final project document-kutadza alimon

7/31/2019 Final Project Document-kutadza Alimon

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KUTADZA ALIMON R082717H Page 1

CHAPTER 1 INTRODUCTION

1.1: BACKGROUND

Amidst intensifying global recession and critical economic meltdown Southern Africa and

Zimbabwe in particular has not been spared from the effects of these worldwide catastropheswith Zimbabwe reeling from the extremes of electrical power shortages both in the rural and

commercial communities. Zimbabwe is currently experiencing serious power shortages and

having excessive load shedding as the order of the day. A wide range of factors are of course

responsible for the power shortages. In as much as the entire nation is blaming the national

utility Zimbabwe Electricity Supply Authority (ZESA) for the power shortages, emphasis

needs to be shifted on how we can find ways that more electricity can be produced as well as

utilising that which is already being produced.

Zimbabwe has an installed capacity to produce about 2100MW of electricity. Electricity is

generated locally at Hwange Thermal power station [920MW], Kariba Hydro power station

[750MW], and three electric power stations at Harare [135MW], Bulawayo [120MW] and

Munyati [120MW].Most of these power stations are now incapacitated to produce electricity

to their full capacity. The installed capacity also falls far short of the national demand which

is about 2500MW.Rural Zimbabwe as well as the low income urban households are highly

dependent on fuel wood. However fuel wood is not legally a commercial fuel as the

collection and sale of fuel wood requires a license from the government. Rural areas are

facing more acute shortages of wood as well as the associated land degradation.

1.2: PROBLEM STATEMENT

The shortage of electricity in Zimbabwe has heightened the need to consider alternative

generation modes. This has resulted in some companies resorting to generating their on

electricity for example Triangle limited and Hippo Valley Estates which fall under the

Tongaat Hullets group of companies. Other companies have also turned to the use of diesel

and petrol generators to supplement the national grid. However this has proved to be very

costly for some companies leading to discussions in some sectors on less costly and more

realistic approaches to electricity generation. Recently three stakeholders have applied for

power generation licenses from the government to produce a total of 125MW between them

from hydro power.

Another alternative is utilising some of the nations idle lying biomass points in the Eastern

highlands and South eastern lowveld. For the points to be utilised correspondingly small scale

electric power plants need to be designed which leads to the Development of a high speed

electric generator as a renewable energy technology (RET) model for rural and commercial

community applications which is the focus of this study. The projects goal is to provide

sustainable biomass energy management for positive economic development in Zimbabwe in

Zimbabwe.


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1.3: JUSTIFICATION

Developments and improvements in technology in our generation has made electrical

machine design more realisable. The impetus is now on the design engineer to arbitrarily

design and perform calculations rather than actually performing them. These latest

developments have enabled the solving of many design problems. The engineers job is

therefore to apply standard solutions to standard problems.

Although renewable energy technologies are already in use in Zimbabwe, their potential

remains largely unexploited. Earlier attempts to disseminate RETs have experienced minimal

success due to unforeseen barriers. It is therefore very important that barrier removal work

like this project that aims at research, design and construction be carried out so that future

will avoid pitfalls experienced by earlier initiatives. If successful it promises to be a model

for the integration of large scale and community/household scale biomass energy

management in a number of developing countries. These projects will kick-start provision of

electricity to rural and commercial communities from biomass to supplement the national

grid.

1.4: RESEARCH OBJECTIVES

The objectives of the project are as listed below.

Learn relevant design skills including production of design drawings using relevant

computer software.

Establish production methods for the RET model with the help of a local company,

organisation and/ or institution using locally available resources.To perform calculations, develop detailed drawings and design sheet for the RET

model.

Construct the RET model and evaluate its performance.

1.5: METHODOLOGY AND TIMELINES

The procedures to be followed are

Research using the internet

Reference to text books Company visits, Zimbabwe Power Company, Relmo, Alstom etc Analysis of captured data: The data accumulated is then interpreted using detailed

drawings and concepts to come with a design

Construction of a laboratory model Recommendations and conclusions: After analysing the process data and design

layout, concepts and evaluation of lab model performance, then relevant

recommendations and conclusions shall be made.

Time lines are shown in the table below.


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TASK DATES

Introduction 10/10/2011-17/10/2011

Literature review 10/10/2011-31/10/2011

Design process 17/10/2011-30/11/2011

Design sheet 05/12/2011-12/12/2011

Lab model design and construction 13/12/2011-28/03/2012Testing of lab model 29/03/2012-05/04/2012

Discussions limitations and

recommendations

05/04/2012-12/04/2012

Table 1.1 Timelines


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CHAPTER 2 LITERATURE REVIEW

2.1: INTRODUCTION

Renewable energy technologies have over the years become an integral part of the energy

supply chain in most developed countries. Recent projections show that 13.5% of theworlds primary energy supply comes from renewable resources and this figure has an

aggregated growth rate of 16%.Wind has the highest annual growth rate of 22% while the

least annual growth rate is that for hydro power. The main push for renewables like wind

in developed countries is environmental concerns and the business aspect in power

generation. The situation is however completely different in African countries where the

thrust for RETs is developmental based.

