Unit I introduction

IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

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DESIGN OF ELECTRICAL MACHINES

1. INTRODUCTION

2Major considerations in Electrical Machine Design -Electrical Engineering Materials-Space factor - Choice ofSpecific Electrical and Magnetic loadings -Thermalconsiderations - Heat flow -Temperature rise - Rating ofmachines -Standard specifications.


3 DESIGN may be defined as a creative physicalrealization of theoretical concepts.

ENGINEERING DESIGN is the application of Science,technology and inventions to produce various machines tosolve specified tasks with optimum economy andefficiency.

Problem of design and manufacture of electric machineryis to build, as economically as possible, a machine whichfulfils a certain set of specifications and guarantees.

Major considerations in Electrical Machine Design


4The major considerations to evolve a good design are:

(i) Cost .

(ii) Durability.

(iii) Compliance with performance criteria as laid down in specifications.


Design Factors Mechanical force required for movement in rotating

electrical machines can be produced both by electrostaticand electromagnetic fields since both the fields storeenergy.

In electrostatic machines, the energy density is limited bythe dielectric strength of the medium used.

In electromagnetic machines, magnetic effect is used forproduction of force and there is no comparable restrictionin magnetic fields.

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Voltages that can be developed and used by normalmeans, the forces produced by electrostatic effects arevery weak.

A small current can produce large mechanical forces byelectromagnetic means and therefore all the modernelectrical machines are electromagnetic type.

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7DESIGN FLOW CHART

START

GIVEN SPECIFICATION

CHOICE OF MATERIALS:

MAGNETIC, CONDUCTING, INSULATING, etc.

ASSUMPTION OF BASIC QUANTITIES SUCH AS, FLUX DENSITY, AMP.COND/m, etc

DESIGN PROCESS: MAGNETIC CIRCUIT, ELECTRIC CIRCUIT, MECHANICAL & THERMAL DESIGN MODIFY

ASSUMPTIONS

BAIFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

8A

PERFORMANCE CALCULATION

COMPARE WITH GIVEN SPECIFICATION

IS

SATISFACTORY?

PRINT DESIGN SHEET

STOP

B

NO

YES


9BASIC STRUCTURAL PARTS OF AN ELECTROMAGNETIC ROTATING MACHINE

MAGNETIC CIRCUIT:

Provides the path for magnetic flux.Consists of air gap, stator& rotor teeth, stator &rotor, cores(yokes)

cores (yokes). ELECTRIC CIRCUIT:

Consists of stator &rotor windings.

DIELECTRIC CIRCUIT :Consists of insulation

THERMAL CIRCUIT:Considered with mode& media for dissipation of

heat.MECHANICAL PARTS:

Frame, bearing and shaft.


Limitations in design

(i) Saturation.(ii) Temperature rise.(iii) Insulation.(iv) Efficiency.(v) Mechanical parts.(vi) Commutation.(vii) Power factor.(viii) Consumer specifications(ix) Standard specifications.

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SATURATION:The maximum allowable flux density is to be determined by the

Saturation level of the ferromagnetic material.

TEMPERATURE RISE: Life of the machine depends on the type of insulating material

used. Life of the insulating material in turn depends upon the

temperature rise of the machine . Proper cooling and ventilation techniques are required to keep the

temperature rise within safe limits.


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INSULATION:

The insulating material should withstand electrical,mechanical and thermal stresses produced in the machine.

The size of the insulation is not only decided by themaximum voltage stress but also by the mechanical stressesproduced.

Eg. For the same operating voltage thicker insulation has tobe used for large sized conductors than for smaller sizedones.


EFFICIENCY:

Should be as high as possible to reduce the operating cost.

Magnetic and electric loadings used should be small and thisrequires large amount of material.

So the capital cost of a machine designed foe high efficiencyis high while its running cost is low.

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MECHANICAL PARTS: Construction of a machine should satisfy numerous

technological requirements.

In I.M length of air gap is small in order to have high p.f.

In large machines, size of the shaft is decided by consideringthe critical speed, which depends upon the deflection of theshaft.

