EET 412Electrical Machine Design

Dina MaizanaE-mail :dina @ kukum.edu.my

Electrical Machine Design

TopicMagnetism

By: By: DINA MAIZANA DINA MAIZANA

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.

ElectronDirection of Line of Magnetic Force

Direction of spin of electron

Orbit of electron

Figure 1 Shows an orbiting electron.Figure 1 Shows an orbiting electron.

A moving electron is accompanied by a line magnetic force.A moving electron is accompanied by a line magnetic force. Forming a complete circle ellipse. Forming a complete circle ellipse. Direction of this force is parallel to the axis of spin .Direction of this force is parallel to the axis of spin . An electron spinning in a counter-clockwise direction. An electron spinning in a counter-clockwise direction. Its magnetic force clockwise when observed from the right.Its magnetic force clockwise when observed from the right. This direction of spin may be term positive. 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 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 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 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 exactly equal to the number of negatively-spinning electrons, so that their magnetic effects cancel. cancel.

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.

Iron filings

Magnetized bar

Patterns assumed by iron filings Patterns assumed by iron filings around a bar magnetaround a bar magnet

The filings arrange themselves in paths which are The filings arrange themselves in paths which are highly concentrated at the two ends, or poles, and highly concentrated at the two ends, or poles, and extend from one end of the strip to the other. The form extend from one end of the strip to the other. The form assumed by these paths is dictated by that of the assumed by these paths is dictated by that of the endless magnetic lines of force established by endless magnetic lines of force established by spinning electrons in the material of which the strip is spinning electrons in the material of which the strip is composedcomposed

(a)

(b)

Powder pattern of domain

Powder patterns formed by magnetic Powder patterns formed by magnetic domainsdomains

(a), distinct groupings visible in many random directions, (a), distinct groupings visible in many random directions, a few in end to end formation. a few in end to end formation.

(b), more of the groupings assuming an end to end (b), more of the groupings assuming an end to end state, the higher the degree of magnetization, the state, the higher the degree of magnetization, the greater the number of such groups.greater the number of such groups.

Finally, when the iron has been magnetized as strongly Finally, when the iron has been magnetized as strongly as possible, substantially the whole surface will show a as possible, substantially the whole surface will show a continuous structure of this kind. Under this condition, continuous structure of this kind. Under this condition, the piece of iron is said to be magnetically saturated.the piece of iron is said to be magnetically saturated.

Scientific investigations show these surface groupings to be characteristic of the atomic inside the 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 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 is formed by a drawing together of atoms whose magnetic forces are all in one direction, that is, either negative or positive. negative or positive.

(a)

Bar magnet

N-pole of compass attracted towards S-pole of magnet

S N S N

S N

N

N

S

(b) (c)

Force of attraction

Force of repulsion

The S pole The S pole repels the S repels the S pole of the pole of the second, but second, but attracts its N attracts its N pole. pole.

If a bar of soft iron is magnetized, it tends to remain strongly magnetic only so long as the magnetizing force 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. 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 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. piece is called a permanent magnet.

S

N

N

S

N

N

S

S

Nail

Bar magnet

(a) (b)

(a), the nail will cling to the magnetic pole. If the pole happens to be the (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. 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 magnetIf 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 (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 magnet, the nail will lose its magnet properties. In other words, the nail is only a temporary magnet. only a temporary magnet.

The method of inducing magnetism by contact with magnetized piece, The method of inducing magnetism by contact with magnetized piece, which is called magnetic induction. which is called magnetic induction.

S N

Some of the various forms of the magnets and the iron Some of the various forms of the magnets and the iron filings indicate the field pattern.filings indicate the field pattern.

Magnets are produced in a variety of shapes Magnets are produced in a variety of shapes and sizes. The pattern assumed by lines of and sizes. The pattern assumed by lines of force is shown by iron filings attracted to the force is shown by iron filings attracted to the poles. These magnets are employed in poles. These magnets are employed in numerous small devices, in direct current numerous small devices, in direct current measuring instruments, and in tachometer measuring instruments, and in tachometer generators for indicating speed of rotation. generators for indicating speed of rotation. Some of them have accompanied satellites Some of them have accompanied satellites into orbit.into orbit.

N

S

(a)

(b)

Stroke constantly in this direction from one end to the other end

Unmagnetized bar of iron

Bar magnet

Magnetizing coil

Ferromagnetic substances can be magnetized in any way .Ferromagnetic substances can be magnetized in any way .

(a) Stroking a piece of unmagnetized iron with a magnet(a) Stroking a piece of unmagnetized iron with a magnet

(b) The unmagnetizd metal being placed inside a direct (b) The unmagnetizd metal being placed inside a direct current magnetizing coilcurrent magnetizing coil

When current flows through an electrical When current flows through an electrical conductor so magnetic lines of force are set conductor so magnetic lines of force are set up at right angles to the direction of current up at right angles to the direction of current flow. They form circles around the wire and flow. They form circles around the wire and are called circles of force.are called circles of force.

