Review of Electromagnetism - PowerPoint PPT Presentation

1 / 25
About This Presentation

Review of Electromagnetism


Review of Electromagnetism – PowerPoint PPT presentation

Number of Views:370
Avg rating:3.0/5.0
Slides: 26
Provided by: mohdrusll


Transcript and Presenter's Notes

Title: Review of Electromagnetism

Review of Electromagnetism
  • 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
  • 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

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

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

Voltage Induced from a time changing magnetic
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
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

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
  • 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
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
Write a Comment
User Comments (0)