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7/28/2019 Dea Two Marks

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

TWO MARKS QUESTION AND ANSWERS

ELECTRICAL MACHINE DESIGN

VELALAR COLLEGE OF ENGINEERING AND TECHNOLOGY


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UNIT I

1. How the design problems of an electrical machine can be classified?

The design problems of an electrical machine are classified as follows1. Electromagnetic design2. Mechanical design3. Thermal design4. Dielectric design.

2. What is magnetic circuit?The magnetic circuit is the path of magnetic flux. The mmf of the circuit creates flux in the

path by overcoming the reluctance of the path. The equation which relates flux, mmf andreluctance is given by,

Flux = MMF

Reluctance

3. What are the constituents of magnetic circuit in rotating machines?The magnetic circuit of rotating machine consists of air-gap, stator and rotor cores, and

stator and rotor teeth.

4. What is loss curve?The loss curve is a graph showing the relation between iron loss and magnetic

field intensity, H. It is used to estimate the iron loss of the magnetic materials and itis supplied by the manufacturers of stampings or laminations.

5. What is leakage flux?The leakage flux is the flux passing through unwanted path. The leakage flux willnot help either for transfer or conversion of energy.

6. What is fringing flux?The bulging of magnetic path at the air-gap is called fringing. The fluxes in the bulged

portion are called fringing flux.

7. What are the factors which modify the reluctance of air-gap?The factor which modifythe reluctance of air-gap are (i) slots.(ii) radial ventilating ducts and

(iii) Non-uniform air-gaps.

8. Define total gap contraction factor, Kg. (AU)The total gap contraction factor, Kg is defined as the ratio of reluctance of slotted

armature with ducts and the reluctance of smooth armature without ducts.


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9. Define field form factor. (AU)The field form factor, Kfis defined as the ratio of average gap density over the

pole pitch to maximum flux density in the air-gap.

10. What is real and apparent flux density? (AU)

The real flux density is due to the actual flux through a tooth. The apparent fluxdensity is due to total flux pass through the tooth. Since some of the fluxpasses through slot the real flux density is always less than the apparent or total fluxdensity.

11. List the methods used for estimating the mmf for teeth. (AU)The following are the three methods used for estimating the mmf for teeth are

1. Graphical method2. Three ordinate method (simpsons rule)3. Bt1/3 method.

12. Define specific permeanceSpecific permeance is defined as the permeance per unit length (or) depth of field.

13. What is unbalanced magnetic pull?The unbalanced magnetic pull is the radial force acting on the rotor due to non

uniform air-gap around armature periphery.

14. List the various techniques employed to overcome unbalanced magnetic pull.The various techniques employed to overcome the unbalanced magnetic pull are

1. The length of the rotor is small and diameter is made higher.2. Ball bearings are employed and rotor is dynamically balanced.3. The combinations of rotor and stator slots which produce vibrations are

avoided.

15. What factor decides the number of turns in a winding?The factors deciding the number of turns in the winding are (i) the emf per turn and

flux density. (ii) the emf per turn depends on the type of insulation employed.

16. What is the effect of salient poles on the air-gap mmf?In salient pole machines the length of air-gap is not constant over the wholepole pitch. Hence the effective air-gap length is given by Kglg where Kg is thegap contraction factor. Also for calculating mmf, the maximum gap density Bgat the centre of the pole is considered instead of average gap density. MMFfor air-gap in salient pole machine = 800,000 BgKglg.

17. What are the problems encountered in estimating the mmf for teeth?The problems encountered in estimating the mmf for teeth are

1. The flux density in different section of a tooth is not uniform.2. The slot provides another parallel path for the flux, shunting the tooth.


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18. What are the factors which modify the reluctance of air-gap?The reluctance of air-gap is modified by slots, radial ventilating ducts and non-

uniform air-gaps.

19. Define leakage coefficient.The leakage coefficient is defined as the ratio of total flux to useful flux.Leakage coefficient Cl = Total flux

Useful flux

20. Define gap contraction factor for ducts.The gap contraction factor for ducts, Kgd isdefined as the ratio of reluctance of air-

gap with ducts to reluctance of air gap without ducts.

Kgd = Reluctance of air-gap with ductsReluctance of air-gap without ducts

21. Define gap contraction factor for slots.The gap contraction factor forslots, Kgs isdefined as the ratio of reluctance of air-gap

of slotted armature to reluctance of air-gap of smooth armature.

