Download File - ganesh subramanian

PAGE: 1
E.G.S.PILLAY ENGINEERING COLLEGE, NAGAPATTINAM
DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING
EE2355- DESIGN OF ELECTRICAL MACHINES
UNIVERSITY EXPECTED TWO MARKS
UNIT 1
1) List Out The Construction Elements Of
Synchronous Machine, Squirrel Cage Induction Motor?
Dc
Machine,
Salient
Pole
(i) Dc Machine
Stator: Yoke, Field Pole, Pole Show, Field Winding, Interpol
Rotor: Armature Core, Armature Winding
(ii) Salient pole Synchronous Machine
Stator: Yoke, Armature Core, Armature Winding
Rotor: Field Pole, Pole Show, Field Winding, Damping Winding
(iii) Squirrel Cage Induction Motor
Stator: Yoke, Stator Core, Stator Winding
Rotor: Rotor Core, Rotor Bars, End Rings
2) Write A Short Note On Standard Specifications?
The Standard Specifications Are The Specifications Issued By The Standards
Organization Of A Country. The Standard Specifications Serve As Guideline For The
Manufactures To Produce Quality Products At Economical Prices. The Standard Specifications
For Electrical Machines Include Ratings, Types Of Enclosure, Dimensions Of Conductors,
Name Plate Details, Performance Indices, Permissible Temperature Rise, Permissible
Loss, Efficiency, Etc.,
3) List The Indian Standard Specifications For Transformer?
*IS 1180-1989: Specifications For Out Door 3-Ø Distribution Transformer Up to 100 kVA.
*IS 2026-1972: Specifications Of Power Transformers.
4) List The Indian Standard Specifications For Induction Motor?
*IS 325-1966 : Specifications For 3-Phase Induction Motor.
*IS 1231-1974: Specifications For Foot Mounted Induction Motor.
*IS 4029-1967: Guide For Testing 3-Phase Induction Motor.
*IS 996-1979: Specifications For 1-Phase Ac And Universal Motor.
5) What Is Meant By General Design Procedure?
The General Design Procedure Is To Relate The Main Dimensions Of The Machine To Its
Rated Power Output. A General Output Equation Can Be Developed For Electrical Machines
Which Relates The Power Output To Volume Of Active Part (D2L), Speed, Magnetic And Electric
Loadings.
6) Define Total Magnetic Loading & Electric Loading?
The Total Magnetic Loading Is Defined As the Total Flux Around The Armature (Or
Stator Inner) Periphery At The Air-Gap.
Total Magnetic Loading= P
Where,
P=No Of Poles; =Flux per Pole
The Total Electric Loading Is Defined As The Total Number Of Ampere Conductors
Around The Armature (Or Stator) Periphery.
Total Electric Loading = IZZ
Where,
IZ=Current Through One Armature Conductor
Z=Total Number of Armature Conductor
PAGE: 2
7) Define Specific Magnetic & Electric Loading?
The Specific Magnetic Loading Is Defined As The Average Flux Density Over The AirGap Of A Machine.
Specific Magnetic Loading = Total Flux around the Air-Gap/Area Of Flux Path At The Air-Gap
Bav=P/Dl
The Specific Electric Loading Is Defined As the Number Of Armature (Or Stator)
Ampere Conductors Per Meter Of Armature (Or Stator) Periphery At The Air-Gap.
Specific Electric Loading=Total Armature Ampere Conductors/Armature Periphery at The AirGap
Ac= IZZ /D
8) What Is Output Equation?
The Equation Which Relates the Power Output to the Main Dimensions (D&L), Specific
Loadings (Bav & Ac) And Speed (N) Of A machine Is Known as Output Equation.
The O/P Equation Of Dc Machine Is, Pa=C0 D2l N, In Kw
The O/P Equation of Ac Machine Is, Q=C0d2l Ns, In Kva
Where,
Pa=Power Developed In Armature of Dc Machine
Q=kVA Rating Of Ac Machine , C0=Co-Efficient.