Type of bearings to be used depends on the inertia forces dueto unbalanced rotors and unbalanced magnetic pull, and thetype of construction whether the machine is mountedvertically or horizontally.


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COMMUTATION: Commutation conditions limits the maximum output of the

machine.

POWER FACTOR: Poor P.F results in larger values of current for the same power,

therefore larger conductor sizes have to be used.

Problem of P.F is particularly important in case if I.M.

Length of air gap is determined by P.F considerations.

Value of flux density depends on the power factor, hence P.Fbecomes a limiting factor.


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CONSUMERS SPECIFICATIONS:

Specifications laid down in the consumers order hasto be met.

Design evolved should also satisfy the economicalconstraints imposed on the manufacturer.

STANDARD SPECIFICATIONS:

Specifications are the biggest strain on the design,since both the consumer as well as the manufacturercannot get away from them without satisfying them.


Modern machine manufacturing techniques

Modern machines are characterised by a very wide rangeof power outputs.

The power range varies from a fraction of a watt toseveral hundreds of megawatt in a single unit.

Thus the ratio of power output of the smallest machine tothat of the largest machine is 1:1010

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Range of rotational speeds of electrical machines is verywide.

One machine may have a speed of few revolutions persecond while that of another may be several thousandrevolutions per second.

The large varied fields of applications, and wide range ofboth power output and speed of operation of electricalmachines has led to a variety of types of construction.

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CLASSIFICATION ON THE BASIS OF MANUFACTURING

1. Small size Machine (up to 750W).

2. Medium size Machine (Few kW-250kW).

3. Large size Machine (250kW-5000kW).

4. Larger size Machine (hundreds of MW).


Small size machines: Electrical machines having power output up toabout 750 W may be called small machines.

Medium size machines: Electrical machines having power outputsranging from a few kilowatt up to approximately 250 kW may beclassified as medium size machines.

Large size machines: Electrical machines with power O/P in the rangeof 250 kW up to about 5000 kW are classified as larger size machines.

The machines are usually designed and manufactured as a series andhave a definite power output range.

Larger machines are designed on individual basis. The power o/p of themachines are hundreds to megawatt.

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The action of Electromagnetic machines can be related to3 BASIC PRINCIPLES namely,

(i) Induction (Faradays law).(ii) Interaction (Biot Savarts law).(iii) Alignment (Production of force due

to alignment-in Reluctance motors).


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BASIC PRINCIPLES:

Faradays law:

This law states that e.m.f induced in a closed electriccircuit is equal to the rate of change of flux linkage.

The direction of induced e.m.f is such that the currentproduced by it opposes the change in flux linkages.


The change in flux linkages can be caused in three ways:

Coil is stationary w.r.t flux and the flux varies in magnitude w.r.ttime- emf induced is called as transformer emf or pulsational emf-used in transformers.

Flux is constant w.r.t time and is stationary and the coil movesthrough it- emf induced is called motional emf- used in rotatingmachines like d.c machines, induction and synchronous machines.

Both the changes mentioned above occur together i.e., the coilmoves through a time varying field- this process involves bothtransfer and conversion of energy- thus used in commutatormachines.

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The electromagnetic force is given by:fe= Bil sin newton

Where B= flux density, Wb/m2;

l = length of conductor, m;I = current carried by conductor, A; = angle between direction of current and

magnetic field.

When the conductor and magnetic field are perpendicular to each other, = 90 degree.

So , fe= Bil newton

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ELECTROMAGNETIC DESIGNSTATOR & ROTOR

ROTATING MACHINE STATIONARY

Core Core & windingTeeth dimensionWinding & air gap


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MECHANICAL DESIGN

ROTATING STATIONARY

Frame TankShaft (Transformer tank)

Bearings


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ELECTRICAL ENGINEERING MATERIALS

It consists of1. Conducting materials2. Magnetic materials3. Insulating materials

Electrical Conducting materials consists of1. High conductivity materials

used for making all types if windings requiredin electrical machines, apparatus, devices .

in transmission and distribution.2. High resistivity materials (alloys).

Used for making resistances and heating devices.


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ELECTRICAL ENGINEERING MATERIALS

ELECTRICAL CONDUCTING MATERIALS:

High conductivity materials. Copper, Aluminium, Iron & Steel, Alloys of copper.

