# Review of Electromagnetism - PowerPoint PPT Presentation

1 / 25
Title:

## Review of Electromagnetism

Description:

### Review of Electromagnetism – PowerPoint PPT presentation

Number of Views:370
Avg rating:3.0/5.0
Slides: 26
Provided by: mohdrusll
Category:
Tags:
Transcript and Presenter's Notes

Title: Review of Electromagnetism

1
Review of Electromagnetism
BEE2123 ELECTRICAL MACHINES
• Ext 2312

2
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.

3
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.

4
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.

5
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.

6
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,
speakers, and so on

7
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

8
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.

9
• 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.

10
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.

11
• 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
12
• 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

13
Voltage Induced from a time changing magnetic
field
14
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.
15
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.
16
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

17
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
18
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
19
Production of a Magnetic Field
Total flux
20
Magnetic Circuit
Analogy Electric circuit Magnetic circuit
Electric circuit equation
Magnetic circuit equation
21
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?

22
Magnetic saturation hysteresis in ac magnetic
field
unmagnetized Material
23
Hysteresis Loss
• During a cycle of variation of i (hence H),
there is a net energy flow from the source to the
• 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.

24
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.

25
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