Top Electrical Machine Design Objective Questions
1.The heated parts of an electrical machine dissipate heat into their surroundings by which of the following modes of heat dissipation ?
Conduction
Convection
Radiation
All ofthe above
ANSWER : : All ofthe above
2.Materials exhibiting zero value of resistivity are known as ______ .
conductors
semiconductors
insulators
superconductors
ANSWER : : superconductors
3.Commercial available medium size machines have a speed range of ______ .
200 to 400 r.p.m.
600 to 1000 r.p.m.
1000 to 1500 r.p.m.
2000 to 2500 r.p.m.
ANSWER : : 2000 to 2500 r.p.m.
4.The action of electromagnetic machines can be related to which ofthe following basic principles ?
Induction
Interaction
Alignment
All of the above
ANSWER : : All of the above
5.Aluminium when adopted as a conductor material in ______ transformers, decreases the overall cost of the transformer
small size
medium size
large size
any of the above size
ANSWER : : small size
6.The heat dissipated by from a surface depends upon its temperature and its characteristics like colour, roughness etc.
conduction
convection
radiation
any ofthe above
ANSWER : : radiation
7.Which ofthe following materials is used in the manufacture of resistance grids to be used in the starters of large motors ?
Copper
Aluminium
Steel
Cast-iron
ANSWER : : Cast-iron
8.Hot rolled sheets have ______ value of permeability
zero
low
high
none ofthe above
ANSWER : : low
9.In induction motors, the length of air gap in kept as small as mechanically possible is order to have
low power factor
high power factor
high over load capacity
any of the above
ANSWER : : high power factor
10.Multiple inlet system of air cooling of turbo-alternators can be used for machines of rating upto
10 MW
30 MW
60 MW
150 MW
ANSWER : : 60 MW
11.The length cf air gap to be provided in ______ is primarily determined by power factor consideration.
d.c. series motor .
d.c. shunt motor
induction motor
synchronous motor
ANSWER : : induction motor
12.The density of hydrogen is _____ times the density of air.
0.07
1.5
2.5
3.5
ANSWER : : 0.07
13.Which ofthe following methods is used for air cooling of turbo-alternators ?
One sided axial ventilation
Two sided axial ventilation
Multiple inlet system
All ofthe above
ANSWER : : All ofthe above
14.The design of mechanical parts is particularly important in case of _____ speed machines.
low
medium
high
any of the above
ANSWER : : high
15.If an insulating material is operated beyond the maximum allowable temperature, its life is
drastically increased
drastically reduced
unaffected
none of the above
ANSWER : : drastically reduced
16.The efficiency of a machine should be as ______ as possible to reduce the operating cost.
high
low
either of the above
none of the above
ANSWER : : high
17.The mcrease in heat dissipation by air blasts is due to increase in
conduction
convection
radiation
any ofthe above
ANSWER : : convection
18.________ has a lowrelative permeability and is used principally in field frames when cost is of primary importance and extra weight is not objectionable.
Cast steel
Aluminium
Soft steel
Cast iron
ANSWER : : Cast iron
19.The change in flux linkages can be caused in which ofthe following ways ?
The flux is constant with respect to time and is stationary and the coil moves through it
The coil is stationary with respect to flux and the flux varies in magnitude with respect to time
Both the changes mentioned above occure together, i.e., the coil moves through a time varying field
All ofthe above
ANSWER : : All ofthe above
20.Electrical machines having power outputs ranging from a few kW upto approximately 250 kW may be classified as
small size machines
medium size machines
large size machines
any ofthe above
ANSWER : : medium size machines
21.Electrical machines having a power output upto about 750 W may be called_______machines.
small size
medium size
large size
any of the above
ANSWER : : small size
22.In ______ machines, the size of the shaft is decided by the critical speed which depends on the deflection of the shaft.
small
medium
large
any of the above.
ANSWER : : large
23.The resistivity of water should not be less than
10 Q m
100 Q m
1000 Q m
2000 Q m
ANSWER : : 2000 Q m
24.Which of the following methods is most accurate ?
Equivalent current method
Equivalent power method
Equivalent torque method
Method of average losses
ANSWER : : Equivalent current method
25.The slot leakage can be calculated by making which of the following assumptions ?
The current in the slot conductors is uniformly distributed over their cross-section
The leakage path is straight across the slot and around the iron at the bottom
The permanence of air paths is only considered. The reluctance of iron paths is assumed as zero
All of the above
ANSWER : : All of the above
26.The noise produced by a ______ cooled machine is less as the rotor moves in a medium of smaller density.
air
hydrogen
either (a) or (b)
none of the above
ANSWER : : hydrogen
27.In self cooled motors the cooling time constant is about ______ than the heating time constant because cooling conditions are worse at standstill.
