The basis of operation of electromagnetic machines

Abstract

All electrical machines are forms of energy converter and the vast majority are electrical—mechanical, arranged to convert from electrical to mechanical energy (motors or sinks) or from mechanical to electrical energy (generators or sources). There will always be some form of coupling field between the electrical and mechanical systems and, in most cases, this takes the form of a magnetic field. The structure on which the field circuit is located may be either stationary or rotating, depending on the particular form of machine. In many cases the field will be electromagnetic and field coils carrying the field current will be wound on a magnetic structure. The iron forming this structure will be laminated in order to reduce the field iron losses if the field current is alternating or contains an alternating component. Two different forms of construction are used for the field circuit. Firstly the so-called ‘salient-pole’ arrangement can be used, in which field coils are concentrated and wound around protruding poles. This form of construction is only used for machines with direct-current (d.c.) field supplies. The second form of construction is that in which the field coils are distributed in slots cut into a cylindrical magnetic structure, and this arrangement is commonly used on certain forms of alternating current (a.c.) generators. It is important to note at this stage that the field structure can, in general, be physically situated on either the stationary member (stator) or rotating member (rotor) on the machine.