Fig. 1,924.—Diagram of elementary step up transformer. As shown the primary winding has two turns and secondary 10 turns, giving a ratio of voltage transformation of 10 ÷ 2 = 5. Since only ⅕ as much current flows in the secondary winding as in the primary, the latter requires heavier wire than the former.
Copper Economy with Step Up Transformers.—To comprehend fully the bearing of the matter, it must be remembered that the energy supplied per second is the product of two factors, the current and the pressure at which that current is supplied; the magnitudes of the two factors may vary, but the value of the power supplied depends only on the product of the two; for example, the energy furnished per second by a current of 10 amperes supplied at a pressure of 2,000 volts is exactly the same in amount as that furnished per second by a current of 400 amperes supplied at a pressure of 50 volts; in each case, the product is 20,000 watts.
Now the loss of energy that occurs in transmission through a well insulated wire depends also on two factors, the current and the resistance of the wire, and in a given wire is proportional to the square of the current. In the above example the current of 400 amperes, if transmitted through the same wire as the 10 amperes current, would, because it is forty times as great, waste sixteen hundred times as much energy in heating the wire. It follows that, for the same loss of energy, the 10 ampere current at 2,000 volts may be carried by a wire having only 1/1,600th of the sectional area of the wire used for the 400 ampere current at 50 volts.
The cost of copper conductors for the distributing lines is therefore very greatly economized by employing high pressures for distribution of small currents.
Fig. 1,925.—Diagram of elementary step down transformer. As shown the primary winding has 10 turns and the secondary 2, giving a ratio of voltage transformation of 2 ÷ 10 = .2. The current in the secondary being 5 times greater than in the primary will require a proportionately heavier wire.
Step Down Transformers.—When current is supplied to consumers for lighting purposes, and for the operation of motors, etc., considerations of safety as well as those of suitability, require the delivery of the current at comparatively low pressures ranging from 100 to 250 volts for lamps, and from 100 to 600 volts for motors.
This involves that the high pressure current in the transmission lines must be transformed to low pressure current at the receiving or distributing points by step down transformers, an elementary transformer being shown in fig. 1,925.
Figs. 1,926 and 1,927.—Core type transformer. It consists of a central core of laminated iron, around which the coils are wound. A usual form of core type transformer consists of a rectangular core, around the two long limbs of which the primary and secondary coils are wound, the low tension coil being placed next the core.