Kgs = Reluctance of air - gapof slotted armature

Reluctance of air-gap of smooth armature

23. State any two methods of cooling of turbo alternators. (AU)The two methods of cooling of turbo alternators are

(a)Air cooled(b)Direct cooled.

22. What is optimum design? (AU)The design is said to be optimum when the following are met.

(i) Min loss (or ) Max efficiency,(ii) Min cost,(iii) Min Volume,(iv) Min Weight.

23. Define stacking factor. (AU)Stacking factor is defined as the ratio of actual length of iron in stacks of

assembled core plates to total axial length of stack.


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24. What is rating?Rating of a motor is the power output or the designated operating power limit

based upon certain definite conditions assigned to it by the manufacturer.

25. Mention the different types of duties of a machine.The following are the types of duty as per IS: 4722 -1968(i) S1 = continuous duty(ii) S 2 = short time duty(iii) S3 = Intermittent periodic duty(iv) S4 = Intermittent periodic duty with starting(v) S5 =Intermittent periodic duty with starting and barking(vi) S6 = continuous duty with Intermittent periodic loading(vii) S7 = continuous duty with starting and barking(viii) S8 = continuous duty with periodic speed changes.

26. What is Carters gap co-efficient?The Carters gap co-efficient (Kcs) is the ratio of slot width to gap length. The

formula which gives value of Kcs directly is Kcs = 11+ 5lg/Ws

for parallel sided open slot is Kcs = 2/[ tan -1 Y 1/ log (1+/2)] where Y = Ws/2lg.

28. Write down the expression for calculation of reluctance of air gap with slotted armature.The expression for calculation of reluctance of air gap with slotted armature is

Sg = lg/ys L = lg /0 L (ys - Kcs Ws )


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UNIT -II

1. What is the range of specific magnetic loading in D.C. machine?

The usual range of specific magnetic loading in D.C. machine is 0.4 to 0.8 Wb/m2

.

2. What is the range of specific electric loading in D.C. machine?The usual range of specific electric loading in D.C. machine is 15000 to 50,000

amp.cond./m.

3.What are the factors to be considered for the selection of number of poles in D.C.machine?

The factors to be considered for the selection of number of poles in D.C. machineare frequency of flux reversals, current / brush arm and armature mmf per pole.

4. What are the parameters that are affected by the number of poles?The parameters that are affected by the number of poles are weight of iron andcopper, length of commutator and dimension of brushes are the parameters

affected by number of poles.

5. Why square pole is preferred?Square pole is preferred because the length of the mean turn of field winding is

minimum. Hence to reduce the copper requirement a square cross-section is preferredfor the poles of D.C. machine.

6. What is square pole and square pole face?In square pole, the width of the pole body is made equal to the length of thearmature. In square pole face, the pole arc is made equal to the length of the armature.

7. Define winding pitch.The winding pitch is defined as the distance between the starts of twoconsecutive coils measured in terms of coil sides.

8. What is back pitch?The distance between top and bottom coil sides of a coil measured around theback of the armature is called the back pitch. The back pitch is measured in terms ofcoil sides.

9. What is front pitch?The front pitch is the distance between two coil side connected to the samecommutator segment. It is measured in terms of coil sides.


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10. Define commutator pitch.The commutator pitch is defined as the distance between the two commutatorsegments to which the two ends (start and finish) of a coil are connected.It is measured in terms of commutator segment.

11. What is equalizer connection?The equalizer connections are low resistance copper conductors employed in lapwinding to equalize the induced emf in parallel paths. The difference in induced emfin parallel paths is due to slight unsymmetrical in flux per pole in dc machines.

12. What is the need for brushes in D.C. machine?The brushes are used in D.C. machines to collect or draw current from the rotatingarmature.

13. What are the effects of armature reaction?

The various effects of armature reaction are reduction in induced emf, increase iniron losses, delayed communication, sparking and ring firing.

14. How the polarities of interpole are decided?The polarities of the interpole must be that of the main pole just ahead (in thedirection of rotation) for a generator and just behind (in the direction of rotation) for amotor.

15.What are standard periods for short time rating of a machine?The standard periods for short time ratings are 10, 30,60 and 90mts.

