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DC motors operate on the principles of basic magnetism. Recall that a coil of wire can be magnetized when current is passed through it. When this principle was used in relay coils, the polarity of the current was not important. When the current is passed through a coil of wire to make a field coil for a motor, the polarity of the current will determine the direction of rotation for the motor.
The polarity of the current flowing through the coil of wire will determine the location of the north and south magnetic poles in the coil of wire. Another important principle involves the amount of current that is flowing through the coil. The amount of current was not important as long as enough current was present to move the armature of the relay or solenoid. In a DC motor, the amount of current in the windings will determine the speed (rpm) of the motor shaft and the amount of torque that it can produce.
Recall from basic magnetic theory that the left-hand rule of current flow through a coil of wire helps one understand that the direction of current flow will determine the magnetic polarity of the coil. The left-hand rule is used to show one a principle from which several facts can be determined.
First, the direction of current flow will determine which end of a coil of wire is negative or positive. This will determine which end of the coil will be the north pole of the magnet and which end will be the south pole. It's also easy to see that by changing the direction of the current flow in the coil of wire, the magnetic poles will be reversed in the coil. This is important to understand because the direction of the motor’s rotation is determined by the changing magnetic field.
Another basic concept about magnets that one should remember is the relationship between two like poles and two unlike poles. When the north poles of two different magnets are placed close to each other, they will repel each other. When the north pole of one magnet is placed near the south pole of another magnet, the two poles will attract each other very strongly.
The third important principle with the coil of wire is that the strength of the magnetic field can be varied by changing the amount of current flowing through the wire in the coil. If a small amount of current is flowing, a small number of flux lines will be created and the magnetic field will be relatively weak. If the amount of current is increased, the magnetic field will become stronger. The strength of the magnetic field can be increased to the point of saturation. A magnetic coil is said to be saturated when its magnetic strength can't be increased by adding more current.
Saturation is similar to filling a drinking glass with water. One can't get the level of the glass any higher than full. Any additional water that is put into the glass when it's full won't increase the amount of water in the glass. The additional water will run over the side of the glass and be wasted. The same principle can be applied to a magnetic coil. When the strength of the magnetic field is at its strongest point, additional electric current won't cause the field to become any stronger.
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