Lecture Srm-unit III

7/23/2019 Lecture Srm-unit III

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Switched Reluctance Motors


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Introduction

The switched reluctance motor (SRM) is an electric motor in

which torque is produced by the tendency of its moveable part to

move to a position where the inductance of the excited winding is

maximied!

SRM is a type of synchronous machine! It has wound field coils

of a "# motor for its stator windings and has no coils or magnets

on its rotor!

It can be seen that both the stator and rotor have salient poles$hence% the machine is a doubly salient% singly excited machine!


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Introduction&cont!

Stator windings on diametrically opposite poles are connected in

series or parallel to form one phase of the motor!

Several combinations of stator and rotor poles are possible% such

as ' (' stator poles and rotor poles)% *% +,' etc!

The configurations with higher number of statorrotor pole

combinations have less torque ripple!


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#onfiguration

Initial classification is made on the basis of the nature of the motion

(i!e!% rotating or linear)!

The linear SRMs (-SRMs) have found application in the mar.etplace

by catering to machine tool servos!

The rotary machine&based SRM is differentiated to radial field SRM

and axial field SRM by the nature of the magnetic field path as to its

direction with respect to the axial length of the machine!

SRMs

Rotary SRMs -inear SRMs

Radial /ield 0xial /ield

-ong flux path machines1 "oubly

Salient with concentric windings%

diametrically opposite windings

are in series to form a phase

Short flux path machines1

0d2acent pole windings

are in series to form a

phase winding

Single&stac. Multi&stac.


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#onfiguration&cont!

Short flux path in a five&

phase radial field SRM

with +,* pole

Radial field SRM:

The magnetic field path

is perpendicular to the

shaft or along the radius

of the cylindrical stator

and rotor!


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#onfiguration&cont!

Axial field SRM: The magnetic fieldpath is along the axial direction!

3hole motor Rotor The short magnetic flux path


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#onfiguration&cont!

LSRM: The motion of the motor is linear!

Structure10 -SRM may have windings either on the stator or translator (the moving

part)! /ixed part is called trac.! Moving part is called translator!

0pplications1 Ideal for machine tool drives

4ne side -SRM

Two sided -SRM with winding

on the translator


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5rinciple of 4peration

#ross sectional model of a three phase SRM%

winding arrangement% and equilibrium positionwith phase + excited


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5rinciple of 4peration&cont!


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5rinciple of 4peration&cont!

Rotor rotation as switching sequence proceeds in a three phaseSRM% the rotation direction is opposite to the direction of the

excited phase!

The switching angle for the phase current is controlled and

synchronied with the rotor position% usually by means of a shaft

position sensor!


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Torque 5roduction

/lux&lin.age

#o&energy

Stored field energy

Magnetiation curve

,#urrent i

"efinition of co&energy and stored field energy


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Torque 5roduction&cont!

The torque production in SRM can be explained using the

elementary principle of electro&mechanical energy conversion! The

general expression for the torque produced by one phase at anyrotor position is

3here Tis the torque

Wis the co&energy

6is the displacement of the rotor

!

7

consti

WT

=

=

The constant&current constraint in the formula ensures that duringsuch a displacement% the mechanical wor. done is exactly equal to

the change in the co&energy!


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Torque 5roduction&cont!

In a motor with no magnetic saturation% the magnetiation curves

would be straight lines! 0t any position% the co&energy and the

stored magnetic energy are equal% which are given by

87

8+LiWWf ==

3hereLis the inductance of a exciting stator phase at a particular

position! In this case the instantaneous torque can be derived as

=

d

dLiT 8

8

+


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9nergy #onversion process

In the real switched reluctance motor% the energy conversion process in an

SRM can be evaluated using the power balance relationship!

The first term represents the stator winding loss$ andThe second term denotes the rate of change of magnetic stored

energy$ The third term is the mechanical output power!

