POTENTIAL & POWERTRANSFORMERS

These transformers are extremely accurate-ratio step-down transformers and are used in conjunction with standard low-range voltmeters whose deflection when divided by transformation ratio, gives the true voltage on the high-voltage side. In general, they are of the shell-type and do not differ much from the ordinary two-winding transformers, discussed so for, except then that their power rating is extremely small. Up to voltages of 5000 to 13,800 volts, they may be either dry type between although for voltages above 13,800 they are always oil-immersed. Since their secondary windings are required to operate instruments or relays or pilots lights, their ratings are usually of 40 to100W. For safety, the secondary should be completely insulated from the high-voltage primary and should be, in addition, grounded for affording protection to the operator.

Power Transformers for normal purposes can be considered to work with an approximately constant mutual or main flux Fm . Since the resistance of the winding is always small, and leakage flux is only a small fraction of the total, the resistance and leakage reactance voltage drops are small compared with the reactance due to the mutual flux. Approximately, therefore, the whole of the applied voltage V1, is absorbed circulating the primary current against the e.m.f. induced by the main flux.

The power transformer is required to pass electrical energy from one circuit to another, via the medium of the pulsating mutual magnetic field. Our knowledge of magnetic materials indicates the use of iron or steel for the conveyance of the flux with much greater case. The coils are therefore made to embrace an iron core, which serves as a good conducting path for the mutual magnetic flux. The use of an iron core permits of much greater freedom in the shape and arrangement of the primary and secondary coils. In core types, to avoid undue leakage flux, it is usual to have half the primary and half the secondary winding side by side or concentrically on each limb; not primary on one limb and secondary on the other.
The three phase shall arrangement is merely three single-phase transformers assembled together. The three phase core type, on the other hand, embodies the principle that the sum of the fluxes in each phase in a given direction along the cores is zero. The core type is more easily repaired on site, by removing the yoke, which permits the inspection of the coils and cores. The shell type is more robust mechanically since the coils are more readily braced. The radial shell type employees simple round coils, and the cooling is good, particularly for the iron. The core winding requires special machinery, but the advantages include reduction of joints and the use of the grain – direction of the steel for the flux – path.