what is transformer?

                           Electrical energy is generated at places where it is easier to get water head, oil or coal for hydroelectric, diesel or thermal power stations respectively. Then energy is transmitted at long distance for use of village, towns and cities load. As we know that transmission of electrical energy at high voltage is economical. Some equipment is required to step up and step down the voltage. Electrical machine use for this purpose is called ‘Transformer’.

Transformer

     Definition

The transformer is a static device that 1)transfers electrical energy from one circuit to other circuits, 2)without a change in frequency, 3)using electromagnetic induction, 4)has electrical circuits that are linked by a common magnetic circuit.

Type 

     based on the voltage level

1.step up Transformers: when the input voltage is lower than output voltage it is called as step up transformers (Vi < Vo)

2.step down Transformers: when the input voltage is higher than output voltage it is called as step down transformers (Vi > Vo)

Operating principle

      Transformer consisting a soft iron or silicon steel core and two winding wound on a core. The windings are insulated from each other and core. The core builds with laminations to give low reluctance path to magnetic flux. The winding connected to supply is called primary and winding connected to load is called secondary winding. The winding connected to high voltage is called ‘hv winding’ and the winding connected to low voltage is called ‘lv winding’. In the case of step-up transformers, lv winding is primary and HV winding is secondary. While in the case of a step-down transformers HV winding is primary and lv winding is secondary.  

The working of a transformer is based on the principle that energy may be transferred by induction from one coil to another by means of a carrying magnetic flux, provided that both the sets of coils are on a common magnetic circuit. In Transformers, all part of coil and core are stationary with respect to one another. The EMFs are induced by variation in the magnitude of flux with time as shown in fig.1

In practical construction, the two windings are usually wound one over the other, for the sake of simplicity, the figures for analyzing transformer theory show the windings on opposite sides of the core as in fig.1

When the primary is connected to AC mains the current flows through the primary winding. Since winding is wound on the core so current flowing through this winding produces an alternating flux(Φ) in the core. Also, this flux links with secondary winding and produce EMFs in the secondary winding. But the frequency of inducing EMFs is same in both windings. The secondary winding is able to feed external load thus energy is transformed from primary winding to secondary winding using electromagnetic induction at the same frequency. Flux produced in primary does not only link with secondary but also link itself and produce EMFs. This EMFs opposes the applies voltage and, therefore, sometimes it is called as back emf. This emf limits the primary current like in dc motor.

Application 

It is not only used to step up/down voltage but also used in

1. One-to-One ratio transformers are used in electrically isolate the two parts of an electrical circuit.
2.    In high voltage laboratories, the transformers are used to provide very high voltages for testing purpose.
3.  In electrical communication circuits transformers are used for a variety of purposes like, as an impedance transformation device to allow maximum transfer power from the input circuit to the output device.
4.  In radio and television circuits input transformers, interstage transformers and output transformers are widely used.
5. They are also used in a telephone circuit, instrumentation circuits and control circuits.

Transformer on dc

A transformer cannot operate on dc supply and never be connect a dc source. If rated voltage is applied to the primary of a transformer, the flux produced in the transformer core will not vary but remain constant in magnitude and therefore no end will be induced in the secondary winding except at the moment of switching on. Thus the transformer is not capable of raising or lowering the dc voltage. Also, there will be no self-induced end in the primary winding which is only possible with varying flux linkage to oppose the applied voltage and since the resistance of primary winding is quite low therefore a heavy current will flow through the primary winding which may result in the burning out of the primary winding. This is the reason that dc is never applied to a transformer.

Ideal transformer

For a better understanding and an easier explanation of a practice transformer, the certain idealizing assumption is made which are close approximations for a practical transformer. A transformer having these ideal properties is hypothetical and is referred to as the ideal transformer. It possesses certain essential features of a real transformer but some details of minor significance are ignored which will be introduced step-by-step while analyzing a transformer. The idealizing assumption made are as follows:

1) No winding resistance i.e. the primary and secondary winding have zero resistance. It means that is ohmic power loss and no resistive voltage drop in an ideal transformer.

2) No magnetic leakage i.e. there is no leakage flux and all the flux set up is confined to the core and link both the windings.

3) No iron loss i.e. hysteresis and eddy current losses in transformer core are zero.

4) Zero-magnetizing current i.e. the core has infinite permeability and zero reluctance so that zero magnetizing current is required for establishing the requisite amount of flux in the core.

From the above discussion, an ideal transformer is supposed to consists of two purely inductive coils wound on a loss-free core.

Efficiency 

Since the transformer is a static device because there is no moving part in it, has very rugged machine required less maintains and minimum amount of repair work. Also, no rotating part so no friction or windage losses. In addition, other losses are relatively low so that the efficiency of a transformer is high. Typical transformer efficiency at full load lie between 96% and 97% and with extremely large capacity transformers the efficiency is as high as 99%.

The cost per kva output of transformers is quite low as compared with other electrical machines. As there are no teeth, slots or rotating parts, and the windings can be immersed in oil, it is not difficult to insulate transformers for very high voltages.

Reference

Theory & Performance of electrical machine by J. B. Gupta