16. State different losses in a D.C. generator.The different losses in a D.C. generator are(a)Copper losses

(i)Armature copper loss(ii) Field copper loss

(b) Magnetic losses (iron or core losses)(i) Hystersis loss (ii) Eddy current loss (iii) Mechanical loss

17. State the relationship between number of armature coils and number ofcommutator segments in a D.C. machine.The relationship between number of armature coils and number ofcommutator segments in a D.C. machine isc = D c

CWhere c = Commutator segment pitch

C = Number of coilsDc = Diameter of commutator


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18. Write the expressions for output coefficients of D.C machine.The expressions for output coefficients of D.C machine is

Co = 2 Bav q x10-3 kW/m3 rps

UNIT - III

1. What are the different losses in a transformer?

The different losses in a transformer are

(a)Core or Iron loss(b)Copper loss.

2. State merits of 3 transformers over 1 transformers.The merits of 3 transformers over 1 transformers.

(i) Less weight(ii) Less cost.

3. What is the range of efficiency of a transformer?The range efficiency of a transformer will be in the range of 94% to 99%.

4. What is transformer bank?A transformer bank consists of three independent single phase transformers with

their primary and secondary windings connected either in star or in delta.

5. Define window space factor (AU )The window space factor is defined as the ratio of copper area in window to total area of

window.

6. What are the factors to be considered for selecting the cooling method of a transformer?The factors to be considered for selecting the cooling method of a transformer (i)KVA

rating of transformer,(ii) size, (iii)application and (iv) the site condition where it has to beinstalled.


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7. How the heat dissipates in a transformer?The heat dissipation in a transformer occurs by (i) Conduction, (ii) Convection and

(iii) Radiation.

8. Why cooling tubes are provided?

Cooling tubes are provided to increase heat dissipating area of the tank.

9. What are the various types of transformers?The various types of transformers based on construction are

(i) Core type(ii) Shell type.

10. Mention the main function of cooling medium used in transformers.The main functions of cooling medium used in transformers are

(i)

Transfer the heat by convection from the heated surface to the tank surface.(ii) Create good level of insulation between various conducting parts.

11. How much heat is dissipated by radiation and convection?The heat dissipated by radiation and convection are

(i) 6 watt/m2/C by radiation .(ii) 6.5 watt/m2/C by convection .

12. Why stepped core are generally used for transformer?The stepped core are generally used for transformer because of the following reason LV

and HV coils are circular, for better utilization of space, for reducing the mean length of LVand HV turns and resulting in saving of copper material.

13. What are the important properties of transformer steel?The important properties of transformer steel are (i) High permeability, (ii) high resistivity,

and (iii) low coercive force.

14. Why is the area of yoke of a transformer usually kept 15 20% more than that of core?The area of yoke of a transformer usually kept 15 20% more than that of core so that the

flux density in the yoke is reduced, resulting into reduction in iron losses for yoke.

15. What is the cause of noise in transformer?The cause of noise in the transformer is mainly because of magnetostriction effect and alsoloosening of stamping and mechanical forces produced during working.


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16. What are the advantages of using higher flux density in the core?The advantages of using higher flux density in the core are (i) reduction in the

core and yoke section for same output,(ii) reduction mean length of LV and HV turns,

resulting in saving of copper material,(iii) reduced overall size and weight oftransformer.

17. List various disadvantages of using higher flux density in design of core.The disadvantages of using higher flux density in design of core (i) Increased

magnetizing current and iron losses, saturation of magnetic material, lower efficiency,because of higher no-load losses,(ii) higher temperature rise of transformer.

18. Why the cross-section of yoke is taken greater than core section?The cross-section of yoke is taken greater than core sectionin order to reduce flux

density in the yoke, thereby reducing iron losses and no-load current in the yokesection.

19. What are the types of winding, commonly used for LV winding?The types of winding, commonly used for LV windingare (i) Cylindrical winding

with rectangular conductors and (ii) helical winding.

20. What are the drawbacks of sandwich winding?The drawbacks of sandwich winding are (i) Requires more labour in its

maintenance,(ii) more difficult to insulate different coils from each other and from yoke.

23. Define Stacking factorStacking factor is defined as the ratio of net iron area to the gross iron area. It is

denoted by Ki.

24. Define iron space factorIron space factor is defined as the ratio of gross core area to the area of

circumscribing circle. It is denoted by Kis.

25. What is leg Spacing?The leg spacing is the distance between the centers of two adjacent limbs. It is

denoted by D.


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UNIT - IV

1. What are the main dimensions of induction motor?The main dimensions of induction motor are stator core internal diameter andstator core length.

2. Why wound rotor construction is adopted?The wound rotor has the facility of increasing the rotor resistance through slip-

rings. Higher values of rotor resistance are needed during starting to get a high valueof starting torque.

3. 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 isequivalent to short circuited secondary of a transformer. In transformer thesecondary is fixed but in induction motor it is allowed to rotate.

