Ac Transformers

By Ron Kurtus (17 March 2001)

A major advantage of alternating current or AC electricity over direct current (DC) is the fact that the potential energy can be changed from one voltage to another through the use of a transformer. This device not only changes high voltages to house current, but it also is used to change the voltage from house current to that used by low voltage items, such as mini-tape recorders.

Questions you may have about AC transformer are:

• How does the transformer work?

• What are the principles of electricity and magnetism involved?

• What are some uses of transformers?

This lesson will answer those questions.

Basic principles

A transformer combines several major characteristics of electricity and magnetism to allow the changing of AC voltages. First of all, you need to know the principles for creating an electromagnet and creating electricity.

Creating an electromagnet

A wire with DC electric current flowing through it has a magnetic field around it. This can be demonstrated by placing a compass near a wire and observing the needle move when the DC current is turned on.

By wrapping the wire around a piece of iron, the magnetic field is increased many times due to the realignment of the iron atoms, each which acts as a tiny magnet. The iron core and wire wrapping is called an electromagnet.

Relation to voltage

The greater the current through the wire (or the higher the voltage) the greater the strength of the electromagnet. The strength is approximately proportional to the voltage used. Double the voltage and you double the strength of the electromagnet.

Relation to turns of wire

The greater the number of turns around the iron core the greater the strength of the electromagnet. The strength is approximately proportional to the number of turns. Triple the number of turns and you triple the strength of the electromagnet.

(Experiment idea: measure the change of strength of an electromagnet by changing the voltage and/or number of turns.)

Direction of magnetism

The direction of the magnetic field is determined by the direction of the current and the direction of the turns around the iron core. If you change the direction of the current, the north and south poles of the electromagnet will switch.

With DC electricity, you must physically change the wires to change the direction of the current. With AC electricity, the direction changes with each cycle.

Thus, one end of an AC electromagnet is switching from north to south and back again 60 times per second in the U.S. or 50 times per second in some other countries.

Creating electricity

Electricity is created either when a wire is moved through a magnetic field or when a magnetic field is moved past a wire. Moving the magnetic field past the wire can be done by physically moving a magnet past the wire or by somehow changing the amount of the magnetic field.

Transforming the voltage

To transform or change the voltage of AC electricity, you use an AC electromagnet and the principles described above. 

AC electromagnet

An AC electromagnet continually changes the direction of its magnetic field. If you would put an AC electromagnet near a wire, then its changing magnetic field should create a current in the wire.

Or better yet, why not wrap the wire around the iron core of the electromagnet? This is how a transformer works.

Transformer

A transformer can be a long piece of iron with wire--with AC current going through it--wrapped around it near one end and wire in which to create current wrapped around it at the other end. A more common configuration is a square or donut shaped iron core with the wire wrapping on both sides.