Although the African continent has abundant renewable energy resources like solar,

biomass, wind and hydro potential, they have remained largely unexploited. Several

efforts have been made to help African countries like Zimbabwe to exploit such

resources. In developing countries small hydro projects producing power outputs in the

ranges of 1-10KW are gaining popularity, particularly as isolated power supply schemes

for village electrification. These small generating plants supply power to remote locations

where utility power supply is well out of reach. Zimbabwe has great potential for

renewable energy development particularly in the South Eastern lowveld and in the

Eastern highlands.

In Zimbabwe studies have been carried out for the design for construction and

manufacture of a 1MVA, 3 phases, low speed hydro electric generator. We also have

Triangle limited and Hippo Valley Estates having a set of generators capable of

producing about 60MW of electricity between them and powered by steam turbines. This

study is aimed at developing a high speed steam turbine generator that is biomass

powered.

2.2: ADVANTAGES OF BIOMASS POWER PLANTS AND RETs

Renewable energy technologies contribute to all important elements of

national/regional development i.e.

(i) Economic growth through business earnings and employment(ii) Import substitution with direct and indirect effects on GDP and trade balance(iii) Security of energy supply and diversification(iv) Supports traditional industries, rural diversification and economic development of

rural societies.

Sources of biomass are commonly available and waste products which would

otherwise have been disposed are used.

Reliable, economical and environmentally stable.

Although specialised staff is needed in the construction stages, the running costs

are very small as only a few experienced people are needed.Plant is simple in construction and requires low maintenance


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2.3: SYNCHRONOUS GENERATORS

Synchronous generators are the primary source of all energy. They are commonly used to

convert mechanical power output of steam turbines, gas turbines, reciprocating engines and

hydro turbines into electrical power. They are known as synchronous machines because they

operate at synchronous speed and the rotor speed always matches supply frequency.

2.3.1: PRINCIPLE OF OPERATION

1 From an external source the field winding is supplied with a DC current excitation

2 Rotor (field) winding is mechanically turned (rotated) at synchronous speed

3 The rotating magnetic field produced by the field current induces voltage in the outer stator

(armature) winding.

Fig 2.1: Principle of operation

2.3.2: ROTOR

This is the rotating member of the generator. It is connected to the turbine by a directcoupling or through a speed increase in cases of low heads. Generally two types of rotors

exist which are salient pole and cylindrical type. The salient pole rotor is mostly used in

hydro generators although it is also applicable to steam turbine generators. The rotor body

provides support for the other parts. The design and type of material used for this part is

usually governed by the rotational speed.


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Fig 2.2 Salient pole rotor

2.3.3: STATOR

The stator carries the armature windings. It consists of a number of slots which carries the

armature windings. The armature windings can be arranged in different ways which include

single layer, concentric and double layer winding but double layer is the most suitable for

generators.

2.3.4: Field excitation and exciters

DC field excitation is an important part of the overall design of a synchronousgenerator.

The field excitation must ensure not only a stable AC terminal voltage, but must alsorespond to sudden load changes.

Rapid field excitation response is important.METHODS OF EXCITATION

(i) Slip ringslink the rotors field winding to an external dc source(ii) DC generator exciter-A DC generator exciter is built on the same shaft as the ac

generator rotor and a commutator rectifies the current that is sent to the field

winding.(iii) Brushless exciter-An AC generator with a fixed field winding and a rotor with a

three phase circuit. Diode/SCR rectification supplies DC current to the field

windings.


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Fig 2.3: Typical brushless exciter system

2.4: SELECTION OF SITE

Biomass energy is the use of crop residues, agro industrial residues, forest and wood

waste, etc to produce energy. Sites for construction of a biomass based power plant

should be carefully selected by considering the factors below.

The plant should be in the middle of biomass availability area

Good all round road connectivity (accessibility)

Near to an appropriate KV grid substationAvailability of adequate and good quality water.

Local supplies of building materials

2.5: ECONOMIC CONSIDERATIONS

The economic considerations when setting up a biomass electric power plant are as follows

(i) Capital cost of transmission system(ii) Capital cost of distribution systems and freight costs to site(iii) Managerial costs(iv) Running costs


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(v) Fixed charges

2.6: DESIGN THEORY

2.6.1: BASIC CONSIDERATIONS

In design, shape is given to a concept with the application of science, technology and

invention to the realisation of a machine so as to satisfy the required performance

characteristics i.e. its specifications with optimum economy and efficiency. A design process

involves the following considerations

Design baseBringing in the latest material technology, limitations in design, convenience in production

line and transportation, working safety and reliability, maintenance and repair, environmentalconditions, cost economy, optimisation.

SpecificationFurnishing data for the manufacturer to suit a given specification, which is meeting with

customers needs, guarantees satisfy the national and international standards.

