Magnetism and Electromagnetism

Magnetism

The principles of magnetism play an important role in the operation of an AC motor. Therefore, in order to understand motors, you must understand magnets. To begin with, all magnets have two characteristics. They attract iron and steel objects, and they interact with other magnets. This later fact is illustrated by the way a compass needle aligns itself with the Earth’s magnetic field (Picture 1).

Picture 1: Magnetism

Magnetic Lines of Flux

The force that attracts an iron or steel object has continuous magnetic field lines, called lines of flux, that run through the magnet, exit the north pole, and return through the south pole. Although these lines of flux are invisible, the effects of magnetic fields can be made visible. For example, when a sheet of paper is placed on a magnet and iron filings are loosely scattered over the paper, the filings arrange themselves along the invisible lines of flux (Picture 2).

Picture 2: Magnetic Lines of Flux

Unlike Poles Attract

The polarities of magnetic fields affect the interaction between magnets. For example, when the opposite poles of two magnets are brought within range of each other, the lines of flux combine and pull the magnets together (Picture 3).

Picture 3: Unlike Poles Attract

Like Poles Repel

However, when like poles of two magnets are brought within range of each other, their lines of flux push the magnets apart (Picture 4). In summary, unlike poles attract and like poles repel. The attracting and repelling action of the magnetic fields is essential to the operation of AC motors, but AC motors use electromagnetism.

Picture 4: Like Poles Repel

Electromagnetism

When current flows through a conductor, it produces a magnetic field around the conductor. The strength of the magnetic field is proportional to the amount of current (Picture 5).

Picture 5: Electromagnetism

Left-Hand Rule for Conductors

The left-hand rule for conductors (Picture 6) demonstrates the relationship between the flow of electrons and the direction of the magnetic field created by this current. If a current carrying conductor is grasped with the left hand with the thumb pointing in the direction of electron flow, the fingers point in the direction of the magnetic lines of flux. The following illustration on Picture 7 shows that, when the electron flow is away from the viewer (as indicated by the plus sign), the lines of flux flow in a counterclockwise direction around the conductor. When the electron flow reverses and current flow is towards the viewer (as indicated by the dot), the lines of flux flow in a clockwise direction.

Picture 6: The left-hand rule for conductors

Picture 7: Magnetic flux direction

Electromagnet

An electromagnet can be made by winding a conductor into a coil and applying a DC voltage. The lines of flux, formed by current flow through the conductor, combine to produce a larger and stronger magnetic field. The center of the coil is known as the core. This simple electromagnet has an air core (Picture 8).

Picture 8: Electromagnet with air core

Adding an Iron Core

Iron conducts magnetic flux more easily than air. When an insulated conductor is wound around an iron core, a stronger magnetic field is produced for the same level of current (Picture 9).

Picture 9: Electromagnet with iron core

Number of Turns

The strength of the magnetic field created by the electromagnet can be increased further by increasing the number of turns in the coil. The greater the number of turns the stronger the magnetic field for the same level of current (Picture 10).

Picture 10: Magnetic field and number of turns

Changing Polarity

The magnetic field of an electromagnet has the same characteristics as a natural magnet, including a north and south pole. However, when the direction of current flow through the electromagnet changes, the polarity of the electromagnet changes. The polarity of an electromagnet connected to an AC source changes at the frequency of the AC source. This is demonstrated in the following illustration on Picture 11.

Picture 11: Changing polarity on electromagnet


At time 1, there is no current flow, and no magnetic field is produced. At time 2, current is flowing in a positive direction, and a magnetic field builds up around the electromagnet. Note that the south pole is on the top and the north pole is on the bottom. At time 3, current flow is at its peak positive value, and the strength of the electromagnetic field has also peaked. At time 4, current flow decreases, and the magnetic field begins to collapse.
At time 5, no current is flowing and no magnetic field is produced. At time 6, current is increasing in the negative direction. Note that the polarity of the electromagnetic field has changed. The north pole is now on the top, and the south pole is on the bottom. The negative half of the cycle continues through times 7 and 8, returning to zero at time 9. For a 60 Hz AC power supply, this process repeats 60 times a second.

Induced Voltage

In the previous examples, the coil was directly connected to a power supply. However, a voltage can be induced across a conductor by merely moving it through a magnetic field. This same effect is caused when a stationary conductor encounters a changing magnetic field. This electrical principle is critical to the operation of AC induction motors. In the following illustration on Picture 12, an electromagnet is connected to an AC power source. Another electromagnet is placed above it. The second electromagnet is in a separate circuit and there is no physical connection between the two circuits.

Picture 12: Induced voltage in electromagnets


This illustration (Picture 12) shows the build up of magnetic flux during the first quarter of the AC waveform. At time 1, voltage and current are zero in both circuits. At time 2, voltage and current are increasing in the bottom circuit. As magnetic field builds up in the bottom electromagnet, lines of flux from its magnetic field cut across the top electromagnet and induce a voltage across the electromagnet. This causes current to flow through the ammeter. At time 3, current flow has reached its peak in both circuits. As in the previous example, the magnetic field around each coil expands and collapses in each half cycle, and reverses polarity from one half cycle to another.

Electromagnetic Attraction

Note, however, that the polarity of the magnetic field induced in the top electromagnet is opposite the polarity of the magnetic field in the bottom electromagnet. Because opposite poles attract, the two electromagnets attract each other whenever flux has built up. If it were possible to move the bottom electromagnet, and the magnetic field was strong enough, the top electromagnet would be pulled along with it (Picture 13).

Picture 13: Electromagnetic Attraction