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

Alternating Current (AC) Transformers

by Ron Kurtus (revised 29 April 2012)

A transformer is an electrical device that is used to change the voltage in alternating current (AC) electrical circuits. The ability of this device to change voltage is a major advantage of AC electricity over direct current (DC).

A simple transformer consists of a dual electromagnet, where there are two sets of wires wrapped around it for the input and output voltages. The electromagnet may be in the form of a straight iron rod, horseshoe, or donut shaped.

The output voltage is a function of the input voltage and the ratio of the number of turns of the wire.

Uses for transformers are to decrease the line voltage to house voltage and the change the voltage for small devices.

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.

Useful tool: Metric-English Conversion

Simple transformer

A simple AC transformer is bascially a dual electromagnet with two sets of wires wound around it for the input and output voltages.

The input AC voltage goes through the primary coil, making the soft iron core and AC electromagnet. Soft iron is used because the direction of magnetism can change rapidly with the change in direction of the current. The strength of the magnetic field is a funciton of the number of turns of the primary coil.

Simple AC transformer

The changing magnetic field activates the secondary coil on the electromagnet, creating an output voltage that is a function of the number of coils.

Formula for output voltage

The output voltage of a transformer is:

Vo = Vi(No/Ni)

where

• Vo is the output AC voltage
• Vi is the input AC voltage
• No is the number of turns in the secondary coil
• Ni is the number of turns in the primary coil

In the example of increasing the voltage, this formula can also written as:

Vo/Vi = No/Ni

such that

110VAC / 5 turns = 220VAC / 10 turns

Uses for 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 voltage from the electrical lines coming to your house is 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 reduces the voltage to a safer 220V AC or 110V AC for the appliances in your house.

Adapters

An adapter is a small device that not only reduces 110V AC to a lower voltage, but it also changes AC to DC. Most people use adapters when they power devices that also use batteries. The adapter changes the 110V AC house current to 12V DC or 9V DC that is used by the devices that employ 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

An AC transformer is an electrical device that is used to change the voltage in alternating current (AC) electrical circuits. A simple transformer consists of a dual electromagnet, where there are two sets of wires wrapped around it for the input and output voltages.

The output voltage of a transformer is according to the formula: Vo = Vi(No/Ni). Uses for transformers include decreasing the line voltage to house voltage and the changing the voltage for small devices.

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Resources and references

Following are some resources on this subject.

Web sites

Elements of AC Electricity - Basic electronics tutorial site

DC and AC Electricity Resources

Physics Resources

Books

Transformers and Motors by George Shultz

Teach Yourself Electricity and Electronics by Stan Gibilisco

What do you think?

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