The amount of this decrease, however, is very small, inasmuch as a very small decrease of the induced pressure in the primary coil greatly increases the difference between the pressure applied to the primary coil and the opposing pressure induced in the primary coil, so that the primary current is greatly increased. In fact, the increase of primary current due to the loading of the transformer is just great enough (or very nearly) to exactly balance the magnetizing action of the current in the secondary coil; that is, the flux in the core must be maintained approximately constant by the primary current whatever value the secondary current may have.
When the load on a transformer is increased, the primary of the transformer automatically takes additional current and power from the supply mains in direct proportion to the load on the secondary.
When the load on the secondary is reduced, for example by turning off lamps, the power taken from the supply mains by the primary coil is automatically reduced in proportion to the decrease in the load. This automatic action of the transformer is due to the balanced magnetizing action of the primary and secondary currents.
Fig. 1,923.—Rear view of Fort Wayne distributing transformer, showing hanger irons for attaching to pole cross arm.
Classification of Transformers.—As in the case of motors, the great variety of transformer makes it necessary that a classification, to be comprehensive, must be made from several points of view, as:
1. With respect to the transformation, as
a. Step up transformers;
b. Step down transformers.
2. With respect to the arrangement of the coils and magnetic circuit, as
a. Core transformers;
b. Shell transformers;
c. Combined core and shell transformers.
3. With respect to the kind of circuit they are to be used on, as
a. Single phase transformers;
b. Polyphase transformers.
4. With respect to the method employed in cooling, as
a. Dry transformers;
b. Air cooled transformers {natural draught;
{forced draught, or air blast;
c. Oil cooled transformers;
d. Water cooled transformers.
5. With respect to the nature of their output, as
a. Constant pressure transformers;
b. Constant current transformers;
c. Current transformers;
d. Auto-transformers.
6. With respect to the kind of service, as
a. Distributing;
b. Power.
7. With respect to the circuit connection that the transformer is constructed for, as
a. Series transformers;
b. Shunt transformers.
Step Up Transformers.—This form of transformer is used to transform a low voltage current into a high voltage current. Such transformers are employed at the generating end of a transmission line to raise the voltage of the alternators to such value as will enable the electric power to be economically transmitted to a distant point.