North Pole of compass

Direction of

current flo

w

Circle of force

North Pole of compass

Direction of

current flo

w

Circle of force

The indication of a compass depends on the direction of current flowThe indication of a compass depends on the direction of current flow

Fingers curve in same direction of magnetic

lines set up by electron flow in conductor

Thumb indicates direction of electron

flow

Circle of force

Left hand rules for determining direction of magnetic lines set up by Left hand rules for determining direction of magnetic lines set up by electron flow in conductors. electron flow in conductors.

If the direction of the circles of force s given, the left hand may be If the direction of the circles of force s given, the left hand may be wrapped around the conductor, the fingers representing the circles wrapped around the conductor, the fingers representing the circles of force, and the thumb will then indicate the direction of current of force, and the thumb will then indicate the direction of current flowflow

Conductor

F1

F2 F3

When there is no current in the conductor, there are When there is no current in the conductor, there are no circles of force surrounding it. When a small no circles of force surrounding it. When a small current flows, there will be a circle of force at F1. If the current flows, there will be a circle of force at F1. If the current increases, the circles will increase in size until current increases, the circles will increase in size until it occupies the position at F2. A further increase in it occupies the position at F2. A further increase in current will cause it to expand until it occupies the current will cause it to expand until it occupies the position at F3.position at F3.

There is another motion of the circle of forceThere is another motion of the circle of force

Compass Needle

S

SN

N

Circle of force

Current

Compass Needle

Circle of force

Compass Needle

Compass Needle

CurrentWire loop

Wire

(a) (b)

(a) A piece of copper wire is (a) A piece of copper wire is bent to form a loop. The loop is bent to form a loop. The loop is connected to an electrical connected to an electrical generator or other source of generator or other source of current so that current flows as current so that current flows as shown by the arrows. Applying shown by the arrows. Applying the left hand rule so that the the left hand rule so that the thumb indicates current flow, the thumb indicates current flow, the fingers will wrap around the wire fingers will wrap around the wire so that they are n the inside of so that they are n the inside of the loop at all times. Thus, the the loop at all times. Thus, the direction of the circles of force direction of the circles of force inside the loop will always be inside the loop will always be downward.downward.

(b) Should the current in the loop be reversed, the left hand rule shows the circles of force moving (b) Should the current in the loop be reversed, the left hand rule shows the circles of force moving upward inside itupward inside it

S

N S

N

Circle of force

Current

Iron core

Circle of force Iron core

CurrentWire

Wire

(a) (b)

The magnetizing effect The magnetizing effect of wire loop may be of wire loop may be greatly increased by the greatly increased by the used of an iron core. used of an iron core.

(a) shows a wire loop around a rectangular core of iron. Since the current in the loop flows (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. 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 (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 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 magnetic lines emerge from the lower face of the core forming an N pole there, and an S pole on top. top.

Four turns of

wire

Iron coreThe coil surrounding the iron core has 4 turn. When current flows in the 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 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 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 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 turns. The current of 1 ampere flowing through a coil of 4 turns produces the same quantity of magnetism as 4 amperes flowing produces the same quantity of magnetism as 4 amperes flowing though a coil of 1 turn. Eight times as much magnetism is will be 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 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 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. amperes, and 20 times as much by a current of 5 amperes.

Strength of electromagnet is determined by number of ampere turns.Strength of electromagnet is determined by number of ampere turns.

1.1. What activities of the electron are involved in producing magnetic effects?What activities of the electron are involved in producing magnetic effects?2.2. State two characteristic of the lines of force emanating from a moving electronState two characteristic of the lines of force emanating from a moving electron3.3. Describe the easiest way for an electron to change its direction of spin.Describe the easiest way for an electron to change its direction of spin.4.4. How many positively spinning electrons will be found in a complete shell of eight?How many positively spinning electrons will be found in a complete shell of eight?5.5. Is iron paramagnetic?Is iron paramagnetic?6.6. Through what kind of material do magnetic lines pass with the greatest ease?Through what kind of material do magnetic lines pass with the greatest ease?7.7. Which atomic shells are usually incomplete in ferromagnetic substances?Which atomic shells are usually incomplete in ferromagnetic substances?8.8. What is a domain?What is a domain?9.9. Are domains visible in a pattern of iron filings?Are domains visible in a pattern of iron filings?10.10. Are domains present only at the surface of an iron bar?Are domains present only at the surface of an iron bar?11.11. State the law of magnetic attraction and repulsion.State the law of magnetic attraction and repulsion.12.12. What is the principle different between a temporary magnet and a permanent one?What is the principle different between a temporary magnet and a permanent one?13.13. Explain magnetic induction.Explain magnetic induction.14.14. How does a diamagnetic substance behave when placed in a strong magnetic field?How does a diamagnetic substance behave when placed in a strong magnetic field?15.15. Is wood diamagneticIs wood diamagnetic substances? substances?

Questions

Thank you for your attention.Thank you for your attention.

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