Control Transformer

Definition of Control Transformers

Control transformers allow the adaptation of machines and equipment to the different mains voltages worldwide. They increase the functional reliability of electrical control systems in the case of constant overvoltage and undervoltage in the mains due to the standard additional taps of +/- 5% of the primary voltage. In the case of a breakdown, control transformers attenuate the magnitude of the short-circuit current and make it possible to operate auxiliary circuits without grounding.

Function of Control Transformers

A transformer consists of a magnetic circuit, this is called the core. The core has two current-carrying windings, called the primary winding and the secondary winding. When 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. Power transformers are usually wound according to galvanic isolation.
The manufacturing technique for the core and the quality of transformer core used 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.
Copper has the best conductance except for silver with γ = 56. Aluminum, on the other hand, has only γ = 36. So aluminum follows with about 35 percent gap. 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.

Construction of Control Transformers

A transformer consists essentially 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. That 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 inductance. The magnetic flux passes through the secondary coil with the help of the iron core. Thus, the output voltage can be taken from the secondary side of the transformer. Corresponding to the primary side, the coil on secondary side, is called secondary coil. The winding ratio of the primary and secondary coils defines whether the output voltage 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. If both coils have the same number of turns with wire wrapped around them, then the output voltage is equal to the input voltage. A power transformer works mainly with alternating voltage. The decisive factor for the change in power or voltage or current is the ratio of the number of windings. N1/N2. It is important to note that the transformer can either increase or decrease the voltage or the amperage. The respective counterpart will then decrease or increase to the same extent.

The construction power of a transformer is expressed in VA or kVA (VA is the name for voltampère and stands for the unit of measurement of apparent electrical power, kVA for kilovoltampère).

Application Areas of Control Transformers

The control transformer has electrically separated windings according to EN61558-2-2, and is used to supply auxiliary circuits. Control transformers have a low voltage drop with inductive load. According to DIN VDE 0113 Part 1, a control transformer must be provided in electrical systems 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.