Introduction to Magnetism
Magnetism is another form of force that causes electron flow or current. A basic understanding of magnetism is necessary to study electricity. Magnetism provides a link between mechanical energy and electricity. The use of magnetism causes an alternator to convert some of the mechanical power developed by an engine to electromotive force (EMF). Magnetism will allow a starter motor to convert electrical energy from a battery into mechanical energy for cranking the engine.
The Nature of Magnetism
Most electrical equipment depends directly or indirectly upon magnetism. There are a few electrical devices that do not use magnetism.
There are three basic types of magnets:
The Chinese discovered magnets about 2637 B.C. The magnets that were used in the primitive compasses were called lodestones. Lodestones were crude pieces of iron ore. The lodestones are known as magnetite. Since magnetite has magnetic properties in the natural state, lodestones are classified as natural magnets.
Man-made magnets are typically produced in the form of metal bars. These bars have been subjected to very strong magnetic fields. All man-made magnets are produced. These man-made magnets are sometimes referred to as artificial magnets.
A Danish scientist named Oersted discovered a relation between magnetism and electric current. Oersted discovered that an electric current flowing through a conductor produced a magnetic field around the conductor.
Every magnet has two points opposite each other which attract pieces of iron. These points are called the poles of the magnet: the north pole and the south pole. Just like electric charges repel each other and opposite charges attract each other, like magnetic poles repel each other and unlike poles attract each other.
A magnet clearly attracts a bit of iron because of some force that exists around the magnet. This force is called a magnetic field. Although it is invisible to the naked eye, the force can be shown by sprinkling small iron filings on a sheet of glass or paper over a bar magnet. In Illustration 1, a piece of glass is placed over a magnet and iron filings are sprinkled on the glass. When the glass cover is gently tapped the filings will move into a definite pattern, which shows the field force around the magnet.
The field seems to be made up of lines of force that appear to leave the magnet at the north pole. The lines of force travel through the air around the magnet. The lines of force continue through the magnet to the south pole to form a closed loop of force. The stronger the magnet is, the greater the lines of force are, and the larger the area covered by the magnetic field.
Lines of Force
To better visualize the magnetic field without iron filings, the field is shown as lines of force in Illustration 3. The direction of the lines outside the magnet shows the path a north pole would follow in the field, repelled away from the north pole of the magnet and attracted to the south pole. Inside the magnet, which is the generator for the magnetic field, the lines are from south pole to north pole.
Lines of Magnetic Flux
The entire group of magnetic field lines is called magnetic flux. The flux density is the number of magnetic field lines per unit of a section perpendicular to the direction of flux. The unit is lines per square inch in the English system. The unit is lines per square centimeter in the metric system. One line per square centimeter is called a gauss.
Magnetic lines of force pass through all materials. There is no known insulator against magnetism. Flux lines pass more easily through materials that can be magnetized than through those that cannot be magnetized. Materials that do not readily pass flux lines are said to have high magnetic reluctance. Air has high reluctance. Iron has low reluctance.
An electric current flowing through a wire creates magnetic lines of force around the wire. Illustration 4 shows lines of small magnetic circles that form around the wire.
Because such flux line are circular, the magnetic field has no north pole or no south pole. Individual circular fields merge when the wire is wound into a coil. The result is a unified magnetic field with north and south poles, as shown in Illustration 5.
As long as current flows through the wire, it behaves just like a bar magnet. The electromagnetic field remains as long as current flows through the wire. The field that is produced on a straight wire does not have enough magnetism in order to do work. To strengthen the electromagnetic field, the wire can be formed into a coil. The magnetic strength of an electromagnet is proportional to the number of turns of wire in the coil, and the current flowing through the wire. Whenever electrical current flows through the coil of wire, a magnetic field, or lines of force, build up around the coil. If the coils are wound around a metal core, like iron, the magnetic force strengthens considerably.
Relays and Solenoids
Types of electromagnets that are typically used in Caterpillar machines are relays and solenoids. Relays and solenoids operate on the electromagnetic principle, but function differently. Relays are used as an electrically controlled switch. A relay is made up of an electromagnetic coil, a set of contacts, and an armature. The armature is a movable device that allows the contacts to open and to close. Illustration 6 shows the typical components of a relay.
When a small amount of electrical current flows in the coil circuit, the electromagnetic force causes the relay contacts to close. This process provides a much larger current path to operate another component, such as, a starter.
A solenoid is another device that uses electromagnetism. Like a relay, the solenoid also has a coil. Illustration 7 shows a typical solenoid. When current flows through the coil, electromagnetism pushes or pulls the core into the coil creating linear, or back and forth movements. Solenoids are used to engage starter motors, or control shifts in an automatic transmission.
The effect of creating a magnetic field with current has an opposite condition. It is also possible to create current with a magnetic field by inducing a voltage in the conductor. This process is known as electromagnetic induction. Electromagnetic induction happens when the flux lines of a magnetic field cut across a wire or any conductor. It does not matter whether the magnetic field moves or the wire moves. When there is relative motion between the wire and the magnetic field, a voltage is induced in the conductor. The induced voltage causes a current to flow. When the motion stops, the current stops.
If a wire is passed through a magnetic field, voltage is induced. The voltage induced strengthens when the wire is wound into a coil. This method is the operating principle that is used in speed sensors, generators, and alternators. In some cases, the wire is stationary and the magnet moves. In other cases, the magnet is stationary and the field windings move.
Movement in the opposite direction causes current to flow in the opposite direction. This causes back and forth motion to produce AC voltage (current).
In practical applications, multiple conductors are wound into a coil. This concentrates the effects of electromagnetic induction and makes it possible to generate useful electrical power with a relatively compact device. In a generator, the coil moves and the magnetic field is stationary. In an alternator, the magnet is rotated inside a stationary coil.
The strength of an induced voltage depends on several factors:
- The strength of the magnetic field
- The speed of the relative motion between the field and the coil
- The numbers of conductors in the coil
Means of Induction
There are three ways voltage can be induced by electromagnetic induction:
- Generated Voltage
- Mutual Induction
Illustration 9 shows a simple DC generator that is used to show a moving conductor that passes a stationary magnetic field to produce voltage and current. A single loop of wire is rotating between the north and south poles of a magnetic field.
Self-induction occurs in a current carrying wire when the current flowing through the wire changes. Since the current flowing through the conductor creates a magnetic field around the wire that builds up and collapses as the current changes, a voltage is induced in the conductor. Illustration 10 shows self-induction in a coil.
Mutual induction occurs when the changing current in one coil induces a voltage in an adjacent coil. A transformer is an example of mutual induction. Illustration 11 shows two inductors that are relatively close to each other. When an AC current flows through coil L1 a magnetic field cuts through coil L2 inducing a voltage and producing current flow in coil L2.