9) What Is Peripheral Speed? Write The Expression For Peripheral Speed Of A Rotating
Machine?
The Peripheral Speed Is A Translational Speed That May Exist At The Surface Of The
Rotor, While It Is Rotating. (It Is Translational Speed Equivalent To The Angular Speed At The
Surface Of The Rotor).
Peripheral Speed, Va=Dr N In M/Sec
Where,
Dr=Diameter of Rotor In M , N= Speed Of the Rotor In Rps.
UNIT 2
1. What is a magnetic circuit?
The magnetic circuit is the part of magnetic flux. the mmf circuit creates flux in
the path against the reluctance of the path. the equation which relates flux ,mmf and reluctance
is given by, Flux=mmf/reluctance
2. Write any two seminaries between magnetic circuits electric circuits?
In electric circuits the emf circulates current in a closed path .similarly in a magnetic
circuit the mmf creates flux in a closed path.
In electric circuit the flow of current is opposed by resistance of the circuit. Similarly in a
magnetic circuit creation of flux is opposed by reluctance of the circuit.
3. What is magnetizing curve?
The magnetizing curve is a graph showing the relation between the magnetic
field intensity ,H and the flux density ,B of a magnetic material.It is used to estimate the mmf
required for flux path in a magnetic material and it is supplied by the manufactures of
stampings or laminations.
4. What is loss curve?
The loss curve is a graph showing the relation between iron loss and magnetic field
intensity, H.
PAGE: 3
It is used to estimate the iron loss of the magnetic material and it is supplied by the
manufactures of stampings or laminations.
5. How the mmf of a circuit of a magnetic circuit is determined?
The magnetic circuit is split into convenient parts which may be connected in series or
parallel .
Then the reluctance, flux density and mmf for every sections of the magnetic circuits is
estimated. the summation of mmf of all sections in series gives the total mmf for the magnetic
circuits.
6. Define gap contraction factor for slots ducts?
SLOTS: the gap contraction factor for slots,kgs is defined as the ratio of reluctance of air
gap in machines with slotted armature to reluctance of air gap in mach ines with smooth
armature .
Kgs=reluctance of air gap in machines with slotted armature/ reluctance of air gap in
machines with smooth armature.
DUCTS: It is defined as kgd,as the ratio of reluctance of air gap in machines with ducts to
reluctance of air gap in machines without ducts.
Kgd=reluctance of air gap in machines with ducts/reluctance of air gap in machines
without ducts
7. Define total gap construction factor kg?
The total gap construction factor kg is defined as the ratio of reluctance of air gap in
machine with slotted armature & ducts to the reluctance of air gap in machines with smoth
armature &with out ducts the total gap contraction factor is equal to the product of gap
constraction factor for slots and ducts
8. What is carter co efficient? what is its usefulness in the design of dc machine?
The carter’s coefficient is a parameter that can be used to estimate the contracted or
effective slot pitch in case of armature with open or semi enclosed slots. it is a function of the
ratio of wo/lg,where wois slot opening & lg is air gap length
The carter’s coefficient is also used to estimate the effective length of armature when
ducts are employed.in this case it will be a function of wo/lg where wd is the width of duct
In electrical machine the carter’s coefficient is used to estimate the air gap expension
factor for slots and ducts the increase in reluctance of air gap due to slotting & ducts in
accounted as additional air gap, which in turn in given by air gap expansion factor
9.what is slot leakage flux, tooth tap leakage flux ,zig -zag flux, overhang leakage flux,
harmonic flux, skew leakage flux, peripheral leakage flux?
SLOT LEAKAGE FLUX: The fluxes the crosses the slot from one tooth to the next and
returning through iron.
TOOTH TAP: The flux flowing from top of one tooth to the top of another tooth.
ZIG-ZAG: The flux passing from one tooth to another in a zig-zag fashion across the air
gap.
