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  • A transformer is a static electromagnetic device designed for the transformation of the (primary) alternating current system into another (secondary) one of the same frequency with other characteristics, in particulars, other voltage and current.
  • As a rule a transformer consists of a core assembled of sheet transformer steel and two or several windings coupled electromagnetically, and in the : case of autotransformer, also electrically.
  • A transformer with two windings is called double-wound transformer; a transformer with three or more windings is termed a triple wound or multi-winding one.
  • According to the kind of current, transformers are distinguished as single-phase, three-phase and poly-phase ones. A poly-phase transformer winding is a group of all phase windings of the same voltage, connected to each other in a definite way.
  • Primary and secondary windings. The transformer winding to which the energy of the alternating curr.ent is delivered is called the primary winding; the other winding from which energy is received is called the secondary winding.
  • In accordance with the names of the windings, all quantities pertaining to the primary winding as, for example, power, current, resistance etc., are also primary, and those pertaining to the secondary winding secondary.
  • h.v. and l.v. windings. The winding connected to the circuit with the higher voltage is called the high-voltage winding (h.v.), the winding connected to the circuit with the lower voltage is called the low-voltage winding. (l.v.), If the secondary voltage is less than the primary one, the transformer is called a step-down transformer and if more-a step-up transformer.
  • A tapped transformer is one whose windings are fitted with special taps for changing its voltage or current ratio.
  • Oil and dry transformers. To avoid the deterimental effect of the air on the winding insulation and improve the cooling conditions of the transformer its core together with the windings assembled on it is immersed in a tank filled with transform oil. Such transformers are called oil transformers. Transformers not immersed in oil are called dry transformers.