Operation of a Transformer

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Transformers can be used to convert an alternating current from one voltage to another.

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Transforming voltage

  • The output voltage of a transformer can be less than, greater than, or equal to the input voltage.
  • This depends on the ratio of the number of loops in their coil.
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Types of transformers

  • There are two classes of transformers:
    • Step-up transformers increase voltage.
    • Step-down transformers decrease voltage.
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Transformer equation

  • Assuming that resistance is negligible, the electrical power output of a transformer equals its input.
    • P=IpVp=IsVsP={I_p}{V_p}={I_s}{V_s}
  • We can combine this with the transformer equation (NpNs=VpVs{N_p}{N_s}={V_p}{V_s}) to get:
    • NsNp=VsVp\frac{{N_s}}{{N_p}}=\frac{{V_s}}{{V_p}}

Efficiency of a Transformer

Modern transformers are very efficient. They waste little energy as heat because of improvements in design over time.

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Eddy currents

  • The core is made of iron, which is a magnetic metal.
  • As the magnetic flux in the core changes, the free electrons in the iron experience a force which causes them to move.
  • The moving electrons mean that a current has been induced in the core.
  • These currents are known as “eddy currents” and cause the core to heat up.
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Reducing eddy currents

  • Eddy currents can be reduced by forming the core out of layers of iron glued together with an insulator.
  • The magnetic properties are not severely reduced but the resistance (and so current in) the core is dramatically reduced.
  • This process is known as “laminating the core”.
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Examples of other losses

  • Energy can still be lost by the changing magnetic field, causing the layers of iron to vibrate.
  • The wire forming the coils has resistance, so will heat up when a current flows.
  • Not all of the flux may pass from the primary to the secondary coil.

Transmission of Electrical Power

Transformers are used to either step-up an a.c. potential difference or to step it down. An important application is in mains transmission via the National Grid.

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National Grid

  • The National Grid produces a.c. in power stations.
    • Typically this is around 15 kV.
  • The step-up transformer steps the p.d. up to an RMS (root mean square - a type of average) amplitude of 330 kV.
  • The p.d. is then stepped back down so that consumers can apply it more safely (and at a higher current).
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Transformer power equation

  • Since the input power = output power for a 100% efficient transformer:
    • VinIin=VoutIoutV_{in}I_{in} = V_{out}I_{out}
    • Iout=VinVoutIinI_{out}=\frac{V_{in}}{V_{out}}I_{in}
  • If the output transformer has more turns on it than the input transformer, causing Vout>VinV_{out} > V_{in}, then the output current is less than the input current.
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Reducing transmission losses

  • A lower current through the transmission cables means less energy is wasted heating the surroundings.
  • This is because the heating loss per metre of cable = I2R, where R is the resistance of 1 metre of cable.

Jump to other topics

1Measurements & Errors

2Particles & Radiation


4Mechanics & Materials


6Further Mechanics & Thermal Physics (A2 only)

7Fields & Their Consequences (A2 only)

8Nuclear Physics (A2 only)

9Option: Astrophysics (A2 only)

10Option: Medical Physics (A2 only)

11Option: Engineering Physics (A2 only)

12Option: Turning Points in Physics (A2 only)

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