Industrial Electrical and Electronics

7/28/2019 Industrial Electrical and Electronics

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INDUSTRIAL ELECTRICAL AND

ELECTRONICS


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UNIT-IV


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PMOS


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MOSFET IN SATURATION


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MOSFET SATURATION

CONDITION


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Large Drain Resistance RD


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DIODE


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DIODE

THE IDEAL DIODE


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Terminal characteristics


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The forward bias region:-

I = Is(ev/nv

T -1) VT =KT/q K = Boltzmann's constant = 1.38x10-23 joules/kelvin

T = the absolute temp in Kelvins 273 + temp in degree centigrade

Q = magnitude of electric charge = 1.60 x 10-19.

The Full wave Rectifier:-


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PIV = 2Vs - VD

Bridge Rectifier:-

Determination of PIV:-

vD3 (reverse) = vo + vD2 (forward)

PIV (diode3) =Vs -2VD +VD


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The rectifier with a filter capacitor:- Assume diode is ideal


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IL = vo/R


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ID = iC +iL = cdvi/dt +iL

Bipolar junction Transistor

Device Structure and Physical Operation

NPN


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PNP

BJT Mode of operation


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OPERATION OF THE NPN TRANSISTOR IN ACTIVE MODE


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Np(o) = npoevBE/vT

l i (0)


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Electron concentration =Np(0),

Where np0 is the thermal equilibrium value of the minority carrier concentration(electron) in

base region

VBE is the forward base emitter bias voltage.VT is thermal voltage = 25mv. At room temperature.

Circuit symbols:-


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Current directions:


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The common Emitter Amplifier(CE):-


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Terminal characteristics of CE Amplifier:-

Input Resistance, Voltage gain, Output Resistance


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Common base amplifier:


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UJT


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A Unijunction transistor is a three terminal

semiconductor switching device. this device has a

unique characteristics that when it is triggered , the

emitter current increases regeneratively until is

limited by emitter power supply the unijunctiontransistor can be employed in a variety of

applications switching pulse generator saw tooth

generator etc

UJT


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Construction


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It consists of an N type silicon bar with an electrical

connection on each end the leads to these

connection are called base leads. Base 1 B1 Base 2B2 the bar between the two bases nearer to B2

than B1. A pn junction is formed between a p type

emitter and Bar.the lead to the junction is calledemitter lead E.

i


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Operation

The device has normally B2 positive w.r.t B1

If lt VBB i li d b t B2 d B1 ith


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If voltage VBB is applied between B2 and B1 with

emitter open. Voltage gradient is established along the

n type bar since emitter is located nearer to B2 morethan half of VBB appears between the emitter and B1.

the voltage V1 between emitter and B1 establishes a

reverse bias on the pn junction and the emitter current

is cut off. A small leakage current flows from B2 to

emitter due to minority carriers

If a positive voltage is applied at the emitter the pn

junction will remain reverse biased so long as theinput voltage is less than V1 if the input voltage to the

emitter exceeds V1 the pn junction becomes forwardbiased.under these conditions holes are injected from


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jthe p type material into the n type bar these holes arerepelled by positive B2 terminal and they are attracted

towards B1 terminal of the bar. This accumulation ofholes in the emitter to B1 region results in the degreesof resistance in this section of the bar the internalvoltage drop from emitter to b1 is decreased henceemitter current Ie increases as more holes are injecteda condition of saturation will eventually be reached atthis point a emitter current limited by emitter powersupply only . the devices is in on state.

If a negative pulse is applied to the emitter , the pn

junction is reverse biased and the emitter current is cutoff. The device is said to be off state.


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Characteristics of UJT


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The curve between Emitter voltage Ve andemitter current Ie of a UJT at a given voltage Vbb


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emitter current Ie of a UJT at a given voltage Vbb

between the bases this is known as emitter

characteristic of UJT Initially in the cut off region as Ve increases from

zero ,slight leakage current flows from terminal

B2 to the emitter the current is due to theminority carriers in the reverse biased diode

Above a certain value of Ve forward Ie begins to flow, increasing until the peak voltage Vp and current Ip


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are rreached at point P.

