Motor

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Motor
Mechanical Energy
ELECTRICAL ENERGY
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Index

• Classification
• Laws of electromagnetism
• Rotating Magnetic Field
• AC Motor
• Induction Motor
• Synchronous Motor
• Maintenance Practices

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Laws of Electromagnetism

• Faradays Law
• Lenzs Law
• Flemings Right Hand rule
• Flemings Left Hand rule
• Interaction of two magnetic fields

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Faradays Law of Electromagnetic Induction

• When the magnetic flux through a circuit is
  changing an induced EMF is setup in that circuit
  and its magnitude is proportional to the rate of
  change of flux
• Simulation

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Lenzs Law

• The direction of an induced EMF is such that
  its effect tends to oppose the change producing
  it
• Simulation

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Flemings Right Hand rule

• Used to measure the direction of induced current
  in a conductor when cut by a magnetic field.

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Flemings Left Hand rule

• Used to measure the direction of motion of a
  current carrying conductor when placed in
  magnetic field.

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Interaction of two magnetic fields

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Production of Three Phase Rotating Magnetic Field

• RMF may be set up in two-phase or three-phase
  machines.
• The number of pole pairs must be the same as the
  number of phases in the applied voltage.
• The poles are displaced from each other by an
  angle equal to the phase angle between the
  individual phases of the applied voltage.

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When Current positive and going into When
Current negative and coming from
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Speed of RMF

• The magnetic field established rotates at a
  speed given by
• N 60 f / P
• where f frequency of stator current
• P Number of pair of poles

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AC Motors

• A machine which converts AC electrical energy
  into Mechanical energy

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Induction Motor

• Introduction
• Construction
• Principle of Induction Motor
• Slip
• Starting Current
• Torque
• Torque Speed characteristics
• Two Phase Induction Motor
• Single phase Induction Motor
• Summary
• Uses in Aircraft

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Induction Motor-Intro.

• The induction motor is the most commonly used
  type of ac motor. It is simple, low cost and
  rugged in construction.
• The induction motor derives its name from the
  fact that ac voltages are induced in the rotor
  circuit by the rotating magnetic field of the
  stator.

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Induction MotorMain Parts
Stator Wound Rotor
Start Resistance
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AC Machine Stator
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Squirrel Cage Rotor
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(No Transcript)
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Principle of Induction Motor
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Slip in Induction Motor
slip speed synchronous speed -
rotor speed measured in RPM
Slip (synchronous speed - rotor speed )
/synchronous speed
expressed as a percentage
The greater the slip speed, the greater is the
force on each conductor and the torque exerted
by the whole.?
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Starting Current

• The starting current is very high which may
  damage the stator winding.
• To reduce this heavy starting current, star-delta
  starting switch is used.
• For starting, the stator winding are connected up
  in star via the switch to the supply so that the
  phase voltage is 1/v3 of the normal voltage. This
  reduced voltage limits the starting current.

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phase voltage is 1/v3 of the normal voltage
phase voltage is equal to the line voltage.
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Torque- Starting

• The resistance of the squirrel cage rotor
• is small and inductance high.
• Thus on starting rotor current and the rotor emf
  are nearly 90 degrees out of phase.
• The lagging rotor current interacts little with
  stator current and therefore the starting torque
  is poor.

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Torque- Running

• As the rotor current come into phase with the
  rotor emf with increased rotor speed (decreased
  slip and inductive reactance) the rotor and the
  stator flux comes more into phase and the torque
  increases.

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Methods Of Improving Starting Torque

• In creasing the resistance of the rotor
  conductors
• Using a combination of high and low resistance
  conductors
• Using a wire wound rotor connected to variable
  resistor

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Wire Wound Rotor Connected To Resistor
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Torque Speed Characteristic
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Two Phase Induction Motor
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Two Phase RMF
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Single Phase Pulsating Field
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Single Phase Induction Motor

• The single phase induction motor produces a
  pulsating field.
• However, if the rotor is rotated forward at a bit
  less than the synchronous speed, It will develop
  some torque.
• If the rotor is started in the reverse direction,
  it will develop a same torque in other direction

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Split Phase Induction Motor

• Two phases are produced by splitting a single
  phase.
• A capacitor is inserted in one of the windings
  and is called a permanent-split capacitor motor.
• The direction of the motor is easily reversed by
  switching the capacitor in series with the other
  winding.