High resistivity materials (Alloys). Nickel, Silver & Iron


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FUNDAMENTAL REQUIREMENTS OF HIGH CONDUCTINGMATERIALS:

1. Highest possible conductivity.2. Least possible temperature coefficient of resistance.3. Adequate mechanical strength i.e., absence of brittleness.4. Rollability and Drawability.5. Good weldability and Solderability which ensures high reliability

and low electrical resistance of the joints.6. Adequate resistance to corrosion.


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ALLOYS OF COPPER:

1. Bronze- copper based alloys containing tin, cadmium, beryllium andcertain other metals are generally called Bronzes. Beryllium copper Cadmium copper

2. Brass- contains 66% Cu, 34%Zn.3. Copper silver alloy- contains 99.1% Cu, 0.06 to 0.1% silver.


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MATERIALS OF HIGH RESISTIVITY:They can be classified into 3 categories:I- GROUP: (MATERIALS USED FOR PRECISION WORKS)

Consists of materials used in precision measuringinstruments & in making standard resistances and resistanceboxes.

Important material used is Manganin (composition ofCu86%, Mn 12%, Ni 2%)

II- GROUP: (MATERIALS USED FOR RHEOSTATS) Consists of materials from which resistance

elements are made for all kinds of rheostats andsimilar control devices.

Principal alloy is constantan consisting of 60 to 65% Cu and 40 to 35% Ni).

II- GROUP: (MATERIALS USED FOR HEATING DEVICES ) Consists of materials suitable for making high

temperature elements for electric furnaces, heatingdevices and loading rheostats.


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Alloys of Nickel, Chromium and Iron called asNichrome.

Alloys of Aluminium, Iron and Chromium.

ELECTRICAL CARBON MATERIALS: manufactured from graphite and other forms of

carbon coal, etc. Carbon brushes are often graphited i.e., heat treated to

increase the size of crystals. This raises the conductivity of the brushes and reduces their

hardness. Carbon brushes should acquire a mirror smooth surface in

order that they does not cause wear of commutator whenmounted on them.


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CLASSIFICATION OF MAGNETIC MATERIALS

Based on the relative permeability, materials may classified as,1. Ferromagnetic materials. (r much>1)2. Paramagnetic materials. (r slightly>1)3. Diamagnetic materials. (r slightly

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SOFT MAGNETIC MATERIALS:i) Solid core materialsii) Electrical sheet & stripiii) Special purpose alloys.

i) SOLID CORE MATERIALS:Iron, low carbon silicon steel, cast iron, gray cast iron,

cast steel, soft steel, ferro-cobalt.ii) SHEET METALS:

Electrical steel sheets (Non-oriented steel), andCold Rolled Grain Oriented steel (CRGO).

iii) SPECIAL PURPOSE ALLOYS:Mumetal, Permalloys, Super permalloy, Perminvar,Permendur.


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INSULATING MATERIALS (or) INSULANTS

ELECTRICAL PROPERTIES:An ideal insulating material should have;

i) high dielectric strength, sustained at elevated temperatures,ii) high resistivity or specific resistance,iii) low dielectric losses,iv) good thermal conductivity,v) high degree of thermal stabilty i.e. it should not

deteriorate at high temperatures.


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CLASSIFICATION:

i. Class Y - cotton, silk, paper, cellulose, wood, etc neitherimpregnated nor immersed in oil.

ii. Class materials of class Y suitably impregnated.iii. Class E- Synthetic resin enamels, cotton, paper laminated with

formaldehyde bonding, etc.iv. Class B- mica, glass fibre, asbestos, etc, with suitable bonding

materials.v. Class F-materials of class B with suitable bonding materials of

higher thermal stability(25C higher than class B).vi. Class H- glass fibre, asbestos & built up mica, with suitable bonding

materials like silicon resins.


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vii. Class C- Mica, ceramics, glass, quartz without binders or withsilicon resins of higher thermal stability.

