TRANSFORMERS

Michael Faraday  propounded  the principle  of electro-magnetic  induction  in 1831. It states that a voltage appears  across the terminals of an electric coil when the flux linked with the same changes. The magnitude  of  the induced  voltage  is  proportional to the rate of  change  of the  flux linkages. This  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.  As the d.c.  power system was in use at that  time,   very  little of    transformer principle was made use of.  In the d.c.   supply  system the generating station and  the load center have  to be necessarily close to each other  due to the requirement  of economic transmission  of power. Also the d.c. generators cannot be scaled up due to the limitations of the commutator.  This made  the world look for other  efficient  methods  for bulk  power generation  and  transmis- sion. During the second half of the 19th century the alternators, transformers and induction motors were invented.  These machines  work on alternating power supply.  The role of the transformers became obvious. The transformer which consisted of two electric circuits linked by a common magnetic circuit helped the voltage and current levels to be changed keeping the power invariant.  The efficiency of such conversion was extremely high.  Thus  one could choose a moderate voltage for the generation of a.c. power, a high voltage for the transmis- sion of this power over long distances and finally use a small and safe operating voltage at the user end.  All these are made possible  by  transformers.  The  a.c. power systems thus got well  established.

Transformers  can link two or more electric circuits.  In its simple form two electric circuits can be linked by a magnetic circuit, one of the electric coils is used for the creation of a time varying  magnetic filed.  The  second coil which is made  to link this field has an induced voltage in the same.  The magnitude of the induced emf is decided by the number of turns used in each coil. Thus the voltage level can be increased or decreased by changing the number  of turns.  This  excitation  winding is called a primary  and the output winding

is called a secondary.  As a magnetic medium forms the link between the primary  and the secondary windings there is no conductive connection between the two electric circuits. The transformer thus provides an electric  isolation between  the two circuits.  The frequency on the two  sides will be the same.  As there  is no change in the nature of the power, the re- sulting machine  is called a ‘transformer’ and  not a ‘converter’.  The  electric power at one voltage/current level is only ‘transformed’ into electric power, at the same frequency, to an- other voltage/current level.

Even though most of the large-power transformers can be found in the power systems, the use of the transformers  is not limited  to the power systems.   The  use of the principle of transformers is universal.   Transformers  can be found operating in the frequency range starting from a few hertz going up to several mega hertz.  Power  ratings vary from a few milliwatts to several hundreds  of megawatts.  The use of the transformers is so wide spread that it is virtually impossible to think of a large power system without transformers.  Demand on electric power generation doubles every decade in a developing country.  For every MVA of generation  the installed  capacity  of transformers  grows by about 7MVA. These  figures show the indispensable  nature of power transformers.