What is a transformer? A transformer is used to change or transform alternating potential difference between (a) the power station and the cables, and (2) between the cables and our home. A typical power station generator produces an alternating potential difference in Kilo Volt(KV) range (like 25 KV). The electricity is delivered via a network of cables called the National Grid. The alternating potential difference(PD) of the cables (the grid potential difference) is typically 132 KV. And Mains electricity in homes is 230 V. These various levels of alternating PD in this electricity generation and distribution network are maintained with the help of transformers.
How a transformer works?
A transformer has two coils of insulated wire, both wound around the same soft iron core, as shown in figure 1. This core gets magnetized and demagnetized easily. The primary coil is connected to an alternating current supply. When alternating current passes through the primary coil, an alternating potential difference is induced in the secondary coil.
This happens because:
- Alternating current passing through the primary coil produces an alternating magnetic field.
- The lines of the alternating magnetic field pass through the secondary coil
- The magnetic field is changing
This creates an alternating potential difference between the terminals of the secondary coil. In other words, an alternating PD is induced in the secondary coil.
If a bulb is connected across the secondary coil, the induced potential difference causes an alternating current in the secondary circuit, so the bulb lights up. Electrical energy thus is transferred from the primary coil to the secondary coil. This happens even though they are not electrically connected in the same circuit.
A Step-up transformer makes the potential difference across the secondary coil higher than the potential difference across the primary coil. Its secondary coil has more turns than its primary coil.
A Step-down transformer makes the potential difference across the secondary coil lower than the potential difference across the primary coil. Its secondary coil has fewer turns than its primary coil. For example, if a low voltage supply is needed for a device, a step-down transformer is used to step the mains potential difference down from 230 V.
Why are step-up and step-down transformers required? | How transformers are used in national grids?
The higher the grid potential difference, the greater is the efficiency of transferring electrical power through the grid. The grid PD is at least 132 kV. If the grid PD were much lower then much more current would be needed to deliver the same amount of power. The grid cables would therefore heat up more and waste more energy. To avoid this step-up transformers are used to step up the PD from a power station to the grid PD. And then again step-down transformers are used to step the grid PD down to the mains voltage. Also study about transformer equation and transformer efficiency here.
What is national power grid? | Diagram of national power grid
Electricity is supplied to homes from power stations via the National Grid. Figure 2 shows how the grid system is used to supply industry as well as homes.