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Motor
ELECTRICAL ENERGY Mechanical Energy
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Index
Classification
Laws of electromagnetism
Rotating Magnetic FieldAC 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 magneticfields
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Faradays Law of
Electromagnetic Induction When the magnetic
flux through a circuit ischanging an induced
EMF is setup in thatcircuit and itsmagnitude isproportional to the rateof change of flux
Simulation
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Lenzs Law The direction of an
induced EMF is suchthat its effect tends tooppose the changeproducing it
Simulation
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Flemings Right Hand rule Used to measure thedirection of inducedcurrent in a conductor
when cut by amagnetic field.
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Flemings Left Hand rule Used to measure the
direction of motion ofa current carrying
conductor when placedin magnetic field.
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Interaction of two magnetic
fields
Motion
Current
Field
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Production of Three Phase Rotating
Magnetic Field
RMF may be set up in two-phase or three-phasemachines.
The number of pole pairs must be the same asthe number of phases in the applied voltage.
The poles are displaced from each other by an
angle equal to the phase angle between theindividual 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 RMFThe magnetic field established rotates at a
speed given by
N = 60* f / Pwhere f = frequency of stator currentP = Number of pair of poles
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A machine which converts AC electrical energy intoMechanical 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 typeof ac motor. It is simple, low cost and rugged inconstruction.
The induction motor derives its name from the factthat ac voltages are induced in the rotor circuit by therotating magnetic field of the stator.
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Stator Wound Rotor Start Resistance
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AC Machine Stator
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Squirrel Cage Rotor
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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 damagethe stator winding.
To reduce this heavy starting current, star-delta
starting switch is used.
For starting, the stator winding are connected up instar via the switch to the supply so that the phase
voltage is 1/3 of the normal voltage. This reducedvoltage limits the starting current.
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phase voltage is 1/3 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 emfare nearly 90 degrees out of phase.
The lagging rotor current interacts little with
stator current and therefore the starting torqueis poor.
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As the rotor current come into phase with the rotoremf with increased rotor speed (decreased slip
and inductive reactance) the rotor and the statorflux comes more into phase and the torqueincreases.
Torque- Running
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Methods Of Improving Starting Torque
In creasing the resistance of the rotor conductors
Using a combination of high and low resistanceconductors
Using a wire wound rotor connected to variableresistor
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Wire Wound Rotor Connected To Resistor
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TorqueSpeed 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 bitless than the synchronous speed, It will developsome torque.
If the rotor is started in the reverse direction, itwill develop a same torque in other direction
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Split Phase Induction Motor
Two phases are produced bysplitting a single phase.
A capacitor is inserted in one of
the windings and is called apermanent-split capacitormotor.
The direction of the motor is
easily reversed by switching thecapacitor in series with theother 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-deltaswitch.
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 Aircraft1. 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 aircraftsuch as aileron trim actuators and in reversible
valve actuators in Fuel, hydraulic, oil, andpneumatic systems etc.
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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 aninduction motor, so same principle is applied tothe synchronous motor rotor.
Field excitation is provided on the rotor by either
permanent or electromagnets with number ofpoles equal to the poles of the RMF caused bystator
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Synchronous Motor-Principle
The rotor acting as a bar magnet will turn to line upwith the rotating magnet field. The rotor gets locked to theRMF and rotates unlike induction motor at synchronous speedunder all load condition
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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 torest. The minimum amount of torque which causes this is called the pull out
torque.
Lightly
loaded
motor
Heavily
loaded
motor
Changing The Load
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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 ahigh speed RMF appears around the stator. ThisRMF rushes past the rotor poles so quickly that therotor is unable to get started. It is attracted first inone 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 inductionmotor.
At synchronous speed the squirrel cage has no part toplay.
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Synchronous Machine Construction
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V curves
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Torque versus Speed
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SummaryThe synchronous motor:1. requires to be started by an external prime mover.
2. Runs only at synchronous speed, this is an advantage where
continuous speed is required but a disadvantage where a variablespeed is required.
3. Can be used to adjust the power factor of a system at the same timeit 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 insatisfactory condition.
Examine for signs of over heating
Check that the motor is secureDo an audible check
Ensure that the motor is not over heating when operating, a rule ofthumb 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