Transformation ratio

The basis of the transformer determines the phenomenon of electromagnetic 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. Windings have insignificant resistance and greater inductance.

We apply to both ends of the winding S₁, which we call the primary, the 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 magnetic flux will also increase in the core, the change of which will excite the electromotive force of self-induction in the coils of the coil. 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 = E.

The magnetic alternating current arising in the 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 = E.

Consequently, it can be concluded 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.

Constant value k is the transformer ratio of the current transformer.

In the event that it is necessary to increase the voltage, arrange a secondary winding with an increased number of turns (the 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 (there is no current in it). 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 absent. This experience makes it possible to know the coefficient of transformation, idling resistance and transformer current.

We load the transformer by closing the secondary winding circuit through the rheostat. It will now be followed by an induction current, denoted 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 value of I₁ and become equal to I + I. Due to the increase in current, the magnetic flux in the core of the transformer will increase to the previous value, and the disturbed balance between U₁ and E₁ will be restored again. Thus, the appearance of a 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 circuit takes place. 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 = I₂U.

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%.