point the forefinger in the direction of the field or flux, the middlefinger in a direction opposite to the electron flow through the conductor, and the thumb ivill point in the direction in which the conductor tends to move. This rule is illustrated in Figure 2. Another convenient method for determining this relation is to make use of the fact that the crowding of the magnetic lines behind the conductor tends to push it along, as shown in Figure 1. Torque When an armature or flywheel is rotating about its center, a tangential force is necessary to produce and maintain motion. This force may be developed within the machine itself, or it may be developed by external means. In a motor this force is developed within the machine; the same is true of a steam engine. The product of this force and the radial distance from the center of the shaft to the line of action of the force is called the torque. In the English system of units, torque is expressed in pounds-feet. It is this torque which provides the turning moment for the motor. In Figure 1 the effect of the distorted flux was shown for only one conductor. In practice, the conductor is replaced by a coil. This coil has two conductors 18 INTERNATIONAL PROJECTIONIST