Single-phase transformer

A transformer consists of a magnetic circuit, known as a core, and has at least two windings with a fixed number of turns through which current and voltage flow. The windings facing the electrical voltage (mains voltage) are referred to as the primary side (primary coil), the side with the consumer and the electrical load is referred to as the secondary side (secondary coil). The incoming power from current and voltage is transformed into an outgoing power from current and voltage.

A transformer essentially consists of two or more coils and a common iron core. In a single-phase transformer, only one coil is often used; for higher outputs, two coils are connected in parallel or in series. The three-phase transformer consists of three coils, which are connected together according to the desired switching group. The windings of a transformer are usually made of insulated enameled copper wire and are wound on the iron core, either on a separate coil former or with spacer rods and insulation in compliance with clearance and creepage distances. This is where the AC voltage is connected, creating an alternating magnetic field. The magnetic flux passes through the secondary coil with the help of the iron core. The AC voltage (induced voltage) on the output side can therefore be drawn from the secondary side of the transformer with the desired AC current. The winding ratio of the primary and secondary coils defines whether the voltage at the output is lower or higher than the input voltage. If the number of turns of the secondary coil is greater than that of the primary coil, the output voltage is greater than the input voltage. However, if the number of turns of the secondary coil is lower, the output voltage is lower than the input voltage. The ratio of the number of turns N1/N2 is decisive for the change in power or alternating voltage and current. The wire thickness used on the coils is defined by the current.

The manufacturing technology for the core and the quality of the transformer core (iron core) used have an effect on the magnetic circuit. Ideally, the magnetic circuit of a transformer (magnetic field) should generate low eddy current losses and have low remagnetization losses (hysteresis losses). Another aspect is the resistances in the winding of a transformer. Winding losses can only be reduced with layered and ordered windings on the primary and secondary coils and the best winding metal. The voltage is regulated by the number of turns on the coil. The current strength determines the diameter of the winding metal.

The power rating of a transformer is expressed in VA or kVA (VA stands for volt-ampère and is the unit of measurement for apparent electrical power, kVA for kilovolt-ampère).
With the exception of silver, copper has the best conductivity value with γ = 56. Aluminum, on the other hand, only has γ = 36. Aluminum therefore follows with a gap of around 35 percent. Copper is therefore the best metal and aluminum “only” the second best of the technically and economically usable conductor materials for electrical energy. All other metals cannot be considered as conductors, and alloys generally have a considerably lower conductivity than pure metals. Silver or gold are ruled out completely due to their high price.

The ideal transformer does not exist. The ideal transformer is lossless and is only used as a model to describe the function of transformers. In an ideal transformer, the voltage across the windings is proportional to the rate of change of the magnetic flux and the number of turns of the transformer winding due to electromagnetic induction. This means that the voltage at the winding is proportional to the number of turns of the transformer. If a machine (load) is connected to the secondary coil, this draws energy from the transformer on the secondary side. The current flow within a transformer works according to Lenz’s rule. The currents in the windings are therefore opposite. The primary current in a transformer flows to the right in relation to the core, the secondary current to the left. In an ideal transformer, the combination of the equations for the voltage transformation shows that the energy supplied to the primary side is equal to the energy removed from the secondary side. This means that, in theory, an ideal transformer is not subject to any heat losses.