INSULATING MATERIALS USED IN MODERN ELECTRICMACHINES:

Mica, Micafolium, Fibrous glass, Asbestos, Cotton fibre,Polyamides, Synthetic-resin enamels, Slot-lining materials, Wood,Silicones, Epoxide thermosetting resins, Synthetic resin,Petroleum based mineral oils, Askarels.

APPLICATIONS:a. Wires for magnetic coils and windings of machines.b. Laminationsc. Machines& Transformers.


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LIST OF SYMBOLS:

SYMBOL MEANING UNITa Number of parallel paths in armature

winding.

az Area of cross section of conductor mm2

ac Specific electric loading Amp.cond./m

Bav Specific magnetic loading Wb/m2 or tesla

Bgm Maximum air gap flux density under load conditions.

Wb/m2 or tesla

b Pole arc m

bp Width of the pole body M

Co Output coefficient Kva/m3-rps

c Cooling coefficient C W-m2

D Armature diameter mIFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

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ds Depth of the slot mm

E Generated emf or back emf V

Ecm Maximum voltage between adjacent segments

V

Eph Induced emf per phase V

f frequency Hz

I Rated current A

Ia Armature current A

Iz Current in each conductor A

Iph Current per phase A

L Armature length m


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N speed rpm

n speed rps

ns Synchronous speed Rps

P Rating of machine kW

Pa Power developed by armature kW

p No. of poles -

Q kVA rating of the machine kVA

Ql Loss dissipated per unit area kW/m3

R resistance ohm

S Dissipating surface m2

Tc Turns per coil -

Tph Turns per phase -


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Va Peripheral speed m/sec

Ws Width of the slot mm

ys Slot pitch mm

Z Total no.of armature or stator conductors -

Magnetic flux Wb

Ratio of pole arc to pole pitch -

Pole pitch m

efficiency -

Temperature rise C

Current density A/mm

Resistivity ohm-m


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CONSTRUCTIONAL ELEMENTS OF ROTATING MACHINES

Every rotating machine has the following three quantities. The presence of any two quantities, will produce the third

quantity. Magnetic field-I (field) Magnetic field-II (armature) Mechanical force.


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CONSTRUCTIONAL ELEMENTS OF A D.C MACHINE:

STATOR - yoke or frame- field pole - pole shoe- field winding- Interpole

ROTOR - armature core- armature winding- commutator

OTHERS - brush-brush holder


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CONSTRUCTIONAL ELEMENTS OF SALIENT POLE SYNCHRONOUS MACHINESTATOR - yoke or frame

- armature core- armature winding

ROTOR - field pole - pole shoe- field winding-damper winding


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CONSTRUCTIONAL ELEMENTS OF CYLINDRICAL ROTOR SYNCHRONOUS MACHINE

STATOR - yoke or frame- armature core- armature winding

ROTOR - solid rotor-field conductors or bars


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CONSTRUCTIONAL ELEMENTS OF CYLINDRICAL ROTOR SYNCHRONOUS MACHINESTATOR - yoke or frame

- stator core- stator winding

ROTOR - rotor core-rotor bars-end rings


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CONSTRUCTIONAL ELEMENTS OF SLIP RING INDUCTION MACHINE

STATOR - yoke or frame- stator core- stator winding

ROTOR - rotor core-rotor winding-slip rings


Space Factor

Space factor (Sf) is the ratio of bare conductor area to total slot area.

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CHOICE OF SPECIFIC ELECTRIC & MAGNETIC LOADING

SPECIFIC ELECTRIC LOADING (ac):Defined as the average flux density over the air-gap of the machine.

SPECIFIC MAGNETIC LOADING (Bav):Defined as the number of armature (or stator) ampere conductors per metre of armature (or stator) periphery at the air-gap.

DLp

gapairtheatpathfluxofAreagapairthearoundfluxTotalBav

DIzZ

gapairtheatperiheryArmatureconductorsamperearmatureTotalac


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FACTORS DECIDING THE CHOICE OF SPECIFIC MAGNETIC LOADING:

Maximum flux density in iron parts of the machine, Magnetizing current, and Core losses.FACTORS DECIDING THE CHOICE OF SPECIFIC

ELECTRIC LOADING:

Permissible temperature rise, Voltage rating of the machine, Size of the machine, and Current density.


