Do you know how to calculate the transformer needed? Correct calculation is the basis for reliable operation, long life and high efficiency. In order for it to work optimally, you need to know exactly its transformation ratio, power, efficiency and losses. However, it is important to understand the basic relationships behind the calculations themselves, whether it is to determine the number of turns, the winding voltage or the power parameters. We therefore bring you a practical step-by-step guide explaining how to calculate a transformer and how to calculate the basic transformer parameters, from transformation ratio to efficiency and losses.

Basic concepts before transformer calculation

Before getting into the actual calculation of the transformer, it is important to understand a few basic concepts and relationships that determine its behavior.

Primary and secondary winding voltage

The transformer has two main parts. A primary winding, to which the input voltage U1 is connected, and a secondary winding, from which the output voltage U2 is taken. The ratio between these two voltages determines the so-called transformation ratio (conversion), which is a basic parameter of every transformer:

• If U1>U2, it is a step-down (voltage) transformer.
• If U1<U2 , we are talking about a step-up transformer.

Transformer current and power

The current in the primary winding is denoted by I1 and in the secondary winding by I2. For an ideal transformer, the power at the input and output is the same. However, in a real transformer, some of the power is lost due to thermal and magnetic losses, so the output power is always slightly less than the input power.

Transformer efficiency

Efficiency (η) expresses how efficiently the transformer converts energy. It is defined as the ratio of output to input power: η =Pout/Pin× 100 %. High quality transformers, such as those manufactured by BEZ TRANSFORMÁTORY, achieve efficiencies of 98-99%, reducing operating costs and thermal loads.

Transformer losses

The losses can be divided into two main groups:

• Stray losses (in iron) occur in the transformer core due to magnetic phenomena (hysteretic and eddy currents).
• Short-circuit losses (in the windings) are due to the resistance of the windings, in which some of the energy is converted to heat.

Accurate calculation of transformer losses is important to determine its efficiency, cooling capacity and economic operation.

Types of transformers by application

Before the actual calculation, it is also necessary to know the purpose of the transformer, because the method of calculation depends on it:

• Single-phase transformers are common in smaller equipment and power supplies.
• Three-phase transformers are used in power and industrial applications.
• Measuring transformers are designed for accurate current or voltage measurements.
• Power transformers transmit high power in electrical networks.

These concepts form the basis without which the transformer calculation would not be accurate or understandable.

Calculation of the transformation ratio

One of the basic parameters that determine the behavior of the transformer is the transformation ratio. This ratio indicates how many times the voltage between the primary and secondary windings changes.

The transformation ratio (k) is determined by the formula: k = U1/U2 =_N1/N2,_where:

• U1₌ primary winding voltage (V)
• U2₌ voltage on the secondary winding (V)
• N1₌ number of turns of the primary winding
• N2₌ number of turns of the secondary winding

This relationship shows that the ratio of stresses is the same as the ratio of the number of turns. So, for example, if we want to reduce the voltage by a factor of ten, the secondary winding must have ten times fewer turns than the primary.

Application of transformation ratio calculation in practice

Calculating the ratio helps determine the number of turns of the windings and the cross section of the core. By verifying the transformation ratio, it can be determined whether the device is in order or has a damaged winding. Modern software and online calculators make it possible to quickly determine the correct transformation ratio without manual calculations.

Almost all transformers also have a tap changer that allows you to vary the voltage on the primary side, normally between -5% and +5%. By changing the tap, you can keep the secondary voltage constant. Such regulation can be in an off load or on load state. Transformers with an OLTC on-load tap changer are installed at locations with frequent voltage changes on the primary side. The demand for these transformers is increasing due to the renewable resources connected to the distribution grids.

Recommendation of the experts from BEZ TRANSFORMÁTORY

When designing or checking the transformer, always compare the calculated transformation ratio with the nominal data on the label. Deviations may indicate wear or a fault in the winding.

Calculation of transformer power

The transformer power rating determines how much electricity the device can transfer from the primary circuit to the secondary circuit without excessive heating or losses. This parameter is critical in both the design and selection of a suitable transformer for a particular application.

The basic relationship for the apparent power of a transformer is S = U × I, where:

• S = apparent power (VA or kVA)
• U = voltage (V)
• I = current (A)

This formula expresses the output of one phase. For a three-phase transformer, the power is calculated as follows.

The result is most often given in kilovolt-amperes (kVA), which represents the rated power of the transformer.

How to determine the required transformer power

When designing or selecting a transformer, it is necessary to take into account:

• the sum of the outputs of all connected devices,
• power reserve: it is recommended to increase the rated power by 10-20%,
• nature of the load: continuous, intermittent or impact,
• power factor and operating conditions ( e.g. ambient temperature, cooling).

When deciding on power, evaluate efficiency, as transformers reach their highest efficiency at loads between 40-60% of rated power.

Properly selected power ensures stable operation and minimizes thermal stress on the windings. Transformer power is a key parameter that affects its size, weight and operating efficiency. Therefore , it is important to perform an accurate calculation of the transformer at the design stage and verify it by practical measurement or software simulation.