1. EET 412 Electrical Machine Design Dina Maizana E-mail :dina @ kukum.edu.my

2. Electrical Machine Design Topic Magnetism By: DINA MAIZANA

3. Introduction Magnetism is essential to the generation of electricity by rotating apparatus and the creation of mechanical power by the electric motor. Scientists have found magnetism to be a property of the atom and it is the result the normal electronic activity.

4. Electron Direction of Line of Magnetic Force Direction of spin of electron Orbit of electron Figure 1 Shows an orbiting electron. A moving electron is accompanied by a line magnetic force. Forming a complete circle ellipse. Direction of this force is parallel to the axis of spin. An electron spinning in a counter-clockwise direction. Its magnetic force clockwise when observed from the right. This direction of spin may be term positive. If the electron caused to turn over, it will appear to spin in the opposite or negative direction when observed from above, its magnetic force now counter-clockwise from the right. When an atomic shell is filled, or complete, the number of positively-spinning electrons is exactly equal to the number of negatively-spinning electrons, so that their magnetic effects cancel.

5. Classes of Materials Found in Nature Nonmagnetic substances are some materials no magnetic effect, ex. Vacuum. Paramagnetic substances possess a low order magnetic quality, ex. Aluminum, or Tungsten. Ferromagnetic substances is a high order magnetic quality, ex. iron, cobalt, or nickel. Diamagnetic substances are certain metals, however, exhibit more or less negative reaction to the presence of magnetic lines. ex. Bismuth.

6. Patterns assumed by iron filings around a bar magnet The filings arrange themselves in paths which are highly concentrated at the two ends, or poles, and extend from one end of the strip to the other. The form assumed by these paths is dictated by that of the endless magnetic lines of force established by spinning electrons in the material of which the strip is composed Powder patterns formed by magnetic domains (a), distinct groupings visible in many random directions, a few in end to end formation. (b), more of the groupings assuming an end to end state, the higher the degree of magnetization, the greater the number of such groups. Finally, when the iron has been magnetized as strongly as possible, substantially the whole surface will show a continuous structure of this kind. Under this condition, the piece of iron is said to be magnetically saturated. Scientific investigations show these surface groupings to be characteristic of the atomic inside the material. These groupings are termed domains, and are found in all ferromagnetic substances. A domain is formed by a drawing together of atoms whose magnetic forces are all in one direction, that is, either negative or positive.

7. The S pole repels the S pole of the second, but attracts its N pole. If a bar of soft iron is magnetized, it tends to remain strongly magnetic only so long as the magnetizing force is present, quickly reverting to its original state. Such a piece is called temporary magnet. If a bar of hard steel is magnetized, it tends to remain so longer after the force has been removed. This piece is called a permanent magnet. (a), the nail will cling to the magnetic pole. If the pole happens to be the N pole of the magnet, an N pole will be found at the lower end of the nail. If the nail is touched to the S pole of the magnet (b), the lower end of the nail will have an S pole. If taken away from the magnet, the nail will lose its magnet properties. In other words, the nail is only a temporary magnet. The method of inducing magnetism by contact with magnetized piece, which is called magnetic induction.

8. Some of the various forms of the magnets and the iron filings indicate the field pattern. Magnets are produced in a variety of shapes and sizes. The pattern assumed by lines of force is shown by iron filings attracted to the poles. These magnets are employed in numerous small devices, in direct current measuring instruments, and in tachometer generators for indicating speed of rotation. Some of them have accompanied satellites into orbit. Ferromagnetic substances can be magnetized in any way. (a) Stroking a piece of unmagnetized iron with a magnet (b) The unmagnetizd metal being placed inside a direct current magnetizing coil

9. When current flows through an electrical conductor so magnetic lines of force are set up at right angles to the direction of current flow. They form circles around the wire and are called circles of force. The indication of a compass depends on the direction of current flow Left hand rules for determining direction of magnetic lines set up by electron flow in conductors. If the direction of the circles of force s given, the left hand may be wrapped around the conductor, the fingers representing the circles of force, and the thumb will then indicate the direction of current flow

10. When there is no current in the conductor, there are no circles of force surrounding it. When a small current flows, there will be a circle of force at F1. If the current increases, the circles will increase in size until it occupies the position at F2. A further increase in current will cause it to expand until it occupies the position at F3. There is another motion of the circle of force (a) A piece of copper wire is bent to form a loop. The loop is connected to an electrical generator or other source of current so that current flows as shown by the arrows. Applying the left hand rule so that the thumb indicates current flow, the fingers will wrap around the wire so that they are n the inside of the loop at all times. Thus, the direction of the circles of force inside the loop will always be downward. (b) Should the current in the loop be reversed, the left hand rule shows the circles of force moving upward inside it

11. The magnetizing effect of wire loop may be greatly increased by the used of an iron core. (a) shows a wire loop around a rectangular core of iron. Since the current in the loop flows clockwise when viewed from the top an N pole would be expected there if it were a plain loop. (b) When the directions of current flow are reversed, polarity of the iron core is reversed also. Since the directions of the magnetic circles is downward inside the loop, domains arrange themselves so magnetic lines emerge from the lower face of the core forming an N pole there, and an S pole on top. The coil surrounding the iron core has 4 turn. When current flows in the coil, 4 times as many circles of force result as in the case of 1 turn. The magnetic force is a result as in the case of 1 turn. The magnetic force for a current of 1 ampere becomes 1 ampere x 4 turns, or 4 ampere turns. The current of 1 ampere flowing through a coil of 4 turns produces the same quantity of magnetism as 4 amperes flowing though a coil of 1 turn. Eight times as much magnetism is will be produced by a current of 2 amperes flowing through 4 turns as b a current of 2 amperes flowing through 4 turns as by a current of 1 amperes, and 20 times as much by a current of 5 amperes. Strength of electromagnet is determined by number of ampere turns.

12. 1.What activities of the electron are involved in producing magnetic effects? 2.State two characteristic of the lines of force emanating from a moving electron 3.Describe the easiest way for an electron to change its direction of spin. 4.How many positively spinning electrons will be found in a complete shell of eight? 5.Is iron paramagnetic? 6.Through what kind of material do magnetic lines pass with the greatest ease? 7.Which atomic shells are usually incomplete in ferromagnetic substances? 8.What is a domain? 9.Are domains visible in a pattern of iron filings? 10.Are domains present only at the surface of an iron bar? 11.State the law of magnetic attraction and repulsion. 12.What is the principle different between a temporary magnet and a permanent one? 13.Explain magnetic induction. 14.How does a diamagnetic substance behave when placed in a strong magnetic field? 15.Is wood diamagnetic substances? Questions

13. Thank you for your attention. Terimakasih atas perhatiannya. Good bye