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Magnetic Diagram of a DC Motor
¡¡It will be easier to understand the operation of the DC motor from a basic diagram that shows the magnetic interaction between the rotating armature and the stationary field¡¯s coils. Figure 12-4 shows three diagrams that explain the DC motor¡¯s operation in terms of the magnetic interaction. In Fig. 12-4a you can see that a bar magnet has been mounted on a shaft so that it can spin. The field winding is one long coil of wire that has been separated into two sections. The top section is connected to the positive pole of the battery and the bottom section is connected to the negative pole of the battery. It is important to understand that the battery represents a source of voltage for this winding. In the actual industrial-type motor this voltage will come from the DC voltage source for the motor. The current flow in this direction makes the top coil the north pole of the magnet and the bottom coil the south pole of the magnet. The bar magnet represents the armature and the coil of wire represents the field. The arrow shows the direction of the armature¡¯s rotation. Notice that the arrow s
FIGURE 12-4 (a) Magnetic diagram that explains the operation of a DC motor. The rotating magnet moves clockwise because like poles repel. (b) The rotating magnet is being attracted because the poles are unlike. (c) The rotating magnet is now shown as the armature coil, and its polarity is determined by the brushes and commutator segments.
hows the armature starting to rotate in the clockwise direction. The north pole of the field coil is repelling the north pole of the armature, and the south pole of the field coil is repelling the south pole of the armature.

As the armature begins to move, the north pole of the armature comes closer to the south pole of the field, and the south pole of the armature is coming closer to the north pole of the field. As the two unlike poles near each other, they begin to attract. This attraction becomes stronger until the armature¡¯s north pole moves directly in line with the field¡¯s south pole, and its south pole moves directly in line with the field¡¯s north pole (Fig. 12-4b).

When the opposite poles are at their strongest attraction, the armature will be 'locked up' and will resist further attempts to continue spinning. For t

he armature to continue its rotation, the armature¡¯s polarity must be switched. Since the armature in this diagram is a permanent magnet, you can see that it would lock up during the first rotation and not work. If the armature is an electromagnet, its polarity can be changed by changing the direction of current flow through it. For this reason the armature must be changed to a coil (electromagnet) and a set of commutator segments must be added to provide a means of making contact between the rotating member and the stationary member. One commutator segment is provided for each terminal of the magnetic coil. Since this armature has only one coil, it will have only two terminals, so the commutator has two segments.

FIGURE 11-57 Diagram that shows the position of the six-pole rotor and four-pole stator of a typical stepper motor. (Courtesy of Parker Compumotor Division.)

Since the armature is now a coil of wire, it will need DC current flowing through it to become magnetized. This presents another problem; since the armature will be rotating, the DC voltage wires cannot be connected directly to the armature coil. A stationary set of carbon brushes is used to make contact to the rotating armature. The brushes ride on the commutator segments to make contact so that current will flow through the armature coil.

In Fig. 12-4c you can see that the DC voltage is applied to the field and to the brushes. Since negative DC voltage is connected to one of the brushes, the commutator segment the negative brush rides on will also be negative. The armature¡¯s magnetic field causes the armature to begin to rotate. This time when the armature gets to the point where it becomes locked up with the magnetic field, the negative brush begins to touch the end of the armature coil that was previously positive and the positive brush begins to touch the end of the armature coil that was negative. This action switches the direction of current flow through the armature, which also switches the polarity of the armature coil¡¯s magnetic field at just the right time so that the repelling and attracting continues. The armature continues to switch its magnetic polarity twice during each rotation, which causes it to continually be attracted and repelled with the field poles.

This is a simple two-pole motor that is used primarily for instructional purposes. Since the motor has only two poles, the motor will operate rather roughly and not provide too much torque. Additional field poles and armature poles must be added to the motor for it to become useful for industry.

FIGURE 11-58 Movement of the stepper motor rotor as current is pulsed to the stator. (a) Current is applied to the top and bottom windings, so the top winding is north, (b) Current is applied to left and right windings, so the left winding is north, (c) Current is applied to the top and bottom windings, so the bottom winding is north, (d) Current is applied to the left and right windings so the right winding is north. (Courtesy of Parker Compumotor Division.)

When power is applied, it is directed to only one of the stator pairs of windings, which will cause that winding pair to become a magnet. One of the coils for the pair will become the north pole, and the other will become the south pole. When this occurs, the stator coil that is the north pole will attract the closest rotor tooth that has the opposite polarity, and the stator coil that is the south pole will attract the closest rotor tooth that has the opposite polarity. When current is flowing through these poles, the rotor will now have a much stronger attraction to the stator winding, and the increased torque is called holding torque.

By changing the current flow to the next stator winding, the magnetic field will be changed 90°. The rotor will only move 30° before its magnetic fields will again align with the change in the stator field. The magnetic field in the stator is continually changed as the rotor moves through the 12 steps to move a total of 360°. Figure 11-58 shows the position of the rotor changing as the current supplied to the stator changes.In Fig. ll-58a you can see that when current is applied to the top and bottom stator windings, they will become a magnet with the top part of the winding being the north pole, and the bottom part of the winding being the south pole. You should notice that this will cause the rotor to move a small amount so that one of its south poles is aligned with the north stator pole (at the top), and the opposite end of the rotor pole, which is the north pole, will align with the south pole of the stator (at the bottom). A line is placed on the south-pole piece that is located at the 12 o¡¯clock position in Fig. ll-58a so that you can follow its movement as current is moved from one stator winding to the next. In Fig. ll-58b current has been turned off to the top and bottom windings, and current is now applied to the stator windings shown at the right and left sides of the motor. When this occurs, the stator winding at the 3 o¡¯clock position will have the polarity for the south pole of the stator magnet, and the winding at the 9 o¡¯clock position will have the north-pole polarity. In this condition, the next rotor pole that will be able to align with the stator magnets is the next pole in the clockwise position to the previous pole. This means that the rotor will only need to rotate 30° in the clockwise position for this set of poles to align itself so that it attracts the stator poles.

In Fig. ll-58c you can see that the top and bottom stator windings are again energized, but this time the top winding is the south pole of the magnetic field and the bottom winding is the north pole. This change in magnetic field will cause the rotor to again move 30° in the clockwise position until its poles will align with the top and bottom stator poles. You should notice that the original rotor pole that was at the 12 o¡¯clock position when the motor first started has now moved three steps in the clockwise position.

In Fig. ll-58d you can see that the two side stator windings are again energized, but this time the winding at the 3 o¡¯clock position is the north pole. This change in polarity will cause the rotor to move another 30° in the clockwise direction. You should notice that the rotor has moved four steps of 30° each, which means the rotor has moved a total of 120° from its original position. This can be verified by the position of the rotor pole that has the line on it, which is now pointing at the stator winding that is located in the 3 o¡¯clock position.

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