direct torque control of induction motor - copy

8/10/2019 Direct Torque Control of Induction Motor - Copy

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DIRECT TORQUE CONTROLOF INDUCTION MOTOR

SUBMITTED BY- RISHABH PURIAME 2NDSEM

POWER ELECTRONICS


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INTRODUCTION

First of all let us start by understanding some importantconcepts related to the induction motor.

An induction motor consists of:

a).Three phase supplyb).Stator windingsc). Rotor windings-could be of two types:

1.squirrel cage rotor2.wound rotor

Let us understsnd these by means of the diagrams:


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Basic Principles of IM

a

b

bc

c

Stator current produce statorflux

s r

Interaction between stator

and rotor fluxes produces

torque

Space angle between stator

and rotor fluxes varies with

load, and speed

Stator flux induces rotor

current produces rotor

flux


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Construction

Squirrel cage rotor

Wound rotor

Notice the

slip rings


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Rotating Magnetic Field

Balanced three phasewindings, i.e. mechanically

displaced 120 degrees form

each other, fed by balanced

three phase source A rotating magnetic field

with constant magnitude is

produced, rotating with a

speed

Where feis the supply

120e

sync

fn rpm

P


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Induction motor speed

So, the IM will always run at a speed lower

than the synchronous speed

The difference between the motor speed

and the synchronous speed is called the

Slip

Where nslip

= slip speed

nsync= speed of the magnetic field

nm = mechanical shaft speed of the

motor

slip sync mn n n


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The Slip

sync m

sync

n ns

n

Wheres is theslip

Notice that : if the rotor runs at synchronous speed

s= 0

if the rotor is stationary

s = 1

Slip may be expressed as apercentageby multiplying the above

eq. by 100, notice that the slip is a ratio and doesnt have units


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Frequency

What would be the frequency of therotors induced voltage at any speed nm?

When the rotor is blocked (s=1) , the

frequency of the induced voltage is equal

to the supply frequency On the other hand, if the rotor runs at

synchronous speed (s= 0), the frequency

will be zero

r ef s f


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Torque While the input to the induction motor is electrical power, its output is

mechanical power and for that we should know some terms and

quantities related to mechanical power

Any mechanical load applied to the motor shaft will introduce a

Torqueon the motor shaft. This torque is related to the motor output

power and the rotor speed

and

.outloadm

P N m

2 /60

mm

n rad s


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Variable frequency drives

control 1)Scalar control

V/f (Volts per Hertz) control

2)Vector control

(i)DTC (Direct torque control)

DSC (Direct self-control)

ISC (Indirect self control )

(ii)FOC (Field-oriented control)

Rotor Flux Oriented

Air gap flux oriented

Stator flux oriented
http://en.wikipedia.org/wiki/Direct_torque_controlhttp://en.wikipedia.org/wiki/Direct_torque_control


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Now let us understand what are

Field oriented control (FOC)

(also called Flux oriented control)

Direct torque control (DTC)


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Field Orientated Control

Field oriented control is also called the

vector control.

Field Orientated Control is based on three

major points:

machine current

voltage space vectors transformation of a three phase speed and

time dependent .


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Technical University Darmstadt's K. Hasse

and Siemens' F. Blaschke pioneered

vectorcontrol of AC motors starting in

1968 and in the early 1970s, Hasse in

terms of proposing indirect vector control,

Blaschke in terms of proposing directvector control. Technical University

Braunschweig's Werner Leonhard further

developed FOC techniques and wasinstrumental in opening up opportunities

forAC drivesto be a competitive

alternative to DC drives.
http://en.wikipedia.org/wiki/Space_vectorhttp://en.wikipedia.org/wiki/Variable-frequency_drivehttp://en.wikipedia.org/wiki/Adjustable-speed_drivehttp://en.wikipedia.org/wiki/Adjustable-speed_drivehttp://en.wikipedia.org/wiki/Variable-frequency_drivehttp://en.wikipedia.org/wiki/Space_vector


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Block diagram from Blaschke's

1971 US patent application


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The Field Orientated Control consists ofcontrolling the stator currents represented

by a vector. This control is based onprojections which transform a three-phase time and speed dependent

system into a two co-ordinate ( d and qco-ordinates) time invariantsystem.

