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WorksheetsELECTRICAL MACHINE - 1
Total questions: 80
Worksheet time: 40mins
Name
Class
Date
1.
A good voltage regulation of a transformer means
a)
output voltage fluctuation from no load to full load is least
b)
output voltage fluctuation with power factor is least
c)
difference between primary and secondary voltage is least
d)
difference between primary and secondary voltage is maximum
2.
A transformer can have zero voltage regulation at
a)
leading power factor
b)
lagging power factor
c)
unity power factor
d)
zero power factor
3.
D.C. generators are installed near the load centres to reduce
a)
iron losses
b)
line losses
c)
sparking
d)
corona losses
4.
D.C. generators are normally designed for maximum efficiency around
a)
full-load
b)
rated r.p.m.
c)
rated voltage
d)
all of the above
5.
D.C. motor is to drive a load which has certain minimum value for most of the time and some peak value for short duration We will select the
a)
series motor
b)
shunt motor
c)
compound motor
d)
any of the above
6.
D.C. series motors are used
a)
where load is constant
b)
where load changes frequently
c)
where constant operating speed is needed
d)
in none of the above situations.
7.
D.C. shunt motors are used for driving
a)
trains
b)
cranes
c)
hoists
d)
machine tools
8.
Differentially compound D.C. motors can find applications requiring
a)
high starting torque
b)
low starting torque
c)
variable speed
d)
frequent on-off cycles
9.
During open circuit test of a transformer
a)
primary is supplied rated voltage
b)
primary is supplied full-load current
c)
primary is supplied current at reduced voltage
d)
primary is supplied rated kVA
10.
For a D.C. shunt motor if the excitation is changed
a)
torque will remain constant
b)
torque will change but power will remain constant
c)
torque and power both will change
d)
torque, power and speed, all will change
11.
For a transformer, operating at constant load current, maximum efficiency will occur at
a)
0.8 leading power factor
b)
0.8 lagging power factor
c)
zero power factor
d)
unity power factor
12.
For constant torque drive which speed control method is preferred ?
a)
Field control
b)
Armature voltage control
c)
Shunt armature control
d)
Mechanical loading system
13.
For starting a D.C. motor a starter is required because
a)
it limits the speed of the motor
b)
it limits the starting current to a safe value
c)
it starts the motor
d)
none of the above
14.
For the same HP rating and full load speed, following motor has poor starting torque
a)
shunt
b)
series
c)
differentially compounded
d)
cumulativelyc’ompounded
15.
For which types of D.C. motor, dynamic braking is generally used ?
a)
Shunt motors
b)
Series motors
c)
Compound motors
d)
All of the above
16.
Hopkinson’s test on D.C. machines is conducted at
a)
no-load
b)
part load
c)
full-load
d)
overload
17.
Hysteresis loss in a transformer varies as CBmax = maximum flux density)
a)
Bmax
b)
Bmax1-6
c)
Bmax1-83
d)
B max
18.
If a D.C. motor is connected across the AC supply it will
a)
run at normal speed
b)
not run
c)
run at lower speed
d)
burn due to heat produced in the field winding by .eddy currents
19.
If a D.C. motor is to be selected for conveyors, which rriotor would be preferred ?
a)
Series motor
b)
Shunt motor
c)
Differentially compound motor
d)
Cumulative compound motor
20.
If a D.C. shunt motor is working at no load and if shunt field circuit suddenly opens
a)
nothing will happen to the motor
b)
this will make armature to take heavy current, possibly burning it
c)
this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses
d)
motor will run at very slow speed
21.
If I2 be the armature current, then speed of a D.C. shunt motor is
a)
independent of Ia
b)
proportional to la
c)
varies as (Ia)
d)
varies as la
22.
If R2 is the resistance of secondary winding of the transformer and K is the transformation ratio then the equivalent secondary resistance referred to primary will be
a)
R2/VK
b)
R2IK2
c)
R22!K2
d)
R22/K
23.
If the field of a D.C. shunt motor gets opened while motor is running
a)
the speed of motor will be reduced %
b)
the armature current will reduce
c)
the motor will attain dangerously high speed 1
d)
the motor will continue to nuvat constant speed
24.
If the speed of a D.C. shunt motor is increased, the back emf of the motor will
a)
increase
b)
decrease
c)
remain same
d)
become zero
25.
If the supply voltage for a D.C. motor is increased, which of the following will decrease ?
a)
Starting torque
b)
Operating speed
c)
Full-load current
d)
All of the above
26.
If the terminals of armature of D.C. motor are interchanged, this action will offer following kind of braking (o) regenerative
a)
plugging
b)
dynamic braking
c)
none of the above
d)
any of the above
27.
In a D.C. generator all of the following could be the effects of iron losses except
a)
Loss of efficiency
b)
Excessive heating of core
c)
Increase in terminal voltage
d)
Rise in temperature of ventilating air
28.