The second term always exceeds the third term! The most effective

use of the energy supplied is to maintain phase current constantduring the positive d-

phd slope% in which way% the second term is

equal to ero

+

+=

d

dLiiL

dt

dRiP

ph

phphphsphin

888

8

+

8

+


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/our&quadrant 4peration


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Torque 5roduction&summary

The torque is proportional to the square of the current and

hence% the current can be unipolar to produce unidirectional

torque!

Since the torque is proportional to the square of the current% it

has a good starting torque!

:ecause the stator inductance is nonlinear% a simple equivalent

circuit development for SRM is not possible!

The torque characteristics of SRM are dependent on the

relationship between flux lin.ages and rotor position as afunction of current!


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9quivalent #ircuit

0n elementary equivalent circuit for the

SRM can be derived neglecting the mutualinductance between the phases asfollowing1

;The first term is the resistive voltage drop

;The second term is the inductive voltage drop% and

;The third one is the induced emf% which can be very high at

high speeds

( )

mph

ph

ph

ph

sph

phphsph

idt

idLiL

dt

diRi

iiL

dt

dRiV

++=

+=

)%()%(

)%(


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Torque&speed #haracteristics

The torque&speed plane of an SRM drive can be divided into threeregions1 constant torque region% constant power region and

constant power


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Torque&speed #haracteristics&cont!

Region+1 The constant torque limit region is the region below the

base speed b% which is the lowest possible speed for the motor to

operate at its rated power! /or the small bac.&emf in this region% thecurrent can be set at any desired level by means of regulators such as

hysteresis controller or voltage 53M controller!

Region81 The constant power limit region is the region where the

controller maintains the torque inversely proportional to the speed! Inthis region% the phase excitation time falls off inversely with speed and

so does the current! :ecause torque is roughly proportional to the

square of the current% the rapid fall in torque with speed can be

countered by ad2usting the conduction angle qdwell! :y advancing the

turn&on angle to increase the conduction angle until it reaches its upper

limit at speed p% the phase current can be increased effectively to

maintain the torque production at a high level!


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Torque&speed #haracteristics&cont!

Region =1 In this region% the qdwellupper limit is reached when

it occupies half the electrical cycle! The torque in this region is

governed by natural characteristics% falling off as 1/2!


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5ower -osses

Stator copper losses

3hen consider the case where phase currents are overlapping with boththe previous and succeeding phases% note that the stator copper

losses at any time are the sum of the copper losses contributed by

the instantaneous phase currents! The resistive losses are the result

of the cumulative effect of all three currents% evaluated as follows1

++=

+8

)(+8>

rsmfrsphlosscu

NNTTRp

wherephis the pea. value of phase current%Rsis the per&phase resistance ofthe stator winding% Trand Tfare the current rise and fall time%NsandNrare

the number of stator poles and rotor poles% and ?mis the rotor speed in

rads!


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5ower -osses&cont!

Core losses

The core losses are difficult to predict in the SRM due to the presence

of flux densities with various frequencies in stator segments for these

flux densities are neither pure sinusoids nor constants! The core

losses consist of hysteresis and eddy current losses! The magnitude of

the hysteresis losses is determined by the frequency of flux reversaland its path! To reduce the eddy current losses% the stator and rotor

cores are laminated!


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SRM "rive System

Switched

Reluctance

Motor


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5osition Sensors

#ommonly used position sensors are

5hototransistors and photodiodes

@all elements

Magnetic sensors

5ulse encoders

Aariable differential transformers


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5ower #onverters for SRM

Since the torque in SRM drives is independent of the excitationcurrent polarity% the SRM drives require only one power switch per

phase winding% for example1

0symmetric bridge converter #&dump converter


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5ower #onverters for SRMSplit link circuit used with even phase

number


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5ower #onverters for SRM

C-ump circuit


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0pplications

/lameproof drive

systems for

potentially explosive

atmospheres

3ashing

machine


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0pplications&cont!