After the peak point P an attempt to increase Ve isfollowed by a sudden increases in emitter current Iewith decrease in Ve is a neagative resistance portionof the curve

The negative portion of the curve lasts until thevalley point V is reached with valley point voltageVv.and valley point current Iv after the valley pointthe device is driven to saturation the difference Vp-

Vv is a measure of a switching efficiency of UJT fallof Vbb decreases


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Digital to Analog

Converters (DAC)

Outline


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109

Outline

Purpose

Types

Performance Characteristics

Applications


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110

Purpose

To convert digital values to analog voltages

Performs inverse operation of the Analog-to-DigitalConverter (ADC)

DACDigital Value Analog Voltage

Reference Voltage

ValueDigitalOUTV

DACs


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111

DACs

Types Binary Weighted Resistor

R-2R Ladder

Characteristics Comprised of switches, op-amps, and resistors Provides resistance inversely proportion to significance of

bit


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112

Binary Weighted Resistor

Rf= R

8R4R2RR Vo

-VREF

iI

LSB

MSB


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113

Binary Representation

Rf= R

8R4R2RR Vo

-VREF

iI

LeastSignificant Bit

Most

Significant Bit


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114

Binary Representation

-VREFLeastSignificant Bit

Most

Significant Bit

CLEAREDSET

( 1 1 1 1 )2 = ( 15 )10

2 dd


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115

R-2R LadderVREF

MSB

LSB


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Common Applications

Digital to Analog Converters

Common Applications


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117

Generic use

Circuit Components

Digital Audio

Function Generators/Oscilloscopes

Motor Controllers

-Common Applications


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The phototransistor A phototransistor is an ordinary transistor that has been


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modified in two ways:

(1) there is a transparent window so that light can shine onthe junctions and

(2) the structure has been modified to maximize the light

capture area. Some phototransistors have an

external base lead; others do not. If there is an external

base lead, it is often left floating or connected to a high

impedance bias source to bias the collector current to a

specific value for the no light condition.


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LDR (Light Dependent Resistor)


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( g p ) An LDR is a component that has a resistance that changes

with the light intensity that falls upon it. They have aresistance that falls with an increase in the light intensity

falling upon the device.

The resistance of an LDR may typically have the following

resistances

Daylight = 5000

Dark = 20000000


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Liquid crystal display A liquid crystal display (LCD) is a flat panel display


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A liquid crystal display (LCD) is a flat panel display,

electronic visual display or video display that uses

the light modulating properties of liquid

crystals(LCs) .LCs do not emit light directly.

They are used in a wide range of applications,

including computer monitors, TV, aircraft, They arecommon in consumer devices such as video players,

gaming devices,etc.


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Plasma Display Panel


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p y

A plasma display is comprised of two parallel sheetsof glass, which enclose a gas mixture usually

composed of neon and xenon (some manufacturers

also use helium in the mix) that is contained inmillions of tiny cells sandwiched in between the

glass.

Electricity, sent through an array of electrodes thatare in close proximity to the cells, excites the gas,

resulting in a discharge of ultraviolet light.

The light then strikes a phosphor coating on theinside of the glass, which causes the emission of

(


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red, blue or green visible light. (Each cell, or pixel,

actually consists of one red, one blue and one greensub-pixel).

The three colors in each pixel combine according to

the amount of electric pulses fed to each sub-pixel,(which varies according to the signals sent to the

electrodes by the plasma displays internal

electronics), to create visible images.

Optocoupler


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p p In electronics, an opto-isolator (or optical isolator,

optocoupler, photocoupler, or photoMOS) is adevice that uses a short optical transmission path to

transfer a signal between elements of a circuit,

typically a transmitter and a receiver, while keepingthem electrically isolated since the signal goes

from an electrical signal to an optical signal back to

an electrical signal, electrical contact along the path

is broken.


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A common implementation involves a LED and aphototransistor, separated so that light may travel

b i b l i l


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across a barrier but electrical current may not.

When an electrical signal is applied to the input ofthe opto-isolator, its LED lights, its light sensor then

activates, and a corresponding electrical signal is

generated at the output. Unlike a transformer, the

opto-isolator allows for DC coupling and generally

provides significant protection from serious

overvoltage conditions in one circuit affecting the

other.

opt interrupter


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The optointerrupter is an electronic device thatconsists of a light emitting diode (LED) and a

phototransistor with a slot between them.

When voltage is applied to the LED it emits light likean electric bulb. However, the LED used in an

optointerrupter emits an infrared light beam which

is invisible. Light emitting diodes are very reliableand consume a relatively small current. Big current

may destroy them, therefore a resistor must be

added to limit the current.

Phototransistors are specially designed transistorswith the base region exposed. These transistors are

li ht iti i ll h i f d f


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light sensitive, especially when infrared source of

light is used. They have only two leads (collectorand emitter). When there is no light the

phototransistor is closed and does not allow a

collector-emitter current to go through. The

phototransistor opens only with the presence of

sufficient light.


