TRANSFORMER ON NO LOAD AND ON LOAD CONDITIONS

In fact the practical transformers are very close to this model and hence no major departure is made in making these assumptions.

Fig. 11 shows a two winding ideal transformer. The primary winding has T1 turns and is connected to a voltage source of V1 volts. The secondary has T2 turns. Secondary can be connected to a load impedance for loading the transformer. The primary and secondary are shown on the same limb and separately for clarity. As a current I0 amps is passed through the primary winding of T1 turns it sets up an mmf of I0T1 ampere which is in turn sets up a flux through the core. Since the reluctance of the iron path given by R =l/µA is zero as µ→∞, a vanishingly small value of current
I0 is enough to setup a flux which is finite. As I0 establishes the field inside the transformer it is called the magnetizing current of the transformer.

This current is the result of a sinusoidal voltage V applied to the primary. As the current through the loop is zero (or vanishingly small), at every instant of time, the sum of the voltages must be zero inside the same. Writing this in terms of instantaneous values we have,

v1 _ e1 = 0                                     (10)

where v1 is the instantaneous value of the applied voltage and e1 is the induced emf due to Faradays principle. The negative sign is due to the application of the Lenz's law and shows that it is in the form of a voltage drop. Kirchoff's law application to the loop will result in the same thing.

The voltages E1 and E2 are obtained by the same mutual flux and hence they are in phase. If the winding sense is opposite i.e., if he primary is wound in clockwise sense and the secondary counter clockwise sense then if the top terminal of the first winding is at maximum potential the bottom terminal of the second winding would be at the peak potential. Similar problem arises even when the sense of winding is kept the same, but the two windings are on opposite limbs (due to the change in the direction of flux). Hence in the circuit representation of transformers a dot convention is adopted to indicate the terminals of the windings that go high (or low) together. (Fig. 12).

This can be established experimentally by means of a polarity test on the transformers. At a particular instant of time if the current enters the terminal marked with a dot it magnetizes the core. Similarly a current leaving the terminal with a dot demagnetizes the core.

So far, an unloaded ideal transformer is considered. If now a load impedance ZL is connected across the terminals of the secondary winding a load current flows as marked in Fig. 11(c).

This load current produces a demagnetizing mmf and the flux tends to collapse. However this is detected by the primary immediately as both E2 and E1 tend to collapse. The current drawn from supply increases up to a point the flux in the core is restored back to its original value. The demagnetizing mmf produced by the  econdary is neutralized by additional magnetizing mmf produces by the primary leaving the mmf and flux in the core as in the case of no-load. Thus the transformer operates under constant induced emf mode. Thus,

An impedance of ZL when viewed `through' a transformer of  turns ratio ( T1/T2) is seen as (T1/T2)2:ZL. Transformer thus acts as an impedance converter. The transformer can be  interposed in between a source and a load to `match' the impedance. Finally, the phasor diagram for the operation of the ideal transformer is shown in Fig. 13 in which θ1 and θ2 are power factor angles on the primary and secondary sides. As

the transformer itself does not absorb any active or reactive power it is easy to see that θ1 = θ2. Thus, from the study of the ideal transformer it is seen that the transformer provides
electrical isolation between two coupled electric circuits while maintaining power invariance at its two ends. This can be used to step up or step down the voltage /current at constant volt-ampere. Also, the transformer can be used for impedance matching. In the case of an ideal transformer the efficiency is 100% as there are no losses inside the device.