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7/30/2019 inductionsynchronousmotor-090330132342-phpapp01

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Motor

ELECTRICAL ENERGY Mechanical Energy
http://h/123/Shahzad%20Khalil.ppt


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Index

Classification

Laws of electromagnetism

Rotating Magnetic FieldAC Motor

Induction Motor

Synchronous Motor Maintenance Practices
http://localhost/var/www/apps/conversion/tmp/scratch_5/MOTOR%20tree.pdfhttp://localhost/var/www/apps/conversion/tmp/scratch_5/Shahzad%20Khalil.ppt


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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
http://localhost/var/www/apps/conversion/tmp/scratch_5/Lenz.avi


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Lenzs Law The direction of an

induced EMF is suchthat its effect tends tooppose the changeproducing it

Simulation
http://localhost/var/www/apps/conversion/tmp/scratch_5/Lenz.avihttp://h/123/Lenz.avi


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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
http://localhost/var/www/apps/conversion/tmp/scratch_5/DC_motor.avihttp://localhost/var/www/apps/conversion/tmp/scratch_5/DC_motor.avihttp://localhost/var/www/apps/conversion/tmp/scratch_5/DC_motor.avihttp://localhost/var/www/apps/conversion/tmp/scratch_5/DC_motor.avihttp://localhost/var/www/apps/conversion/tmp/scratch_5/DC_motor.avihttp://localhost/var/www/apps/conversion/tmp/scratch_5/DC_motor.avihttp://localhost/var/www/apps/conversion/tmp/scratch_5/DC_motor.avi


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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

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