Design transferDrawings, processes, instructions, job flow, meeting deadlines. Talking about optimisation in

design several aspects are to be considered. It is not just minimizing the cost; a designer has

to identify a criterion that gives best design to meet a given specification or a given duty. It is

therefore the purpose of the design to achieve four basic requirements namely: Lower costs,

smaller size, wider temperature operatibility and lower weight.

2.6.2: STANDARDISATION AND STANDARDS

The world is full of standards; Standards regulate, simplify and make possible an extensive

division of labour which should be recognized as a necessary basis for far reaching

modernisation process. The aim of design is to achieve design parameters which permit use

of standard materials and equipment. This presents the advantage of using infrastructure and

materials that is already in existence which will result in cost reduction; this however results

in interchange ability for the user and rigidity for the designer.

2.6.3: SPECIFICATIONS

Initiation of design of the high speed electric generator requires specification of performance.

Specifications for the should provide the following basic information

Type of machine KVA rating Number of phases


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Rated output voltage Type of cooling

2.6.4: Generated terminal voltage

The economical terminal voltages for generators of different KVA ranges are predeterminedas tabulated below. The choices of these are limited because of the need to use existing

infrastructure (only voltages listed can be successfully fed into the national grid)

Up to 750KVA 751KVA-2500KVA 2501KVA-5000KVA Above 5000KVA

400V 3.3KV 6.6KV 11KV

Table 2.1: Terminal voltages

2.6.5: CHOICE OF BASIC MATERIALS

The first step in the design procedure is the choice of basic materials to use. This choice

follows from a study of available materials to use, cost and material characteristics. The

materials used in design of electrical machines are divided into three categories namely

conducting, magnetic and insulating material.

Magnetic material

Magnetic materials can be classified into three broad categories that is diamagnetic materials,

paramagnetic materials and ferromagnetic materials. From the electrical engineering point of

view both diamagnetic and paramagnetic materials could be considered as non magnetic.

Ferromagnetic materials (such as nickel, cobalt, iron, steel, and silicon steel, perm alloy) are

further divided into two broad groups from hysteresis point of view i.e. hard magnetic and

soft magnetic materials.

The electric sheet steel is most commonly used for electrical machines. It has a steeply rising

magnetisation curve, relative small and narrow hysteresis loop and consequently small energy

loss per cycle of magnetisation. The electric sheet steel is widely is widely used for the

construction of cores of electrical rotating machines, transformers and for making

electromagnets, reactors, relays etc. Addition of silicon 1.8 to 3 percent to the iron has the

following advantages

Increase the resistivity in steel almost in direct proportion to the silicon content. Reduces hysteresis and eddy current losses. Increase magnetic permeability of steel in weaker magnetic fields, but reduces it in

stronger fields.

Abates aging of steel.


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Addition of steel has the drawback that it impairs certain properties of steel especially its

machinability. For synchronous machines the desirable characteristics of the magnetic circuit

are shown below.

PART MATERIAL NORMALLY

USED

NORMAL MAX FLUX

DENSITYStator core Silicon steel 0.8-1.2

Stator tooth Silicon steel 1.8-2.2

Gap Air (in air cooled machines) 0.5-0.65

Pole Silicon steel (for salient pole

machines)

0.8-1.2

Rotor core Silicon steel 0.8-1.2

Table 2.2: Magnetic circuit characteristics

Conducting material

Silver, copper, aluminium are some of the important conducting materials. Silver has

conductivity of about 10% but it is never used in making electrical machines due to its

excellent electrical and mechanical properties. The next important conductivity material is

aluminium which is being increasingly used in place of copper because copper deposits are

fast exhausting and copper prices fluctuate consistently. The table below compares the

properties of copper and aluminium and explains why both can be used interchangeably.

Table 2.3: Properties of copper and aluminium

Insulating material

SI

NO//

ITEM COPPER ALUMINIUM (times

copper)

1 Resistivity 0.0214 1.64

2 Specific weight 8.89 0.33

3 Thermal conductivity 350 0.57

4 Specific heat capacity 400 2.3

5 Coefficient of linear expansion 1.7 X 10

1.35

6 Melting point 1083 0.6


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The life of a generator is determined by the type of insulation and temperature of operation.

Knowledge of the operating temperatures together with the temperature rises involved is thus

required in order to determine the most suitable insulation for the generator. There are quite a

large variety of insulating materials available having vastly different properties. The

fundamental needs of a good insulating are:

High dielectric strength

High insulating resistance

Low dielectric loss

Good mechanical strength

Good thermal conductivity

High degree of thermal stability

Good machinability to mass production. It must also be easily and economically

available.

Insulating materials normally used in electrical machinery and apparatus according to their

thermal stability in service are grouped into seven classes as summarised.

Table 2.4: Classes of insulating materials

After choices of design materials have been made, the next step in the design is the

commencement of the real design.

