Transformator consist of two coil electricity insulated from one another and wound on the same iron.
An alternating current in one winding sets up an alternating magnetic flux in the core. Most of this flux links with the other winding and induces in it an alternating electromotive force (emf).
Power is thus transferred from one winding to the other via the flux in the core. The winding to which power is supplied is called the primary that from which power is delivered is called secondary.
The power output of a transformer is necessarily less than power input because of unavoidable losses in the form of heat. The losses consist of I^2.R heating in the primary and secondary winding (the copper losses) and hysteresis and eddy current heating in the core (the core losses). In the spite of these losses, transformer efficiency are usually well over than 90% and in large installation may reach 99%.
For simplicity we shall first consider and idealized transformer in which there are no losses and no leakage flux. Let the secondary circuit be open. The primary winding then functions merely as an inductor. The primary current which is small lags the primary voltage by 90degree and is called the magnetizing current, Im.
Since the same flux links both primary and secondary, the induced emf per turn is the same in each. The ratio of primary to secondary induced emf is therefore to the ratio primary to secondary turns έ2/έ1 = N2/N1
In the idealized case assumed, the induce emf’s έ1 and έ2 are numerically equal to corresponding terminal voltage V1 and V2. Hence by properly choosing the turn ratio N2/N1, any desired secondary voltage may be obtained from a given primary voltage. If V2 > V1, we have step-up transformer; if V2 < V1, a step down transformer.
The vector diagram of the idealized transformer is given picture beside for a turn ratio of N2/N1 =2. The induced emf’s in both primary and secondary, since they proportional to the negative rate of change of flux, will be lag 90degree behind the flux, but since the induce emf in the primary έ1 is back emf, the primary terminal voltage V1 is opposite to it phase (V1 = -έ1).
Consider next the effect of closing the secondary circuit. The secondary current I2 (picture beside) and its phase angle έ2 will, of course, depend on the nature of the secondary circuit. It has been assumed that the load is inductive and hence I2 lags V2. As soon the secondary circuit is closed, some power must be deliver by the secondary (except when έ2 = 90degree) and from energy consideration an equal amount of power must be supplied to the primary.
The process by which the transformer is enable to draw the requisite amount of power is as follows. When the secondary circuit is open, the core flux is produced by the primary current only. But when the secondary circuit is closed, both primary and secondary current step up a flux in the core. The secondary current by Lenz’s law, tends to weaken the core flux and there fore to decrease the back emf in the primary. But (in the absence of losses) the back emf in the primary must equal the primary terminal voltage which is assumed to be fixed.
The primary current therefore increases until the core flux is restored to its original no load magnitude. The vector I1’ represent the change in the primary current that takes place when the secondary delivers the current I2. It is opposite in phase to the secondary current I2 and of such magnitude that its magnetomotive force (N1 I1’) is equal and opposite to the magnetomotive force of the secondary current (N2 I2) that is
N2 I2 = N1 I1’ or
I2 / I1’ = N1 / N2
The resultant primary current I1 is the vector sum of I1’ and magnetizing current Im, But in practice the magnetizing current is never more than a few percent of the full loaf current. Hence I1 and I1’ are practically equal and one may write approximately
I2 /I1 = N1 / N2
That is the primary current and secondary current are inversely proportional to the primary and secondary turn.
The effect of leakage flux and resistance of the winding requires that the primary terminal voltage shall be somewhat large than the primary induced emf and not exactly 180degree out of phase with it. Similarly the secondary terminal voltage is somewhat smaller than and out of phase with the secondary emf.
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As the transformer is basically a linear device, a ratio now exists between the number of turns of the primary coil divided by the number of turns of the secondary coil. This ratio, called the ratio of transformation, more commonly known as a transformers “turns ratio”, ( TR ).
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