A DC motor is the simplest motor that can be found. For this reason, they are used in a myriad of applications. Such examples of their use are in radio-controlled vehicles, electric cars, and fans. How does a DC motor work?
A DC motor works by converting electric power into mechanical work. This is accomplished by forcing current through a coil and producing a magnetic field that spins the motor. The simplest DC motor is a single coil apparatus, used here to discuss the DC motor theory. The process can be explained in further detail by observing the diagram below. In this diagram, the voltage source forces voltage through the coil via sliding contacts or brushes that are connected to the DC source. These brushes are found on the end of the coil wires and make a temporary electrical connection with the voltage source. In this motor, the brushes will make a connection every 180 degrees and current will then flow through the coil wires. In the 0 degrees diagram, the brushes are in contact with the voltage source and current is flowing. The current that flows through wire segment C-D interacts with the magnetic field that is present and the result is an upward force on the segment. The current that flows through segment A-B has the same interaction, but the force is in the downward direction. Both forces are of equal magnitude, but in opposing directions since the direction of current flow in the segments is reversed with respect to the magnetic field. In the 180 degrees diagram, the same phenomenon occurs, but segment A-B is forced up and C-D is forced down. In the 90 and 270-degree diagrams, the brushes are not in contact with the voltage source and no force is produced. In these two positions, the rotational kinetic energy of the motor keeps it spinning until the brushes regain contact.
Problems with a DC Motor
One drawback to the motor shown in the diagram is the large amount of torque ripple that it has. The reason for this excessive ripple is because of the fact that the coil has a force pushing on it only at the 90 and 270 degree positions. The rest of the time the coil spins on its own and the torque drops to zero. The torque curve produced by this single coil DC motor is represented in Figure 1 below. As more coils are added to the motor, the torque curve is smoothed out as seen in Figure 2, the diagram of the torque curve for a two-coil DC motor.
Figure 1.
In the diagram below, the resulting torque curve never reaches the zero point and the average torque for the motor is greatly increased. As more and more coils are added, the torque curve approaches a straight line and has very little torque ripple and the motor runs much more smoothly. Another method of increasing the torque and rotational speed of the motor is to increase the current supplied to the coils. This is accomplished by increasing the voltage that is sent to the motor, thus increasing the current at the same time.
Figure 2.
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