Review of Electromagnetism

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Review of Electromagnetism
BEE2123 ELECTRICAL MACHINES

• Muhamad Zahim
• Ext 2312

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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.

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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.

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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.

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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.

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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

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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

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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.

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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.

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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.

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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
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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

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Voltage Induced from a time changing magnetic
field
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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.
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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.
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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

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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
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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
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Production of a Magnetic Field
Total flux
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Magnetic Circuit
Analogy Electric circuit Magnetic circuit
Electric circuit equation
Magnetic circuit equation
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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?

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Magnetic saturation hysteresis in ac magnetic
field
unmagnetized Material
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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.

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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.

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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