Electromagnetism and Motor Principles

Understanding the fundamental laws of magnetism is the necessary first step in gaining a clear concept of the principles upon which the electric motor operates. It it is this relationship that is crucial to operating motors of all kinds.

A simple, yet effective example that demonstrates a magnet is a compass. The earth is a permanent magnet where the north magnetic pole points to somewhere in northern Canada. The compass needle always points toward this magnetic north pole. The compass, therefore, is an instrument that can give an indication of magnetism.

Magnetic Attraction and Repulsion

A typical magnet typically has two spots of attraction where one points to the north and the other points to the south. We will call the north-seeking pole "N" for short and the south-seeking pole "S". Observe the diagram below:

If N of a magnet is brought near the N of a stationary magnet, as shown above, the poles repel each other. This same concept applies if two S poles are brought together. However, if an N pole is brought near the S pole of a magnet in motion, the two unlike poles will attrat each other. The same holds true if the S pole is brought near the N pole. Therefore, we can establish that the like poles will repel one another while the opposite poles will attract.

Effects of an Electric Current

Expanding on this topic, see the diagram below that shows a coil connected to a battery:

The compass points to one end of the coil, but if the battery connections are reversed the compass then points away from that end. Thus, the direction of the current through the coil affects the compass in the same way as the permanent magnet in the previous experiment.

In the early 1800's, Hans Christian Oersted, the Danish physicist, discovered the relationship between electricity and magnetism. He observed that when a conductor connecting the poles of a battery was held over a compass needle, the north pole of the needle went in a certain direction. If the wire was placed under the compass, the needle deflected in the opposite direction.

Magnetic Field of an Electric Current

Oersted's experiments also proved something else: Electric current sets up a magnetic field at right angles to the conductor. If a strong current is sent through a vertical wire that passes through a horizontal piece of plywood on which iron fillings are placed, a gentle tap of the plywood results in the iron fillings forming a ring around the vertical wire. And if a compass was placed on this board, it would indicate the direction of the lines of force, even if it is placed in various locations.

Principles of Electromagnetic Induction

A magentic field that moves past a stationary conductor (coil) causes a reversal in electric flow. This changing magnetic field that causes a potential difference (voltage) in a conductor is known as Electromagnetic Induction. The electric current that is produced by moving the coil in a magnetic field is referred to as induced current. And current carrying wires experience force in a magnetic field.

Electric induction does much more than create an electric field. It is used in generators, motors and the construction of all sorts of machinery. Perhaps the most common use of electromagnetic induction are transformers. These devices are used to transform low voltage current into high voltage current and vice versa. In transformers, a copper wire is wrapped around an iron bar which creates a coil of many turns. When electricity flow through coils, it creates a magnetic field that expands outward. The iron bar in this case becomes an electromagnet.

One final but important thing to note is that the current created in this conductor is alternating current because it flows back and forth. This is the result of the conductor being raised and lowered in the magnetic field. This is why 99% of electrical distribution is alternating current, not direct current.

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