Operation of a Transformer

A transformer transforms electric energy from one electrical circuit to another. More specifically, a step-up transformer increases the voltage while it is being transferred from a primary circuit to a secondary one. A step-down transformer, on the other hand, decreases the voltage while it is being transferred from a primary circuit to a secondary one. In order to understand how a transformer does this, you need to learn about the operation of a transformer.

Operation of a transformer: Principle

The transformation of energy is done by mutual induction between the windings. The simplest form of a transformer is shown in figure 1, which depicts a transformer that consists of two inductive coils, primary and secondary windings. The two coils are connected through a laminated steel core that allows the flow of magnetic flux through the laminated path.

Figure 1. Diagram of a transformer. Source: Georgia Panagi, Vaia.

When the primary winding is connected to an external source of alternating voltage, magnetic flux is induced in the wingdings according to Faraday’s Law.

When an alternating current passes through the primary winding, the magnetic field changes, thus inducing an electromotive force. The resulting magnetic field cuts the winding of the secondary coil, which generates an alternating voltage in that winding through electromagnetic induction.

Most of the magnetic flux becomes linked with the secondary winding, which is called ‘main flux’, while the remaining flux does not get linked with secondary winding and is known as ‘leakage flux’.

The induced EMF is known as mutually induced EMF, and its frequency is the same as the supplied electromotive force.

When the secondary winding is a closed circuit, the mutually induced current flows through the circuit, transferring electrical energy from the primary circuit to the secondary one.

The core of a transformer

The core of a transformer consists of laminated sheets of steel that are positioned in such a way that there is a minimum air gap between each sheet. This is done to provide a continuous path for the magnetic flux. The type of steel that is used provides high permeability and reduces eddy current losses and low hysteresis losses.

Types of transformers

There are various types of transformers with different geometrical variations.

Core-type transformer

In a core-type transformer, windings are in a cylindrical shape and positioned in the core, as shown in figure 2 below. The cylindrical coils have different layers, with each layer being insulated from the other. Core-type transformers exist in both small and large-size versions. The effective core area of the transformer can be reduced with the use of lamination and insulation.

Shell-type transformer

In a shell-type transformer, the coils are mounted in layers and stacked with insulation between them. A shell-type transformer may have a simple rectangular form, as shown in figure 3 (left), or can have a distributed configuration (right).

Figure 3. Shell-type rectangular transformer (left) and shell-type distributed transformer (right). Source: CircuitsToday.

Zigzag or interconnecting star type transformer

A zigzag transformer has a zigzag connection, in which the currents in the windings on the core department flow in opposite directions to avoid saturation.

Figure 4. Zigzag transformer configuration.

The uses and purposes of a transformer

Transformers are classified based on their uses, purposes, and supply. There are two main purposes for which transformers are used:

• Step-up transformers are used to increase the voltage at the secondary winding. A step-up transformer has more turns on the secondary coil than on the primary one.
• Step-down transformers are used to decrease the voltage at the secondary winding. A step-down transformer has fewer turns on the secondary coil than on the primary one.

The transformer ratio equation specifies the ratio between the secondary and primary voltages V₁ and V₂ measured in Volts, currents I₁ and I₂ measured in Amperes, and the number of turns in the coils n₁ and n₂. This ratio can be used to decrease or increase a quantity proportionally to the second or primary winding.

\[a = \frac{n_1}{n_2} = \frac{V_1}{V_2} = \frac{I_2}{I_1}\]

The ideal transformer equation

The ratio of voltages is equal to the ratio of the number of turns, as shown in the previous equations. In an ideal transformer without any electrical energy losses, which includes core losses, eddy current losses, or hysteresis losses, the input power is equal to the output power.

Therefore, the efficiency of the transformer is 100%, or the ratio of the output to input power is 1. This is also shown in the equation of the ideal transformer below, where I₁ and V₁ are the current and voltage of the primary winding, respectively, while I₂ and V₂ are the current and voltage of the secondary winding, respectively.

\[\text{Input Power = Output Power} \qquad V_1 \cdot I_1 = V_2 \cdot I_2\]

Single and three-phase transformers

Transformers can also be classified in terms of their type of supply. There are two types of supply:

• Single-phase transformers contain one conductor and one neutral wire. They work using a voltage cycle that operates in a time phase and are used widely in today’s technology to convert AC values to the desired ones.
• Three-phase transformers are widely used for electrical distribution and network applications. They work with the same principle as single-phase transformers. However, there are three conductors, each of which contains a set of primary and secondary winding and one neutral wire.

The current in a three-phase transformer has three peaks and troughs for each period. Hence, the maximum amplitude is reached many times, which helps provide power at a constant rate.