unit 4 section 4 transformers


what's a transformer?

transformer: a device that makes use of electromagnetic induction to change the size of the voltage of an alternating current.

1. A.C. flowing in primary/input coil causes the core to magnetise and demagnetise continously in opposite directions.

2. this produces a rapidly changing magnetic fluxacross then core.

3. because of this, a magnetically soft material is needed - usually iron or a special alloy.

4. the rapidly changing magnetic flux in the iron core passes through the secondary/output coil, where it induces an A.V. of the same frequency but different voltage, assuming the number turns is different.

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what's a transformer?

faradays law says the voltage in the primary and secondary coils can be calculated.

- primary coil is: Vp = Np x rate of change of flux

- secondary coil: Vs = Ns x rate of change of flux

- rearranging this gives: Ns/Np = Vs/Vp

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step-up and step-down transformers

step-up transformers increase the voltage by having more turns on the secondary coil than the primary.

step-down transformers reduce the voltage by having fewer turns on the secondary coil.

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in/efficiency in a transformer

if a transformer was 100% efficient input would equal output but in practice this is not true and there will be small losses of power mostly as heat.

metallic core as continously changing flux which induces an e.m.f. in the core, this causes eddy currents, which cause it toheat up and energy is lost.

this loss can be reduced by laminating the core - having layers of the core seperated by thin layers of insulation.

heat is also generated in the coils, so wires with low resistance are used like thick copper wire because it has low resistivity and a larger diameter means smaller resistance.

energy is needed to magnetise and demagnetise the core, this energy is wasted as it heats the core. to reduce this a magnetically soft material that magnetises and demagnetises easily should be used.

all the magnetic flux from the primary coil should cut through the secondary coil, but in practice this isn't the case.

to reduce this magnetic loss, a core design in which the cores are as close as possible is used - this can include winding the coils on top of each other around the same part of the core. 

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calculating the efficiency of a transformer

P = I x V


Ip x Vp = Is x Vs


Vp/Vs = Is/Ip

Efficiency = Is x Vs/Ip x Vp

E = P x t

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transformers in the national grid

- electricity is sent around the country through the national grid at the lowest current possible

- high currents cause greater energy losses due to heating in in the cables.

- the energy loss is proportional to P = I^2 x R

- so if the current is doubled, you quadruple the power lost.

- using low resistance cables also reduces power loss.

- P = I x V so a low current means a high voltage for the same amount of power.

- transformers allow for a 400,000V step-up

- has to be stepped down to 230V before home use for safety and insulation issues.

- power station, 25,000V to step-up to pylons, 400,000V to step-down to home, 230V.

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