4. What type of starter cannot be used for squirrel-cage motors?The starter which cannot be used for squirrel cage motor is rotor resistance

starter.

5. What are the materials used for slip-rings and brushes in induction motor?The slip-rings are made of brass on phosphor bronze. The brushes are made of

metal graphite which is an alloy of copper and carbon.

6. What are the factors to be considered for the choice of specific magneticloading?

The choice of specific magnetic loading depends on power factor, iron loss andover load capacity.

7. What are the factors to be considered for the choice of specific electric loading?The choice of specific electric loading depends on copper loss, temperature rise,

voltage rating and over load capacity.

8. How the induction motor can be designed for best power factor?The induction motor can be designed for best power factor by choosing

pole pitch, = 0.18L.

where = pole pitchL = length of the armature

9. What are the different types of stator windings in induction motor?The different types of stator windings are

(i) mush winding, (ii) lap winding and (iii) wave winding.


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10. What is full pitch and short pitch or chorded?

When the coil span is equal to pole pitch (180e). the winding is called full

pitched winding. When the coil span is less than the pole pitch (180e). thewinding is called short pitched or chorded.

11. Why short chorded windings are employed in induction motor?For short chorded windings the length of mean tum will be lesser than the fullpitch coils. Hence it results in reduction of copper. Also the short chordedwindings eliminates certain harmonic magnetic fields.

12. What type of slots are preferred in induction motor?Semi enclosed slots are preferred for induction motor. It results in less air gapcontraction factor giving a small value of magnetizing currents, low tooth pulsation

loss and much quieter operation (less noise).

13. What is slot space factor?The slot space factor is the ratio of conductor (or copper) area per slot and slotarea. It gives an indication of the space occupied by the conductors and the spaceavailable for insulation. The slot space factor for induction motor varies for 0.25 to0.4.

14. Write the expression for length of mean turn of stator winding.

The expression for length of mean turn of stator, Lmts = 2L + 2.3 + 0.24

15. Which part of induction motor has maximum flux density? What is themaximum value of flux density in that part?The teeth of the stator and rotor core will have maximum flux density.The maximum value of flux density in the teeth is 1.7 Sb/m2.

16.What are the factors to be considered for estimating the length of air-gap ininduction motor?

The following factors are to be considered for estimating the length of air-gap.1. Power factor 2. Overload capacity 3. Pulsation loss4. Unbalanced magnetic pull 5. Cooling 6. Noise

17. List the undesirable effects produced by certain combination of rotor andstator slots.The following problems may develop in induction motor with certain

combination of rotor and stator slots.1. The motor may refuse to start, (cogging)2. The motor may run at sub synchronous speed (crawling)3. Severe vibration may develop and the noise will be excessive.


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18. What is crawling and cogging?Crawling is a phenomena in which the induction motor runs at a speed lesser

than sub synchronous speed.

Cogging is a phenomena in which the induction motor refuses to start.

19. Explain the phenomena of cogging.When the number of stator and rotor slots are equal, the speeds of all theharmonics produced by stator slotting coincide with the speed of corresponding,rotor harmonics. This harmonics of every order would try to exert synchronoustorques at their corresponding synchronous speeds and the machine would refuse tostart This is known as cogging.

20. What are the methods adopted to reduce harmonic torques?The methods used for reduction or elimination of harmonic torques are chording,

integral slot winding, skewing and increasing the length of air-gap.

21. What are the different types of windings used for rotor of induction motor?The different types of windings employed in induction motor rotor are mushwinding the double layer bar type winding.


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UNIT V

1. Name the two types of synchronous machines.Based on construction the synchronous machines may be classified as,1. Salient pole machines. 2. Cylindrical rotor machines.

2. What is runaway speed?The runaway speed is defined as the speed which the prime mover would have,

if it is suddenly unloaded, when working at its rated load.

2. What are the factors to be considered for the choice of specific magneticloading?The factors to be considered for the choice of specific magnetic loading are1. Iron loss 2. Temperature rise 3. Voltage rating

4. Synchronous reactance 5. Stray load losses.

4. What are the two types of poles used in salient pole machines?The two types of poles used in salient pole machines are Round poles and

Rectangular poles.

5. List the factors to be considered for the choice of number of slots .The factors to be considered for the choice of number of slots are1. Balanced winding 2. Cost 3. Host spot temperature in winging 4. Leakagereactance 5. Tooth losses 6. Tooth flux density.