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CHOICE OF SPECIFIC ELECTRIC LOADING(ac)

i) Temperature rise () depends on Q (losses), which in turn depends on ac. Allowable depends on insulating material used.

ii) Cooling coefficient (C) is also proportional to the cooling coefficient; a machine with a better ventilation has a lower C and then higher ac can

be used.iii) Operating voltage (V)

In high voltage machines, the slot space factor, Sf is less and so only smaller ac can be used. It also depends on the shape of the conductors, circular or rectangular in cross section.

iv) Current Density ()Choice of depends on cooling; higher C higher value in the choice of ac.


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THERMAL CONSIDERATIONS

MODES OF HEAT TRANSFER: Conduction. Convection. Radiation.

EQUATION OF HEAT FLOW FOR CONDUCTION :Qcon = 1- 2

Rwhere,

Qcon =heat dissipated by conduction, W; 1,2 = temperatures of two bounding surfaces ,C;R =thermal resistance of the conducting medium,

thermal ohm.

Natural

Artificial


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NEWTONS LAW OF COOLING:Q=Qrad + Qconv

=radS+ conv S Q = S watt

Where = rad + conv = specific heat dissipation or emissivity due to

radation plus convection.THERMAL RESISTANCE:

It is defined as the resistance which causes a drop of 1C per watt of heat flow.


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HEATING TIME CONSTANT: It is the index of time taken by the machine to attain its final

steady temperature rise.

COOLING TIME CONSTANT: It is defined as time taken by the machine for its temperature

rise to fall to 0.368 of its initial value.

RATING OF MACHINES: Rating of machines refers to the whole of the numerical values

of electrical and mechanical quantities with their duration andsequences assigned to the machines by the manufacturers and stated onthe rating plate, the machine complying with the specified dimensions.


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ENCLOSURES FOR ROTATING ELECTRICAL MACHINE

1. Open machine2. Open pedestal machine3. Open end- bracket machine4. Protected machine5. Screen protected machine6. Drip-proof machine7. Splash-proof machine8. Hose-proof machine9. Pipe-ventilated (or) duct-ventilated machine10. Totally enclosed machine


Heat flow

Heat flow in two dimensions:

Heat does not travel along parallel paths and the dissipating surfaces arenot homogenous.

In actual practice the heat flow is in different directions and the windingsand cores have insulation in addition to copper and iron respectively.

Thermal resistivity of built up windings and cores depends upon relativethickness of insulation to copper and iron.

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Consider a coil having a large axial length:

l-length of coil m,W-width of coil m,T- thickness of coil,m.py-thermal resistivity along aa, ohm mpx-thermal resistivity along bb, ohm mq- heat produced per unit volume.

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Thermal resistance of horizontal and vertical elementary stripsTwo thermal paths are in parallel.

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lxtwdx

CDABltdxw yy

4)(/

( ) 4

xx

wdx tdxtl AB CD lwx


Total thermal resistance of walls

Q be the total heat produced in the coil

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224 twlxtwdx

xy

yx

yx wt

twl

Q

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TEMPERATURE RISE

Q = Power loss (heat produced ), J/s or WG = weight of the active material of the Machine, kgh = specific heat, J/kg-C S = cooling surface area, m2 = specific heat dissipation, W/ m2 -Cc = 1/ = cooling coefficient, m2 -C / Wm = final steady temperature rise, C


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HEATING CURVE

m

0.0632

m

Th Time,t

Temp rise

= m(1-e-t/Th)


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COOLING CURVE

Time,t

0.368

i

Tc

Temp rise = ie-t/Tc


Rating of Machines

Rating of machines refers to the whole of the numerical values ofelectrical and mechanical quantities with their duration and sequence assigned to the machines by the manufactures and stated on the ratingplate, the machine complying with specified conditions.

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OVER MOTORING:(using a motor of higher rating than is required by load) leads to highercapital costs and increased losses because of lower efficiency atreduced load.

In a.c drives, motors working at reduced loads lead to poor powerfactor leading to uneconomic loading of supply circuits and apparatus.