2.6.6: Design process

Class Limiting working

temperature

Typical material

Y 90 Organic fibre materials on cellulose base

A 105 Class Y fibre material impregnated with lacquers

E 120 Enamelled wire on base of polyvinyformal, poly urethaneand epoxy resins moulding, powder plastics and phenolic

formaldehyde

B 130 Inorganic material (mica, glass, asbestos)

F 155 Inorganic material impregnated or glued with epoxy or

other vanish

H 180 Mica, glass, asbestos, with silicon binder and silicon resin

C above Inorganic material


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Knowing the specifications that a machine has to satisfy, the designer can develop the design

based on knowledge of certain basic data of primary importance. Choice of materials for

generator is restricted to conducting, magnetic and insulating materials as mentioned before.

The choice of these is dependent on;

Material characteristics Material availability And the cost of the material

The main areas of design are;

The magnetic circuit The electric circuit (the windings) The heating and cooling circuit (thermal design) Insulation and mechanical construction

The design process starts with assumption of certain basic quantities such as

Flux density Magnetic loading Electrical loading

From the designed data the parameters of the apparatus are determined so that its

performance characteristics are calculated and compared with given specifications. If no

satisfactory result is obtained, the basic assumed quantities are suitably modified until the

result is up to satisfaction. The components that result as a result of breaking down the design

are shown below.

Fig 2.3: Diagrammatic representation of design components

Performance

calculation

Design sheetDesign problem

Mechanical design

Thermal design

Magnetic circuit

Electric circuit


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During the design process, the following flow chart was used

Design flow chart

Air Gap diameter

Main dimensions

Gross stator core length

Exciter designRotor designStator design

1. Voltage per pole

winding

2. Resistance of mean

turn

3. Estimated length

4. Winding specs

5. Turns per layer

6. Number of layers

7. Turns per coil

8. Length of coil

9. Resistance of coil

10. Current Iac

11. Watts per coil

12. Total watts lost in

excitation

No load usefulflux

No loadleakagecoefficient

Total no loadflux

Resistancevoltage

drop/phase on

full load

Synchronousreactance per

phase

EMF requiredon full load

Air gap lengthand tooth flux

Electric loading Magnetic loading Flux per pole Size of conductor Slot dimensions Tooth dimensions Tooth flux Stator core external diameter

Stator core losses

Copper loss


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CHAPTER 3 DESIGN PROCESS

3.1: MAIN DIMENSIONS

The number of poles on the electric generator is given by the formula

Where Np is the number of polesf is the frequency

N is the rpm rating of machine

The output equation of a synchronous machine is given as

Where CO is the output coefficient defined as Where KW is the winding factor initially assumed, Bav is the average magnetic loading of

machine and ac is the average electrical loading of the machine.

3.1.1: Electrical loading

The number of armature or stator ampere conductors per metre of armature periphery at the

air gap is known as specific electric loading, ac and is given by

, IZ is current, Dg is gap diameter and Z is total number of statorconductors.The factors governing the choice of electrical loading are

Heating or temperature rise-Use of higher ac creates a problem of heat dissipation.

Speed of the machine-For higher speed machines the ventilation is obviously better

and so permits use of higher ac

Voltage-In high voltage machines, space for copper is reduced due to requirement of

large space for insulation and so does not permit the use of higher ac

Size of machine-In large size machine, more space available for accommodating

copper permits use of higher ac

Armature reaction-Restrict the value ofac used

Current density

3.1.2: Magnetic loading

The average flux density over the air gap of a machine is known as specific magnetic loading

and is given as

Bav = Total flux around the air gap/Area of flux path at the air gap

The factors governing the choice of average gap flux density are


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The maximum flux density in iron parts of machine Iron losses and Magnetising current

The value of air gap flux density should be so chosen that the flux density at the root of the

teeth (where the tooth section is minimum) does not exceed 2.2T.The maximum gap fluxdensity Bg varies from 0.64 to 1.1T and the corresponding values of Bav are 0.45 to 0.75. The

design of electric generators with higher magnetic loadings has the advantages of

Reduced size of the machine Higher stability limit Satisfactory parallel operation Reduced cost of machine

It however also have the following disadvantages:

Higher iron losses Increased transient short circuit current Decreased efficiency Higher temperature rise

As mentioned previously, the output coefficient is related to output KVA and hence the main

dimensions of the electric machine in the following formula

Where Q is the output KVA, Dg is the air gap diameter, LC is the gross stator core length and

ns is the speed of generator in revs/sec.

The factors affecting the size of the electrical machine are

Specific electric loading Magnetic loading Speed

The product Dg2LC thus obtained, a means of separating the product is provided by

considering the peripheral velocities, costs, efficiency and resultant reactance. A good

compromise is achieved when the ratio of LC to is in the given range

Where is the pole pitchThe relation between the stator internal diameter and axial length can be found by assuming a

suitable value of aspect ratio.


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There are some guidelines for choosing a particular value of aspect ratio. It cannot be chosenarbitrarily due to its importance for cooling of the machine. In summary the guidelines for

choosing the right aspect ratio are:

For minimum overall cost-1.5 to 2.0

For good efficiency -1.4 to 1.6

For good overall design-1.0 to 1.1

For good power factor-1.0 to 1.3

By properly choosing the value of the ratio, the values of the gross stator core length LC and

gap diameter Dg can be successfully settled. These are the basis for the main dimensions of

the stator frame.