6. What is the limiting factor for the diameter of synchronous machine?The limiting factor for the diameter of synchronous machine is the peripheralspeed. The limiting value of peripheral speed is 175 m/sec for cylindrical rotormachines and 80m/sec for salient pole machines.

7. Write the expression for air-gap length in cylindrical rotor machine.The expression for air-gap length in cylindrical rotor machine is

Length of air-gap, lg = 0.5 (SCR)(ac) Kfx 10-6

KgBav

8. 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 byskewing. In order to eliminate the effect of any harmonic, the rotor bars should beskewed through an angle so that the bars lie under alternate harmonic poles of thesame polarity or in other words the bars much be skewed through two pitches.


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9. What are the advantages and disadvantages of large air-gap length in inductionmotor?The advantages and disadvantages of large air-gap length in inductionMotor are

Advantage

A large air-gap length results in higher overload capacity, better coolingreduction in noise and reduction in unbalanced magnetic pull.Disadvantage

The disadvantage of large air-gap length is that it result in high value ofmagnetizing current.

10. What are the different types of induction motor? How they differ from eachother?

The two different types of induction motor are squirrel cage and slip ring type.The stator is identical for both types but they differ in the construction of rotor.The squirrel cage rotor has copper or aluminum bars mounted on rotor slots and

short circuited at both ends by end rings.The slip ring rotor carries a three phase winding. One end of each phase isconnected to a slip ring and other ends are star connected.

11. 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 resistances are connected to slip-rings. While starting the full resistance isincluded in the rotor circuit to get high starting torque. Once the rooterstarts rotating, the resistances are gradually reduced in steps. At runningcondition the slip-rings are shorted and so it is equivalent to squirrel cage rotor.

12. What are the ranges of specific magnetic loading and electric loading ininduction motor?

The ranges of specific magnetic loading and electric loading ininduction motor are

Specific magnetic loading = 0.3 to 0.6 Wb/m2Specific electric loading = 5000 to 4500 amp. Cond/m.


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13. What is integral slot winding and fractional slot winding?In integral slot winding the total number of slots is chosen such that the slots per

pole is an integer. The integer should also be a multiple of number of phases.In fractional slot winding the total number of slots is chosen such that the slots perpole is not an integer.

14. What type of connection is preferred for stator of induction motor?Under running condition the stator of induction motor is normally connected in

delta. (In delta connection the torque developed will be higher than the starconnection). But for reducing the starting current, the stator can be connected instar while starting and then changed to delta.

15.What is Short Circuit Ratio (SCR)?The Short Circuit Ratio (SCR) is defined as the ratio of field current required to

produce rated voltage on open circuit to field current required to circulate ratedcurrent 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 polealternator SCR varies from 1.0 to 1.5.

16. What is magnetization curve?The magnetization 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 toestimate the mmf required for flux path in the magnetic material and it is suppliedby the manufacturer of stampings or laminations.

17. What is leakage flux?The leakage flux is the flux passing through unwanted path. The leakage flux

will not help either for transfer or conversion of energy.

18. What is leakage coefficient?The leakage coefficient is defined as the ratio of total flux to useful flux.

Leakage coefficient, Cl = Total fluxUseful flux

19. What are the differences between leakage flux and fringing flux?The leakage flux is not useful for energy transfer or conversion. But the

fringing flux is useful flux.The leakage flux flows in the unwanted path. But the fringing flux flows in the

magnetic path.The effect of leakage flux on machine performance is accounted by leakage

reactance. The fringing flux increases the slot reactance.


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.20. How the tertiary winding is connected? Why?

The tertiary winding is normally connected in delta. When the tertiary isconnected in delta, the unbalance in phase voltage during unsymmetrical faults inprimary or secondary is compensated by the circulating currents flowing in the

closed delta.

21. How the heat dissipation is improved by the provision of cooling tubes.The cooling tubes will improve the circulation of oil. The circulation of oil isdue to more effective pressure heads produced by columns of oil in tubes. The .improvement in cooling is accounted by taking the specific heat dissipation due toconvection as 30% more than that without tubes.

22. Why wound rotor construction is adopted?The wound rotor has the facility of increasing the rotor resistance through slip-rings. Higher values of rotor resistance are needed during starting to get a high value

of starting torque.

23. What type of connection is preferred for stator of induction motor?Under running condition the stator of induction motor is normally connected indelta. (In delta connection the torque developed will be higher than the starconnection). But for reducing the starting current, the stator can be connected in starwhile starting and then changed to delta.

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