Fundamentals of Electric Machines: A Primer with MATLAB

A Primer with MATLAB

Warsame Hassan Ali, Matthew N. O. Sadiku, Samir Abood

• Publisher: CRC Press
• Year: 2019
• ISBN:
  9781000012026
• Edition: 1
• Topic: Tecnologia e ingegneria
• Subtopic: Ingegneria elettronica e telecomunicazioni

An electric machine is a device that converts mechanical energy into electrical energy or vice versa. It can take the form of an electric generator, electric motor, or transformer. Electric generators produce virtually all electric power we use all over the world. Electric machine blends the three major areas of electrical engineering: power, control and power electronics. This book presents the relation of power quantities for the machine as the current, voltage power flow, power losses, and efficiency. This book will provide a good understanding of the behavior and its drive, beginning with the study of salient features of electrical dc and ac machines.

1

Basic Concepts of Magnetism

I am a slow walker, but I never walk backwards.

Abraham Lincoln

Magnetism is a force generated in the matter by the motion of electrons within its atoms. Magnetism and electricity represent different aspects of the force of electromagnetism, which is one part of nature’s fundamental magnetic force. The region in space that is penetrated by the imaginary lines of magnetic force describes a magnetic field. The strength of the magnetic field is determined by the number of lines of force per unit area of space. Magnetic fields are created on a large scale either by the passage of an electric current through magnetic metals or by magnetized materials called magnets. The elemental metals—iron, cobalt, nickel, and their solid solutions or alloys with related metallic elements—are typical materials that respond strongly to magnetic fields. Unlike the all-pervasive fundamental force field of gravity, the magnetic force field within a magnetized body, such as a bar magnet, is polarized—that is, the field is strongest and of opposite signs at the two poles of the magnet.

1.1 History of Magnetism

The history of magnetism was dated to earlier than 600 B.C., but it is only in the twentieth century that scientists have begun to understand it and develop technologies based on this understanding. Magnetism was most probably first observed in a form of the mineral magnetite called lodestone, which consists of an iron oxide—a chemical compound of iron and oxygen. The ancient Greeks were the first known to have used this mineral, which they called a magnet because of its ability to attract other pieces of the same material and iron.

The British physicist William Gilbert (1600 B.C.) explained that the earth itself is a giant magnet with magnetic poles that are somewhat distracted from its geographical poles. The German scientist Gauss then studied the nature of earth’s magnetism, followed by the French scientist Koldem (1821 A.C.) known that the magnet is a ferrous material only.

Quantitative studies of magnetic phenomena initiated in the eighteenth century by Frenchman Charles Coulomb (1736–1806), who established the inverse square law of force, which states that the attractive force between two magnetized objects is directly proportional to the product of their individual fields and inversely proportional to the square of the distance between them. Danish physicist Hans Christian Oersted (1777–1851) first suggested a link between electricity and magnetism. Experiments involving the effects of magnetic and electric fields on one another were then conducted by Frenchman Andre Marie Ampere (1775–1836) and Englishman Michael Faraday (1791–1869), but it was the Scotsman, James Clerk Maxwell (1831–1879), who provided the theoretical foundation to the physics of electromagnetism in the nineteenth century by showing that electricity and magnetism represent different aspects of the same fundamental force field. Then, in the late 1960s, American Steven Weinberg (1933–) and Pakistani Abdus Salam (1926–), performed yet another act of theoretical synthesis of the fundamental forces by showing that electromagnetism is one part of the electroweak force.

The modern understanding of magnetic phenomena in condensed matter originates from the work of two Frenchmen: Pierre Curie (1859–1906), the husband and scientific collaborator of Madame Marie Curie (1867–1934) and Pierre Weiss (1865–1940). Curie examined the effect of temperature on magnetic materials and observed that magnetism disappeared suddenly above a certain critical temperature in materials like iron. Weiss proposed a theory of magnetism based on an internal molecular field proportional to the average magnetization that spontaneously aligns the electronic micro magnets in the magnetic matter. The present-day understanding of magnetism based on the theory of the motion and interactions of electrons in atoms (called quantum electrodynamics) stems from the work and theoretical models of two Germans, Ernest Ising (1900–) and Werner Heisenberg (1901–1976). Werner Heisenberg was also one of the founding fathers of modern quantum mechanics.

1.2 The Cause of Magnetism

The prime reason of magnetism is the movement of electrons in a particular object. The movement of electrons in specific orbits around the nucleus is as same as the motion of the planets around the earth and the rotation of the earth around its axis. In this case, there are two movements: orbital and rotational. Due to these movements, a magnetic moment is produced on each electron that behaves like an intricate magnet. In a particular matter, the numbers of all electrons of an atom are equal. In each orbit, there are many pairs of electrons that are usually remaining as many pairs in each orbit and each electron is rotating in two opposite direction. As a result, the total torque that was created by electrons in each orbit is zero.

The metallic bond is usually seen in pure metal and some metalloids. Most of the metal is symbolized by high electrical and thermal conductivity as well as by malleability, materials contain transition atoms. On the other hand, there are some metal that contains an odd number of an electron at their outer shell such as—iron, cobalt, and nickel. These metals have their own specific value for the sum of the torque. Some of which contain an odd number of electrons such as iron, cobalt, and nickel. The sums of torque in these orbits contain a specific value. The positively charged particles of the magnetic materials gain the magnetic effect generated by the electrons which are revolving around them. Positively charged ions or group of atoms (about 10⁵ atoms) that are directed toward the same direction, is called a domain. Each field has its magnetic field with its northern and southern poles. Non-magnetized field of iron pieces is oriented in random, non-uniform directions. When placing the iron piece within a relatively weak external magnetic field, some iron fields will align with some and with the outer magnetic field. As the external magnetic field increases, lined and outward-facing fields will increase remarkably. As the strength of the outer field increases, we retain a situation in what all the fields of the iron segment are heading toward this area. Consequently, the object is perceived as being saturated magnetically, whereas all the fields have been lined up in the desired direction. In either case, lifting the effect of the magnetic field outside from the iron piece, the fields will return to the previous random states.

1.3 Types of Magnets

Magnets are divided into two types according to the composition method, namely, natural and artificial magnets. There are confined to the natural magnets in those present in the form of natural rocks such as the ancient Greeks and China, and the characteristics of these magnets to the rocks produced by nature and no human being. Natural rock or magnets are currently present in the United States of America, Sweden, and Norway, but it is not practical to use in known electrical appliances because of the ease and economy of making very powerful and efficient magnets. Synthetic magnets produce magnetic materials such as iron, nickel, and cobalt in various shapes and sizes.

To obtain very strong magnets, special alloys are used for iron or electrically magnetized steel. This is done by placing the magnetized iron piece inside a coil of insulated wire through which an electric current or pack of electrons pass through as shown in Figure 1.1, and the force that turns cutting iron into a magnet is a line of magnetic forces quite like the lines of the electric field. Synthetic magnets are classified into both temporary and permanent depending on their ability to retain magnetic properties after the magnetization is removed.

FIGURE 1.1
Magnetized piece of iron (a) When Magnetized piece of iron In direction case. (b) When Magnetized piece of iron Out direction case.

Magnetic materials such as iron, iron-silicon alloys, and nickel-iron alloys, used in the manufacture of transformers, motors, and generators have highly reluctance, which are easy to magnetize and give temporary magnets. However, these materials keep a small amount of magnetism after removing the external magnetization force. This magnetism is called residual, which is essential in the work of electric machines. Another criterion for measuring the susceptibility of a material to the formation of a permanent or temporary magnet is the ease with which these materials allow the lines of magnetic forces to be distributed within it. This is cal...