Most devices in an airplane, from the starter to the automatic pilot, depend upon mechanical energy furnished by direct current motors. A direct current motor is a rotating machine, which transforms direct current energy into mechanical energy. It consists of two principal parts — a field assembly and an armature assembly. The armature is the rotating part in which current carrying wires are acted upon by the magnetic field.
Whenever a current carrying wire is placed in the field of a magnet, a force acts on the wire. The force is not one of attraction or repulsion; however, it is at right angles to the wire and also at right angles to the magnetic field set up by the magnet. The action of the force upon a current carrying wire placed in a magnetic field is shown in Figure 10-276. A wire is located between two permanent magnets. The lines of force in the magnetic field are from the north pole to the south pole. When no current flows, as in Figure 10-276A, no force is exerted on the wire, but when current flows through the wire, a magnetic field is set up about it, as shown in Figure 10-276B. The direction of the field depends on the direction of current flow. Current in one direction creates a clockwise field about the wire, and current in the other direction, a counterclockwise field.
Since the current carrying wire produces a magnetic field, a reaction occurs between the field about the wire and the magnetic field between the magnets. When the current flows in a direction to create a counterclockwise magnetic field about the wire, this field and the field between the magnets add or reinforce at the bottom of the wire because the lines of force are in the same direction. At the top of the wire, they subtract or neutralize, since the lines of force in the two fields are opposite in direction. Thus, the resulting field at the bottom is strong and the one at the top is weak. Consequently, the wire is pushed upward as shown in Figure 10-276C. The wire is always pushed away from the side where the field is strongest.
If current flow through the wire were reversed in direction, the two fields would add at the top and subtract at the bottom. Since a wire is always pushed away from the strong field, the wire would be pushed down.
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