In a D.C. generator, the iron losses mainly take place in
a)
yoke
b)
commutator
c)
armature conductors
d)
armature rotor
29.
In a D.C. shunt motor, speed is
a)
independent of armature current
b)
directly proportional to the armature current
c)
proportional to the square of the current
d)
inversely proportional to the armature current
30.
In a D.C. shunt motor, under the conditions of maximum power, the current in the armature will be
a)
almost negligible
b)
rated full-load current
c)
less than full-load current
d)
more than full-load current
31.
In a differentially compounded D.C. motor, if shunt field suddenly opens
a)
the motor will first stop and then run in opposite direction as series motor
b)
the motor will work as series motor and run at slow speed in the same direction
c)
the motor will work as series motor and run at high speed in the same direction
d)
the motor will not work and come to stop
32.
In a given transformer for a given applied voltage, losses which remain constant irrespective of load changes are
a)
hysteresis and eddy current losses
b)
friction and windage losses
c)
copper losses
d)
none of the above
33.
In a transformer the resistance between its primary and secondary should be
a)
zero
b)
10 Q
c)
1000 Q
d)
infinity
34.
In case of D.C. shunt motors the speed is dependent on back emf only because
a)
back e.m.f. is equal to armature drop
b)
armature drop is negligible
c)
flux is proportional to armature current
d)
flux is practically constant in D:C. shunt motors
35.
In case-the conditions for maximum power for a D.C. motor are established, the efficiency of the motor will be
a)
100%
b)
around 90%
c)
anywhere between 75% and 90%
d)
less than 50%
36.
In D.C. machines fractional pitch winding is used
a)
to improve cooling
b)
to reduce copper losses
c)
to increase the generated e.m.f.
d)
to reduce the sparking
37.
In D.C. motor which of the following can sustain the maximum temperature rise?
a)
Slip rings
b)
Commutator
c)
Field winding
d)
Armature winding
38.
In D.C. shunt motors as load is reduced
a)
the speed will increase abruptly
b)
the speed will increase in proportion to reduction in load
c)
the speed will remain almost/constant
d)
the speed will reduce
39.
Open circuit test on transformers is conducted to determine
a)
hysteresis losses
b)
copper losses
c)
core losses
d)
eddy current losses
40.
Short circuit test on transformers is conducted to determine
a)
hysteresis losses
b)
copper losses
c)
core losses
d)
eddy current losses
41.
The magnetising current of a transformer is usually small because it has
a)
small air gap
b)
large leakage flux
c)
laminated silicon steel core
d)
fewer rotating parts
42.
The main reason for generation of harmonics in a transformer could be
a)
fluctuating load
b)
poor insulation
c)
mechanical vibrations
d)
saturation of core
43.
The secondary of a current transformer is always short-circuited under operating conditions because it
a)
avoids core saturation and high voltage induction
b)
is safe to human beings
c)
protects the primary circuit
d)
none of the above
44.
The transformer oil should have _____ volatility and _____ viscosity
a)
low,low
b)
high,high
c)
low,high
d)
high,low
45.
Transformer breaths in when
a)
load on it increases
b)
load on it decreases
c)
load remains constant
d)
none of the above
46.
What will happen if the transformers working in parallel are not connected with regard to polarity ?
a)
The power factor of the two trans-formers will be different from the power factor of common load
b)
Incorrect polarity will result in dead short circuit
c)
The transformers will not share load in proportion to their kVA ratings
d)
none of the above
47.
Which D.C. motor is generally preferred for cranes and hoists ?
a)
Series motor
b)
Shunt motor
c)
Cumulatively compounded motor
d)
Differentially compounded motor
48.
Which method of braking is generally used in elevators ?
a)
Plugging
b)
Regenerative braking
c)
Rheostatic braking
d)
None of the above
49.
Which motor has the poorest speed control?
a)
Differentially compounded motor
b)
Cumulatively compounded motor
c)
Shunt motor
d)
Series motor
50.
Which of the following application requires high starting torque ?
a)
Lathe machine
b)
Centrifugal pump
c)
Locomotive
d)
Air blower
51.
Which of the following is the main advantage of an auto-transformer over a two winding transformer ?
a)
Hysteresis losses are reduced
b)
Saving in winding material
c)
Copper losses are negligible
d)
Eddy losses are totally eliminated
52.
Which of the following law/rule can he used to determine the direction of rotation of D.C. motor ?
a)
Lenz’s law
b)
Faraday’s law
c)
Coloumb’s law
d)
Fleming’s left-hand rule
53.
Which of the following load normally needs starting torque more than the rated torque?
a)
Blowers
b)
Conveyors
c)
Air compressors
d)
Centrifugal pumps
54.
Which of the following loss in a D.C. generator varies significantly with the load current ?
a)
Field copper loss
b)
Windage loss
c)
Armature copper loss
d)
None of the above
55.
Which of the following losses are significantly reduced by laminating the core of a D.C. generator ?
a)
Hysteresis losses
b)
Eddy current losses
c)
Copper losses
d)
Windage losses
56.
