Control transformer

Control transformers enable machines and systems to be adapted to different mains voltages and types worldwide. They increase the functional reliability of electrical control systems in the event of constant overvoltage and undervoltage in the mains due to the standard additional taps of +/- 5% of the primary voltage. In the event of a fault, control transformers attenuate the level of the short-circuit current and enable auxiliary circuits to be operated unearthed.

A transformer consists of a magnetic circuit, which is referred to as the core. The core has two windings through which current flows, the so-called primary winding and the secondary winding. If an alternating electrical voltage is applied to the primary winding, an alternating current begins to flow through the primary winding. This alternating current generates a magnetic field in the winding, which changes its strength and frequency with the alternating current source. The magnetic flux of the transformer changes with the frequency of the AC voltage. Mains transformers are usually wound with galvanic isolation.
The manufacturing technology for the core and the quality of the transformer 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.
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.

A transformer essentially consists of two or more coils and a common iron core. The windings of a transformer are usually made of insulated copper wire and are wound on the iron core. The input voltage is applied to the primary winding of the transformer. This is why the coil on the primary winding is often called the primary coil. The alternating voltage on the primary coil creates an alternating magnetic field due to inductivity. The magnetic flux passes through the secondary coil with the help of the iron core. The output voltage can therefore be taken from the secondary side of the transformer. Corresponding to the primary side, the coil on the secondary side is called the secondary coil. The winding ratio of the primary and secondary coil defines whether the output voltage 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. If both coils have the same number of turns of wire wound around them, the output voltage is equal to the input voltage. A mains transformer works mainly with alternating voltage. The ratio of the number of windings is decisive for the change in power, voltage or current. N1/N2. It is important to note that the transformer can either increase or decrease the voltage or current. The respective counterpart will then decrease or increase to the same extent.
The power rating of a transformer is expressed in VA or kVA (VA stands for volt-ampere and is the unit of measurement for apparent electrical power, kVA for kilovolt-ampere).

The control transformer has electrically separate windings in accordance with EN61558-2-2 and is used to supply auxiliary circuits. Control transformers have a low voltage drop with an inductive load. According to DIN VDE 0113 Part 1, a control transformer must be provided in electrical installations if: Machines and systems have more than 5 electromagnetic actuating coils, relays, contactors etc., control and signaling devices are installed outside control cabinets and machines or if electronic control or signaling circuits are to be supplied.