Understanding transformer losses

We divide transformer losses into two main groups, which have different sources and require different design solutions.

1. Idle losses (iron losses)

These losses occur in the magnetic core of the transformer. They are constant and do not depend on the load. They are consumed by the transformer whenever it is connected to the grid, even if it is not transmitting any power. Hysteresis losses are caused by the constant change in the direction of the magnetic field (remagnetization) of the core material. They are related to the quality of the magnetic sheets and their loss characteristics. Eddy current losses are due to induced currents directly in the core material, which are generated by the variable magnetic flux. They are reduced by dividing the core into thin insulated sheets (lamination).

2. Short-circuit losses (losses in windings)

Short-circuit losses mainly occur in the transformer winding (in copper or aluminum). These losses are directly related to the magnitude of the current flowing through the transformer and are therefore load dependent. They are caused by the winding resistance. As the load on the transformer increases, the current increases and the losses increase with the square of the current.

Innovative materials for loss reduction in low-loss transformers

The key to radically reducing both types of loss lies in the selection and processing of superior materials. BEZ TRANSFORMÁTORY, we use top-quality steel with a grain-oriented structure (CRGO) that ensures easy passage of the magnetic flux. These cores also utilize advanced sheet metal shearing and folding technology, such as step-lap, which minimizes transient resistive losses at the corners.

The amorphous metal material has an unstructured atomic structure, similar to glass. The lack of a crystalline structure reduces the energy required to remagnetize the material, resulting in extremely low hysteresis losses. Low-loss transformers with an amorphous metal core can reduce no-load losses by up to 66% compared to conventional CRGO steel, delivering unmatched standby efficiency. We use either copper, which has an extremely low resistivity, or aluminium, which offers a favourable price/performance ratio, in the manufacture of the windings. The key is to use conductors with the ideal cross-section with respect to the current flowing through them. A larger cross-section naturally reduces resistance and therefore heat loss. Optimizing the cross-section of the conductors becomes the most important design element in the fight against load losses.

Advanced technologies and design solutions for low-loss transformers

Low losses are not just a matter of materials, but also require precise design and manufacturing technologies to ensure optimum use of these superior raw materials. We use advanced shearing lines with Step-Lap technology to produce magnetic cores. This method ensures extremely precise lamella joints at the corners of the core, minimizing air gaps and reducing local eddy current and hysteresis losses at magnetic flux bending points.

The winding design, especially for power transformers, must minimize stray losses. These are additional losses caused by leakage magnetic flux that induces currents in the surrounding metal parts of the transformer (e.g. clamping frames). We use special shielding and optimize the spatial arrangement of the windings to direct the flux and suppress these unwanted losses.

Low-loss dry-type transformers with cast insulation represent a modern alternative to traditional oil-cooled transformers and bring significant low-loss and safety benefits. Instead of flammable oil, they use epoxy resin ( EP) to pot the windings. This resin is environmentally friendly, flame retardant and reduces the risk of fire, thereby increasing the safety of operation. Cast insulation technology allows more precise placement of windings, minimizing internal resistance and increasing mechanical stability.

Proper heat dissipation is also key. Higher operating temperature increases winding resistance, which directly leads to an increase in short-circuit losses. We therefore use efficient natural or forced cooling systems to keep the transformer at optimum operating temperature. We ensure maximum flow of coolant (air or oil) around the windings and core, thus achieving a stable low temperature and the transformer consistently operates under the lowest possible loss conditions.

Eco-design and the future of low-loss transformers

For BEZ TRANSFORMÁTORY, meeting the EU’s Ecodesign requirements is a minimum standard, not a target. Our innovations in materials and design ensure that our low-loss transformers not only meet the strict loss limits in place from 2021 (Stage 2), but many even exceed them, providing our clients with future-ready solutions.

Článok Low-loss transformers: modern technologies and materials je zobrazený ako prvý na BEZ Transformátory.

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.

Článok How to calculate transformer parameters je zobrazený ako prvý na BEZ Transformátory.