Electrical Apparatus

7/29/2019 Electrical Apparatus

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NOORUL ISLAAM COLLEGE OF ENGINEERINGKUMARACOIL

ELECTRICAL AND ELECTRONICS DEPARTMENT

DESIGN OF ELECTRICAL APPARATUSSHORT QUESTIONS AND ANSWERS

1. What are the constructional elements of a transformer?The constructional elements of a transformer are core, high and low

voltage windings, cooling tubes or radiators and tank.2. List the constructional elements of a dc machine.

The major constructional elements of a dc machine are stator, rotor,brushes and brush holders. The various parts of stator and rotor are listedbelow,

Stator- Yoke (or) Frame Rotor - Armature core- Field pole - Armature winding- Pole shoe - Commutator- Field winding- Interpole

3. List the constructional elements of salient pole synchronous machine.The various constructional elements of salient pole synchronous machineare,Stator - Frame Rotar - Field pole

- Armature core - Pole shoe- Armature winding - Field winding

- Damper winding4. What are the constructional elements of cylindrical rotor synchronousmachine?

The constructional elements of cylindrical rotor synchronousmachine are,Stator - Frame Rotor - Solid rotor

- Armature core - Field conductors- Armature winding (or) bars

5. List the constructional elements of squirrel cage induction motor.The constructional elements of squirrel cage induction motor are,

Stator - Frame Rotor - Rotor core- Stator core - Rotor bars- Stator winding - End ring

6. List the constructional elements of slip-ring induction motor.The constructional elements of squirrel cage induction motor are,

Stator Frame Rotor Rotor core- Stator core - Rotor bars- Stator winding - Slip rings.

7. How the design problems of an electrical machine can be classified?1. Electromagnetic design2. Mechanical design

3. Thermal design


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4. Dielectric design8. What are the main dimensions of a rotating machine?

The main dimensions of a rotating machine are the armaturediameter (or) stator bore, D and armature (or) stator core length, L.9. What is equation?

The equation describing the relationship between output and maindimensions (D and L), specific loadings (Bav and ac) and speed (n) of amachine is known as output equation.10. List the factors that influences the separation of D and L of a dcmachine.

In dc machines the separation of D and L depends on1. Pore proportions 3. Peripheral speed2. Moment of inertia 4. Voltage between adjacent segments

11. What are the factors to be considered for the separation of D and L ofsynchronous machine?

In synchronous machine the separation of D and L depends on

Peripheral speed, number of poles and short circuit ratio.12. What are the factors that affect the size of rotation machine?

The factors affecting the size of rotating machines are speed,specific electric and magnetic loadings.13. What re the factors that decide the choice of specific magneticloading?

The value of magnetic loading is determined by1. Maximum flux density in iron parts of machine.2. Magnetizing current and3. Core losses.

14. What is magnetic circuit?The magnetic circuit is the path of magnetic flux. The mmf of thecircuit creates fluz in the path by overcoming the reluctance of the path.

The equation which relates flux, mmf and reluctance is given by,Flux = MMF

Reluctance15. What are the constituents of magnetic circuit in rotating machines?

The magnetic circuit of rotating machine consists of air-gap, statorand rotor cores, stator and rotor teeth.16. 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 themagnetic materials and it is supplied by the manufacturers of stampingsor laminations.17. What is leakage flux?

The leakage flux is the flux passing through unwanted path. Theleakage flux will not help either for transfer or conversion of energy.18. What is fringing flux?

The bulging of magnetic path at the air-gap is called fringing. Thefluxes in the bulged portion are called fringing flux.19. What are the factors which modify the reluctance of air-gap?


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The reluctance of air-gap is modified by slots, radial ventilating ductsand non-uniform air-gaps.


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20. Define total gap construction factor, KgThe total gap contraction factor, Kg is defined as the ratio of

reluctance of slotted armature with ducts and the reluctance of smootharmature without ducts.21. What is the effect of salient poles on the air-gap mmf?

In salient pole machines the length of air-gap is not constant overthe whole pole pitch. Hence the effective air-gap length is given by Kglgwhere Kg is the gap contraction factor. Also for calculating mmf, themaximum gap density Bg at the centre of the pole is considered instead ofaverage gap density.

MMF for air-gap in salient pole machine = 800,000 BgKglg.22. Define field form factor.

The field form factor, Kf is defined as the ratio of average gapdensity over the pole pitch to maximum flux density in the air-gap.23. What are the problems encountered in estimating the mmf for teeth?

