The transformer principle was demonstrated in 1831 by Michael Faraday, although he used it only to demonstrate the principle of electromagnetic induction and did not foresee its practical uses. The first widely used transformer was the induction coil, invented by Irish clergyman Nicholas Callan in 1836. He was one of the first to understand the principle that the more turns a transformer winding has, the larger EMF it produces. Induction coils evolved from scientists efforts to get higher voltages from batteries. They were powered not by AC, but DC from batteries which was interrupted by a vibrating 'breaker' mechanism. Between the 1830s and the 1870s efforts to build better induction coils, mostly by trial and error, slowly revealed the basic principles of transformer operation. Efficient designs would not appear until the 1880s but within less than a decade, the transformer was instrumental during the "War of Currents" in seeing alternating current systems triumph over their direct current counterparts, a position in which they have remained dominant.
Russian engineer Pavel Yablochkov in 1876 invented a lighting system based on a set of induction coils, where primary windings were connected to a source of alternating current and secondary windings could be connected to several "electric candles". The patent claimed the system could "provide separate supply to several lighting fixtures with different luminous intensities from a single source of electric power". Evidently, the induction coil in this system operated as a transformer.
William Stanley, an engineer for Westinghouse, built the first commercial device in 1885 after George Westinghouse had bought Gaulard and Gibbs' patents. The core was made from interlocking E-shaped iron plates. This design was first used commercially in 1886. The transformer is shown below………
Fig1: A historical Stanley transformer
The transformer is a device ,that transfers electric energy from one electrical circuit to another electrical circuit through the medium of magnetic field and without a change in the frequency.
In a transformer the electrical energy transfer from one circuit to another circuit takes place without the use of moving parts –it has therefore the highest posssible efficinency out of all the electrical machine and almost requires negligible amount of maintenance and supervision.
Fig 2: Ideal transformer showing magnetic flux
The transformer is based on two principles: firstly that an electric current can produce a magnetic field (electromagnetism) and secondly that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction).
By changing the current in the primary coil, it changes the strength of its magnetic field; since the changing magnetic field extends into the secondary coil, a voltage is induced across the secondary.
In addition to the secondary winding there may be third winding on the same iron core.the emf induced in the secondary or tertiary winding is usaually reffered to as the emf due to transformer action.
Ideal two winding transformer
- Winding resistance are negligible.
- Flux are confined i.e no magnetic likage.
- The core losses are negligible
- The core has constant permeability i.e the magnetization curve is linear.
But in actual practice no transformer is ideal.All show deviation from the assumption.
Types of Transfarmer
On the basis of voltage up and down transformer are two types.
- Step - Up
- Step - Down
In step up transformer the secondary voltage is higher than the primary voltage ,so by the above describe equation the secondary winding has more number of turns than the primary winding have.
In step down transformer the secondary voltage is lower than the primary voltage ,so by the above describe equation the secondary winding has less number of turns than the primary winding have.
The alternating current in the primary coil induces an alternating magnetic flux that 'flows' around the ferromagnetic core, changing direction during each electrical cycle. The alternating flux in the core in turn induces an alternating current in each of the secondary coils. The voltage at each of the secondary coils is directly related to the primary voltage by the turns ratio, or the number of turns in the primary coil divided by the number turns in the secondary coil.
The changing magnetic field induces an electromotive force (EMF) across each winding. Since the ideal windings have no impedance, they have no associated voltage drop, and so the voltages VP and VS measured at the terminals of the transformer, are equal to the corresponding EMFs. The primary EMF, acting as it does in opposition to the primary voltage, is sometimes termed the "back EMF". This is due to Lenz's law which states that the induction of EMF would always be such that it will oppose development of any such change in magnetic field.
VP = primary voltage
VS = secondary voltage
NS = secondary no. of turns
VS / VP = NS / NP
Fig. 3: Shows the primary and secondary winding.
A transformer may have multiple secondary coils to feed a number of electrical loads; however, power must be conserved, so the sum of the output power must equal the sum of the input power minus losses.
Transformer universal EMF equation
If the flux in the core is sinusoidal, the relationship for either winding between its rms Voltage of the winding E, and the supply frequency f, number of turns N, core cross-sectional area a and peak magnetic flux density B is given by the universal EMF equation:
In the above describe eqution E Iis emf induced ,f is frequency,Nis no of turns, a is area of the core,B is magnetic field.
Note: Transformer does not operate in dc.why ?
There is simple reason behind this.in the transformer the main phenomena is induction which is due to the chaning nature of the current,but in the dc there is constant cureent so no emf is induced that’s why it does not operate in dc.
- For increasing or decreasing voltage and current levels from one circuit to another circuit.
- For matching the impedance of source and its load for maximum power transfer in electronic and control circuit.
So it is an essential piece of apparatus both for and low current circuit.