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Transformer Basics

Transformers are crucial electrical components that transfer electrical energy between different circuits.

They have made significant progress over the years, revolutionising the energy sector and facilitating the effective transmission and distribution of electricity.

The history of transformers, from their modest beginnings to present times, proves human ingenuity and the constant pursuit of innovation in the field of electrical engineering.


Certainly! The origins of transformers can be traced back to the 19th century when their fundamental principles were first revealed.

In 1831, Michael Faraday, an English scientist, made a significant breakthrough in the field of electromagnetism when he discovered electromagnetic induction. This discovery demonstrated that by moving a magnet inside a coil of wire, an electric current could be generated within the wire. This pivotal moment in scientific history laid the groundwork for the development of transformers.

The initial functional transformer was devised by Hungarian engineer Ottó Bláthy in 1885 during his tenure at the Ganz Works corporation in Budapest. Bláthy's design incorporated an iron core alongside distinct primary and secondary windings.

This particular transformer facilitated the distribution of alternating current (AC) over extensive distances, leading to a transformative impact on the power sector.

Consequently, transformer technology witnessed consistent improvements during the early 20th century. Oil-filled transformers were introduced, utilising mineral oil as both insulator and coolant. This led to improved efficiency and reliability and their adoption in numerous applications in power generation, transmission, and distribution.

Continuing advancements included the use of silicon steel cores in transformer design, resulting in decreased energy loss due to magnetic hysteresis and eddy currents, thereby enhancing overall efficiency.

With the emergence of computer technology in the late 20th century, transformers played a pivotal role in powering electronic devices like computers, televisions, and telecommunications equipment. The need for smaller, more efficient transformers escalated, which led to the creation of lightweight and streamlined designs.

Today, transformers remain integral to numerous industries and applications. Beyond power transmission and distribution, they function within renewable energy systems, such as solar and wind power, where they convert and distribute electricity generated from these sources.

In summary, the evolution of transformers, from Faraday's discovery of electromagnetic induction to modern, high-efficiency designs, has facilitated our interconnected world's power.

  • A transformer only works with alternating current (AC), not with direct current (DC)!
  • The AC supply is connected to the primary side --> the alternating current creates a alternating magnetic field --> the voltage induced by the magnetic field in the primary winding keep balance with the supply voltage (=induction law)
  • Through the iron core the magnetic flux is conducted to the secondary side
  • Now the secondary winding is also permeated with a changing magnetic field --> this induces an AC voltage in the secondary coil
  • The induced voltage is the same in each turn / the induced voltage of both coils is proportional to the number of their turns = transformer ratio

Basic construction of a transformer

  • Consists of two or more coils on a common iron core
  • The core is built from a stack of individual sheets, with electrically insulation layers in between --> thus reducing eddy currents
  • Primary winding = input winding --> voltage connection
  • Secondary winding = output winding --> here is the electrical load connected

Different construction types

Shell type

Both windings are on the middle leg, either one above the other ore side by side.


  • Higher efficiency
  • More power


  • Maintenance work is very hard
  • Forced cooling indispensable


Core type

The middle leg is missing, the core forms the shape of a rectangle in side view. Usually, the windings are located separately on the two outer legs. 


  • Cheaper
  • Easier maintenance


  • Higher flux leakage
  • Consists of only one coil which has one or more taps to draw the output voltage
  • Primary coil and secondary coil are combined
  • No galvanic separation


  • Smaller construction with the same power
  • More efficient compared to two windings transformer


  • If a fault like a short circuit occurs on the primary side, this fault is also on the secondary side, because they are combined

Area of application:
Wherever deviating voltages have to be provided and galvanic isolation is not required.

Unloaded transformer

  • If no load is connected to the secondary winding, there is no load --> the transformer is unloaded
  • In no-load operation, the iron losses are due to the small input current much bigger than the cooper losses due to the no-load current in the primary coil
  • It does not normally exist in practice

Loaded transformer

  • If a consumer is connected to the secondary winding, it takes electrical energy from the secondary coil
  • A current arises on the secondary side and the primary current gets bigger
  • The currents are directed in opposite directions in the windings
  • If the primary current flows right-handed through the coil, the secondary current flows left-handed and vice versa
  • Transformers can be heavily overloaded for a short time

Short-circuit voltage

  • The short-circuit voltage depends on the construction of the core and the position of the coils in relation to each other
  • High stray fluxes lead to high short-circuit voltages
  • Stray fluxes are part of a magnetic flux which are coupled with one of the windings only, so lost from the energy transfer. Stray fluxes can be taken into account by the impedance representing their magnetic path. More stray flux, larger impedance, higher voltage drop.

Short-circuit current

  • This is the name given to the current that flows in the event of a short circuit on the secondary side and at nominal voltage supply on the primary side.
  • It is much higher than the rated current and can destroy the transformer within a short time
  • The short-circuit current is higher the lower the short-circuit voltage is
  • Short-circuit proof --> Name for transformers, which are designed in such a way that they are not destroyed in the event of a short circuit

In reality there is no loss-free transformer

  • Losses due to resistances in the windings and parasitic capacitances
  • Losses in the metal structure caused by scattering of the magnetic flux
  • Eddy currents and magnetic reversal losses in the iron core

This results in energy losses.

The efficiency indicates the ratio of the power which leaves the transformer on the secondary side and the power which flows in on the primary side. That means how much energy gets "lost".

The higher the efficiency, the fewer the lossesand the more efficient the transformer works.

The efficiency can, because of the iron and copper losses never be 1. It is expressed in %.

Formula for calculation

Typical efficiency of different transfomers:

  • Transformers with high rated power: 99%
  • Small transformers: 80%
  • Miniature transformers: 50%


The voltages, currents and the windings can be calculated with the formula below.

V = Voltage in Volt (V)
I = Current in Ampere (A)
N = Winding turn number

The ratio of the input and output voltage corresponds to the ratio of the winding turn numbers of the input and output sides.

V1 = input voltage; V2 = output voltage; I1 = current primary side; I2 = current secondary side; N1 = Number of windings primary side; N2 = Number of windings secondary side

Contact our Experts

Herbert Schubert

Area Sales Manager

Phone +49 941 4090 7453
Mobile +49 175 2948 706
E-Mail h.schubert@reinhausen.com

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