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Answers to Readers' Questions on Physical Science:

AC Transformers

The following 51 comments and questions have been sent in. They are listed according to date.

List of first 10 items

• Do transformers only work with AC?
• Wants practical experience on transformers
• How is wire size determined in an ac transformer?
• What type of windings used in a DC transformer?
• Why does current decrease in a transformer?
• What is a transformer made up of?
• Is 12 V DC different from different AC voltages?
• Why are transformers in multiples of 11?
• How could Faraday invent the transformer before AC?
• What is Inductor?

Next 10

Do transformers only work with AC?

Question

September 22, 2009

i just like to ask if transformers only work with an alternating current, or with the direct current? and on what circumstances?

neil - Philippines

18362

Answer

Transformers require AC, because the changing current in one coil creates a magnetic field which creates AC in the second coil. DC cannot do that directly. Instead, the DC must be converted to an AC signal by what is called an inverter. The AC signal goes to a transformer and the new signal is then converted back to DC by a rectifier device. Thus, DC requires a number of steps to have its voltage transformed.

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Wants practical experience on transformers

Question

April 6, 2009

I would like to have a practical experience on transformers?

tusiime - Uganda

17559

Answer

Most practical experience on transformers are large ones that your local electric company uses or those used in a college Electrical Engineering course.

You could get some experience by speaking with an electrician or electrical repairman in your community.

There is nothing online in our site for that experience.

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How is wire size determined in an ac transformer?

Question

February 24, 2009

how is wire size determined in an ac transformer?

nelson - USA

17326

Answer

In a transformer, the wire is wound around a core. A thick wire would make the transformer too bulky, so you would want the wire to be somewhat thinner.

But a thin wire has more electrical resistance and will heat up more. Overheating is a major problem at high voltages.

The resistance of a wire of a given material is proportional to its length divided by cross-section area. The increase in heat is proportional to the resistance of the wire.

Thus, for a given voltage or current, you would need to calculate the best diameter of wire that would allow the transformer no to be too bulky and not to heat up too much, but yet to transform the voltages that you want.

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What type of windings used in a DC transformer?

Question

November 11, 2007

what is the difference between the windings of an AC transformer and a DC transformer? Are they wound differently?

Reid - USA

14751

Answer

A standard transformer needs alternating current (AC) to operate. A DC transformer doesn't use windings. It must use electronics to transform and change the voltage. Obviously, that isn't as easy as how an AC transformer works.

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Why does current decrease in a transformer?

Question

November 6, 2007

respected sir,
my question is, according to ohm's law if voltage increases then current will also increase because both are directly proportional but in case of transformer when we increase voltage then current will decrease in step up transformer why? does ohm's law works for transformers?

gurjinder - India

14715

Answer

Another way of stating Ohm's Law is that for a given resistor, the voltage and current are proportional. That is V = IR.

But in a transformer, the electrical power going into the transformer must equal the output power. This is due to the Law of Conservation of Energy. Thus, if the power is P = VI, then VI = vi, which requires the output current to decrease if the output voltage increases.

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What is a transformer made up of?

Question

June 5, 2007

What transformer made up of

Bulelani - South Africa

13879

Answer

A transformer is made of an iron core with windings of wire around it.

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Is 12 V DC different from different AC voltages?

Question

January 30, 2007

Is 12 V DC current produced from 110-120 V AC same as that produced by 220-240 V AC? or is there a difference between the two?

- USA

13010

Answer

The 12 V DC should be the same, but it may be able to draw more current when changed from 220 V AC. That depends somewhat on the transformer.

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Why are transformers in multiples of 11?

Question

December 12, 2006

Q:- why transformers in india rated as 11KV/33kv/110kv/220kv etc. i.e.multiples of 11 only ?

sadanand - INDIA

12605

Answer

The starting point with generators is usually at 11,000V. The step-down transformer works in factors of 2, 5 or 10. For example, 11000V/25 = 440V. 440V/2 = 220V and so on.

Since the starting point is a multiple of 11, all others are also when divided by 10.

Also note there is a form factor in AC voltages that is the RMS value divided by the Average value of the since wave. This is 0.707/0.637 = 1.11.

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How could Faraday invent the transformer before AC?

Question

December 4, 2006

respected sir,
my question is that the first battery was formed by volta in 1800 and it is come to my knowledge that ac is invented in 1880. But transformer is invented in 1831 by faraday. so tell me sir, is ac is invented with the help of transformer? if not then tell me how ac was invented?

gurjinder - India

12542

Answer

The transformer that Faraday invented was really a device to demonstrate electromagnetic induction. It showed that the magnetism will change with a changing DC current. The name given can be confusing.

The power transformer that can change voltage was invented in 1881 by Lucien Gaulard and John Dixon Gibbs. That used the secondary coil to transform the voltage. They sold the idea to the Westinghouse company that worked with Tesla on developing AC.

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What is Inductor?

Question

August 14, 2006

What is Inductor?Tell about the Features and applications?

Vinod - India

11702

Answer

An inductor is a conductor that is wrapped in a coil. Its resistance or impedance increases with the increase in AC frequency through it. Inductors are used in radio tuners and devices to block out certain frequencies.

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