3.2: VENTILATION DUCTS

The number of ventilation ducts ndfor generators are obtained from the formula below

Radial ventilation ducts are provided if core length exceeds about 0.12 to 0.14. A duct isprovided for approximately every 70 to 80 mm of core length. The ventilation duct width

usually varies from about 8mm to 10 mm. The number of ducts is also a function of the

required cooling. The effective stator core length is also determined after settling the number

of ducts and their width. In order to settle the effective stator core length,Le the number of

ventilation ducts and thickness of laminations together with inter lamina insulation required.

The space taken by the inter lamina insulation for lamination thickness of 0.33mm and

0.35mm is about 3% of the core length resulting in a space factor of 97% such that effective

core length becomes

Where Wris the width of duct, Kndis the space or staking factor.3.3: POLE PITCH

The ratio pole arc to pole pitch should be as large as possible to obtain as large a flux as

possible. It is however limited by the space required by the interpoles and leakage. For good

designs, this ratio should be as follows

From the above ratio the pole arc can thus be evaluated.


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3.4: ARMATURE WINDING

Armature winding should ensure the following

(i) Temperature rise should not exceed the specified limit.(ii) Load loss should be within specified limits(iii) Percent impedance should be within specified limits(iv) Minimum costs

Armature windings in rotating machines consist of coils uniformly distributed in slots along

the armature periphery. The best winding for electrical generators is the double layer winding

which has the following advantages.

(a) The possibility of shortening the winding pitch thus obtaining a better emf waveform

(b) Short end connections due to shortening of the pitch thereby saving copper.

(c) The possibility of forming a large number of absolutely equivalent parallel paths

(d) Simplicity of manufacturing the windings since all the coils are of the same shape and

therefore can be form wound.

However the double layer winding comes with its own disadvantages as listed below

(a) Difficult in laying the last coils of the winding along a coil pitch due to necessity of lifting

and suspending the upper sides of the coils first land in along the same path.

(b) The necessity of lifting the coils of a whole pitch to get to a damaged lower coil side.

(c) The impossibility of making a split stator without having to lift the coils out of the slots.

However the disadvantages are outweighed by the advantages such that a good design of the

system steam turbine generator will only need to be double layer.

The next step is to determine the number of turns per phase which is given by the basic emf

equation.

Where is given by Number of slots is given by the formula below and will depend on the assumed winding type.

Where NP is the number of parallel paths in winding, TC is the number of turns per coil; Tph is

turns in series per phase.

Stator slot pitch s is given by


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3.5: ARMATURE

3.5.1: Specific electrical loading

The formula below is used to calculate the specific electrical loading

Where Iz is current, Z is the total number of stator conductors, Dg is gap diameter

3.5.2: Specific magnetic loading

The size of the conductor to use in the stator will depend on the desired current density which

is estimated based on temperature rise, resistivity and cooling methods. For copper the

current density ranges from 2.0 to 7Amm-2

.The higher the current density the smaller the

conductor and the higher the resistance. The phase full load current is given by

Where VL is the output line voltage, Area of conductor is then calculatedusing the formula

A is assumed current density, having determined the area of the conductor required to carry

the full load current, it is now possible to calculate the slot dimensions.

3.6: SLOT AND TOOTH DIMENSIONS

In deciding the number of slots the guidelines to be kept in view are

(i) The number of armature slots should be such that a balanced winding is obtained.(ii) The slot pitch usually lies between 25 to 35mm for all except very small machines

where it can be less than 20mm.

(iii) The slot loading, i.e. the number of ampere conductors per slot should not exceedabout 1500.

The design of a slot is an important aspect in the design overall design of a generator. The

slot should be designed to carry a double layer winding with 6 turns per coil. An assumption

is made such that the stator slots and stator teeth are equal in size such that


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Where Ws is the stator slot width, the width of tooth is given by

, where ta is the radial thickness of slot clearance. The tooth depth isequal to the slot depth.3.7: MEAN LENGTH OF TURN (MLT)

Mean length of turn is determined is determined as follows: AC+Bd are estimates depending on rated voltage and usually range from 10 to 120cm for

voltage ranges of 0.4 to 18KV.

Y is given by

,

Where , Xc is coil pitch at end winding = (width ofinsulated coil + clearance).After calculating the mean length of coil, the resistive losses in the

coil can then be evaluated.

3.8: CORE LOSSES

The loss in the laminated stator core is usually the largest single loss in a generator and the

design of the core particularly the choice of, type and grade of steel is thus important. The

core losses are mainly the Eddy current and Hysteresis losses.

Hysteresis losses

Magnetising and demagnetising involves storage and release of energy. The amount of

energy stored is not equal to amount of energy released on the B-H curve. Specific hysteresis

loss is calculated from

Where Bm is the peak value of the sinusoidal flux, Kh = 0.63 forrotating machines, and quite often n=2 is usually used for estimating lossesin electrical machines. The use of high quality steel reduces these losses.

