NOORUL ISLAAM COLLEGE OF ENGINEERING
KUMARACOIL
ELECTRICAL AND ELECTRONICS DEPARTMENT
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,
StatorYoke (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,  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, 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  + 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 airgap 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) 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.