Low Frequency Electromagnetic Design

Perry

• Editore: CRC Press
• Anno: 2019
• ISBN:
  9781351434560
• Edizione: 1
• Argomento: Physical Sciences
• Categoria: Electromagnetism

In an historical context, the development of electromagnetic theory and anaylsis has undergone many evolutionary changes since the 19th century. Faraday's 1831 discovery of the magnetic induction principle was at first a scientific curisoity, then a subject of intense intellectual activity resulting in the infication of the macroscopic electromagnetic principles through Maxwell's equations. One of the subdisciplines created by the discovery of electromagnetic induction and its theoretical foundation was the anaylsis of specific arrangements of ponderable bodies, including conductors which interact with electromagnetic fields to produce the measurable phyiscal effects which we call heat and mechanical force. This book is intended neither as a suppliment or replacement for previous texts, however, a number of conductor arrangements are covered here which are not done elsewhere. It is primarily for industrial use; where insight into the physical processes may be of practical value.

1

INTRODUCTION

1.1 MOTIVATION

A great variety of labor saving devices which we take for granted in our daily activities utilize low frequency alternating current either directly, or indirectly through the electric utility industry power generation and delivery system. Examples of alternating current phenomena are found in most household appliances (refrigerator motors, heating control units, etc.), radio and television receivers, and automotive electrical systems. Remarkably, the same electrical distribution system available for domestic use also supplies primary power for all types of industrial processes (roll mills, injection moulding, welding machines, etc.). In power systems engineering, generators and transformers rated into the millions of volt-amps rely on the same principles of operation as subfractional horsepower motors found in small appliances.

The ensemble of electrical and electromechanical devices in these categories accounts for a large fraction of the primary energy sources consumed in the United States. This fact, coupled with the real increase in fuel prices during the 1970’s, has focused renewed attention on design considerations for all forms of electrical apparatus. Present-day thermodynamic processes used to produce electrical power result in a net heat to electrical energy conversion factor of no more than 40%, and much less than this in many installations. Therefore, a joule of electrical energy saved in the electrical load can be worth at least three times this energy content in the form of fossil fuels. It is in this environment that the electromagnetic laws are studied today in universities and industrial research centers throughout the world.

Low frequency phenomena in many cases is manifest as “eddy current” effects in current carrying conductors. In a traditional device such as the power frequency transformer, eddy current losses create additional heating in the windings as well as the flux concentrating core. The latter problem has necessitated the use of iron or steel laminations in the core to promote flux penetration and reduce losses. In addition to the extra fuel cost associated with alternating current (ac) resistance of conductors, excess power consumption results in higher operating temperatures in comparison with those which would exist if power system apparatus carried dc current. This may require the use of passive or active cooling apparatus in certain cases, further increasing the cost of electrical power. In transformers for example, insulating oil is employed for cooling. This oil is generally circulated through external cooling fins to dissipate heat, and occasionally is pumped using active devices which also consume and dissipate energy.

On the other hand, eddy current engineering is by no means limited in scope to unwanted additional losses in power frequency devices such as generators, transformers, and cables. Research over the years has progressed to the extent that many processes which rely on low frequency phenomena have been developed. This includes induction heating, nondestructive evaluation, and many for...