Transformer Efficiency (AQA A Level Physics): Revision Note

Transformer Efficiency

• In reality, transformers are not 100% energy efficient

  • This means their power output is less than the power input

• For an ideal (100% efficient) transformer, there is no electrical energy and power loss:

Power in = Power out

I_pV_p = I_sV_s

• Where:

  • I_p = current in the primary coil (A)

  • V_p = voltage in the primary coil (V)

  • I_s = output current from the secondary coil (A)

  • V_s = output voltage from the secondary coil (V)

• This is from the power equation:

P = IV

• The ideal transformer equation can also be written as:

• Where:

  • N_p = number of turns in the primary coil

  • N_s = number of turns in the secondary coil

• The efficiency of a transformer is the ratio of the power out to the power in:

• This efficiency is given as a decimal

  • To find this as the percentage, multiply by 100

Eddy Currents

• Eddy currents are a key source of energy loss in a transformer, which makes them unlikely to be 100% efficient

• They arise from:

  • A changing magnetic field (and flux) from the alternating current or voltage

  • Which creates a changing magnetic field in the core that acts against the field that induced them

  • An e.m.f is therefore induced

  • A current flows, as the core is made from a conducting material

• The current also dissipates energy by generating heat in the wires

• Eddy currents are reduced by:

  • Laminating the iron core with layers of insulation

  • Having a core made from a high resistivity metal

Eddy currents in a solid v laminated core

•  Laminating the core consists of building it with thin layers of metal, instead of a solid metal

  • The eddy currents are therefore reduced significantly

• The laminations are insulated from each other, so the current doesn't flow between them