core of the transformer determines the phenomenon of electromagnetic induction.A transformer core consists of individual steel plates, assembled in a closed frame of some form.At the core are placed two windings S₁ and S₂ with the number of turns and w₁ w₂.The windings have little resistance and a large inductance.
attach to both ends of the winding S₁, which we call the primary alternating voltage U₁.On the winding pass AC current I, which magnetize the steel core, creating in him a magnetic alternating current.Magnetizing effect of the current proportional to the number of ampere-turns (Iw₁).
With the current rise will increase the magnetic flux in the core, a change which will arouse in the turns of the coil self-inductance electromotive force.Once it reaches the applied voltage, the rise of the current in the primary circuit stops.Thus, in the circuit of the primary winding of the transformer will act U₁ applied voltage and electromotive force of self-induction E₁.The voltage E₁ U₁ greater the magnitude of the voltage drop in the coil, which is very little.Therefore, we can write approximately:
U₁ = E₁.
alternating magnetic flux occurring in the transformer core, passes also on its secondary winding coils, each coil exciting coil such that the largest electromotive force as a primary winding of each coil.
Based on the fact that the number of primary turns equals w₁, and secondary - w₂, is inducted into these forces will therefore equal:
E₁ = w₁e,
E₂ = w₂e,
where e - electromotive forcearising in one turn.
U₂ same voltage at the ends of the open coil equals the electromotive force therein, ie .:
U₂ = E₂.
Therefore, we can conclude that the voltage on both ends of the primary winding of the transformer provides as to the magnitude of voltage across the second winding, as the number of primary turns relates to the number of turns in the secondary winding:
(U₁ / U₂) = (w₁ / w₂) = k.
Constant k - factor of the current transformer.
In that case it is necessary to increase the voltage, arrange the secondary winding with the 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 takes fewer turns (step-down transformer).A transformer can act as both a step-up transformation ratio and the like decrease, depending on what is used as the primary winding.
secondary winding is open (the current it does not).The transformer is idling.At the same time it consumes little energy as current magnetizing the iron core, due to the high inductance of the coil is very small.Energy transfer to the secondary circuit from the primary while offline.This experience gives you the opportunity to learn the transformation ratio, the resistance of the idle and the current transformer.
load transformer, hooked through the resistor circuit of the secondary winding.As it is now the induction current will be denoted by the letter I₂.This current, according to Lenz's law will cause a decrease in magnetic flux in the core.However, the weakening of the magnetic flux in the core will reduce the self-induction electromotive force in the primary winding and to an imbalance between this force and the voltage E₁ U₁, given by the generator to the primary winding.As a result, the primary winding current increases to some value equals I₁ and I₁ + I.Due to the increase of the current magnetic flux in the transformer core to increase to the same size, and the imbalance between U₁ E₁ and again restored.Thus, the appearance of the secondary current I₂ causes an increase in current in the primary winding on the value I₁, which will determine the load current of the transformer primary.
Under load the transformer is continuous transfer of energy in the secondary circuit from the primary.According to the law of conservation and transformation of energy output current in the primary circuit is equal to the power of the current in the secondary circuit;therefore, must act equality:
I₁ U₁ = I₂U₂.
In reality, this equality is not respected, since the transformer there are losses, though small.The transformation ratio is about 94-99%.