LOAD DIAGRAM(or) TIME SEQUENCE GRAPHS:which show the variation of motor torque, power and load current asfunction of time.

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TYPES OF DUTIES AND RATINGS

The following are the types of duty as per IS:4722-1968 Specification for rotating electric machinery:

S1: Continuous duty S2: Short time duty S3:Intermittent periodic duty S4:Intermittent periodic duty with starting S5:Intermittent periodic duty with starting & braking S6:Continuous duty with intermittent periodic loading S7:Continuous duty with starting & braking S8:Continuous duty with periodic speed changes.


In the descriptions and diagrams for duty types S1 through S9 the following symbols are used:

P = power in kW Pv = losses in kW max = maximum temp. in C n = speed/min = temperature in C JM = moment of inertia of the

motor in kgm2

Jext = moment of inertia of the load referenced to the motor shaft in kgm2

tA = starting time in s, min tB = load period in s, min tBr = Braking time in s, min tS = cycle duration in seconds tSt = stop period in s, min, or h tr = relative duty cycle (%) t = time in s, min, or h T = thermal time constant in

minutes


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CONTINUOUS DUTY(DUTY TYPE S1 ):

Duration of load is for a sufficiently long time such that all parts of the motor attain thermal equilibrium.

(i.e) The motor will attain its maximum final steady state temperature rise.

E.g continuously running fans, pumps and other equipmentwhich operate for several hours.

The simplified load diagram is a horizontal straight line. Continuous rating may be defined as the load that may be

carried by the machine for an infinite time without thetemperature rise of any part exceeding the maximumpermissible value.


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SHORT TIME DUTY(DUTY TYPE S2 ):

The motor operates at a constant load for some specified time which is then followed by a period of rest.

Period of load is so short that the machine cannot reach its thermal equilibrium i.e steady state temperature rise.

Period of rest is so long that that the motor temperature drops to the ambient temperature.E.g. Railway turntable, navigation lock gates.

Short time rating of a motor may be defined as its output at which it may be operated for a specified time without exceeding the maximum permissible value of temperature rise.


S1: Continuous duty S1: Short time duty


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The period of operation is so short that the temperature rise of the motor does not reach its final steady state value.

And the period of rest is so long that the motor returns to cold conditions. Standard short time ratings are: 10,30,60 & 90 minutes

SHORT TIME RATING:Short time rating of a motor may be defined as its output at which it

may be operated for a certain specified time without exceeding themaximum permissible value of temperature rise.

INTERMITTENT RATING:It applies to an operating condition during which short time load

periods alternate with periods of rest or no load without the motorreaching the thermal equilibrium & without the maximum temperaturerising above the maximum permissible value .

DUTY FACTOR (or) LOAD FACTOR (or) CYLIC DURATIONFACTOR):

It is defined as the ratio of the heating (working) period to the periodof whole cycle.


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INTERMITTENT PERIODIC DUTY(DUTY TYPE S3 ): The period of constant load and machine de-energized alternate. The load periods are too short to allow the motor to reach its final

steady state value. Periods of rest are also too small to allow the motor to cool down to

ambient temperature. E.g Cranes , lifts and certain metal cutting machine tool drives.


S3: Intermittent periodic duty-type without starting


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INTERMITTENT PERIODIC DUTY WITH STARTING:Consists of sequence of identical duty cycles each consisting of aperiod of starting, a period of operation at constant load and a restperiod.

The operating period and rest period are too short to obtain thermalequilibrium during one duty.

The intermittent rating of the motor applies to an operating conditionduring which short time load periods alternate with periods of rest orno load without the motor reaching the thermal equilibrium andwithout the maximum temperature rising above the maximumpermissible value.


S4: Intermittent periodic duty with starting


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INTERMITTENT PERIODIC DUTY WITH STARTING &BRAKING:

Consists of a sequence of identical duty cycles each consisting of aperiod of starting, a period of operation at constant load, a period ofbraking and a rest period.

The operating period and rest period are too short to obtain thermalequilibrium during one duty cycle.


S5: Intermittent periodic duty with starting and electrical braking


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CONTINUOUS DUTY WITH INTERMITTENT PERIODIC DUTY: Consists of a sequence of identical duty cycles each consisting of a

period of operation at constant load and a period of operation at noload.