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Avalanche Photodiode (APD)


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Attributes: high speed and internal gain

Good for communications

A thin side layer is exposed through a window to achieve

illumination. 3 ptype layers follow this and terminate at the electrode.

These p-type layers have different doping levels in order to modify the

field distribution across the diode.

1st p-type region is a thin layer

2nd p-type region is a thick, lightly dope layer. (almost intrinsic)

3rd p-type region is heavily doped layer.

n

p

APD


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The diode operates in the reverse bias mode in order toincrease the field in the depletion regions.

Applying an adequate R.B. will force the depletion region in

the p-layer to reach-through to layer.

The field ultimately extends from -side depletion layer tothe - side depletion layer.

Absorption of photons and therefore photogeneration takes

place in the long layer.

It is a uniform field in the layer due to the small net spacecharge density.

n

p

APD


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The E-field is at a maximum at the - side and a minimum atthe - side.

Drifting electrons arriving at the p-layer experience elevated

fields and acquire enough kinetic energy (greater than Eg) to

impact-ionize some of the Si covalent bonds and releaseEHPs.

These EHPs can be accelerated by high fields to high kinetic

energies to cause further impact ionization releasing even

more EHPs leading to an avalanche of impact-ionization

process.

n

p

p

SiO2Electrode

R

E

Iph


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p+

ne t

x

x

E(x)

h >Eg

p

e h+

Absorption

region

Avalancheregion

(a)

(b )

(c)

(a) A schematic illustration of the structure of an avalanche photodiode (APD) biasedfor avalanche gain. (b) The net space charge density across the photodiode. (c) Thefield across the diode and the identification of absorption and multiplication regions.

Electrode

1999 S.O. Kasap,Optoelectronics (Prentice Hall)

n+

Power supply A power supply is a device that supplies electricalenergy to one or
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more electric loads.

The term is most commonly applied to devices that convert one formof electrical energy to another, though it may also refer to devices that

convert another form of energy (e.g., mechanical, chemical, solar) to

electrical energy. A regulated power supply is one that controls the

output voltage or current to a specific value; the controlled value is

held nearly constant despite variations in either load current or the

voltage supplied by the power supply's energy source.

Every power supply must obtain the energy it supplies to its load, as

well as any energy it consumes while performing that task, from an

energy source. Depending on its design, a power supply may obtainenergy from:

Electrical energy transmission systems. Common examples of thisinclude power supplies that convert AC line voltage to DC voltage.

Energy storage devices such asbatteries and fuel cells.
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Electromechanical systems such as generators and alternators.

Solar power. A power supply may be implemented as a discrete, stand-alone device

or as an integral device that is hardwired to its load. In the latter case,

for example, low voltage DC power supplies are commonly integrated

with their loads in devices such as computers and householdelectronics.

Switched-mode power supply In a switched-mode power supply (SMPS), the AC mains input is

directly rectified and then filtered to obtain a DC voltage. The
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directly rectified and then filtered to obtain a DC voltage. The

resulting DC voltage is then switched on and off at a high frequency

by electronic switching circuitry, thus producing an AC current thatwill pass through a high-frequency transformer or inductor. Switching

occurs at a very high frequency (typically 10 kHz 1 MHz), thereby

enabling the use oftransformers and filter capacitors that are much

smaller, lighter, and less expensive than those found in linear powersupplies operating at mains frequency. After the inductor or

transformer secondary, the high frequency AC is rectified and filtered

to produce the DC output voltage. If the SMPS uses an adequately

insulated high-frequency transformer, the output will be electrically

isolated from the mains; this feature is often essential for safety.
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Types of Power Supply There are many types of power supply. Most are designed to convert

high voltage AC mains electricity to a suitable low voltage supply for


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electronics circuits and other devices. A power supply can by broken

down into a series of blocks, each of which performs a particularfunction.

Transformer

Transformers convert AC electricity from one voltage to another with

little loss of power. Transformers work only with AC and this is one ofthe reasons why mains electricity is AC. Step-up transformers increase

voltage, step-down transformers reduce voltage. Most power supplies

use a step-down transformer to reduce the dangerously high mains

voltage (230V in UK) to a safer low voltage.

The input coil is called the primary and the output coil is called the

secondary. There is no electrical connection between the two coils,

instead they are linked by an alternating magnetic field created in the

soft-iron core of the transformer. The two lines in the middle of the

circuit s mbol re resent the core

Transformers waste very little power so the power out is (almost)equal to the power in. Note that as voltage is stepped down current is

stepped up.


