Design of electrical apparatus question and answers

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Design of electrical apparatus question and answers

DESIGN OF ELECTRICAL APPARATUS
SHORT 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 listed below,
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 machine are,
Stator         -        Frame                  Rotar          -        Field pole
-        Armature core                        -        Pole shoe
-        Armature winding                   -        Field winding
-       Damper winding
4. What are the constructional elements of cylindrical rotor synchronous machine?
The constructional elements of cylindrical rotor synchronous machine 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 design
2. Mechanical design
3. Thermal design
4. Dielectric design
8. What are the main dimensions of a rotating machine?
The main dimensions of a rotating machine are the armature diameter (or) stator bore, D and armature (or) stator core length, L.
9. What is equation?
The equation describing the relationship between output and main dimensions (D and L), specific loadings (Bav and ac) and speed (n) of a machine is known as output equation.
10. List the factors that influences the separation of D and L of a dc machine.
In dc machines the separation of D and L depends on
1. Pore proportions                 3. Peripheral speed
2. Moment of inertia           4. Voltage between adjacent segments
11. What are the factors to be considered for the separation of D and L of synchronous 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 magnetic loading?
The value of magnetic loading is determined by
1. Maximum flux density in iron parts of machine.
2. Magnetizing current and
3. Core losses.
14. What is magnetic circuit?
The magnetic circuit is the path of magnetic flux. The mmf of the circuit 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
Reluctance
15. What are the constituents of magnetic circuit in rotating machines?
The magnetic circuit of rotating machine consists of air-gap, stator and 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 the magnetic materials and it is supplied by the manufacturers 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 fringing flux?
The bulging of magnetic path at the air-gap is called fringing. The fluxes in the bulged portion are called fringing flux.
19. 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.

20. Define total gap construction factor, Kg
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.
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 over the whole pole pitch. Hence the effective air-gap length is given by Kglg where Kg is the gap contraction factor. Also for calculating mmf, the maximum gap density Bg at the centre of the pole is considered instead of average 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 gap density 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 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.
24. List the methods used for estimating the mmf for teeth.
The following are the three methods used for estimating the mmf for teeth.
1. Graphical method
2. Three ordinate method (simpson’s 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. The apparent 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 always less 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 rotor due 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 and flux density. The emf per turn depends on the type of insulation employed.

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 of poles in dc machine?
The factors to be considered for the selection of number of poles in dc machine are frequency of flux reversals, current / brush arm and armature 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 of brushes 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 copper requirement a square cross-section is preferred for the poles of dc machine.
35. What is square pole and square pole face?
In square pole, the width of the pole body is made equal to the length of the armature. In square pole face, the pole arc is made equal to the length of the armature.
36. What factor decides the minimum number of armature coils?
The maximum voltage between adjacent commutator segments decides the minimum number of coils.
37. State the difference between the armature winding of dc machine and the 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 of two consecutive coils measured in terms of coil sides.
39. What is back pitch?
The distance between 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.
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 the two commutator segments to which the two ends (start and finish) of a coil are connected. It is measured in terms of commutator segment.
42. What is equalizer connection?
The equalizer connections are low resistance copper conductors employed in lap winding to equalize the induced emfs in parallel paths. The difference in induced emf in parallel paths is due to slight unsymmery in 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 from the 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 ring firing.
45. How the polarities of interpole are decided?
The polarity of the interpole must be that of the main pole just ahead (in the direction of rotation) for a generator and just behind (in the direction 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 edge and demagnetize the trailing edge of main pole. In case of motor the interpoles will demagnetize the leading edge and magnetize the trailing edge of main pole.
47. What is the range of efficiency of a transformer?
The efficiency of a commercial transformer will be in the range of 94% to 99%. Among the available electrical machines the transformers have the highest efficiency.
48. 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.
49. What is window space factor?
The window space factor is defined as the ration of copper area in window to total area of window.
50. What are the factors to be considered for selecting the cooling method of 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 the tank.
53. What are the main dimensions of induction motor?
The main dimensions of induction motor are stator core internal diameter and stator core length.
54. 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.
55. 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.
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 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.
58. What are the factors to be considered for the choice of specific magnetic 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 specific electric 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, i is chosen such that, i = Ö0.18L.
61. What are the different types of stator windings in induction motor?
The different types of stator windings are mush winding, lap winding and wave winding.
62. 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.
When the coil span is less than the pole pitch (180°e). The winding is 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 lesser than the full pitch coils. Hence it results in reduction of copper. Also the short 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 in less air gap contraction factor giving a small value of magnetizing currents, low tooth pulsation loss and much quieter operation (less noise).
65. 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 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 i + 0.24
67.Which part of induction motor has maximum flux density? What is the maximum 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.