2 to 3 times greater
3 to 4 times greater
4 to 5 times greater
none of the above
ANSWER : : 2 to 3 times greater
28.The distance between the starts of two consecutive coils measured in terms of coil sides is called
front pitch
winding pitch
commutator pitch
back pitch
ANSWER : s: b
29.The value of exciting or magnetizing current depends upon which of the following factors ?
Total m.m.f. required
The number of turns in the exciting winding
The way in which the winding is distributed
All of the above
ANSWER : : All of the above
30.With conventional hydrogen cooling it is possible to increase the rating of a single unit to
50 MW
100 MW
200 MW
none ofthe above
ANSWER : : 200 MW
31.With direct water cooling it is possible to have ratings of about
200 MW
300 MW
400 MW
600 MW
ANSWER : : 600 MW
32.The time taken by the machine to attain 0.632 of its final steady temperature rise is called
heating time constant
cooling time constant
either (a) or (b)
none of the above
ANSWER : : heating time constant
33.Dummy coil should not be used in
small machines
large machines
either (a) or (b)
none of the above
ANSWER : : large machines
34.Tractive magnets are operated from
a.c. supply
d.c. supply
either a.c. or d.c. supply
none of the above
ANSWER : : either a.c. or d.c. supply
35.The winding where dummy coils are used is sometimes called
duplex winding
triplex winding
forced winding
none of the above
ANSWER : : forced winding
36.By which of the following methods the temperature rise of windings and other parts may be determined ?
Thermometer method
Resistance method
Embedded temperature detector method
Any of the above
ANSWER : : Any of the above
37.Direct water cooling of rotor winding presents
no mechanical difficulties
lesser mechanical difficulties
greater mechanical difficulties
none of the above
ANSWER : : greater mechanical difficulties
38.Machines cooled by direct cooling method may be called
“supercharged”
“inner cooled”
“conductor cooled”
any of the above
ANSWER : : any of the above
39.The thermal conductivity of hydrogen is ______ times that of air
2
3
5
7
ANSWER : : 7
40.Which of the following is the commonly used type of electromagnets ?
Flat-faced armature type
Horse shoe type
Flat-faced plunger type
All of the above
ANSWER : : All of the above
41.In direct cooled system using hydrogen both stator and rotor conductors are made
solid
hollow
perforated
any ofthe above
ANSWER : : hollow
42.When the two coil sides forming a coil are spaced exactly one pole pitch apart they are said to be of
short pitch
full pitch
either of the above
none of the above
ANSWER : : full pitch
43.Which of the following methods does not take into account the maximum temperature rise under variable load conditions ?
Equivalent power method
Equivalent current method
Method of average losses
Equivalent torque method
ANSWER : : Method of average losses
44.Transformers with a capacity of upto _____ have a cooling radiator system with natural cooling
2 MVA
5 MVA
7.5 MVA
10 MVA
ANSWER : : 10 MVA
44.The forced oil and air circulation method is usual one for transformers of capacities
upto 5 MVA
upto 10 MVA
upto 20 MVA
30 MVA upwards
ANSWER : : 30 MVA upwards
45.D.C. servomotors are used in
purely D.C. control systems
purely AC. control systems
both D.C. and AC. control systems
none of the above
ANSWER : : purely D.C. control systems
46.For 275 kV transformers, using cold rolled grain oriented steel, which of the following values of flux density may be used?
1.0 Wb/m2
1.1 Wb/m2
1.3 Wb/m2
1.6 Wb/m2
ANSWER : : 1.6 Wb/m2
47.D.C. windings are
sometimes 2-layer type
never 2-layer type
always 2-layer type
none of the above
ANSWER : : always 2-layer type
48.Cylindrical windings using circular conductors, employed in transformers, are
single layered
double layered
multi-layered
none of the above
ANSWER : : multi-layered
49.Which of the following is the basic consideration in the design of insulation ?
Electrical considerations
Mechanical considerations
Thermal considerations
All of the above
ANSWER : : All of the above
50.The high voltage winding is usually which of the following type ?
Cylindrical winding with circular conductors
Cross-over winding with either circular or small rectangular conductors
Continuous disc type winding with rectangular conductors
All of the above types
ANSWER : : All of the above types
51.Helical windings are used in
distribution transformers
power transformers
shell type transformers
none of the above
ANSWER : : power transformers
52.Yokes with rectangular cross-section are used for
small capacity transformers
medium capacity transformers
large capacity transformers
any of the above
ANSWER : : small capacity transformers
53.The insulation between windings and grounded core and the insulation between the windings of the same phase is called
minor insulation
major insulation
either of the above
none of the above
ANSWER : : major insulation
54.A current density of _____ is ilsed for large power transformers with forced circulation of oil or with water cooling coils
1.5 to 2.5 A/mm2
3.5 to 4.5 A/mm2
4.0 to 5.0 A/mm2
5.4 to 6.2 A/mm2
ANSWER : : 5.4 to 6.2 A/mm2
55.The cylindrical windings using circular conductors are used for current rating of
upto 20 A
upto 40 A
upto 60 A
upto 80 A
ANSWER : : upto 80 A
56.The cold rolled grain oriented steel has ______ permeability in the direction of the grain orientation.
minimum
maximum
nil
none of the above
ANSWER : : maximum
57.The voltage control in electric supply networks in required on account of which of the following reasons ?
