Single-Phase Transformer Anatomy: Core & Winding Construction

A look back in the History

Who invented the transformer?

Otto Blathy, Miksa Deri, Karoly Zipernowsky first designed Later on Lucien Gaulard, Sebstian Ferranti, and William Stanley perfected the design

When was the transformer invented? In 1886, William Stanley, built the first reliable commercial transformer

Where were the first transformers used? Great Barrington, Massachusetts in 1886 In 1891, Mikhail Dobrovsky designed and demonstrated his 3 phase transformers at Frankfurt, Germany

• Michael Faraday: principle of electro-magnetic induction in 1831.

• The finding forms the basis for many magneto-electric machines.

• The earliest use of this phenomenon was in the development of induction coils.

• These coils were used to generate high voltage pulses to ignite the explosive charges in the mines.

• d.c. power system was in use at that time

• Limitation of d.c. system:

  • for economic transmission of power the generating station and the load center have to be necessarily close to each other

  • the d.c. generators cannot be scaled up due to the limitations of the commutator

• world look for other efficient methods for bulk power generation and transmission.

• During the second half of the 19th century the alternators, transformers and induction motors were invented.

• These machines work on alternating power supply

• With the invention of transformers, it was possible to choose a moderate voltage for generation of a.c power, a high voltage for transmission of this power over long distance, and finally use a small and safe operating voltage at the user end

Transformer: The heart of the AC system

• Transformer is a static device

• Change the voltage and current levels keeping the power invariant

• Consists of two electrical circuit linked by a common magnetic circuit

• One of the electrical coils is used for generation of time varying magnetic field

• Second coil links to the magnetic field and has induced voltage in it

• Magnitude of the induced emf is decided by the number of turns in each coil

• Voltage level can be changed by changing the number of turns

• Excitation winding is called primary winding and output winding is called secondary

• No conductive connection between two electrical circuits and thus provides electrical isolation

• Frequency on the two sides will be the same

• Efficiency of the conversion is extremely high

• No change in the nature of power so ’Transformer’ and not ’Converter’

• Transformers are not limited to power systems

  • frequency operating from few Hz to several MHz

  • power rating from few milliwatts to several hundred of Megawatts

• Electric power generation demand doubles every decade in a developing country

• For every MVA of generation, the installed capacity of transformers grows by about 7 MVA

Construction

• The construction of the transformer differs based on their end use

• Apart from use in power systems, special transformers are used in application like, electronic supplies, rectification, furnaces, traction, etc.

• Principle of operation remains the same

• Focus on power transformer

• Constructional aspects will be discussed under three categories:

  • Core construction

  • Winding arrangements

  • Cooling aspects

Core Construction

• Material used for core should be highly permeable (\(\mu_r > 1000\))

• High permeability will give low reluctance for the path of the flux

• Flux line will confine to the iron core

• Silicon steel in the form of thin laminations is used

• Over the years, for better magnetic properties, Hot rolled non-oriented to Hot rolled grain oriented steel is used

• Later better lamination in Cold Rolled Grain Oriented Steel (CRGOS) became available

• The thickness of the laminations progressively got reduced from over 0.5 mm to the present 0.25 mm per lamination

• Laminations are coated with a thin layer of insulating varnish, oxide or phosphate

• Magnetic material is required to have

  • high permeability \(\mu\)

  • high saturation flux density

  • small area under the B-H loop-to permit high flux density of operation with low magnetizing current and low hysteresis loss

• The resistivity of the iron sheet itself is required to be high to reduce the eddy current losses

• The eddy current itself is highly reduced by making the laminations very thin

• If the lamination is made too thin then the production cost of steel laminations increases

• For very small transformers (few VA to few KVA) hot rolled silicon steel laminations in the form of \(E \& I, C \& I\) or \(O\) are used and core cross-section should be square or rectangle

  

• Silicon content in steel is 3.5%. Above this steel becomes very brittle and also very hard to cut

• Saturation flux density of the present steel laminations is about 2%