The problems encountered in estimating the mmf for teeth are1. 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.24. List the methods used for estimating the mmf for teeth.

The following are the three methods used for estimating the mmf forteeth.

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

25. What is real and apparent flux density?

The real flux density is due to the actual flux through a tooth. Theapparent flux density is due to total flux has to be pass through the tooth.Since some of the flux passes through slot the real flux density is alwaysless than the apparent or total flux density.26. Define specific permeance.

Specific permeance is defined as the permeance per unit length (or)depth of field.27. What is unbalanced magnetic pull?

The unbalanced magnetic pull is the radial force acting on the rotordue to non uniform air-gap around armature periphery.28. List the various techniques employed to overcome unbalanced

magnetic pull.The techniques employed to overcome the unbalanced magnetic

pull are the following.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.29. What factor decides the number of turns in a winding?

The number of turns in a winding is decided by the emf per turn andflux density. The emf per turn depends on the type of insulationemployed.


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30. What is the range of specific magnetic loading in dc machine?The usual range of specific magnetic loading in dc machine is 0.4 to

0.8 Wb/m2.31. What is the range of specific electric loading in dc machine?

The usual range of specific electric loading in de machine is 15000

to 50,000 amp.cond./m.32. What are the factors to be considered for the selection of number ofpoles in dc machine?

The factors to be considered for the selection of number of poles indc machine are frequency of flux reversals, current / brush arm andarmature mmf per pole.33. What are the parameters that are affected by the number of poles?

Weight of iron and copper, length of commutator and dimension ofbrushes are the parameters affected by number of poles.34. Why square pole is preferred?

If the corss-section of the pole body is square then the length of the

mean turn of field winding is minimum. Hence to reduce the copperrequirement a square cross-section is preferred for the poles of dcmachine.35. What is square pole and square pole face?

In square pole, the width of the pole body is made equal to thelength of the armature. In square pole face, the pole arc is made equal tothe length of the armature.36. What factor decides the minimum number of armature coils?

The maximum voltage between adjacent commutator segmentsdecides the minimum number of coils.

37. State the difference between the armature winding of dc machine andthe stator winding of ac machine.The armature winding of dc machine has closed coils but the stator

winding of ac machine has open coils.38. Define winding pitch.

The winding pitch is defined as the distance between the starts oftwo consecutive coils measured in terms of coil sides.39. What is back pitch?

The distance between top and bottom coil sides of a coil measuredaround the back of the armature is called the back pitch. The back pitch ismeasured in terms of coil sides.

40. What is front pitch?The front pitch is the distance between two coil sided connected to

the same commutator segment. It is measured in terms of coil sides.41. Define commutator pitch.

The commutator segment is defined as the distance between thetwo commutator segments to which the two ends (start and finish) of a coilare connected. It is measured in terms of commutator segment.42. What is equalizer connection?

The equalizer connections are low resistance copper conductorsemployed in lap winding to equalize the induced emfs in parallel paths.


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The difference in induced emf in parallel paths is due to slight unsymmeryin flux per pole in dc machines.43. What is the need for brushes in dc machine?

The brushed are used in dc machines to collect or draw current fromthe rotating armature.

44. What are the effects of armature reaction?The various effects of armature reaction are reduction in induced

emf, increase in iron losses, delayed communication, sparking and ringfiring.45. How the polarities of interpole are decided?

The polarity of the interpole must be that of the main pole justahead (in the direction of rotation) for a generator and just behind (in thedirection of rotation) for a motor.46. What is the effect of interpole on main pole?

In case of generator the interpole will magnetize the leading edgeand demagnetize the trailing edge of main pole. In case of motor the

interpoles will demagnetize the leading edge and magnetize the trailingedge of main pole.47. What is the range of efficiency of a transformer?

The efficiency of a commercial transformer will be in the range of94% to 99%. Among the available electrical machines the transformershave the highest efficiency.48. What is transformer bank?

A transformer bank consists of three independent single phasetransformers with their primary and secondary windings connected eitherin star or in delta.

49. What is window space factor?The window space factor is defined as the ration of copper area inwindow to total area of window.50. What are the factors to be considered for selecting the cooling methodof a transformer?

The choice of cooling method depends on KVA rating of transformer,size, application and the site condition where it has to be installed.51. How the head dissipates in a transformer?

The heat dissipation in a transformer occurs by Conduction,Convection and Radiation.52. Why cooling tubes are provided?

