Transformation ratio

The basis of the transformer determines the phenomenonelectromagnetic induction. The core of the transformer consists of separate steel plates assembled in a closed frame of one form or another. Two windings S₁ and S₂ with the number of turns w₁ and w поме are placed on the core. The windings have insignificant resistance and large inductance.

Apply to both ends of the winding S₁, whichlet's call the primary, alternating voltage U₁. The alternating current I will pass through the winding, which will magnetize the steel of the core, creating a magnetic alternating stream in it. The magnetizing effect of the current is proportional to the number of ampere-turns (Iw₁).

As the current rises, the magneticflux and in the core, the change of which will excite in the turns of the coil the electromotive force of self-induction. As soon as it reaches the value of the applied voltage, the current growth in the primary circuit will cease. Thus, in the circuit of the primary winding of the transformer, the applied voltage U₁ and the electromotive force of self-induction Е будут will act. At the same time, the voltage U₁ is greater than E₁ by the voltage drop in the winding, which is very small. Consequently, we can approximately write:

U₁ = E₁.

The magnetic variable flow arising inthe core of the transformer, also passes through the windings of its secondary winding, exciting in each winding of this winding the same electromotive force as in each turn of the primary winding.

Proceeding from the fact that the number of turns of the primary winding is w₁, and the secondary winding is w₂, the forces induced in them will be, respectively, equal:

E₁ = w₁e,

E₂ = w₂e,

where e is the electromotive force arising in one revolution.

The voltage U₂ at the ends of the open winding is equal to the electromotive force in it, ie:

U₂ = E₂.

Consequently, we can conclude that the quantityvoltage at both ends of the primary winding of the transformer refers to the voltage value 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.

The constant value k is the transformer ratio of the current transformer.

In the event that you need to increase the voltage,arrange a secondary winding with an increased number of turns (so-called step-up transformer); in the case where it is necessary to lower the voltage, the secondary winding of the transformer is taken with a smaller number of turns (a step-down transformer). One transformer can act both as a step-up conversion factor and as a step-down transformer, depending on which winding is used as the primary.

The secondary winding is still open (current in itno). The transformer is idle. At the same time, it consumes little energy, since the current, the magnetizing steel core, is very small due to the large inductance of the coil. The transfer of energy to the secondary circuit from the primary is not present at all. This experience makes it possible to know the coefficient of transformation, the resistance of idling and the current of the transformer.

We load the transformer, closing it through the rheostatsecondary winding circuit. Now an induction current will flow along it, we denote it by the letter I₂. This current, according to Lenz's law, will cause a decrease in the 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, in the primary winding the current will increase by some amount I₁ and become equal to I + I₁. Due to the increase in current, the magnetic flux in the transformer core will increase to the previous value, and the disturbed equilibrium 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,continuous transmission of energy into the secondary circuit from the primary. According to the law of conservation and transformation of energy, the current in the primary circuit is equal to the current in the secondary circuit; therefore, equality must work:

I₁ U₁ = I₂U₂.

In fact, this equality is not respected, since when the transformer is operating there are losses, albeit small ones. The transformation ratio is about 94-99%.