Transformers

Transformer Structure

A transformer is an electrical device used to change the voltage of an alternating current (AC). It consists of two coils of wire called the primary coil and the secondary coil, which are wrapped around a soft iron core.

• Primary coil: Connected to the input voltage source.
• Secondary coil: Connected to the output circuit.
• Iron core: Provides a path for the magnetic field and increases the efficiency of the transformer by concentrating the magnetic flux.

Transformers come in two types:

• Step-up transformer: Increases voltage from primary to secondary coil. It has more turns on the secondary coil than the primary coil.
• Step-down transformer: Decreases voltage from primary to secondary coil. It has fewer turns on the secondary coil than the primary coil.

How Transformers Work

Transformers operate based on electromagnetic induction. When an alternating current flows through the primary coil, it creates a changing magnetic field in the iron core.

This changing magnetic field induces an alternating voltage in the secondary coil. The voltage induced depends on the number of turns in each coil.

Key points:

• Transformers only work with alternating current (AC) because the magnetic field must be constantly changing to induce a voltage.
• The voltage and current in the secondary coil can be different from the primary coil depending on the coil turns.

For example, if the primary coil has 100 turns and the secondary coil has 200 turns, the voltage in the secondary coil will be twice the voltage in the primary coil (step-up transformer).

If the voltage is increased, the current decreases, and vice versa, to keep the power approximately constant (ignoring losses).

For instance, if the primary voltage is 12 V and the secondary coil has twice as many turns, the secondary voltage will be:

$V_s = 2 \times V_p = 2 \times 12 = 24 \text{ V}$

Transformer Equations

The relationship between the voltages and the number of turns in the coils is given by the transformer equation:

$\frac{V_p}{V_s} = \frac{N_p}{N_s}$

• $V_p$ = voltage across the primary coil (volts)
• $V_s$ = voltage across the secondary coil (volts)
• $N_p$ = number of turns on the primary coil
• $N_s$ = number of turns on the secondary coil

For an ideal transformer (no energy losses), the power input equals the power output:

$V_p I_p = V_s I_s$

• $I_p$ = current in the primary coil (amperes)
• $I_s$ = current in the secondary coil (amperes)

This means if the voltage increases, the current decreases proportionally, and vice versa.

Example: A transformer has 500 turns on the primary coil and 100 turns on the secondary coil. The primary voltage is 240 V. Calculate the secondary voltage.

Using the formula:

$\frac{V_p}{V_s} = \frac{N_p}{N_s}$

Rearranged to find $V_s$:

$V_s = V_p \times \frac{N_s}{N_p} = 240 \times \frac{100}{500} = 240 \times 0.2 = 48 \text{ V}$

Applications of Transformers

Transformers are essential in the National Grid for efficient transmission of electricity over long distances.

• Electricity generated at power stations is at a relatively low voltage.
• A step-up transformer increases the voltage to hundreds of thousands of volts for transmission along power lines.
• High voltage reduces the current for the same power, which reduces energy lost as heat in the cables (because power loss $P = I^2 R$).
• Near homes and businesses, step-down transformers reduce the voltage to safer, usable levels (e.g., 230 V in the UK).

Using transformers in this way makes the National Grid much more efficient and safer.