3.3.2 Transformers

Topic 9.3

Transformers

A transformer is an electrical device that can convert an alternating electrical supply at one voltage to a different (higher or lower) voltage.

Transformers that increase voltage are said to be step-up transformers.

Transformers that decrease voltage are said to be step-down transformers.

Transformers

A transformer has two windings or coils wound on to an iron core.

The core is laminated to reduce eddy currents and energy losses.

The iron core acts to transmit and trap the magnetic flux generated by the primary winding.

Transformers

An alternating current generates a magnetic field in the primary winding.

The magnetic flux is transmitted through the iron core.

This changing magnetic flux induces an emf in the secondary winding.

Transformers

It can be shown that:

If 100% efficiency is assumed:

Transformers and Energy

A transformer works using the principle of conservation of energy.

The energy flowing into the primary coil flows must flow out of the transformerThis is mostly in the form of electrical energy from the secondary coil.

Transformer Losses

A real transformer is not 100% efficient.Eddy currents are formed in the iron core due to induction. This is minimised by laminating the iron core to reduce the area of induction and hence the size of the induced current.

Winding resistance. The windings of the transformer are usually made of thick copper to reduce heating and loses to a minimum

Hysteresis losses are caused when magnetising and demagnetising the iron core. This causes heating and energy loss.

Laminating

Transformers are made out of laminated iron rather than solid iron.

This decreases the space available for eddy currents and hence increase the resistance of the core.

The increased resistance causes a lower eddy current and hence less heating.

Transformer Problems

A step-up transformer converts 10kV into 400kV for transmission. If the primary coil has 200Turns, how may turns are on the secondary coil?

A transformer converts 240V into 5V to charge an iPod. The secondary coil has 400T, how many turns are on the primary coil?

Transformer Problems

A particular transformer is 95% efficient and is designed to turn 240V into 12V and a maximum current of 5A. The primary winding has 1000T. How many turns are on the secondary coil?

What current flows in the primary coil when the transformer is running at maximum power?

Using Transformers

Transformers are everywhere:

They are used in substations to step-up or down the voltage to or from the grid.

These transformers are so big and generate so much heat that they usually need to be cooled with oil.

Using Transformers

Small transformers are found in plug tops.

These step down the mains voltage from 240V to a more usable level for a device to use.

They often also have a device in them called a rectifier that converts AC to DC.

Rectifier

Electrical components called diodes allow electricity to flow in one direction only.

Using 4 diodes allows the negative half of AC to be flipped positive.

A capacitor stores energy and releases is slowly, smoothing out the bumps in the AC.

Why are Transformers useful for Society?

Transformers allow for voltage to be changed at will to suit a particular application.

Transformers increase efficiency for power transmission.

The use of transformers allows more efficient AC generators to be used.

The output of a transformer can be converted to DC as required using only 5 components.