Transformers are electrical devices that are designed to convert high voltage and current to lower values and vice versa. They do not have any moving parts and use only magnetic cores to transfer the electrical energy from the circuit. They operate on the principle that magnetic induction is created in a circuit that has electrical current flowing through it.

On their own, transformers cannot convert DC to AC or vice versa, change the voltage or current of DC or change the AC supply frequency. Transformers, however, are components of the systems that can perform all of these duties. According to the law of conservation of energy, the power that is put out by a transformer cannot be higher than the power that is fed into it. The power that is dissipated in a load at any time is equal to the product of the voltage across it and the (in phase) current that is passing through it.

Transformers are among the most basic electrical machines. The design and materials made to use these devices haven’t really changed much over the last century. Transformers have played an important role in the development of high-voltage power transmission over the years. Transformers enable the transmission of power over large distances. This lets generating stations to be located further away from the sites of electricity demand and nearer to their fuel sources.

Transformers are manufactured in a variety of sizes from thumbnail sized coupling transformers that are hidden inside stage microphones to gigawatt devices that are used to connect large sections of power grids. All sizes of transformers perform their duties with the same basic principles and with many similarities in their parts.

A transformer has a primary and a secondary coil and a core. The core can be made of materials such as: air, iron, steel, toroidal, coil with tickler and center tap. The basic types of transformers are single phase and three phase transformers.

Transformers can be classified as:

  • Step down: This is where the output voltage is lesser than the input voltage.
  • Step up: This is where the output voltage is greater than the input voltage.
  • Isolating: This is where the voltage is transformed from one to another of the same value.
  • Variable: This is where the value can be varied. Having a different number of turns in the primary and the secondary coil achieves this. 

While selecting a transformer you need to consider the primary frequency, the maximum primary/secondary voltage, current rating, maximum power rating and the type of output required (AC or DC). Other characteristics are toroidal and laminated.

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