Theory of Transformer Action.— A transformer may be defined as a piece of apparatus without continuously moving parts, which by electromagnetic induction transforms alternating voltage and current in one winding into alternating voltage and current in one or, more other windings, usually at different values of voltage and current. It consists essentially of an iron core on which are wound the primary and secondary windings. When an alternating voltage is applied to the terminals of the primary winding, the secondary being open-circuited, the apparatus behaves like a choking coil. An alternating magnetic flux is set up and this induces a back e.m.f. in the primary winding. Neglecting losses, this back e.m.f. exactly neutralizes the applied e.m.f., each turn of the winding providing its own proper proportion of the total voltage. The alternating magnetic flux also induces an e.m.f. in the turns of the secondary winding, the volts per turn being the same for both windings. In actual practice, the induced e.m.f. in the primary windings is very slightly less than the applied voltage, on account of voltage drops in the circuit. Similarly, the induced e.m.f. in the secondary windings is very slightly greater than the secondary terminal voltage, when the transformer is delivering a load current, for the same reason. The voltage ratio is therefore slightly greater than the turn’s ratio. An actual transformer may be represented, for purposes of explanation, as consisting of an ideally perfect transformer, having no losses or magnetizing current, together with various additions to allow for these effects. Fig.13. represents such an ideal transformer having a resistance, R1 and a reactance X1 in series with its primary winding, and a resistance, R2, and a reactance, X2, in series with its secondary winding. The no-load current has both an active and a reactive component. The latter is the magnetizing current of the iron core, Im' and is represented in the diagram by the current flowing through an additional reactance while the active component, Ic, supplying the iron losses in the core' is represented by the current flowing through an additional resistance. The vector resultant of Im and Ic is I0 , which is the total no-load current of the transformer. The secondary current is I2, and the component of the primary current that neutralizes I2 is I'1. The total primary current, I’1, is the vector sum of this component, I2, and the no-load current, I2. The turns in the primary and secondary windings are T1 and T2, respectively, and are so related that   The flux Ф is the useful flux linking both primary and secondary windings. The induced secondary e.m.f. is E2, this being slightly greater than the secondary terminal voltage, V2 since the secondary resistance, R2, and the secondary reactance, X2,cause voltage drops of I2R2 and I2X2 in phase and in quadrature with the current, I2, respectively. The "back induced e.m.f., E1, is related to E2 by the formula   The primary applied voltage, Vt, is again slightly larger than E1 ,on account of the voltage drops caused by the primary resistance, R1 , and the primary reactance, X1 . These again bring about voltage drops of I1R1 and I1Xl in phase and in quadrature with the primary current, I1 , as before.   М.А. Беляева и др. «Сборник технических текстов на англ. языке»