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The ratio of the number of turns on each coil, called the turns ratio,

determines the ratio of the voltages. A step-down transformer has alarge number of turns on its primary (input) coil which is connected to

the high voltage mains supply, and a small number of turns on its

secondary (output) coil to give a low output voltage.

turns ratio = VP/VS = NP/NS


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Bridge rectifier A bridge rectifier can be made using four individual diodes, but it is

also available in special packages containing the four diodes required.


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p p g g q

It is called a full-wave rectifier because it uses all the AC wave (both

positive and negative sections). 1.4V is used up in the bridge rectifierbecause each diode uses 0.7V when conducting and there are always

two diodes conducting, as shown in the diagram below. Bridge

rectifiers are rated by the maximum current they can pass and the

maximum reverse voltage they can withstand (this must be at leastthree times the supply RMS voltage so the rectifier can withstand the

peak voltages). Please see the Diodes page for more details, including

pictures of bridge rectifiers.
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Zener diode regulator For low current power supplies a simple voltage regulator can be

made with a resistor and a zener diode connected in reverse as

h i h di Z di d d b h i b kd


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shown in the diagram. Zener diodes are rated by their breakdown

voltage Vz and maximum power Pz (typically 400mW or 1.3W). The resistor limits the current (like an LED resistor). The current

through the resistor is constant, so when there is no output current

all the current flows through the zener diode and its power rating Pz

must be large enough to withstand this.


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UNIT-V


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BASIC DIGITAL CONCEPTS

NC and CNC machines and Control Programming


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Introduction to NC and CNC machines

CNC controls and RS274 programming


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Conventional milling machines


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Vertical milling machine

Vertical Milling machine architecture



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Vertical Milling machine architecture

Horizontal Milling machine architecture



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How does the table move along X- Y- and Z- axes ?

NC machines

Motion control is done b ser o controlled motors


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Motion control is done by: servo-controlled motors

~

Servo Controller

Counter Comparator

Encoder A/C Motor

Input (converted from analog to digital value)

TableLeadscrew

CNC terminology


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BLU: basic length unitsmallest programmable move of each axis.

Controller: (Machine Control Unit, MCU)

Electronic and computerized interface between operator and m/c

Controller components:

1. Data Processing Unit (DPU)

2. Control-Loops Unit (CLU)


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Open loop control of a Point-to-Point NC drilling machine

NOTE: this machine uses stepper motor control


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173

The z-Transform

z-Transform


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174

The z-transform is the most general concept for thetransformation of discrete-time series.

The Laplace transform is the more general concept for thetransformation of continuous time processes.

For example, the Laplace transform allows you to transform

a differential equation, and its corresponding initial andboundary value problems, into a space in which the equationcan be solved by ordinary algebra.

The switching of spaces to transform calculus problems intoalgebraic operations on transforms is called operationalcalculus. The Laplace and z transforms are the mostimportant methods for this purpose.


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Some Special Functions


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179

First we introduce the Dirac delta function (or unit samplefunction):

0,1

0,0)(

n

nn

This allows an arbitrary sequence x(n) or continuous-time functionf(t) to be expressed as:

dttxxftf

knkxnx

k

)()()(

)()()(

or

0,1

0,0)(

t

tt

Convolution, Unit Step


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180

These are referred to as discrete-time or continuous-timeconvolution, and are denoted by:

)(*)()(

)(*)()(

ttftf

nnxnx

We also introduce the unit step function:

0,0

0,1)(or

0,0

0,1)(

t

ttu

n

nnu

Note also:

k

knu )()(

Poles and Zeros


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181

When X(z) is a rational function, i.e., a ration of polynomials in z, then:

1. The roots of the numerator polynomial are referred to as the zerosofX(z), and

2. The roots of the denominator polynomial are referred to as the

poles ofX(z).

Note that no poles ofX(z) can occur within the region of convergencesince the z-transform does not converge at a pole.

Furthermore, the region of convergence is bounded by poles.


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Inverse z-Transform

The inverse z transform can be derived by using Cauchys


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184

The inverse z-transform can be derived by using Cauchy s

integral theorem. Start with the z-transform

n

nznxzX )()(

Multiply both sides by zk-1 and integrate with a contour integralfor which the contour of integration encloses the origin and lies

entirely within the region of convergence ofX(z):

transform.-zinversetheis)()(2

1

21)(

)(2

1)(

2

1

1

1

11

nxdzzzXi

dzzi

nx

dzznxi

dzzzXi

C

k

n C

kn

Cn

kn

C

k

Properties


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185

z-transforms are linear:

The transform of a shifted sequence:

Multiplication:

But multiplication will affect the region of

convergence and all the pole-zero locationswill be scaled by a factor of a.