Cooling tubes are provided to increase heat dissipating area of thetank.53. What are the main dimensions of induction motor?

The main dimensions of induction motor are stator core internaldiameter and stator core length.54. Why wound rotor construction is adopted?

The wound rotor has the facility of increasing the rotor resistancethrough slip-rings. Higher values of rotor resistance are needed duringstarting to get a high value of starting torque.55. What is rotating transformer?


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The principle of operation of induction motor is similar to that atransformer. The stator winding is equivalent to primary of a transformerand the rotor winding is equivalent to short circuited secondary of atransformer. In transformer the secondary is fixed but in induction motorit is allowed to rotate.

56. 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.57. What are the materials used for slip-rings and brushes in inductionmotor?

The slip-rings are made of brass on phosphor bronze. The brushesare made of metal graphite which is an alloy of copper and carbon.58. What are the factors to be considered for the choice of specificmagnetic loading?

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

59. What are the factors to be considered for the choice of specificelectric loading?

The choice of specific electric loading depends on copper loss,temperature rise, voltage rating and over load capacity.60. How the induction motor can be designed for best power factor?

For best power factor the pole pitch, is chosen such that, =

0.18L.61. What are the different types of stator windings in induction motor?

The different types of stator windings are mush winding, lapwinding and wave winding.

62. What is full pitch and short pitch or chording?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). The windingis called short pitched or chorded.63. Why short chorded windings are employed in induction motor?

For short chorded windings the length of mean tum will be lesserthan the full pitch coils. Hence it results in reduction of copper. Also theshort chorded windings eliminates certain harmonic magnetic fields.64. What type of slots are preferred in induction motor?

Semi enclosed slots are preferred for induction motor. It results inless air gap contraction factor giving a small value of magnetizingcurrents, low tooth pulsation loss and much quieter operation (lessnoise).65. What is slot space factor?

The slot space factor is the ratio of conductor (or copper) area perslot and slot area. It gives an indication of the space occupied by theconductors and the space available for insulation. The slot space factorfor induction motor varies for 0.25 to 0.4.66. Write the expression for length of mean turn of stator winding.

Length of mean turn of stator, Lmts = 2L + 2.3 + 0.24


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67.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 fluxdensity. The maximum value of flux density in the teeth is 1.7 Sb/m2.


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68. What are the factors to be considered for estimating the length of air-gap in induction motor?

The following factors are to be considered for estimating the lengthof air-gap.

1. Power factor 2. Overload capacity 3. Pulsation loss

4. Unbalanced magnetic pull 5. Cooling 6. Noise69. List the undesirable effects produced by certain combination of rotorand stator slots.

The following problems may develop in induction motor withcertain combination of rotor and stator slots.

1. The motor may refuse to start, (cogging)2. The motor may run at subsynchronous speed (crawling)3. Severe vibration may develop and the noise will be excessive.

70. What is crawling and cogging?Crawling is a phenomena in which the induction motor runs at t

speed lesser than subsynchronous speed.

Cogging is a phenomena in which the induction motor refuses tostart.71. Explain the phenomena of cogging.

When the number of stator and rotor slots are equal, the speeds ofall the harmonics produced by stator slotting coincide with the speed ofcorresponding, rotor harmonics. This harmonics of every order would tryto exert synchronous torques at their corresponding synchronous speedsand the machine would refuse to start. This is known as cogging.72. 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 ofair-gap.73. What are the different types of windings used for rotor of inductionmotor?

The different types of windings employed in induction motor rotorare mush winding the double layer bar type winding.74. Name the two types of synchronous machines.

Based on construction the synchronous machines may be classifiedas,

1. Salient pole machines. 2. Cylindrical rotor machines.75. What is runaway speed?

The runaway speed is defined as the speed which the primemoverwould have, if it is suddenly unloaded, when working at its rated load.76. What are the factors to be considered for the choice of specificmagnetic loading?

The factors to be considered for the choice of specific magneticloading are

1. Iron loss 2. Temperature rise 3. Voltage rating4. Synchronous reactance 5. Stray load losses.

77. 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.


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78. List the factors to be considered for the choice of number of slots are1. Balanced winding 2. Cost 3. Host spot temperature in winging4. Leakage reactance 5. Tooth losses 6. Tooth flux density.