Eddy current losses

These are a result of circulating stray current found to exist in closed paths within the body of

the ferromagnetic material and cause undesirable heat loss. Specific eddy current loss is given

by t = thickness of lamination in metres

Ked = 0.005 for core and 0.008 for teeth.

This equation will lead to the equation where the eddy current losses in the core are given by


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), PC = Specific resistance of insulation

Eddy current in the stator teeth is given by

Wt3 is the width of the tooth at 1/3 depth 0f its depth from the gap. The factor 4 in the

equation of the core caters for the fact that laminations are short circuited at the back of the

core.

3.9: STATOR CORE OUTSIDE DIAMETER

The outside diameter of the core is chosen to give a flux density at the back of the slots BC

not more than 1.15Wb/m2, a guarantee to ensure reasonable core losses. Half the flux that

crosses the air gap goes to the left of the core whilst the other half goes to the right such that

the equation that gives the stator core outside diameter, DO is

3.10: THE MAGNETIC CIRCUIT

Armature reaction magnetising force Where Ka = amplitude factor generally assumed 1.05

Ma is related to air gap ampere turns using the formula

Where , x1 = leakage reactance per unit assumed 0.15,xd= direct axissynchronous reactance pu assumed 1.2.

The effective radial air gap over pole arc is given by Evaluation of air gap area

The value of the pole shoe depth is usually estimated and lies between 4 and 10cm, the height

of the pole hpis a function of pole pitch such that Kh is related to other parameters as


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Mr= rotor winding magnetising force,

Width of the pole Wp,

Then , Width of copper strip Outside diameter of rotor rim, , gmin = minimum gap diameterAir gap

Length of air gap has a profound effect on the electric generator performance. The air gap is

chosen so that the machine runs satisfactorily under normal operation and faulty conditions.

Formulae used for calculation of air gap dimensions is given below

Where Ke and Kd are estimated quantities depending on geometry of gap and pole and Kd

varies as shown on graph.

Nd is number of ducts, wr is width of duct, wane is assumed 1.8cm


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Reduction in effective gap area is given by Carters coefficient such that air gap area is given

by the equation above. Then gap flux density is given by giving

A compromise is made in choosing the right length of the air gap. The compromise is made

choosing between advantages and disadvantages of a large air gap compared to a small air

gap. A small air gap causes the following

1. Small value of intrinsic regulation2. Higher value of stability limit3. Higher synchronising power which makes the generator less sensitive to load

variations

A large air gap results in

1. Low noise2. Better cooling3. Lower unbalanced magnetic pulling,

Tooth flux density Where

Now that the values of the flux densities for core have been found for the no load condition,

the corresponding values for the gap flux mmf Mg, tooth mmf, and core mmf are determined

from respective B-H curves. The average length of core path that caters for the curvature at

back of tooth is given by The length of the tooth flux path is the tooth depth. Having settled these, the total ampere

turns are calculated.

Interpolar leakage flux

Diagram [Key to symbols]

Interpolar leakage flux is given by so that flux from rotor is givenby . For steady state conditions total permeance is given by

Where

,


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, ,

Cross section of pole body

The length of laminated rotor body needs to be multiplied by a space factor approximately o.97 to cater for inter lamina insulation. Solid steel end plates cross sectional

area is given by . The factor 1.05 on the flux path along pole caters for the poleshoe. The next step after calculating rotor flux density and pole path length the rotor mmf Mr

is determined.

Total no load ampere turns

3.11: EXCITATION SYSTEM

3.11.1: Excitation voltage

Excitation voltage is taken to be 75% of terminal voltage. It is calculated as

3.11.2: Resistance of mean turn

This is the resistance when the machine is running and is calculated as follows

3.11.3: Estimated mean length of turn

To calculate the mean length of turn estimation, relationships are used as shown below

3.11.4: Turns per layer

3.11.5: Number of layers


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3.11.6: Length of coil

Length of coil is given by Where l = length of mean turn, T = number of turns per pole pair.

3.11.7: Excitation current

3.12: CALCULATION OF LOSSES

Rotor winding

Resistance of field winding, Full load field loss Stator

DC resistance , AP = total cross sectional area of copper.Total stator loss Efficiency,


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CHAPTER 4 DESIGN CALCULATIONS

4.1: GENERATOR SPECIFICATIONS

Type of machine Synchronous generator

Output KVA 250KVAOutput power (watts) 200KW

Generator speed 1500rpm

Connection star

Number of poles 4

Power factor 0.8

Line current 360A

Line current 400V

Frequency 50Hz

Table 4.1: Generator specifications

(i)

(ii)

(iii) Terminal voltage

Line to line voltage = chosen terminal voltage

, Voltage per phase,

Say (iv) Line current

Line current =


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Say line current = 360A

(v) Generator speed =

(vi) A fixed frequency of 50 Hz will be used

(vii) A power factor of 0.8 will also be used

(viii) A star connection will also be used

4.2: CHOICE OF BASIC MATERIALS

4.2.1: Magnetic material

Stator core Silicon steel 0.3mm thickness

Stator tooth Silicon steel 0.3mm thickness

Gap Air

Pole Silicon steel 0.3mm thickness

Rotor core Silicon steel

Table 4.2: Magnetic material

4.2.2: Conducting material

Copper

4.2.3: Insulating material

Class E insulation, Enamelled wire on base of polyvinyformal, poly urethane and epoxy

resins, moulding powder plastics and phenolic formaldehyde.

