Transformers

Transformers

1. Mains electricity to homes is 230 volts.

   • Energy to run mains supply comes from a power station - National Grid. Aleternating potential difference of cables is typically 132000 volts.
     • Transformer is used to change size of alternating potential difference.
       • Use step-up transformers to step-up the potential difference from power stations. As higher the potential difference at which electrical energy is transmitted across the Grid, smaller the energy wasted in the cables.

2. A transformer has two coils of insulated wire both wound around same iron core. Primary coil is attached to alternating current supply and when this supply passes through primary coil, an alternating potential difference is induced in secondary coil.

   • Primary coil produces a magnetic field which stays within iron core. Nearly all of it passes through to secondary coil.
     • As there is alternating current in the primary coil, field in iron core is constantly changing direction - changing magnetic field.
       • Rapidly changing magnetic field is felt by secondary coil. This induces an alternating potential difference across secondary coil.
         • Relative number of turns on two coils determines whether potential difference induced in secondary coil is greater or less than potential difference in primary.
           • If you supplied direct current to the primary coil you'd get nothing out of secondary coil. There would be a magnetic field in the iron core, but wouldn't be constantly changing. There would be no induction in secondary as need a changing field to induce a potentialdifference.
   • Primary and secondary coils are both wound round the same part of the core. Core is layered to cut out induced currents in the iron layers. If not layered the efficiency of transformer would be reduced.

3. Step-up transformers step the voltage up and have more turns on secondary coil than primary coil so the potential difference across secondary coil is less than that of the primary coil.

4. Step-down transformers step the voltage down and have more turns on primary coil than secondary so potential difference across secondary coil is less than that of primary coil.

   • They are reduce the potential difference so it's safe to be used by consumers.

5. The iron core is for transferring the changing magnetic field from primary to secondary coil so no electricity flows here.

6. Switch mode transformers aree a type of transformer that operate at higher frequencies than traditional transformers. Usually operate between 50kHz and 200kHz.

   • They work at higher frequencies so they can be made much lighter and smaller than traditional transformers that work from a 50Hz mains supply.
   • Makes them useful in things like mobile phone chargers and power supplies.
   • More efficient than other types of transformer which use very little power when they're switched on but no load is applied.

7. The higher the grid potential difference, the greater the efficiency of transferring electrical power through the grid.

8. Transformers are almost 100% efficient. When a device is connected to secondary coil, almost all electrical power supplied to transformer is delivered to device.