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 length of air-gap.
1. Power factor     2. Overload capacity       3. Pulsation loss
4. Unbalanced magnetic pull   5. Cooling   6. Noise
69. List the undesirable effects produced by certain combination of rotor and stator 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 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 to start.
71. Explain the phenomena of cogging.
When the number of stator and rotor slots are equal, the speeds of all the harmonics produced by stator slotting coincide with the speed of corresponding, rotor harmonics. This harmonics of every order would try to exert synchronous torques at their corresponding synchronous speeds and 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 of air-gap.
73. What are the different types of windings used for rotor of induction motor?
The different types of windings employed in induction motor rotor are mush winding the double layer bar type winding.
74. 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.
75. What is runaway speed?
The runaway speed is defined as the speed which the primemover would have, if it is suddenly unloaded, when working at its rated load.
76. What are the factors to be considered for the choice of specific magnetic loading?
The factors to be considered for the choice of specific magnetic loading are
1. Iron loss           2. Temperature rise        3. Voltage rating
4. 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.
78. List the factors to be considered for the choice of number of slots are
1. Balanced winding       2. Cost 3. Host spot temperature in winging         4. 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 for cylindrical 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)i Kf x 10-6
KgBav
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 even entirely eliminated by skewing.
In order to eliminate the 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 much be skewed through two pitches.
82. What are the advantages and disadvantages of large air-gap length in induction motor?

Advantage

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

Disadvantage

The disadvantage of large air-gap length is that it result in high value of magnetizing current.
83. What are the different types of induction motor? How they differ from each 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 the construction of rotor.
The squirrel cage rotor has copper or aluminium 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 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 protection circuits. The resistances are connected to slip-rings. While starting the full resistance is included in the rotor circuit to get high starting torque. Once the rooter starts rotating, the resistances are gradually reduced in steps. At running condition the slip-rings are shorted and so it is equivalent to squirrel cage rotor.
85. What are the ranges of specific magnetic loading and electric loading in induction motor?
Specific magnetic loading = 0.3 to 0.6 Wb/m2
Specific electric loading = 5000 to 4500 amp. Cond/m.

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 of number 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 normally connected in delta. (In delta connection the torque developed will be higher than the star connection). But for reducing the starting current, the stator can be connected in star while starting and then changed to delta.
88.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 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.
89. 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 to estimate the mmf required for flux path in the magnetic material and it is supplied by the manufacturers of stampings or laminations.
90. 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.
91. What is leakage coefficient?
The leakage coefficient is defined as the ratio of total flux to useful flux.
Leakage coefficient, Cl = Total flux
Useful flux
92. 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 of reluctance of air-gap of slotted armature to reluctance of air-gap of smooth armature.
Kgs = Reluctance of air-gap with slotted armature
Reluctance of air-gap with smooth armature.
94. How the tertiary winding is connected? Why?
The testing winding is normally connected in delta. When the teritary is connected in delta, the unbalance in phase voltage during unsymmetrical faults in primary or secondary is compensated by the circulating 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 oil in tubes. The improvement in cooling is accounted by taking the specific heat 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 resistance through slip-rings. Higher values of rotor resistance are needed during starting 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 be higher than the star connection). But for reducing the starting current, the stator can be connected in star while starting and then changed to delta.
98. 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 slot is chosen such that 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 current required to produce rated voltage on open circuit to field current required to circulate rated at short circuit.
It is also given by the reciprocal as synchronous reactance, X d 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.
100. What are the main dimensions of induction motor?
The main dimensions of induction motor are stator core internal diameter and stator core length.

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