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To understand motor theory, we need to cover the underlying principles of magnetic fields, current flow, and induced motion.
NOTE: There are two theories regarding the flow of current. Electron Flow Theory states that current flows from negative to positive. Conventional Flow Theory states that current flows from positive to negative. This module uses Electron Flow Theory. For more information on these theories, see Module 2, Fundamentals of Electricity.
Magnetic Fields Between the poles of a magnet, there exists a magnetic field. The direction of the magnetic field is called Magnetic Flux. Magnetic flux moves from the north pole to the south pole, as shown in Figure 2.
Figure 2. Lines of Magnetic Flux Flow from North Pole to South Pole Current Flow Now, let’s consider a wire (conductor) with an electric current flowing through it. A magnetic field surrounds the wire , as shown in Figure 3.
Figure 3. Left Hand Flux Rule: Lines of Magnetic Flux Surround a Conductor Understanding the direction of the magnetic flux around the conductor is critical to understanding motor motion. The direction of the magnetic flux can be determined using the Left Hand Flux Rule. Imagine grasping the wire with your left hand, making sure your thumb points in the direction of the current flow. Your fingers will curl around the wire in the direction of the magnetic flux. In Figure 3, the current is flowing into the page, so the lines of flux rotate counterclockwise around the wire. Induced Motion When this current-carrying conductor is placed between the poles of a magnet, both magnetic fields distort. In Figure 4, the conductor will tend to move upward because the current is flowing into the page. The force exerted upward depends on the strength of the magnetic field between the poles of the magnet, and the strength of the current through the conductor. A simple method for determining the direction of motion is the Right Hand Motor Rule. In Figure 4, the index finger points in the direction of the magnetic flux (N to S), the middle finger points in the direction of current flow through the conductor, and the thumb points in the direction of the conductor movement.
Figure 4. Right Hand Rule: Wire is Moved Upward This means that if you know the direction the current is flowing, and the orientation the poles, you can determine which way the conductor will move through the magnetic field . Applying the right hand motor rule to Figure 4, the conductor will move upward through the magnetic field. If the current through the conductor were to be reversed, the conductor would move downward. Note that the conductor current is at a right angle to the magnetic field . This is required to bring about motion because no force is felt by a conductor if the current and the field direction are parallel. Now, suppose we change the single conductor into a simple coil or loop of wire. This coil is called an Armature, and is shown in Figure 5.
Figure 5. Armature Rotating Both sections of the armature AB and CD have a force exerted on them. Why does the coil want to move in a counterclockwise motion? Recall that the magnetic flux rotates around the conductors. Armature sections AB and CD have the current flowing in opposite directions. This means the magnetic flux flows around them in opposite directions, as shown in Figure 6.
Figure 6. Magnetic Flux Around the Armature Sections When the magnetic field of the magnets are put in the picture, the two magnetic fields distort. A turning force, or Torque, acts on the coil. The lines of force act like stretched rubber bands that tend to contract. The result is that the armature rotates in a counterclockwise direction. Figure 7 illustrates a cross-sectional view of the induced motion.
Figure 7. Creating Torque: A Cross Section The interaction between the two magnetic fields causes a bending of the lines of force. Where the lines straighten out, they cause the armature to rotate. The left conductor (AB) is forced downward, and the right conductor (CD) is forced upward, causing a counterclockwise rotation. Commutator As we mentioned earlier, when the armature is positioned so that the loop sides are at right angles to the magnetic field, a turning force is exerted. But what happens when the coil rotates 180°? A problem arises here. The magnetic field in the conductor is now opposite that of the field, and this will tend to push the armature back the way it came, stopping the rotating motion. To solve this problem, some method must be employed to reverse the current in the armature every one-half rotation so that the magnetic fields will work together to maintain a positive rotation. A device called a Commutator performs this task. Two stationary Brushes, one supplied with positive DC current, the other with negative DC current, supply current to the two rotating commutator segments. As the armature and commutator rotate together, the commutator reverses the direction of the current through the armature. In this way, magnetic fields are always running in the direction needed to contribute to a continuing turning effort. Figure 8. The Commutator Reverses the Current Through the Armature
Now we are getting somewhere. With the armature continuously rotating through the magnetic field, mechanical energy is created from electrical energy.
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