MUZAIDI BIN OTHMAN @ MARZUKI

Chapter 4:Electromagnetic

Objective

• Define and explain Faraday’s Law, Lenz’s Law, Flemming Law, magnetic field, magnetic material, and Magnetization curve.

• Explain and analyze reluctance, magnetic equivalent circuit, air gap, electromagnetic induction, Sinusoidal excitation.

• Define and explain magnetic losses, eddy current, hysteresis.

What is Electromagnet?

• An electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current is turned off.

• Electromagnets are widely used as components of other electrical devices, such as motors, generators, relays, loudspeakers, hard disks, MRI machines, scientific instruments, and magnetic separation equipment, as well as being employed as industrial lifting electromagnets for picking up and moving heavy iron objects like scrap iron.

What is Electromagnet?

A simple electromagnet consisting of a coil of insulated wire wrapped around an iron core. The strength of magnetic field generated is proportional to the amount of current.

Iron Core

Insulated wire

Magnetic Field• In the region surrounding a permanent magnet

there exits a magnetic field, which can be represented by magnetic flux lines similar to electric flux lines.

• Magnetic flux lines do not have origins or terminating points as do electric flux lines but exits in continues loops.

• The magnetic flux lines radiate from north pole to the south pole and returning to the north pole through the metallic bar.

Magnetic Field (Cont…)• Continues magnetic flux lines will strive to

occupy a small an area as possible.• The strength of magnetic field in a given

region is directly related to the density of flux lines in that region.

• If unlike poles of 2 permanent magnet are brought together the magnet will attract & flux distribution will shown:

Magnetic Field (Cont…)• If like poles of 2 permanent magnets are

brought together, the magnets will repel and the flux distribution will be as shown:

Magnetic Field (Cont…)• If a nonmagnetic material (glass or copper) is

place in the flux paths surrounding a permanent magnet, there will be an almost unnoticeable change in the flux distribution.

• However, if a magnetic material (soft iron) is placed in the flux path, the flux lines will pass through the soft iron rather than the surrounding air.

Magnetic Field (Cont…)• So, there is 4 properties of lines in magnetic

flux:

Magnetic Field (Cont…)• This principle is put to use in the shielding of

sensitive electrical elements and instruments that can be affected by stray magnetic fields.

Magnetic Field (Cont…)• A magnetic field is present around every wire that carries an

electric current.• The direction of the magnetic flux lines can be found using

Right Hand Rule:

- thumb: Direction of conventional current flow

- other fingers: Direction of magnetic flux

Magnetic Field (Cont…)• If the conductor is wound in a single turn coil,

the resulting flux will flow in a common direction through the center of the coil.

Magnetic Field (Cont…)• A coil of more than one turn would produce a

magnetic field that would exist in continuous path through and around the coil.

Magnetic Field (Cont…)• The flux distribution around the coil is quite

similar to the permanent magnet.• The flux lines leaving the coil from the north

and entering to the south pole.• The concentration (field strength) of flux lines

in a coil is less than that of a permanent magnet.

Magnetic Field (Cont…)• The field concentration (field strength) increased by

placing a core made of magnetic materials (iron, steel, cobalt) within the coil.

• The field strength of an electromagnet can be varied by varied one of the component value (current, turn, core material)

Magnetic Field (Cont…)

• Flux and Flux Density– In the SI system of units, magnetic flux is measured in webers

(Wb) and is represented using the symbol ϕ.– The number of flux lines per unit area is called

flux density (B). Flux density is measured in teslas (T).

)( TA

B

B = flux density (Tesla,T)

ø = magnetic flux (Wb)

A = area (m2) 1 tesla = 1 T = 1 Wb/m2

Magnetic Field (Cont…)

Magnetic Field (Cont…)• The flux density of an electromagnet is directly

related to:• the number of turns of coil, N

• the current through the coil, I

The product is the magnetomotive force, :

At turns,-ampereNI

is any physical driving (motive) force that produces magnetic flux.

Magnetic Field (Cont…)Permeability• Another factor affecting the field strength is the

type of core used.• If cores of different materials with the same

physical dimensions are used in the electromagnet, the strength of the magnet will vary in accordance with the core used.

• The variation in strength is due to the number of flux lines passing through the core.

• Permeability () is a measure of the ease with which magnetic flux lines can be established in the material.

Magnetic Field (Cont…)• Permeability of free space 0 (vacuum) is

• Materials that have permeability slightly less than that of free space are said to be diamagnetic and those with permeability slightly greater than that of free space are said to be paramagnetic.

Wb/A.m104 7 o

Magnetic Field (Cont…)• Magnetic materials, such as iron, nickel, steel

and alloys of these materials, have permeability hundreds and even thousands of times that of free space and are referred to as ferromagnetic.

• The ratio of the permeability of a material to that of free space is called relative permeability:

or

Simplified comparison of permeabilities for: ferromagnetic (μf), paramagnetic (μp), free space(μ0) and diamagnetic (μd)

Magnetic Field (Cont…)

• In general for ferromagnetic materials,

• For nonmagnetic materials,

• Relative permeability is a function of operating conditions.

100r

1r

Induced Voltage• If a conductor is moved through a magnetic field

so that it cuts magnetic lines of flux, a voltage will be induced across the conductor.

Induced Voltage

• The magnitude of the induced voltage, e is directly related to the speed of movement (i.e. at which the flux is cut).

• Moving the conductor in parallel with the flux lines will result in zero volt of induced voltage.

dt

de

Induced Voltage• If a coil of conductor instead of a straight

conductor is used, the resultant induced voltage will be greater

Faraday’s law of electromagnetic induction

• If a coil of N turns is placed in the region of the changing flux, as in the figure, a voltage will be induced across the coil as determined by Faraday’s Law.

Induced Voltage

Induced Voltage

• Changing flux also occurs in a coil carrying a variable current.

• Similar voltage will be induced, the direction of which can be determined by Lenz’s Law.

Induced Voltage

Lenz’s law

• An induced effect is always such as to oppose the cause that produced it.

• The magnitude of the induced voltage is given by:

• L is known as inductance of the coil and is measured in Henry (H)

dt

diLevL ind

di

dNL