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Review of Electromagnetism

BEE2123 ELECTRICAL MACHINES

- Muhamad Zahim
- Ext 2312

Learning Outcomes

- At the end of the chapter, students should be

able to - Understand the fundamental laws in the dynamic

magnetic systems and their relation to the

electrical machines.

Introduction to Electrical Machines

- An electric machine is a device which converts

electrical power (voltages and currents) into

mechanical power (torque and rotational speed),

and/or vice versa. - A motor describes a machine which converts

electrical power to mechanical power a generator

(or alternator) converts mechanical power to

electrical power.

Introduction to Electrical Machine

- Many electric machines are capable of performing

both as motors and generators - The capability of a machine performing as one or

the other is often through the action of a

magnetic field, to perform such conversions.

Introduction to Electrical Machine

- To understand how an electrical machines works,

the key is to understand how the electromagnet

works. - The principles of magnetism play an important

role in the operation of an electrical machines.

Review of Electromagnetism

- The basic idea behind an electromagnet is

extremely simple a magnetic field around the

conductor can be produced when current flows

through a conductor. - In other word, the magnetic field only exists

when electric current is flowing - By using this simple principle, you can create

all sorts of things, including motors, solenoids,

read/write heads for hard disks and tape drives,

speakers, and so on

Magnetic Field

- Unlike electric fields (which start on q and end

on q), magnetic field encircle their current

source.

field is perpendicular to the wire and that the

field's direction depends on which direction the

current is flowing in the wire

A circular magnetic field develops around the

wire follows right-hand rules

- The field weakens as you move away from the wire
- Amperes circuital law - the

integration path length is longer

Example of Electromagnetic

- An electromagnet can be made by winding the

conductor into a coil and applying a DC voltage. - The lines of flux, formed by current flow through

the conductor, combine to produce a larger and

stronger magnetic field. - The center of the coil is known as the core. In

this simple electromagnet the core is air.

Adding an Iron Core

- Iron is a better conductor of flux than air. The

air core of an electromagnet can be replaced by a

piece of soft iron. - When a piece of iron is placed in the center of

the coil more lines of flux can flow and the

magnetic field is strengthened.

Strength of Magnetic Field (Cont)

- Because the magnetic field around a wire is

circular and perpendicular to the wire, an easy

way to amplify the wire's magnetic field is to

coil the wire - The strength of the magnetic field in the DC

electromagnet can be increased by increasing the

number of turns in the coil. The greater the

number of turns the stronger the magnetic field

will be.

Faradays Law and Lenzs Law

- Faradays Law If a magnetic flux, ?, in a coil

is changing in time (n turns), hence a voltage,

Vab is induced - Lenzs Law if the loop is closed, a connected

to b, the current would flow in the direction to

produce the flux inside the coil opposing the

original flux change. (in other words, Lenzs Law

will determine the polarity of the induced

voltage)

V induced voltage N no of turns in coil ??

change of flux in coil ?t time interval

If no turns

Faradays Law

- The effect of magnetic field
- Induced Voltage from a Time Changing Magnetic

Field - Production of Induced Force on a Wire
- Induced Voltage on a Conductor Moving in a

Magnetic Field

Voltage Induced from a time changing magnetic

field

Voltage Induced in a conductor moving in a

magnetic field

- Faradays Law for moving conductors For coils

in which wire (conductor) is moving thru the

magnetic flux, an alternate approach is to

separate the voltage induced by time-varying flux

from the voltage induced in a moving conductor. - This situation is indicates the presence of an

electromagnetic field in a wire (conductor). This

voltage described by Faradays Law is called as

the flux cutting or Electromotive force, or emf. - The value of the induced voltage is given by
- E Blv
- where
- E induced voltage (V)
- B flux density (T)
- l active length of the conductor in the

magnetic field (m) - v relative speed of the conductor (m/s)

The polarity of induced voltage is given by the

right-hand rule.

Induced Force

- The electrical circuit consists of

battery, resistor, two stationary rails, and

movable bar that can roll or slide along the

rails with electrical contact. - When switch is closed
- Current will not start immediately as inductance

of the circuit. (However time constant L/R is

very small). Hence, current quickly reach V/R. - Force is exerted on the bar due to interaction

between current and magnetic flux to the right

and made the bar move with certain velocity. The

mechanical power out of the bar.

Force induced on the conductor

F ilB

Unit (N)

The direction of force is given by the right-hand

rule.

Induced Force (Cont)

- The motion of the bar produces an electromagnetic

force. The polarity of the emf is ve where the

current enters the moving bars. The moving bar

generates a back emf that opposes the current. - The instantaneous electrical power into the bar

mechanical output power

Production of a Magnetic Field

- The production of a magnetic field by a current

is determine by Amperes law

H magnetic field intensity dl differential

element of length along the path of integration

Magnetic field intensity

lc mean path length

Production of a Magnetic Field

- The strength of the magnetic field flux produced

in the core also depends on the material of the

core.

Magnetic flux density

u magnetic permeability of material

u0 permeability of free space ur relative

permeability of material

Production of a Magnetic Field

Total flux

Magnetic Circuit

Analogy Electric circuit Magnetic circuit

Electric circuit equation

Magnetic circuit equation

Example

- A ferromagnetic core is shown in Figure. Three

sides of this core are of uniform width, while

the fourth side is somewhat thinner. The depth of

the core (into the page) is 10cm, and the other

dimensions are shown in the figure. There is a

200 turn coil wrapped around the left side of the

core. Assuming relative permeability is 2500, how

much flux will be produced by a 1 A input current?

Magnetic saturation hysteresis in ac magnetic

field

unmagnetized Material

Hysteresis Loss

- During a cycle of variation of i (hence H),

there is a net energy flow from the source to the

coil-core assembly and return to the source. - Energy flowing is greater than energy returned.
- This energy loss goes to heat the core.
- The loss of power in the core due to the

hysteresis effect is called hysteresis loss.

Eddy Current Loss

- Voltage will be induced in the path of magnetic

core because of time variation of flux enclosed

by the path. - A current, known as an eddy current will flow

around the path. - Because core has resistance, a power loss will

be cause by the eddy current and appear as heat

in the core.

Eddy Current Loss

- Eddy current can be reduced in 2 ways
- Adding a few percent of silicon to iron to

increase the resistivity. - Laminate core with thin laminations and

insulated from each other. - Hysteresis loss eddy current loss Core loss