4.3: MAIN DIMENSIONS

(i) 4 pole generator

(ii) Output coefficient = Taking , CO = (iii)


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Since choosing

Take Dg =330mm

Take LC = 410mm

(iv) Ventilation ducts

Number of ventilation ducts,

Say nd = 5, each 10mm width, Effective core length,

Say Le =350mm

(v) Pole pitch =


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(vi) Pole arc

Choosing pole arc =0.8

, Pole arc = 20.73cm

(vii) Flux per pole,

(viii) Turns per phase,

Take Tph =24, that is 48 conductors per phase,

Corrected flux per pole =

(ix) Number of slots,

NP = 4 parallel paths, TC =6 = number of turns per coil, Tph = 24

NS = 48 slots

(x)

(xi) By choosing double layer winding, in that case, the number of conductors per slot should

be even and a multiple of 3X2, that is 6, using 6 turn coils, i.e. 12 conductors per slot.therefore 48 conductors per phase or 3 effective conductors per slot.


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Modified electrical loading,

AC = 25kAt/m

Modified magnetic loading,

Bav = 0.6Wb /m2

(xii) After choosing 12 conductors per slot, to reduce the size of conductors use 4 parallel

paths or circuits to reduce the size of sub conductors so that they can get into the slot

opening.

Thus winding chosen is:

Total number of coils on the armature =48

Coil pitch = 1-6

Number of turns per coil = 6

4 parallel star

Class E insulation

Double layer winding

(xiii) Size of conductor

For class E insulation, the current density ranges between 5-7Amm-2. Choosing a current

density of 6A mm-2

, total current =360A and therefore current in each parallel circuit,

Therefore area of each conductor A =15mm

2, Area required by conductors per slot =15x12

=180mm2

Choose a 10mm x 1.5mm conductor with 1mm thick insulation = 15mm2 conductor.


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(xiv) Slot dimensions

Slot width, Ws

Conductors 12 x1.5mm =18mm

Slot insulation =1mm

Slack insulation =0.5mm

Total =19.5mm

Say Ws =1.2cm

Slot depth, ds

Conductors 12 x 2.5 = 30mm

Slot insulation =3mm

Insulation between =1mm

Insulation between layers =1mm

Insulation at bottom of slot =0.5mm

Slack =0.5mm

Wedge =1.5mm

Total =37mm

Say slot depth, ds=3,8cm

(xv) Stator core outside diameter

Assume BC = 0.8Wb/m2,

Stator core external diameter, DO=0.52m

4.4.1: Mean length of turn of stator winding

With a percentage coil pitch of 90%, Assume coil clearance, w =0.50cm,

, Pd =28.70cm


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Total length of wire = 2.47 x 24 x 3

= 177.84m

4.4.2: WINDING FACTOR

Having 48 slots


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Number of slots per phase =

Number of slots per pole =

Number of slots per pole per phase = Angle between adjacent slots Distribution factor

, Pole pitch = 12 slots, take coil span = 9 slots so that coil throw is from 1-10, that is 9 slots

Pitch or chording factor, where =3 x 15 = 45 electrical degrees.

Winding factor

4.5: MAGNETIC CIRCUIT

(i) Armature reaction magnetising force , Ka = amplitude factor generally assumed 1.05

(ii) ,

(iii) Gap density


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(iv) Gap diameter

Say ge = 0.70cm (v) Height of pole

Sayhp=10cm

(vi) Width of pole

Say wp = 8cm

Depth of pole shoe taken asdp = 2cm

(vii) Outside diameter of rotor rim

( )

(viii) Stator tooth width at 1/3 depth


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(ix) Ideal pole arc that caters for flux fringing

Ideal pole arc =

Percentage effective pole arc

(x) Tooth flux density

(xi) Length of gap

Air gap area

nd =5, newa= 1.8cm, ge =0.70cm, C =1.1, Kd =0.56, wr =0.4, Ke =2.85, wr = 0.4

Ag = Ag = 0.1149m

2

(xii) Air gap flux density,

Bg = 0.55T

(xii) Average length of core path at back of teeth


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(xiv) Air gap ampere turns

(xv) Armature core ampere turns as obtained from B-H curve

Stator tooth ampere turns = 4 x ds

Total ampere turns =Mg + Atc + Att

= 3063 + 81.6 +15.2

= 3159.8At

(xvi) Rotor ampere turns calculation

Interpolar leakage flux, so that flux from rotor body is given by For steady state conditions

