CHAPTER 4 - TRANSFORMERS

Abstract

This chapter explains the principles of an elementary two-coil transformer, waveform of no-load current, and transformer on load. The transformer is a straightforward application of Faraday's Law of Electromagnetic Induction. The basic transformer consists of two coils in close proximity. One coil of N1 turns is excited with alternating current (AC) and therefore establishes a flux ϕ11 that alternates with the current. The other coil is linked by most of this flux and thus has a mutually induced electromotive force (EMF) of value e2 = N2dϕ21/dt. This EMF would drive a load current through any circuit connected to the terminals of the second coil. Energy would then be transferred through the medium of the magnetic field from coil 1 to coil 2. The transformation could be from any convenient input voltage to any convenient output voltage. This simple function of the transformer makes it vital to modern industry. The chapter discusses the problem of cooling in transformers. On small transformers of up to a few kVA, natural air cooling is satisfactory. For larger sizes, the transformers are usually immersed in an oil-filled tank. The heat is passed to the oil that circulates around the tank by natural convection, thus carrying the heat to the tank walls where it is dissipated. A three-phase transformer is lighter, smaller, cheaper, and more efficient than three single-phase units. Therefore, the choice of transformer arrangement is governed by the relative importance of these various factors in particular cases.