Which of the following method of speed control of D.C. machine will offer minimum efficiency ?
a)
Voltage control method
b)
Field control method
c)
Armature control method
d)
All above methods
57.
Which of the following motor has the poorest speed regulation ?
a)
Shunt motor
b)
Series motor
c)
Differential compound motor
d)
Cumulative compound motor
58.
Which one of the following is not necessarily the advantage of D.C. motors over AC motors ?
a)
Low cost
b)
Wide speed range
c)
Stability
d)
High starting torque.
59.
Which one of the following is not the function of pole shoes in a D.C. machine ?
a)
To reduce eddy current loss
b)
To support the field coils
c)
To spread out flux for better uniformity
d)
To reduce the reluctance of the magnetic path
60.
Why are the D.C. motors preferred for traction applications ?
a)
Torque and speed are inversely proportional to armature current
b)
Torque is proportional to armature current
c)
Torque is proportional to square root of armature current
d)
The speed is inversely proportional to the torque and the torque is proportional to square of armature current
61.
Scott connections are used in _______________
a)
three-phase to single phase transformation
b)
three-phase to two-phase transformation
c)
single phase to three-phase transformation
d)
all phase transformations
62.
The developed electromagnetic force and/or torque in electromechanical energy conversion system, acts in such a direction that tends to ___________
a)
increase the stored energy at constant mmf
b)
decrease the stored energy at constant mmf
c)
decrease the co-energy at constant mmf
d)
increase the stored energy at constant flux
63.
An electro-mechanical energy conversion device is one which converts _______
a)
Electrical energy to mechanical energy only
b)
Mechanical energy to electrical energy only
c)
All of the mentioned
d)
None of the mentioned
64.
For a linear electromagnetic circuit, which of the following statement is true?
a)
Field energy is less than the Co-energy
b)
Field energy is equal to the Co-energy
c)
Field energy is greater than the Co-energy
d)
Co-energy is zero
65.
For a toroid to extract the energy from the supply system, the flux linkages of the magnetic field must be ________
a)
zero
b)
changing or varying
c)
constant
d)
any of the mentioned
66.
Less current through the field of a shunt motor will cause a speed increase.
a)
true
b)
false
67.
No-load current in a transformer
a)
lags behind the voltage by about 75°
b)
leads the voltage by about 75°
c)
lags behind the voltage by about 15°
d)
leads the voltage by about 15°
68.
Number of parallel paths in wave winding are ______
a)
Equal to P
b)
Equal to P/2
c)
2
d)
Depends on other parameters
69.
Primary winding of a transformer
a)
is always a low voltage winding
b)
is always a high voltage winding
c)
could either be a low voltage or high voltage winding
d)
none of the above
70.
Resultant pitch in the lap winding is__________
a)
Depends on Yb value
b)
Depends on Yf value
c)
Depends on Yb and Yf value
d)
Always equal to 2
71.
The degree of mechanical vibrations produced by the laminations of a transformer depends on
a)
tightness of clamping
b)
gauge of laminations
c)
size of laminations
d)
all of the above
72.
The delta-delta connections are used in applications of
a)
large l.v. transformers
b)
small h.v. transformers
c)
large h.v. transformers
d)
small l.v. transformers
73.
The energy stored in a magnetic field is given by ____________ where L=self-inductance and Rl=reluctance.
a)
1/2 Li2
b)
1/2 (mmf*Rl)2
c)
1/2∅Rl
d)
1/2 φ2i
74.
The no-load current drawn by transformer is usually what per cent of the full- load current ?
a)
0.2 to 0.5 per cent
b)
2 to 5 per cent
c)
12 to 15 per cent
d)
20 to 30 per cent
75.
The path of a magnetic flux in a transformer should have
a)
high resistance
b)
high reluctance
c)
low resistance
d)
low reluctance
76.
The transformer which is more feasible to use in the distribution ends should be
a)
star-delta
b)
delta-star
c)
scott
d)
delta-delta
77.
When a current of 5A flows through a coil of linear magnetic circuit, it has flux linkages of 2.4 wb-turns. What is the energy stored in the magnetic field of this coil in Joules?
a)
6
b)
12
c)
1.2
d)
2.4
78.
Which of the following is not a part of transformer installation ?
a)
Conservator
b)
Breather
c)
Buchholz relay
d)
Exciter
79.
Which of the following statements regarding an idel single-phase transformer having a turn ratio of 1 : 2 and drawing a current of 10 A from 200 V A.C. supply is incorrect ?
a)
Its secondary current is 5 A
b)
Its secondary voltage is 400 V
c)
Its rating is 2 kVA
d)
Its secondary current is 20 A (e) It is a step-up transformer
80.
Which winding in a transformer has more number of turns ?
a)
Low voltage winding
b)
High voltage winding
c)
Primary winding
d)
Secondary winding
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