Field orientated controlled machines needtwo constants as input references: the

torque component (aligned with the q co-ordinate) and the flux component (alignedwith d co-ordinate)


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FOC of IM drive

)ii(L

L

2

p

2

3T sdrqsqrd

r

me

Choose a frame such that:

rrd

r

0rrq

qs

ds

si

r

sqi

rq

sdi rd

As seen by stator reference frame:


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FOC of IM drive

r

sqrr

m

e iL

L

2

p

2

3

T

si

Choose a frame such that:

rrd

r

0rrq

qs

ds

rdr

qr

r

sdi

r

sqi

)ii(L

L

2

p

2

3

T sdrqsqrdr

m

e

Rotating reference frame:


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Direct torque control

DTC was patented by Manfred

Depenbrock in the US and in Germany,

the latter patent having been filed on

October 20, 1984, both patents havingbeen termed direct self-control (DSC).

The first major successful commercial

DTC products, developed byABB

ACS800 and ACS880 drives.
http://en.wikipedia.org/wiki/ABB_Asea_Brown_Boverihttp://en.wikipedia.org/wiki/ABB_Asea_Brown_Boveri


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Direct torque control(DTC) is one

method used in variable frequency drives

to control the torque(and thus finally the

speed) of three-phaseAC electric motors.

This involves calculatingan estimate of

the motor's magnetic fluxand torque

based on the measured voltageandcurrentof the motor.
http://en.wikipedia.org/wiki/Variable_frequency_drivehttp://en.wikipedia.org/wiki/Torquehttp://en.wikipedia.org/wiki/Speedhttp://en.wikipedia.org/wiki/Three-phasehttp://en.wikipedia.org/wiki/Electric_motorhttp://en.wikipedia.org/wiki/Calculationhttp://en.wikipedia.org/wiki/Magnetic_fluxhttp://en.wikipedia.org/wiki/Torquehttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Torquehttp://en.wikipedia.org/wiki/Magnetic_fluxhttp://en.wikipedia.org/wiki/Calculationhttp://en.wikipedia.org/wiki/Electric_motorhttp://en.wikipedia.org/wiki/Three-phasehttp://en.wikipedia.org/wiki/Three-phasehttp://en.wikipedia.org/wiki/Three-phasehttp://en.wikipedia.org/wiki/Speedhttp://en.wikipedia.org/wiki/Torquehttp://en.wikipedia.org/wiki/Variable_frequency_drive


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METHOD OF DTC..

Stator flux linkage is estimated byintegrating the stator voltages.

Torque is estimated as a cross product of

estimated stator fluxlinkage vector andmeasured motor current vector.

The estimated flux magnitude and torque

are then compared withtheir referencevalues.


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If either the estimated flux or torque deviates from the

reference more thanallowed tolerance, the transistors of

the variable frequency drive are turnedoff and on in sucha way that the flux and torque will return in their

tolerancebands as fast as possible

Thus direct torque control is one form of the hysteresiscontrol.

The direct torque method performs very well even without

speed sensors.


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PROBLEMS WITH DTC

The problems associated with the DTC drives are:- Variable switching frequency due to the hysteresis

comparators usedfor the torque and flux comparators

This problem can be easily done away with adopting

space vectormodulation. This technique is adopted to maintain a constant

average switchingfrequency.

Inaccurate stator flux estimations which can degrade

the driveperformance. The inaccurate flux approximation can be avoided by

using a low passfilter instead of using an integrator

because of which the problem of driftarises.


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Disadvantages

Vector control:

Greater parameter

sensitivity

Commissioning problems

with the setting up of

current control loops.

Complex architecture.

DTC

Loss of flux at low

speed/loads.

Higher torque and current

ripples.

Uncontrolled current

transients.


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References

Field-Oriented Control of Induction Machine Dr. Nik Rumzi Nik Idris

Department of Energy Conversion,

Faculty of Electrical Engineering Universiti Teknologi Malaysia.

DIRECT TORQUE CONTROL OF MULTILEVEL INVERTERFEDINDUCTION MACHINESA SURVEY

1NAZIHA AHMAD AZLI, 2NORJULIA MOHD NORDIN, 3NIK RUMZI NIK IDRIS

Universiti Teknologi Malaysia, Faculty of ElectricalEngineering, Department of Energy

Conversion, UTM

Skudai, Johor, Malaysia.

Wikipedia.

Google images.

direct torque control of induction motor - copy

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