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Explanation of how an AC transformer works. Also refer to physics, Alternating Current, DC, direct current, magnetism, voltage, electromagnet, house current, high voltages, Ron Kurtus, School for Champions. Copyright © Restrictions

Alternating Current (AC) Transformers

by Ron Kurtus (revised 12 August 2005)

A major advantage of alternating current (AC) electricity over direct current (DC) is the fact that the potential energy can be readily changed from one voltage to another through the use of a transformer. An AC transformer changes power line high voltages to house current. It also is used to change the voltage from house current to that used by low voltage devices. Often the AC in these small transformers or adapters is also changed to DC.

Questions you may have include:

• 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. There is a mini-quiz near the end of the lesson.

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Time = 7 min. 36 sec.

Basic principles

A transformer combines several major characteristics of electricity and magnetism to change 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. Placing a compass near a wire and observing the needle move when the DC current is turned on can demonstrate this.

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. This means the field goes from zero to N to zero to S and so on. 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 having with AC current going through it and wrapped around the piece of iron near one end. It also has wire that creates electrical 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.

Transformer changes voltage

Output proportional to turns

The strength of the magnetic field is proportion to the input voltage and the number of turns around the core (called the primary coil). By reversing the rule, the output voltage is proportional to the strength of the changing magnetic field and the number of turns (called the secondary coil).

For example, if you wanted to increase your house voltage from 110 volts (110V) to 220V in order to power your electric stove, you could use a transformer with twice the turns in the secondary coil as in the primary coil.

The relationship is written as:

input volts / input turns = output volts / output turns

110V / 5 turns = 22 = 220V / 10 turns

Using transformers

Transformers are used to lower voltage to be safer to use in your house. You may also use an adapter to lower the voltage even more for some devices you use. DC transformers are now available, but they won't replace AC transformers.

House voltage

Normally, the current in the electrical lines outside your house are around 1100V AC. The reason it is so high is that the electricity travels more effectively over long distances at higher voltages. High voltage lines carry up to 10,000 volts.

The transformer near to top of the electrical pole changes the voltage to a safer 110V for your house.

Adapters

Most people use adapters when they power devices that also use batteries. An adapter is a transformer that changes the 110V AC house current to 12V DC or 9V DC that is used by the device. It also changes the AC to DC, because the device works on batteries.

Changing AC to DC is done by electronic circuitry called a rectifier. It essentially chops off 1/2 of the AC current to make it similar to DC. Some of the lost AC current is turned into heat. That is one reason your adapters sometimes get warm.

DC transformers

You can see that DC voltages could not be changed with the configuration of the transformer. This is because the DC current would not be changing the magnetic field the way AC current does. And this was the reason that AC won over DC when electricity started to be used around the world.

Since then, electrical engineers have developed DC transformers, primarily using special circuitry. Since most everyone now uses AC, it is too late to change the system.

Summary

The principles for creating an AC electromagnet and how a changing magnetic field leads to how a transformer works. The output voltage of a transformer is proportional to the ratio of the number of turns of the coils. Transformers reduce the high voltage to a safer home voltage, as well as to reduce 110V AC to what can be used in some battery-powered devices.

Readers Questions and Feedback

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Resources

Following are some resources on this subject.

Web sites

Elements of AC Electricity - Basic electronics tutorial site

DC and AC Electricity Resources

Physical Science Resources

Books

Basic Electricity by Bureau of Naval Personnel; Dover Pubns; (1970) $14.95 - Provides thorough coverage of the basic theory of electricity and its applications

Miscellaneous

Mini-quiz to check your understanding

1. What is a basic principle of an AC transformer?

2. Why does an alternating magnetic field create electricity in a nearby wire?

3. Why does an adapter get warm?

If you got all three correct, you are on your way to becoming a Champion in Physical Science. If you had problems, you had better look over the material again.

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Alternating Current (AC) Transformers