)()()()( zbYzaXnbynax

)()( 00 zXznnxn

)()( 1zaZnxan


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Definition. Stable System. A system is stable if

kh )(


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189

k

kh )(

Which means that a bounded input will not yield an

unbounded output.

Definition. Causal System. A causal system is one in

which changes in output do not precede changes in input.

In other words,

.for)()(then

for)()(If

021

021

nnnxTnxT

nnnxnx

Linear, shift-invariant systems are causal iffh(n) = 0 forn < 0.

is,That.sinusoidalbe)(let)()()(Given

k

k nxnhkxny


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190

.)()(

thatso)()(Let

)()()(

Then.for)(let

)(

nii

k

kii

k

kini

k

kni

ni

k

eeHny

ekheH

ekheekhny

nenx

Here H(ei) is called the frequency response of the system

whose impulse response is h(n). Note that H(ei) is the Fourier

transform ofh(n).

We can generalize this state that:

ii


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191

.offunctioncontinuousatouniformlyconverges

andconvergentabsolutelyistransformthethen,)(

)(2

1)(

)()(

n

nii

n

nii

nxIf

deeXnx

enxeX

This implies that the frequency response of a stable system always

converges, and the Fourier transform exists.

These are the Fouriertransform pair.

Ifx(n) is constructed from some continuous functionxC(t) by

sampling at regular periods T(called the sampling period),

then x(n) = x (nT) and 1/T is called the sampling frequency or


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192

thenx(n) =xC(nT) and 1/Tis called the sampling frequencyor

sampling rate.

If0 is the highest radial frequency of sinusoids comprising

x(nT), then

2

1or

2 00

w

TT

Is the sampling rate required to guarantee thatxC(nT) can be

used to fully recoverxC(t), This sampling rate 0 is called the

Nyquist rate (or frequency). Sampling at less than this rate will

involve losing information from the time series.

Assume that the sampling rate is at least the Nyquist rate.


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195

Bilinear transformation with pre-warping Example

The bilinear transform (also known as Tustin's method) is used in digital signal

processing and discrete-time control theory to transform continuous-time system

representations to discrete-time and vice versa.
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Critical frequency

0s j


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May 14, 2007Feedback Control Systems (II) Douglas

Looze200

0j

Define

0

0

tan22

T

s s

T

Then apply bilinear transformation tos

2 11

zsT z

0tan

T


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Looze201

Note:

0

tan

22 2 11

zsT T z

0

0

1

1tan

2

z

s zT

Bilinear

transformation

with pre-warping

00 j Ts j z e

00 j Tc dbK j K e


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Use symmetric optimum design

1 sin75

0.0173


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Looze204

1 sin75 1

g

T

1

12.8 0.0173 0.593

0.0102T

0.593 1

0.0102 1c cp

s

K s k s


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Continuous-time design

0.593 1

25 7s

K s


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Looze206

Bilinear transformation with pre-warping

0 12.8 rad/sg

0

0

1

1tan

2

zs

zT

12.8 1

1tan .64

z

z

117.2

1

zs

z

120

1

zs

z

Bilinear:

25.70.0102 1

cK s

s

Continuous-time design

0.593 1

25 7s

K s


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Looze207

Bilinear controller Matlab commands

Kdb =

c2d(Kc,T,tustin) 0.844

274.2 0.659dbz

K z z

Bilinear controller with pre-warp at 12.8

rad/s Kdm =c2d(Kc,T,prewarp,0)

0.822244.5

0.700dbp

zK z

z

25.70.0102 1

cK s

s

103

104

e(abs)

Bode Diagram

Controllers


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Looze208

101

102

Magnitude

Frequency (rad/sec)

100

101

0

30

60

90

Phase(deg)

Continuous-time

Bilinear

Bilinear w ith Prew arp


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Step Response

1.2

1.4

Step Response


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Looze210

Time (sec)

Amplitude

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.2

0.4

0.6

0.8

1

Continuous-time

Bilinear

Bilinear with Prew arp


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PLD Advantages


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213

Short design time Less expensive at low

volume

Volume

Nonrecurring engineering cost

PLD

ASIC


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Programmable ROM (PROM)


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215

2N

x M

ROM

N input M output

Address: N bits; Output word: M bits

ROM contains 2N

words of M bits each

The input bits decide the particular word that becomes availableon output lines

Logic Diagram of 8x3 PROM


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216

Sum of minterms

Combinational CircuitImplementation using PROM


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217

0 0 0 1 0 0

0 0 1 0 1 0

0 1 0 0 1 10 1 1 1 0 0

1 0 0 0 1 0

1 0 1 0 0 1

1 1 0 1 0 0

1 1 1 0 1 0

I0 I1 I2 F0 F1 F2

F0 F1 F2


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PROM: Advantages andDisadvantages