OVERGANG: The fluxes produce by overhang portion of the armature winding .
HAR MONIC: The fluxes produced due to the difference in stator and rotor harmonic
contents.
SKEW: The reduction in mutual flux due to skewing of rotor in induction motor.
PAGE: 4
PERIPHERAL: The fluxes flowing circumferentially round the air gap without linking
with any of the windings.
Unit-3
1. State the factors which limits armature diameter of a DC machine ?
 peripheral speed
 induced emf per conductor
 pole pitch
 power output
 specific magnetic loading
2. List the advantages &dis advantages of large number of poles?
 Weight of armature core
 overall length& diameter of
&yoke
machine
 Cost of armature & filed
 length of commentator
conductors
 distortion of field from
under load condition
3. Define:
winding pitch
 The winding pitch is defined as the distance b/w the starts of two consecutive
coils measured in terms of coil sides.
back pitch
 The distance b/w top and bottom coil sides of a coil measured around the back
of the armature is called the back pitch. The back pitch is measured in terms of
coil sides.
front pitch
 front pitch is the distance b/w two coil sides connected to the same
communtator segment. it is measured in terms of coil sides.
commutator pitch
 The commutator pitch is defined as the distance b/w the two commutator
segments to which the two ends (start&finish) of a coil are connected. It is
measured in terms of commutator segment.
4. State the uses of yoke in a DC machine ?
The yoke serve as a path for flux in dc machine and it also serve as an
enclosure for the machine.
Unit-4
1. What are the various types of transformers?
The transformers can be classified based on construction, applications,
frequency range, number of windings and type of connection
 Based on construction
Core type
Shell type
 Based on applications:
Distribution transformers
Power transformers
Special transformers
Instrument transformers
Electronic transformers
 Based on frequency range
Power frequency transformers
Audio frequency transformers
UHF transformers
Wide band transformers
Narrow band transformers
Pulse transformers
 Based on number of windings
Auto transformers
Two winding transformers
 Based on type of connection
Single phase transformers
Three
phase
transformers
2. What are salient features of distribution transformers ?
 The distribution transformers will have low iron loss and higher value of copper
loss.
 The capacity of transformers will be up to 500 KVA.
 The leakage reactance and regulation will be low.
 The transformers will have plain walled tanks or provided with cooling tubes or
radiators.
PAGE: 5
3. Mention the uses
transformers?
of
distribution
transformers,
what
are
distribution
The distribution transformers are used at load centres to step down the
transmission line voltage to a standard service voltage required for consumer.
The transformers used at load centres to step down distribution voltage to
a standard service voltage required for consumer are called distribution transformers.
4. In transformer, why the low voltage winding is placed near the core?
The winding &core are both made of metals and so insulation have to be
placed in b/w them. The thickness of insulation depends on the voltage rating of the
winding. In order to reduce the insulation required the low voltage winding is placed near
the core.
5. What is window space factor?
The window space factor is defined as the ratio of copper area in window
to total area of window.
6.write down the output equation of 1&3 phase transformer?
The equation which relates the rated kva output of the transformer to core &window area is
called output equation
Output equation of single phase transformer ,=2.22fBmAiAw$*10-3
Output equation of the three phase transformer=3.33f BmAiAw$*10-3
7.what do you meant by stacking factor (or) iron space factor?what is usual value?
In transformer the core is made of laminations and the laminations are insulated from
each other by a thin coating of varnish .Hence when the laminations are stacked to form the core
the actual iron area will be less than the core area . The ratio of iron area and total core area is
called stacking factor
Stacking factor,Sf=Area of cross-section of iron in the core/ Area of cross-section of the
core including the insulation area. The usual value of stacking factor is 0.9
8. Why stepped cores are used in transformers?
When stepped core are used the diameter of the circumscribing circle is
minimum for a given area of a core. This helps in reducing the length of mean turn of the
winding with consequent reduction in both cost of copper & copper loss.
