Transformer

4.5.6 The Transformer

A transformer is a device that changes the voltage of an alternating current (AC). It works on the principle of electromagnetic induction.

Principle of Operation

A transformer consists of two or more coils of wire wound around a common iron core. These coils are called the primary coil and the secondary coil.

When an alternating current flows through the primary coil, it creates a constantly changing magnetic field in the iron core.

This changing magnetic field then induces an electromotive force (EMF) in the secondary coil. The magnitude and direction of this induced EMF depend on the number of turns in the secondary coil compared to the number of turns in the primary coil.

Voltage Transformation

The ratio of the number of turns in the primary coil (N_p) to the number of turns in the secondary coil (N_s) determines whether the voltage is increased (stepped-up) or decreased (stepped-down). This ratio is known as the turns ratio (N_p/N_s). The relationship between the primary voltage (V_p) and the secondary voltage (V_s) is:

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

• If N_s > N_p (more turns in the secondary coil), the voltage is stepped up.
• If N_s < N_p (fewer turns in the secondary coil), the voltage is stepped down.

Current Transformation

Ideally, the power in the primary coil is equal to the power in the secondary coil (ignoring losses). Therefore:

V_pI_p = V_sI_s

This means that if the voltage is stepped up, the current is stepped down, and vice versa.

Types of Transformers

Based on the number of turns in the primary and secondary coils, transformers are classified into two main types:

• Step-up Transformer: N_s > N_p (Increases voltage, decreases current)
• Step-down Transformer: N_s < N_p (Decreases voltage, increases current)

Applications of Transformers

Transformers are essential components in many electrical systems, including:

• Power transmission: Step-up transformers are used at power stations to increase the voltage for efficient long-distance transmission.
• Power distribution: Step-down transformers are used in substations and near homes to reduce the voltage to safe levels for domestic use.
• Electronic devices: Transformers are used in many electronic devices to provide the required voltage levels.

Summary Table

Feature	Step-up Transformer	Step-down Transformer
Turns Ratio (Np/Ns)	> 1	< 1
Voltage Change	Increases	Decreases
Current Change	Decreases	Increases
Typical Use	Power stations, long-distance transmission	Substations, domestic use