Transformers and the Turns Rule

Transformers and the Turns Rule

The great thing about a.c. electricity is that you can transform it! For instance, you can step its voltage up or down.

Transformers work on the principles of electromagnetism and electromagnetic induction.

Here's how they work...

Transformers and the Turns Rule

A.c. in a coil will set up a changing magnetic field in the coil, which will mean that the core becomes a constantly changing magnet.

Transformers and the Turns Rule

Put a second coil around this changing magnet (the core) and you induce an alternating emf in the coil.

We name the first coil the primary coil and the second coil the secondary coil.

The arrangement in transformers is similar.

For Example:

Transformers and the Turns Rule

Remember: Emf is induced if a conductor is in a changing magnetic field. The larger the number of turns on the coil, the greater the flux linkage and therefore the greater the induced emf.

If you change the number of turns in the coils you change the induced emf. This allows you to change (transform) the voltage from the primary to the secondary coil.

The Turns Rule is:

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Where:

Ns = number of turns on the secondary coil

Np = number of turns on the primary coil

Vs = voltage across the secondary coil

Vp = voltage across the primary coil

So if number of turns on the secondary coil is greater than on the primary coil, the output voltage will be greater than the input voltage. This is called a step up transformer.

Of course, you can't create energy by stepping up voltage.

If a transformer is perfectly efficient, then the energy put into a transformer by the primary each second = the energy removed by the secondary each second.

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So when you step up the voltage, you step down the current.

One of the main reasons is efficiency. Let's say you have to deliver 10kW at 100V to a factory through wire (see below) which has resistance, R = 100Ω

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To deliver 10kW at 100V, the current needed is:

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Well, pushing 100A through wire will produce very hot pieces of wire. In fact, energy lost each second (power) as heat in the wire is:

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That's 1 million watts!

To supply 10kW to the factory you need to generate 1,010,000W at the power station (the a.c. supply in our diagram) and lose 1,000,000w on the way. But, use a transformer to step up the voltage through the wire to 10kV and you step down current in the wire to 1A.

Calculate power loss in wires with this new stepped down current.

P= I2R = 12 x 100 = 100W. That's much better. Much less energy is lost.

Transformers, however, are not 100 % efficient. Energy is lost in the wires in the coils and in the iron core itself. Small eddy currents are induced in the iron core, which waste energy heating up the core. Eddy currents can be reduced by laminating the core - for instance, by making the core out of thin slices of metal, which are 'glued' together. This reduces the area the eddy currents can flow through, hence increasing the resistance and reducing the size of the eddy currents.

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