Adjustment of voltage at consumers premises within statutory limits
Control of active and reactive power
Adjustment of short period daily and seasonal voltage variations in accordance with variations of load
All of the above
ANSWER : : All of the above
58.The surge phenomenon is particularly important in case of
low voltage transformers
medium voltage transformers
high voltage transformers
any of the above
ANSWER : : high voltage transformers
59.The flash point of transformer oil should be higher than
40�C
60�C
80�C
104�C
ANSWER : : 104�C
60.The usual values of maximum flux densities for distribution transformers using hot rolled silicon steel are
0.5 to 0.8 Wb/m2
0.8 to 1.0 Wb/m2
1.1 to 1.35 Wb/m2
1.4 to 1.8 Wb/m2
ANSWER : : 1.1 to 1.35 Wb/m2
61.A practical formula for determining the thickness of insulation between low voltage and high voltage windings is
1 + 0.2 kVmm
2 + 0.5 kVmm
4 + 0.7 kV mm
5 + 0.9 kV mm
ANSWER : : 5 + 0.9 kV mm
62.Disc windings are primarily used in
short capacity transformers
medium capacity transformers
high capacity transformers
any of the above
ANSWER : : high capacity transformers
63.In transformers using hot rolled steel, the cross-section of the yoke is made about _____ greater than that of the core
5 percent
10 percent
15 percent
30 percent
ANSWER : : 15 percent
64.Power transformers should be designed to have maximum efficiency
at one-fourth load
at one-half load
at or near full load
any of the above
ANSWER : : at or near full load
65.For large power transformers, self oil cooled type or air blast type which of the following values of current density may be used ?
1.0 to 1.2 A/mm2
1.5 to 2.0 A/mm2
2.2 to 3.2 A/mm2
3.2 to 4.2 A/mm2
ANSWER : : 2.2 to 3.2 A/mm2
66.Multi-layer helical windings are commonly used in the transformers as high voltage windings
upto 20 kV
upto 50 kV
upto 80 kV
for 110 kV and above
ANSWER : : for 110 kV and above
67.In a D.C. machine, the value of peripheral speed should not, normally, exceed
10 m/s
20 m/s
30 m/s
40 m/s
ANSWER : : 30 m/s
68.In a D.C. machine the current per brush arm should not be more than
100 A
200 A
300 A
400 A
ANSWER : : 400 A
69.In a D.C. machine the number of brush arms is _____ the number of poles.
less than
equal to
greater than
none of the above
ANSWER : : equal to
70.In D.C. machines, ir order to prevent excessive distortion of field form by the armature reaction, the field mmf must be made
equal to that of armature mmf
less in comparison with the arma�ture mmf
large in comparison with the arma-ture mmf
none of the above
ANSWER : : large in comparison with the arma-ture mmf
71.______ brushes are fragile and cause excessive wear of commutator,
Natural graphite
Hard carbon
Electro graphitic
Metal graphite
ANSWER : : Natural graphite
72.In D.C. machines the number of slots per pole usually lies
between 2 to 4
between 6 to 8
between 9 to 16
between 20 to 30
ANSWER : : between 9 to 16
73.The laminations of the armature of a D.C. machine are usually _____ thick.
0.1 to 0.2 mm
0.2 to 0.3 mm
0.3 to 0.4 mm
0.4 to 0.5 mm
ANSWER : : 0.4 to 0.5 mm
74.The weight of copper of both armature and field windings decreases with _____ in number of poles.
increase
decrease
either of the above
none of the above
ANSWER : : increase
75.Which of the following brushes can be used for high values of current density ?
Metal graphite brushes
Hard carbon brushes
Electro-graphitic brushes
Natural graphite brushes
ANSWER : : Metal graphite brushes
76.In D.C. machines the usual limit of slot pitch is
between 5 to 10 mm
between 10 to 15 mm
between 15 to 20 mm
between 25 to 35 mm
ANSWER : : between 25 to 35 mm
77.The operation of a D.C. machine with large air gap lengths is comparatively
quiet
noisy
either of the above
none of the above
ANSWER : : quiet
78.In D.C. machines, the ____ in field mmf results in increase in size and cost of machines.
increase
decrease
either of the above
none of the above
ANSWER : : increase
79.In D.C. machines the width of the duct is usually
4 mm
6 mm
8 mm
10 mm
ANSWER : : 10 mm
80.D.C. machines designed with a large value of air gap length have
worst, ventilation
poor ventilation
better ventilation
none of the above
ANSWER : : better ventilation
81.is usually used for brush rockers
Mild steel
Copper
Aluminium
Cast-iron
ANSWER : : Cast-iron