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219

Widely used to implement functions withlarge number of inputs and outputs

Design of control units (Micro-programmed

control units) For combinational circuits with lots of dont

care terms, PROM is a wastage of logic

resources


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PLA 4 X 6 X 2


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221


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Programmable Array Logic (PAL)


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224

Programmable AND array Fixed OR array

Each output line permanently connected to a

specific set of product terms

Number of switching functions that can be

implemented with PAL are more limited than

PROM and PLA

PAL Logic Diagram


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225


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Design with PAL


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227

CPLD


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228

Logic

Block

Logic

Block

Logic

Block

Logic

Block

I/OI/O

Programmable

Interconnect

CPLD Logic Block

Simple PLD


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229

p

Inputs

Product-term array

Product term allocation function

Macro-cells (registers)

Logic blocks executes sum-of-product expressions and storesthe results in micro-cell registers

Programmable interconnects route signals to and from logic

blocks


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Con igura e Logic B oc CLB


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232


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Configuring FPGA


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234

Configure CLB and IOB Configure interconnect

Interconnect technology

SRAM Anti-fuse (program once)

EPROM / EEPROM


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PLD Logic Capacity


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237

SPLD: about 200 gates CPLD

Altera FLEX (250K logic gates)

Xilinx XC9500 FPGA

Xilinx Vertex-E ( 3 million logic gates)

Xilinx Spartan (10K logic gates) Altera


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Doubly fed Induction machine


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Switched reluctance motor


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AC Induction


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Source: AC, Brushless, Switched Reluctance Motor Comparisons James R. Hendershot, Magna Physics Corporation


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Switched Reluctance


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Source: AC, Brushless, Switched Reluctance Motor Comparisons James R. Hendershot, Magna Physics Corporation


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In Summary

Due to the absence of rotor windings, SRM is very simple to construct, has a low inertia and allowsan extremely high-speed operation. SRM operates in constant torque from zero speed up to the

rated speed Above rated speed up to a certain speed the operation is in constant power The


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rated speed. Above rated speed up to a certain speed, the operation is in constant power. Therange of this constant power operation depends on the motor design and its control. Designing amotor with high constant power range to base speed (e.g. at least 4:1), is not hard to achieve withSRM, and has a great effect in designing a lower power motor that can produce significant torque.

The absence of rotor copper loss eliminates the problem that the induction motor has associatedwith rotor cooling due to its poor thermal effects. The absence of permanent magnets on the rotoreliminates the problem that the Brushless DC motor has with high temperature environments

whereby the magnets can lose their magnetization.

The SRM has many advantages, mostly resulting from its simple structure. SRM is normally low costbecause of its extremely simple construction. Moreover, The SRM operation is extremely safe andthe motor is particularly suitable for hazardous environments. The SRM drive produces zero orsmall open circuit voltage and short circuit current.

Furthermore most SRM converters are simple because the current is unipolar. The SRM drive is

immune from shoot through faults, unlike the inverters of induction and brushless dc motors. Dueto the inductive nature of the motor, the power factor of the SRM is lower and requires a higherrated converter when compared to induction or BLDC motors.


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Brushless dc machine


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Unit-III


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Permanent Magnet Synchronous

and Variable Reluctance Motors

Introduction

Permanent magnet synchronous motors have the

rotor winding replaced by permanent magnets These


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rotor winding replaced by permanent magnets. These

motors have several advantages over synchronous

motors with rotor field windings, including:

Elimination of copper loss Higher power density and efficiency

Lower rotor inertia

Larger airgaps possible because of larger coerciveforce densities.

Introduction (contd)

Some disadvantages of the permanent magneth


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Some disadvantages of the permanent magnetsynchronous motor are:

Loss of flexibility of field flux control

Cost of high flux density permanent magnets is high Magnetic characteristics change with time

Loss of magnetization above Curie temperature

Permanent Magnets

Advances in permanent magnetic materials over the last

several years have had a dramatic impact on electric


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several years have had a dramatic impact on electric

machines. Permanent magnet materials have special

characteristics which must be taken into account in

machine design. For example, the highest performance

permanent magnets are brittle ceramics, some have

chemical sensitivities, all have temperature sensitivity,

and most have sensitivity to demagnetizing fields.

Proper machine design requires understanding the

materials well.