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Summary

• The three phase induction motor
• Is very robust in construction
• No need for slip rings and therefore less
  maintenance.
• Has a high starting current reduced by star-delta
  switch.
• Has a poor starting torque.
• Runs at a speed less than synchronous speed.

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• Direction of rotation can be reversed by
  interchanging any two stator phases.
• Is of two types depending on motor construction
  Squirrel Cage or Slip Ring

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Uses in Aircraft

1. Constant speed with varying loads and require
   smoother torque e.g. fuel booster pumps,
   hydraulic systems Electric Motor Driven pumps.
2. Systems which need high torque and reversing e.g.
   Flap Power units (for alternate flap drives),
   Stabilizer Trim Actuator.
3. Two phase induction motors also used in aircraft
   such as aileron trim actuators and in reversible
   valve actuators in Fuel, hydraulic, oil, and
   pneumatic systems etc.

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Induction Motor Example
A 100 hp, 8 pole, 60 Hz, 3 phase induction motor
runs at 891 rpm under full load. Determine the
synchronous speed in rpm, slip speed and Slip.
Solution on white board
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Synchronous Motor

• Synchronous Motor-Intro
• Synchronous Motor-principle
• Changing the Load
• Starting Torque
• Improvement of starting torque
• Synchronous Machine Construction
• V curves
• Torque versus Speed
• Summary

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Synchronous Motor- Intro

• The synchronous motor rotates at the synchronous
  speed i.e. the speed of the RMF.
• Stator is similar in construction to that of an
  induction motor, so same principle is applied to
  the synchronous motor rotor.
• Field excitation is provided on the rotor by
  either permanent or electromagnets with number
  of poles equal to the poles of the RMF caused by
  stator

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Synchronous Motor-Principle
The rotor acting as a bar magnet will turn to
line up with the rotating magnet field. The rotor
gets locked to the RMF and rotates unlike
induction motor at synchronous speed under all
load condition
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Changing The Load
An increase in the load will cause the rotor to
lag the stator field but still maintain
synchronous speed. Increase in load has increased
the torque component, but the field strength has
decreased due to the increase in length of the
air gap between the rotor and the stator. If the
synchronous motor is overloaded it pulls out of
synchronism and comes to rest. The minimum amount
of torque which causes this is called the pull
out torque.
Lightly loaded motor
Heavily loaded motor
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Starting Torque

• It cannot be started from a standstill by
  applying ac to the stator. When ac is applied to
  the stator a high speed RMF appears around the
  stator. This RMF rushes past the rotor poles so
  quickly that the rotor is unable to get started.
  It is attracted first in one direction and then
  in the other and hence no starting torque.

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Improvement of starting torque

• It is started by using a squirrel cage within a
  rotor construction and therefore starts as an
  induction motor.
• At synchronous speed the squirrel cage has no
  part to play.

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Synchronous Machine Construction
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Synchronous Machine Phasor Diagram
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Excitation and Stator induced voltage
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V curves
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Torque versus Speed
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Summary

• The synchronous motor
• requires to be started by an external prime
  mover.
• Runs only at synchronous speed, this is an
  advantage where continuous speed is required but
  a disadvantage where a variable speed is
  required.
• Can be used to adjust the power factor of a
  system at the same time it is driving a
  mechanical load.

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Maintenance Practices-A.C. Motors

• Clean, but dont forget to inspect before and
  after cleaning
• Check electrical connections for security, the
  insulation to be in satisfactory condition.
• Examine for signs of over heating
• Check that the motor is secure
• Do an audible check
• Ensure that the motor is not over heating when
  operating, a rule of thumb is that if it is too
  hot for the hand, it is too high.
• When replacing a motor always ensure that the
  load, valve has not seized.
• Also ensure that the motor operates in the
  correct direction
• End of Motors
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