9. How to design the winding of a transformer?
The winding design consists of estimation of number of turns and area of cross-section
of winding conductors.uslly the number of turns in low voltage winding is estimated by
assuming emf per turn and the turns of high voltage winding are estimated from the voltage
ratio
Number of turns in low voltage winding=rated voltage of low voltage winding/Emf per
turn
Number of turns in high voltage winding=(no. of turns in lv winding)*rated voltage of hv
winding/rated voltage of lv winding
PAGE: 6
The area of cross section of the winding conductors are determined by assuming a suitable
current density
Area of cross section of winding conductor=Rated current of the winding/current
density
10.List the different method of cooling of transformers?
The different types of transformer are,




Air natural
Air blast
Oil natural air forced
Oil natural water
forced




Oil forced
Oil forced air natural
Oil forced air forced
Oil forced water
forced
Unit-5
1. What is rotating transformer?
The principle of operation of induction motor is similar to that a transformer. The stator
winding is equivalent to primary of a transformer and the rotor winding is equivalent to short
circuited secondary of a transformer. In transformer the secondary is fixed but in induction
motor it is allowed to rotate. Hence the induction motor is also called rotating transformer.
2. How the slip-ring motor is started?
The slip-ring motor is started by using rotor resistance starter. The starter consists of
star connected variable resistances and protection circuits. The resistance is connected to sliprings. While starting the full resistance Is included in the rotor circuit to get high starting torque.
Once the rotor starts rotating, the resistance are gradually reduced in steps.at running condition
the slip rings are shorted and so it is equivalent to squirrel cage rotor.
3. What are the special features of the cage rotor of induction machine?




The cage rotor can adopt itself for any number of phases and poles
It is suitable for any type of starting method except using rotor resistance starter
It is cheaper and rugged
Rotor overhang leakage reactance is lesser which results in better power factor,
greater pull out torque and overload capacity.
4. What is the advantage of cage induction motor over slip ring induction motor?
 It is cheaper than slip ring induction motor.
 It does not have any wear and tear parts like slip rings, brush gear and short
circuiting devices. Hence the construction will be rugged.
 The rotor slots can be fully occupied by the conductor due to absence of insulation.
Hence the rotor bars may have low resistance. Also there is no overhang (or small
overhang) in rotor winding. Due to these two factors the rotor copper loss will be
lesser than slip ring motors and so efficiency will be slightly higher.
 Due to smaller overhang leakage reactance, the motor will have better power factor,
a greater pull out torque and over load capacity.
5. Write the expression for output equation and output coefficient of induction motor?
The input KVA,Q=Co D^2 L ns in kva
Output coefficient, Co=11 Kws Bav ac * 10^-3 in KVA/ m-3-rps
PAGE: 7
6.What are the ranges of efficiency and power factor in induction motor?
Squirrel cage motors
Slip ring motor
Efficiency= 0.72 to 0.91
Efficiency= 0.84 to 0.91
Power factor = 0.66 to 0.9
Power factor = 0.7 to 0.92
The ISI specifications say that the product of efficiency and power factor shall be in the
range of 0.83 to 0.88.
7. What is full pitch and short pitch or chording?
When the coil span is equal to pole pitch (180 e). the winding is called full pitched
winding.
oil span is less than the pole pitch(180 e). The winding is called short pitched or
chorded.
8. What is slot space factor?
The slot space factor is the ratio of conductor (or copper) area per slot and slot area. It
gives an indication of the space occupied by the conductors and the space available for
insulation. The slot space factor for induction motor varies from 0.25 to 0.4.
9. List out the methods to improve the power factor of induction motor?
The power factor of the induction motor can be improved by reducing the magnetizing
current and leakage reactance.
The magnetizing current can be reduced by reducing the length of air-gap. The leakage
reactance can be reduced by reducing the depth of stator & rotor slots, by providing short
chorded winding and reducing the overhang in stator winding.
