Figure 4 illustrates one method by which the commutation function might be accomplished. Rather than hard wiring the current source to the coil, the current is conducted through sliding contacts (brushes) connected to the current source. The brushes ride on the ends of the coil wires, thus conducting current through the coil. In this simplified motor, the brushes switch coil connections about once every 180¡ of rotation. Therefore, the direction of current flow remains fixed with respect to the magnetic field.
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| Figure 4 |
Figure 5 illustrates the coil of the single-coil motor at angular positions of 0 degrees, 90 degrees, 180 degrees, and 270 degrees. At 0 degrees, the ends of the coils are contacting the "brushes", and current flows through the coil. The interaction of the current flow in coil segment CD with the magnetic field produces a force upward on segment CD. The current flowing through segment AB interacts with the magnetic field to produce a force downward. The two forces are identical in magnitude but opposite in direction since the direction of the current flow in the two segments are reversed with respect to the magnetic field. As mentioned earlier, there is no net force produced on coil segment AC. As a result of the forces on segments AB and CD, there is a net rotational force (torque) clockwise on the coil.
As the coil reaches the 90 degree position, the coil ends lose contact with the "brushes", and there is no current flowing in the coil. Therefore, there is no force produced on any of the coil segments, and the crude motor depends upon the rotational kinetic energy of the coil to rotate it past the 90 degree position.
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| Figure 5 |
As the coil approaches the 180 degree position, the coil ends contact the "brushes" again, and current flow is reestablished. The current flow in segment AB interacts with the magnetic field to produce a force upward in segment AB. The current flow in segment CD produces a downward force on that coil segment. This pair of forces produces an angular acceleration of the coil in the clockwise direction.
At the 270 degree position, there is, once again, no current flow in the coil, and the coil continues to rotate only due to its own inertia.
The crude motor developed in the preceding paragraphs has several design flaws which prevent its use in most practical applications. Perhaps its most limiting feature is the large amount of torque ripple produced during operation. Figure 6 is an illustration of the motor torque output as a function of angular position.
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| Figure 6 |
At each 90 degree and 270 degree position, the generated torque drops momentarily to zero. If a second coil is added to the structure of the first so that the two coils are 90 degrees apart, the torque generated from the two coils would be represented by the curves in Figure 7.
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| Figure 7 |
A simplified curve for the output of the two segment motor is shown in Figure 8. With this construction, the output torque never reaches zero but is smoothed out at some nominal value.
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| Figure 8 |
The number of coil segments can be (and usually is) increased well beyond two coil segments. The resulting motor has very little torque ripple.
A second problem with the motor previously developed is that it is modeled using a single conductor in each coil. In practice, the motor coils contain multiple conductors, each adding to the torque production as the single conductor model has been shown to do. The use of multiple conductors in the coils is discussed further in the next section.
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