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Principle of Induction Motor

When three phase supply is given to the three phase stator winding of the

induction motor,a rotating magnetic field is developed around the stator

which rotates at synchronous speed.This rotating magnetic field passes

through the air gap and cuts the rotor conductors which werestationary.Due to the relative speed between the stationary rotor

conductors and the rotating magnetic field,an emf is induced in the rotor

conductors.As the rotor conductors are short circuited, current starts

flowing through it.And as these current carrying rotor conductors are

placed in the magnetic field produced by the stator, they experiences a

mechanical force i.e. torque which moves the rotor in the same direction

as that of the rotating magnetic field.

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Principle Contd.

The induction motor can't run at the

synchronous speed because at synchronous

speed the induction motor can not develop

any torque to move the rotor from its

stationary position.

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Methods of speed control of

Induction Motor There are the following methods of speed

control of I.M.

1. Stator voltage control2. Supply frequency control

3. Pole changing schemes

4. Rotor resistance control

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Stator voltage control

By reducing stator voltage speed of a high slipinduction motor can be reduced.

Torque is proportional of voltage squred andcurrent is proportional to voltage.

Therefore, as voltage is reduced to reducespeed, for the same current motor develops

lower torque. Method suitable for app. where torque

demand reduces with speed.

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Stator voltage control contd.

Method is suitable for Fan and Pump drives.

Efficiency falls with decrease in speed.

Also power dissipated in rotor itself, whichmay overheat the rotor.

Because of these reasons , this method is

employed in fan and pump.

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Supply freq. control

Synchronous speed(Ns)=120f/p.

And Rotor speed (Nr)=(1-s)Ns.

Now, it is evident that varying synchronous speed,which can vary by varying the supply frequency, canvary the motor speed. Voltage induced in stator isproportional to the product of supply frequency fs andair gap flux m.

V=4.44.kw.fs. m.T

Any reduction in the supply frequency keeping thesupply voltage constant causes the increase of air-gap

flux m. 64/30/2011

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Supply freq. control contd.

The increase in flux will saturate the motor.

This will increase the magnetizing current anddistort the line current and voltage, increase in

core loss and stator I2R loss and produce a high-pitch acoustic noise

Therefore, variable frequency control below ratedfrequency is generally carried out at rated air gap

flux by varying supply voltage with frequency soas to maintain V/f ratio constant at the ratedvalue.

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Pole changing

For a given frequency speed is inverselyproportional to number of poles. Synchronousspeed, and therefore, motor speed can bechanged by changing the number of poles.

In squirrel cage motor the number of poles aresame as the Stator winding. So there is noprovision for changing the number of poles. Butfor wound rotor arrangement for changing the

number of poles in rotor is required, whichcomplicates the machine. So it is only used forSquirrel cage induction motor.

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Pole changing contd.

A simple but expensive arrangement for

changing number of stator poles is to use two

separate winding which are wound for two

different pole numbers.

An economical and common alternative is to

use single stator winding divided into few coil

groups.

Changing the connections of these coil groups

change number of poles.

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Rotor resistance control

In the rotor resistance control method, the speedvariation in a motor can be achieved by alteringthe slip the motor can operate.

This method is applicable for slipring inductionmotors, as it involves addition of the externalresistance in the rotor circuit of the motor.

The principle employed in the rotor resistancecontrol is changing the internal motor circuitparameters, by adding external rotor resistance.This in turn changes the torque-speedcharacteristics of the motor.

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Rotor resistance control contd.

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Rotor resistance control contd.

With increasing resistance, the slope of the

motor curve decreases, shifting the stable

operating point for the given load curve to a

point with higher slip.

Thus the speed control is achieved in the

rotor resistance control.

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What is ANN

Artificial neural networks are inspired by the learningprocesses that take place in biological systems.

Artificial neurons and neural networks try to imitate theworking mechanisms of their biological counterparts.

Biological neural learning happens by the modification ofthe synaptic strength. Artificial neural networks learn in thesame way.

A neural network acquires knowledge through learning.

A neural network's knowledge is stored within inter-neuron

connection strengths known as synaptic weights.

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ANNs The basics

ANNs incorporate the two fundamental

components of biological neural nets:

1.Neurones (nodes)

2.Synapses (weights)

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Graphical representation of MLP

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Introduction to direct torque control

method

This method was proposed by I Takahashi and T

Noguchi.

This method, appeared in second half of the

eighties competing with conventional methods.

Several studies are planned to decrease the

harmonics on the level of the torque and flux. For

that, we developed an intelligent technique toimprove the dynamic performances of the direct

torque control.

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Introduction

For all these reasons, it is interesting to

examine the removal of the mechanical sensor

and to replace it by estimators or observers of

the speed and position, based on the measure

of the electric quantities of the machine.

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Principle of direct torque control

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Principle contd.

The methods of direct torque control DTC

consist in controlling directly the opening or

closing the inverter switches from the

computed values of stator flux and torque.

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ANN BASEDDIRECTTORQUE

CONTROL

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ANN BASEDDIRECTTORQUE

CONTROL

A neuronal controller is used to replace

hysteresis comparators and switching table.

where the inputs are the error of d flux, ofd.

The output is the impulses allowing the

control of the inverter switches.

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Conclusion

This is a intelligent approach of speed control of

induction motor.

This method replaces the switching table and two

hysterisis comparators.

They allow a significant reduced torque and

stator flux ripples and a good starting behavior.

Using the intelligent techniques, the selection ofthe voltage vector becomes much convenient and

the switching state can be obtained when the

error of the torque and stator flux is attained.234/30/2011

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???Any query???

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Thank you

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