The basis of the transformer is determined by the phenomenon of electromagnetic induction. The core of the transformer consists of individual steel plates assembled in a closed frame of one form or another. Two windings S₁ and S₂ with the number of turns w₁ and w₂ are placed on the core. The windings have low resistance and high inductance.
Apply to both ends of the winding S₁, which we will call primary, an alternating voltage U₁. An alternating current I will pass through the winding, which will magnetize the steel of the core, creating a magnetic alternating flux in it. The magnetizing effect of the current is proportional to the number of ampere turns (Iw₁).
As the current increases, the magnetic flux will also increase in the core, the change of which will excite the electromotive force of self-induction in the turns of the coil. As soon as it reaches the value of the applied voltage, the current growth in the primary circuit will stop. Thus, the applied voltage U₁ and the electromotive self-induction force E₁ will act in the transformer primary winding circuit. In this case, the voltage U₁ is greater than E₁ by the magnitude of the voltage drop in the winding, which is very small. Therefore, approximately you can write:
U₁ = E₁.
The magnetic alternating flux arising in the core of the transformer also passes through the turns of its secondary winding, exciting in each turn of this winding the same magnitude electromotive force as in each turn of the primary winding.
Based on the fact that the number of turns of the primary winding is w₁ and the secondary is w -, the forces induced in them will be, respectively, equal to:
E₁ = w₁e,
E₂ = w₂e,
where e is the electromotive force arising in one turn.
The voltage U₂ at the ends of the open winding is equal to the electromotive force in it, i.e.:
U₂ = E₂.
Therefore, we can conclude that the magnitude of the voltage at both ends of the primary winding of the transformer refers to the magnitude of the voltage at the ends of the second winding, as the number of turns of the primary winding refers to the number of turns of the secondary winding:
(U₁ / U₂) = (w₁ / w₂) = k.
A constant value k is the transformation coefficient of the current transformer.
In the event that you need to increase the voltage, arrange a secondary winding with an increased number of turns (the so-called step - up transformer); in the case when it is necessary to lower the voltage, the secondary winding of the transformer is taken with fewer turns (step-down transformer). One transformer can act both as an increase in the transformation ratio and as a decrease, depending on which winding is used as the primary one.
The secondary winding is still open (there is no current in it). The transformer is idling. At the same time, it consumes little energy, since the current magnetizing the steel core is very small due to the large inductance of the coil . There is no transfer of energy to the secondary circuit from the primary one. This experience makes it possible to find out the transformation coefficient, idling resistance and current of the transformer.
We load the transformer by closing the secondary circuit through the rheostat. The induction current will go along it, we denote it by the letter I₂. This current, according to Lenz’s law, will cause a decrease in magnetic flux in the core. But the weakening of the magnetic flux in the core will lead to a decrease in the electromotive force of self-induction in the primary winding and to an imbalance between this force E₁ and the voltage U₁ given by the generator to the primary winding. As a result of this, in the primary winding, the current will increase by some value I₁ and become equal to I + I₁. Due to the increase in current, the magnetic flux in the core of the transformer will increase to the previous value, and the imbalance between U₁ and E₁ will be restored again. Thus, the appearance of the secondary current I₂ causes an increase in the current in the primary winding by an amount I₁, which will determine the load current of the primary winding of the transformer.
When the transformer is loaded, a continuous transfer of energy to the secondary circuit from the primary takes place. According to the law of conservation and conversion of energy, the current power in the primary circuit is equal to the current power in the secondary circuit; therefore, equality must apply:
I₁ U₁ = I₂U₂.
In reality, this equality is not observed, since there are losses, although small, when the transformer is operating. The transformation ratio is about 94-99%.