The machines with excited windings hence normal no load voltageexcitation during the load period.

The operation and no load voltage excitation during the load period are too short to attain thermal equilibrium during one duty cycle .


S6: Continuous-operation periodic duty


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CONTINUOUS DUTY WITH STARTING & BRAKING:

Consists of a sequence of identical duty cycles each having a period of starting, a period of operation at constant load and a period of electrical braking.

There is no rest or de-energised period.

The duty factor for this duty cycle is 1.


S7: Continuous-operation duty with starting and electrical braking


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CONTINUOUS DUTY WITH PERIODIC SPEED CHANGES:

Consists of a sequence of identical duty cycles each consisting of aperiod of operation at constant load corresponding to a predeterminedspeed of rotation, followed immediately by a period of operation atanother load corresponding to a different speed of operation.

The operating period is too short to attain thermal equilibrium duringone duty cycle, there being no rest and de=energized period.


S8: Continuous-operation periodic duty with relatedload/speed changes


Temperature rise

Q = Power loss (heat produced ), J/s or WG = weight of the active material of the Machine, kgh = specific heat, J/kg-C S = cooling surface area, m2 = specific heat dissipation, W/ m2 -Cc = 1/ = cooling coefficient, m2 -C / Wm = final steady temperature rise while heating , Cn = final steady temperature rise while cooling, Ci = initial temperature rise over ambient medium, CTh = heating time constantTc = Cooling time constant, St = time, S


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METHODS FOR DETERMINATION OF MOTOR RATINGS FOR VARIABLE LOAD DRIVES:

I. Method of average losses,II. Equivalent current method,III. Equivalent torque method, andIV. Equivalent power method.

METHODS OF MEASUREMENT OF TEMPERATURE RISE:i. Thermometer method.ii. Resistance method.iii. Embedded temperature detector method.


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COOLING OF ROTATING ELECTRICAL MACHINES:In most cases, the cooling electrical machines is carried out by air

flow and this cooling is called ventilation. In high speed machines such as turbo alternators, hydrogen is

used for cooling.ADVANTAGES OF HYDROGEN COOLING:

Compared with air, hydrogen has the following properties:-i) (1/14) th density thereby the windage losses and

noise reduced .ii) 14 times specific heat and 1.5 times heat

transfer leading to improved cooling.


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iii) 7 times thermal conductivity resulting in reducedtemperature gradient,

iv) reduced corona effect,v) will not support combustion so long as the

hydrogen/air mixture exceeds 3/1. In operation, the fans mounted on the rotor circulate

hydrogen through the ventilating ducts and internallymounted gas coolers.

The required gas pressure is maintained by a regulator. The precaution to be observed is the stator frame must be

gas tight and explosion proof and oil film gas seals at therotor shaft ends are essential.


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INDUCED AND FORCED VENTILATION:

In induced ventilation, the fan produces decreased air pressure inside ofthe machine, causing air to be sucked into the machine under theexternal atmospheric pressure .

In the forced ventilation, the air is forced into the fan by the fansmounted internally or externally.

The ventilation can also be classified as,i) Radial,ii) axial andiii) combined radial and axial.


Standard Specifications

Standard ratings of machines Types of enclosure Standard dimensions of conductors to be used Method of marking ratings and name plate details Performance specifications to be met Types of insulation and permissible temperature loss Permissible loss and range of efficiency Procedure for testing of machine parts and machines Auxiliary equipments

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Name Plate Details KW or KVA rating of machine Rated working voltage Operating speed Full load current Class of insulation Frame size Manufacturers name Serial number of the machine

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Standard Specifications

IS 325-1966 : Specifications for 3ph induction motor IS 4029-1967 : Guide for testing 3ph induction motor IS12615-1986 : Specifications for energy efficient

induction motor IS13555-1993 : Guide for selection & application of 3ph

induction motor for different types of driven equipment

IS8789-1996 : Values of performance characteristic for3ph induction motor

IS 12066-1986 : 3ph induction motors for machine tools

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Unit I introduction

Documents

Transcript of Unit I introduction