B-H Loop

A typical B-H loop for a permanent magnet is shown

below The portion of the curve in which permanent


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below. The portion of the curve in which permanentmagnets are designed to operate in motors is the top

left quadrant. This segment is referred to as the

demagnetizing curve and is shown on the next

slide.

Demagnetizing Curve


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Demagnetizing Curve (contd)

The remnant flux density Br

will be available if the

magnet is short circuited However with an air gap


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magnet is short-circuited. However, with an air gap

there will be some demagnetization resulting in the

no-load operating point, B. Slope of no-load line is

smaller with a larger air gap. With current flowing in

the stator, there is further demagnetization of the

permanent magnet causing the operating point to

shift to C at full load.

Demagnetizing Curve (contd)

Transients or machine faults can lead to a worst-cased ti ti h hi h lt i t


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Transients or machine faults can lead to a worst casedemagnetization as shown which results in permanent

demagnetization of the permanent magnet. The recoil

line following the transient is shown and shows a

reduced flux density compared to the original line. It isclearly important to control the operation of the

magnets to keep the operating point away from this

worst-case demagnetization condition.

Permanent Magnetic Materials

Alnico - good properties but too low a coercive force

and too square a B H loop => permanent


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and too square a B-H loop => permanent

demagnetization occurs easily

Ferrites (Barium and Strontium) - low cost, moderately

high service temperature (400C), and straight linedemagnetization curve. However, Br is low => machine

volume and size needs to be large.

Permanent Magnet Materials(contd)

Samarium-Cobalt (Sm-Co) - very good properties

but very expensive (because Samarium is rare)


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but very expensive (because Samarium is rare)

Neodymium-Iron-Boron (Nd-Fe-B) - very good

properties except the Curie temperature is only

150C

Permanent Magnet Materials(contd)


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PM Motor Construction

There are two types of permanent magnet motor

structures:


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structures:

1) Surface PM machines

- sinusoidal and trapezoidal

2) Interior PM machines- regular and transverse

Circuit Model of PM Motor (contd)

Based on the recoil line, we can write:


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Based on the recoil line, we can write:

where Prc, the permeance, is the slope of

the line. From this equation we can write:

0

0

( )rcP

F F

0r rcP F

Equivalent Circuit Model of PMMotor

Rearranging the slope equation, we get:


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This equation suggests the following equivalent circuit

for a permanent magnet:

0

rc

F FP

Equivalent Circuit Model of PMMotor (contd)

It can be shown that the mmf, flux and permeance

are the mathematical duals of current voltage


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are the mathematical duals of current, voltage,

and inductance, respectively. Therefore, the

following electrical equivalent circuits can be used

to represent the magnetic circuit:


We can now use this equivalent circuit of thepermanent magnets on the rotor and the


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We can now use this equivalent circuit of thepermanent magnets on the rotor and the

previous equivalent equivalent circuits of the

synchronous motor to develop a set of qd0

equivalent circuits for the permanent magnetsynchronous motor. Assuming the PM

synchronous motor has damper cage windings

but no g winding, the qd0 equivalent circuits are

as shown on the next slide.



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Here the PM magnet inductance Lrc can be lumpedwith the common d axis mutual inductance of the


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g pwith the common d-axis mutual inductance of the

stator and damper windings, and the combined d-

axis mutual inductance indicated by Lmd. Also, the

current im is the equivalent magnetizing current forthe permanent magnet referred to the stator side.

qd0 Equations for PermanentMagnet Synchronous Motor

The qd0 equations for a permanent magnet motor are

given in the table below:


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given in the table below:

qd0 Equations for Permanent

Magnet Synchronous Motor

(contd)


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(contd)The developed electromagnetic torque expression has

three components:


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p

1) A reluctance component (which is negative for Ld


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p y

The winding currents can be expressed (as before) as:

'( )mq mq q kqL i i

' '( )md md d m kd L i i i

q mq

q

ls

i

L

d mdd

ls

iL

'

'

'

kq mq

kq

lkq

i

L

''

'

kd md kd

lkd

iL



(contd)Combining these equations gives:


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where .

Similar expressions for mq and LMQcan be written

for the q-axis.

''

'

d kdmd MD m

ls lkd

L iL L

'

1 1 1 1

MD ls lkd mdL L L L



(contd)Under steady state conditions where =e as in the case

of Ef in the wound field synchronous motor, we can i b E h


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expressem or xmdim by Em, the permanent magnets

excitation voltage on the stator side. If the stator

resistance is neglected and the Efterm in the earlier

torque expression replaced by Em, the torque of apermanent magnet synchronous motor in terms of the

rms phase voltage Va at its terminal can be written as:

2 1 13 sin sin 22

a me a

e d q d

V EPT V

X X X

Simulation of PM SynchronousMotor

A line-startpermanent magnet motor has magnetsembedded in the rotor to provide synchronous


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p g gembedded in the rotor to provide synchronous

excitation and a rotor cage provides induction

motor torque for starting. Thus it is a high

efficiency synchronous motor with self-startcapability when operated from a fixed frequency

voltage source.