79. What is the limiting factor for the diameter of synchronous machine?The limiting factor for the diameter of synchronous machine is the

peripheral speed. The limiting value of peripheral speed is 175 m/sec forcylindrical rotor machines and 80m/sec for salient pole machines.80. write the expression for air-gap length in cylindrical rotor machine.

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

81. What is skewing?Skewing is twisting either the stator or rotore core. The motor

noise, vibrations, cogging and synchronous cusps can be reduced or evenentirely eliminated by skewing.

In order to eliminate the effect of any harmonic, the rotor barsshould be skewed through an angle so that the bars lie under alternateharmonic poles of the same polarity or in other words the bars much beskewed through two pitches.82. What are the advantages and disadvantages of large air-gap length ininduction motor?Advantage

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

The disadvantage of large air-gap length is that it result in highvalue of magnetizing current.

83. What are the different types of induction motor? How they differ fromeach other?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 theconstruction of rotor.

The squirrel cage rotor has copper or aluminium bars mounted onrotor slots and short circuited at both ends by end rings.

The slip ring rotor carries a three phase winding. One end of eachphase is connected to a slip ring and other ends are star connected.84. 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 protectioncircuits. The resistances are connected to slip-rings. While starting thefull resistance is included in the rotor circuit to get high starting torque.Once the rooter starts rotating, the resistances are gradually reduced insteps. At running condition the slip-rings are shorted and so it isequivalent to squirrel cage rotor.85. What are the ranges of specific magnetic loading and electric loadingin induction motor?

Specific magnetic loading = 0.3 to 0.6 Wb/m2

Specific electric loading = 5000 to 4500 amp. Cond/m.


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86. 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 ofnumber of phases.

In fractional slot winding the total number of slots is chosen such

that the slots per pole is not an integer.87. What type of connection is preferred for stator of induction motor?

Under running condition the stator of induction motor is normallyconnected in delta. (In delta connection the torque developed will behigher than the star connection). But for reducing the starting current,the stator can be connected in star while starting and then changed todelta.88.What is Short Circuit Ratio (SCR)?

The Short Circuit Ratio (SCR) is defined as the ratio of field currentrequired to produce rated voltage on open circuit to field current requiredto 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. forsalient pole alternator SCR varies from 1.0 to 1.5.89. What is magnetization curve?

The magnetization curve is a graph showing the relation betweenthe magnetic field intensity, H and the flux density, B of a magneticmaterial. It is used to estimate the mmf required for flux path in themagnetic material and it is supplied by the manufacturers of stampingsor laminations.90. What is leakage flux?

The leakage flux is the flux passing through unwanted path. Theleakage flux will not help either for transfer or conversion of energy.91. What is leakage coefficient?

The leakage coefficient is defined as the ratio of total flux to usefulflux.Leakage coefficient, Cl = Total flux

Useful flux92. 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.93. Define gap contraction factor for slots.

The gap contraction factor for slots, Kgs is defined as the ratio ofreluctance of air-gap of slotted armature to reluctance of air-gap ofsmooth armature.

Kgs = Reluctance of air-gap with slotted armatureReluctance of air-gap with smooth armature.

94. How the tertiary winding is connected? Why?


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The testing winding is normally connected in delta. When theteritary is connected in delta, the unbalance in phase voltage duringunsymmetrical faults in primary or secondary is compensated by thecirculating currents flowing in the closed delta.95. 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 is due to more effective pressure heads produced by columns of oilin tubes. The improvement in cooling is accounted by taking the specificheat dissipation due to convection as 30% more than that without tubes.96. Why wound rotor construction is adopted?

The wound rotor has the facility of increasing the rotor resistancethrough slip-rings. Higher values of rotor resistance are needed duringstarting to get a high value of starting torque.97. 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 behigher than the star connection). But for reducing the starting current,the stator can be connected in star while starting and then changed todelta.98. What is integral slot winding and fractional slot winding?

In integral slot winding the total number of slots is chosen such thatthe slots per pole is an integer. The integer should also be a multiple ofnumber of phases.

In fractional slot winding the total number of slot is chosen suchthat the slots per pole is not an integer.

99. What is Short Circuit Ratio (SCR)?The Short Circuit Ratio is defined as the ratio of field currentrequired to produce rated voltage on open circuit to field currentrequired to circulate rated at short circuit.

It is also given by the reciprocal as synchronous reactance, X d inp.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 1.5.100. What are the main dimensions of induction motor?

The main dimensions of induction motor are stator core internaldiameter and stator core length.

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