9nvironmentallyfriendly air

conditioning

system for

passenger trains

Servo systems for

advanced technology

weaving machine


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M0T-0:SIMB-ICD Simulation

;Librar!

;Machines

;escription


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;The Switched Reluctance Motor (SRM) bloc. represents three most commonswitched reluctance motors1 three&phase ' SRM% four&phase *' SRM% five&phase

+,* SRM% as shown in the following figure!


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;The electric part of the motor is represented by a nonlinear model based on the

magnetiation characteristic composed of several magnetiing curves and on the

torque characteristic computed from the magnetiation curves! The mechanic part

is represented by a state&space model based on inertia moment and viscous friction

coefficient!

;To be versatile% two models are implemented for the SRM bloc.1 specific andgeneric models! In the specific SRM model% the magnetiation characteristic of the

motor is provided in a loo.up table! The values are obtained by experimental

measurement or calculated by finite&element analysis!


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;In the generic model% the magnetiation characteristic is calculated usingnonlinear functions and readily available parameters!


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"ialog :ox and 5arameters

;#onfiguration Tab


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;"!peSpecifies a three&phase ' motor% four&

phase *' motor% or a five&phase +,* motor!

;Machine modelSelect Eeneric model or

Specific model! The #arameters tab is

modified accordingly!


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;5arameters Tab1 Eeneric Model


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;Stator resistanceThe resistance Rs (F) of each statorphase winding!

;$nertiaThe inertia momentum G (.g!m8)!

;%rictionThe friction coefficient : (C!m!s)!

;$nitial speed and positionThe initial rotation speed w,

(rads) and initial rotor position Theta, (rad)!

;&nali'ned inductanceThe stator inductance when the

rotor is in unaligned position -q(@)!

;Ali'ned inductanceThe unsaturated stator inductance

when the rotor is in aligned position -d(@)!

;Saturated ali'ned inductanceThe saturated stator

inductance when the rotor is in aligned position -dsat(@)!

;Maximum currentThe stator maximum current Im(0)!

;Maximum flux linka'eThe maximum flux lin.age Hm

(3b or A!s) corresponding to Im!


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;5arameters Tab1 Specific Model


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;Stator resistanceThe resistance Rs (F) of eachstator phase winding!

;$nertiaThe inertia momentum G (.g!m8)!

;%rictionThe friction coefficient : (C!m!s)!

;$nitial speedThe initial rotation speed w, (rads)

and initial rotor position Theta, (rad)!;Rotor an'le vectorThe rotor position (deg) for

which the flux lin.age is specified!

;Stator current vectorThe stator current Is(0) for

which the flux lin.age is specified!

;Ma'neti(ation characteristicThe 8&" loo.uptable containing the flux lin.age as a function of

stator current and rotor position!


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;0dvanced Tab


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;#lot ma'neti(ation curvesIf selected% themas. plots the magnetiation curves

corresponding to the loo.up table provided!

The magnetiation curves represent the

machine flux lin.age versus the stator

current with the rotor position as a

parameter!

;Sample time )-* for inherited+Specifies the

sample time used by the bloc.! To inherit the

sample time specified in the 5owergui bloc.%

set this parameter to &+!


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Inputs and 4utputs

;T-1 The bloc. input is the mechanical load torque (in C!m)!T- is positive in motor operation and negative in generator

operation!

;M1 The bloc. output m is a vector containing several

signals! Jou can demultiplex these signals by using the :us

Selector bloc. from Simulin. library!


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9xample

;The power>SwitchedReluctanceMotor demo illustrates the simulation of the

Switched Reluctance Motor!


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5hase Inductance and #urrent Ideal 3aveforms

;To develop positive torque% the currents in the phases of a SRM must be

synchronied to the rotor position! The following figure shows the ideal waveforms

(5hase 0 inductance and current) in a ' SRM! Turn&on and turn&off angles refer

to the rotor position where the converter7s power switch is turned on and turned off%

respectively!

Documents

Lecture Srm-unit III