10. Why the air-gap of an induction motor is made as small as possible?
The mmf and magnetizing current are primarily decided by the length of air-gap. If airgap is small then mmf and magnetizing current will be low, which in turn increase the value
of power factor. Hence by keeping small air-gap, high power factor is achieved.
11.What Is Harmonic Induction Torque And Harmonic Synchronous Torque?
Harmonic Induction Torque Are Torques Produced By Harmonics Field Due To Stator
Winding And Slots.
Harmonic Synchronous Torque Are Torques Produced Bycombined Effect Of Same
Order Of Stator And Rotor Harmonic Fields.
Due To Both The Harmonic Torquesthe Machine May Crawl,But Ther Will Be Difference
In The Crawling Speeds.In Case Of Harmonic Induction Torque Crawling Speed Is Slightly
Lesser Than The Subsynchronous Speed.But In Case Of Harmonic Synchronous Torque The
Crawling Speed Is Same As That Of Subsynchronous Speed And This Is Also Called
Synchronous Cusps
12.What Is Crawling And Cogging?
Crawling Is A Phenomena In Which The Induction Motor Runs At A Speed Lesser Than
Subsynchronous Speed.
Cogging Is A Phenomena In Which The Induction Motor Refuse To Start.
PAGE: 8
13.What Is Skewing?
Skewing Is Twisting Either The Stator Or Rotor Core.The Motor
Noise,Vibrations,Cogging And Synchronous Cusps Can Be Reduced Or Even Entirely Eliminated
By Skewing.
In Order To Eliminatethe Effect Of Any Harmonic, The Rotor Bars Should Be Skewed
Through An Angle So That The Bars Lie Under Alternate Harmonic Poles Of The Same Polarity
Or In Other Words The Bars Must Be Skewed Through Two Pitches.
14.Define Dispersion Coefficient?
The Dispersion Coefficient Is Defined As The Ratio Of Magnetizing Current To Ideal
Short Circuit Current. Dispersion Coefficient , б =Im/Isci ; Where, Im= Magnetizing Current
; Isci = Ideal Short Circuit Current
15.What Is Runaway Speed?
The Runaway Speed Is Defined As The Speed Which The Prime Mover Would Have, If It
Is Suddenly Un Liaded, When Working At Its Rated Load.
16.Define Pitch Factor And Distribution Factor.
The Pitch Factor Is Defined As The Ratio Of Vector Sum Of Emf Induced In A Coil To
Arithmetic Sum Of Emf Induced In The Coil
Pitch Factor,Kc= Vector Sum Of Emf Induced In A Coil / Arithmetic Sum Of Emf Induced
In The Coil
The Distribution Factor Is Defined As The Ratio Of Vector Sum To Arithmetic Sum Of Emf
Induced In The Conductors Of One Phase Spread.
Distribution Factor,Kd = Vector Sum Of Emf Induced In The Conductors Ofa Phase Under A
Pole/ Arithmetic Sum Of Emf Induced In The Conductors Of A Phase Under A Pole.
17. What Is Short Circuit Ratio (Scr)?
The Short Circuit Ratio Is Defined As The Ratio Of Field Current Required To Produce
Rated Voltage On Open Circuit To Field Current Required To Circulate Rated Current At Short
Circuit.
It Is Also Given By The Reciprocal Of Synchronous Reactance,Xd In P.U(Per Unit). For
Turbo Alternators Scr Is Normally Between 0.5 To 0.7. For Salient Pole Alternator Scr Varies
From 1.0 To 1.5.
18. Mention The Factors That Govern The Design Of Field System Of Alternator.
The Following Are The Factors To Be Considered For The Design Of Field System In Alternator.





Number Of Poles And Voltage Across Each Field Coil
Amp-Turn Per Pole(Or Mmf Per Pole)
Copper Loss In Field Coil
Dissipating Surface Of Field Coil
Specific Loss Dissipation And Allowable Temperature rise