Assuming K1 = 3.5, K2 = 6.2, f1 = 0.85


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Total permeance, , which gives a leakage flux,

Therefore flux in rotor,

With pole end plate 2.0 thick, area of rotor pole body ( )

Area

Length of pole body


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Rotor body flux density,

=1.40T

Giving mmf of1131.9At

Hence total ampere turns required = 1131.9 + 3159.8

= 4291.7At

Leakage reactance estimated as

Therefore ampere turns to produce rated current on short circuit =1.17 x 3063

= 3583.71At


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Assuming a rotor winding thickness = 3mm, width = 15mm. Insulation flanges on top and

bottom side of coil =0.5cm. Each inter turn insulation =0.01mm.

Maximum current through coil

Minimum current through coil

With rotor geometry designed, MLT of field winding:

[ ]

4.6: EXCITATION CIRCUIT

4.6.1: Excitation voltage

Assume 75% of terminal voltage Area of conductor

( )

4.6.2: Estimated length of mean turn

Obtained from winding specifications

4.6.3: Winding specifications

Winding depth (bare) = 30mm


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Insulation = 1.5mm

Inside diameter of coil = 30.15cm

Total depth = 31.5mm

4.6.4: Turns per layer

4.6.5: Turns per pole

Current density , therefore Therefore turns per pole

4.6.6: Number of layers

Number of layers

Say number of layers = 10

4.6.7: Length of coil

4.6.8: Resistance of mean coil


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4.6.9: Excitation current

4.6.10: Watts per coil

4.7: EFFICIENCY AND LOSSES

4.7.1: Resistance of field winding

Full load field loss

4.7.2: Stator

Stator winding length for the three phases = 177.84m

DC resistance

Total stator

4.7.4: Efficiency


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Efficiency Total watts generated = 200KW

Watts available in load circuit

Efficiency

Loss Quantity/KW

Core 3.273

teeth 1.195

Stator winding 1.2069

Field winding 3.473

Total watts generated 200Watts available in load circuit 191

Efficiency 96%

Table 4.3: Efficiency and losses


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CHAPTER 5: LAB MODEL DESIGN

5.1: GENERATOR SPECIFICATIONS

Type of machine Synchronous generator

Output KVA 2.0KVA

Output power (watts) 1.7KW

Generator speed 1500rpm

Connection star

Number of phases 3

Power factor 0.8

Line current 60A

Line voltage 28V

Frequency 50Hz

Table 5.1: Generator specifications

5.2: Basic materials

The model would be constructed using available materials and design is made to demonstrate

the basic principles of the generator. The mega-steam turbine generator design used a

brushless excitation arrangement. However the excitation circuit of the model is made to

comprise of slip rings.

RotorRotor coresilicon steel sheets

Shaftmild steel

Class E insulation

Field winding -Copper winding wire of gauges 24 AWG (42m) / gauge 23 SWG

Brass/carbon brushes

Stator

Armature/ stator winding 24 AWG (52m) / gauge 23 SWG

Stator core and Stator toothsilicon steel

Class E insulation

Slot wedge

Insulation cloth

Vanish

Bearings [Single row 02 series deep groove bearing: Bore 30mm]

Frame

5.3: Main dimensions


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Assumptions chosen noting the previous considerations in Design theory

This gives

Since

Choosing

(it should be low as possible) gives

Pole pitch


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Therefore number of slots

,


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Calculation of rotor winding wire

Length of wire 42m

Height of winding surface 0.02m

Diameter of wire 0.00051Number of turns per layer 0.02/0.00051 = 39

For 0.5 packing factor 39X0.5 = 19

Number of layers needed 300 turn/19 = 16

Table 5.2: calculation of rotor winding wire

5.3: Model Design Sheet

Stator outside diameter = 92.1mm

Rotor inside diameter = 35mm Rotor inside diameter = 91.6mm Type of windinggroup basket Grouping- 3-3 Number of slots 36 Number of coils 12(single layer winding) Number of turns per coil- 40 Coil pitch - 1-10 Conductor gauge 24 diameter 0.0008 Conductors in parallel -1 Leads out -3 Connected2 direction series


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CHAPTER 6: RESULTS ANALYSIS

6.1: RESULTS

Graph of line voltage against DC excitation voltage

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 10 20 30 40 50 60 70 80

Line voltage

DC Voltage

0

0.5

1

1.5

2

2.5

0 10 20 30 40 50 60 70 80

Phase Voltage

DC Voltage


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Graph of Phase Voltage against DC voltage

6.2: DISCUSSION

6.3: CONCLUSION

0

0.5

1

1.5

2

2.5

0 1 2 3 4 5 6 7

Opencircuitvoltage

Field current

Open circuit characteristics

Field current


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APPENDIX A


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APPENDIX B : PROCESS OF CONSTRUCTION (STATOR,ROTOR,EXCITER)

CONSTRUCTION OF ROTOR AND STATOR


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final project document-kutadza alimon

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