Single-Phase Transformer

A transformer consists of a magnetic circuit, called the core, and has at least two windings with a fixed number of turns through which current and voltage flows. The windings facing the electrical voltage (line voltage) is called the primary side (primary coil), and the side with the consumer and electrical load is called the secondary side (secondary coil). The incoming power from current and voltage is transformed into outgoing power from current and voltage.

A transformer essentially comprises two or more coils and a shared iron core. A single-phase transformer often uses only one coil, but at higher powers, two coils are connected in parallel or in series. The three-phase transformer consists of three coils, each of which is interconnected according to the desired vector group. The windings of a transformer are usually made of insulated copper enamelled wire and are wound on the iron core, either on a separate coil former or with spacer bars and insulation in compliance with clearance and creepage distances. The AC voltage is connected there, and an alternating magnetic field is created. The magnetic flux passes through the secondary coil with the help of the iron core. Thus, at the secondary side of the transformer, the output AC voltage (induced voltage) can be taken with the desired AC current. The winding ratio of the primary and secondary coils defines whether the voltage at the output is smaller or larger 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 less, then the output voltage is less than the input voltage. The ratio of the number of turns N1/N2 is decisive for the change in power or AC voltage and current. The wire gauge used on the coils is defined by the current.

The manufacturing technique for the core and the used quality of the transformer core (iron core) affects the magnetic circuit. The magnetic circuit of a transformer (magnetic field) should ideally produce low eddy current losses and have low remagnetization losses (hysteresis losses). Another aspect is the resistances in the winding of a transformer. Only with layered and ordered windings on the primary coil and the secondary coil and the best winding metal can the winding losses be reduced. The voltage is controlled with the number of turns on the coil. The current determines the diameter of the winding metal.
The design power of a transformer is expressed in VA or kVA (VA is the term for voltampère and represents the unit of measurement of apparent electrical power, kVA for kilovoltampère).
Except for silver, copper has the best conductance with γ = 56. Aluminum, on the other hand, has only γ = 36. Aluminum thus follows with a gap of about 35 percent. Thus, copper is 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 of electricity, and alloys generally have considerably lower conductivity than pure metals. Silver or gold are ruled out altogether because of 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 as well as the number of turns of the transformer’s winding due to electromagnetic induction. This means that the voltage on the winding is proportional to the number of turns of the transformer. If a machine (consumer) is connected to the secondary coil, it draws energy from the transformer on the secondary side. The current flow within a transformer works according to Lenzschen rule. The currents in the windings are therefore opposite. The primary current in a transformer flows to the right with respect 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 an ideal transformer in theory is not affected by heat losses.

The differences to a real transformer can be defined as follows. A real transformer has resistances in the winding, which lead to energy losses. In addition, with a real transformer it must always be expected that the re-magnetization as well as eddy currents also lead to energy losses. Thus, copper losses (resistances in the winding), hysteresis losses (re-magnetization) and eddy current losses (loss due to eddy currents) occur. In addition, leakage flux always occurs (the magnetic flux flowing through the primary side does not flow proportionally through the secondary side). In addition, the permeability of an iron core depends on the strength of the magnetic flux (magnetic flux density).

In general, transformers are distinguished according to their galvanic isolation. Isolating transformers have no connection between the input side and the output side. These two windings are separated from each other. In the case of autotransformers, the secondary side taps its voltage on the primary winding part, hirt there is no separate independent secondary winding, so there is no galvanic isolation here. The advantage of autotransformers is their smaller size compared to isolating transformers. The use of autotransformers is only possible to a limited extent and must be checked in each individual case.

The transformer can change the AC voltage and current between input and output, but it cannot change the frequency. The incoming frequency is always equal to the outgoing frequency. Transformers can also be calculated for high frequencies.