Simulation of PM SynchronousMotor (contd)

The simulation equations for the PM synchronous

motor are given below:


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The Simulink file s4 in Ch.7 Ong implements a simulation of a

line-start 3 PM synchronous motor connected directly to a


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60Hz, 3 supply of rated voltage. The overall block diagram

is:


This slide and the next few slides show the internal

blocks of the Simulink model.


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Trapezoidal Surface MagnetMotor

A trapezoidal surface permanent magnet motor is

the same as a sinusoidal PM motor except the 3i di h t t d f ll it h di t ib ti


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winding has a concentrated full-pitch distribution

instead of a sinusoidal distribution.

Trapezoidal Surface Magnet Motor(contd)

This 2-pole motor has a gap in the rotor magnets to

reduce flux fringing effects and the stator has 4 slots


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per phase winding per pole. As the machine rotates

the flux linkage will vary linearly except when the

magnet gap passes through the phase axis. If themachine is driven by a prime mover, the stator

phase voltages will have a trapezoidal wave shape as

shown on the next slide.


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Trapezoidal Surface Magnet Motor(contd)

An electronic inverter is required to establish a six-

step current wave to generate torque. With the


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help of an inverter and an absolute-position sensor

mounted on the shaft, both sinusoidal and

trapezoidal SPM motors can serve as brushless dcmotors (although the trapezoidal SPM motor gives

closer dc machine-like performance).

Synchronous Reluctance Motor

A synchronous reluctance motor has the same

structure as that of a salient pole synchronoust t th t it d t h fi ld i di


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motor except that it does not have a field winding

on the rotor.

Synchronous Reluctance Motor(contd)

The stator has a 3, symmetrical winding which creates

a sinusoidal rotating field in the air gap. This causes a


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reluctance torque to be created on the rotor because

the magnetic field induced in the rotor causes it to align

with the stator field in a minimum reluctance position.

The torque developed in this type of motor can be

expressed as:

2( )

3 sin 22 2

ds qs

e s

ds qs

L LPT

L L


The reluctance torque stability limit can be seen to occur

at (see figure below). / 4


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Iron laminations separated by non-magnetic materialsincreases reluctance flux in the qe-axis. With proper


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increases reluctance flux in the q axis. With proper

design, the reluctance motor performance can approach

that of an induction motor, although it is slightly heavier

and has a lower power factor. Their low cost androbustness has seen them increasingly used for low

power applications, such as in fiber-spinning mills.

Variable Reluctance Motors

A variable reluctance motor has double saliency, i.e. both

the rotor and stator have saliency. There are two groupsof variable reluctance motors: stepper motors and


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of variable reluctance motors: stepper motors and

switched reluctance motors. Stepper motors are not

suitable for variable speed drives.

Ref: A. Hughes, Electric Motors and Drives, 2nd. Edn. Newnes

Switched Reluctance Motors

The structure of a switched reluctance motor is shown

below. This is a 4-phase machine with 4 stator-polepairs and 3 rotor-pole pairs (8/6 motor) The rotor has


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pairs and 3 rotor pole pairs (8/6 motor). The rotor has

neither windings nor permanent magnets.

Switched Reluctance Motors(contd)

The stator poles have concentrated winding ratherthan sinusoidal winding. Each stator-pole pair


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winding is excited by a converter phase, until the

corresponding rotor pole-pair is aligned and is then

de-energized. The stator-pole pairs are sequentiallyexcited using a rotor position encoder for timing.


The inductance of a stator-pole pair and corresponding

phase currents as a function of angular position isshown below.


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shown below.


Applying the stator pulse when the inductance profile

has positive slope induces forward motoring torque.


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Applying the stator pulse during the time that the

inductance profile has negative slope induces

regenerative braking torque.

A single phase is excited every 60 with four

consecutive phases excited at 15 intervals.


The torque is given by:


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where m=inductance slope and

i=instantaneous current.

21

2e

T mi


Switched reluctance motors are growing in popularity

because of their simple design and robustness ofconstruction They also offer the advantages of only


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construction. They also offer the advantages of only

having to provide positive currents, simplifying the

inverter design. Also, shoot-through faults are not an

issue because each of the main switching devices isd h d h

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