7.10.3 The Operation of a Transformer

• A transformer is

A device that changes high alternating voltage at low current to low alternating voltage at high current, and vice versa

• This is designed to reduce heat energy lost whilst electricity is transmitted down electrical power lines from power stations to the national grid
• A transformer is made up of:
  • A primary coil
  • A secondary coil
  • An iron core

• The primary and secondary coils are wound around the soft iron core
  • The soft iron core is necessary because it focuses and directs the magnetic field from the primary to the secondary coil
  • Soft iron is used because it can easily be magnetised and demagnetised

A step-up transformer has more turns in the secondary coil than primary

• In the primary coil, an alternating current producing an alternating voltage is applied
  • This creates an alternating magnetic field inside the iron core and therefore a changing magnetic flux linkage

• A changing magnetic field passes through to the secondary coil through the iron core
  • This results in a changing magnetic flux linkage in the secondary coil and from Faraday's Law, an e.m.f is induced

• An e.m.f produces an alternating output voltage from the secondary coil
• The output alternating voltage is at the same frequency as the input voltage

• The transformer equation is:

• Where:
  • N_s = number of turns in the secondary coil
  • N_p = number of turns in the primary coil
  • V_s = output voltage from the secondary coil (V)
  • V_p = input voltage in the primary coil (V)

• There are two types of transformers:
  • Step-up transformer (increases the voltage of the power source) where N_s > N_p
  • Step-down transformer (decreases the voltage of the power source) where N_p > N_s

• A step-up transformer is used between power stations and transmission wires
• A step-down transformer is used between the transmission wires and buildings