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WorksheetsDC MACHINE
Total questions: 118
Worksheet time: 3600secs
Name
Class
Date
1.
A D.C. series motor is that which
a)
has its field winding consisting of thick wire and less turns
b)
has a poor torque
c)
can be started easily without load
d)
has almost constant speed
2.
A three point starter is considered suitable for
a)
shunt motors
b)
shunt as well as compound motors
c)
shunt, compound and series motors
d)
all D.C. motors
3.
As -the load is increased the speed of D.C. shunt motor will
a)
reduce slightly
b)
increase slightly
c)
increase proportionately
d)
remains unchanged
4.
As there is no back emf at the instant of starting a D.C. motor, in order to prevent a heavy current from flowing though the armature circuit
a)
a resistance is connected in series with armature
b)
a resistance is connected parallel to the armature
c)
armature is temporarily open circuited
d)
a high value resistor is connected across the field winding
5.
Buses, trains, trolleys, hoists, cranes require high starting torque and therefore make use of
a)
D.C. series motor
b)
D.C. shunt motor
c)
induction motor
d)
all of above motors
6.
By looking at which part of the motor, it can be easily confirmed that a particular motor is D.C. motor?
a)
Frame
b)
Shaft
c)
Commutator
d)
Stator
7.
D.C. generators are installed near the load centres to reduce
a)
iron losses
b)
line losses
c)
sparking
d)
corona losses
8.
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
9.
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
10.
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.
11.
D.C. shunt motors are used for driving
a)
trains
b)
cranes
c)
hoists
d)
machine tools
12.
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
13.
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
14.
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
15.
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
16.
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
17.
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
18.
Hopkinson’s test on D.C. machines is conducted at
a)
no-load
b)
part load
c)
full-load
d)
overload
19.
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
20.
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
21.
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
22.
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
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 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
33.
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%
34.
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
35.
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
36.
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
37.
In the D.C. motor the iron losses occur in
a)
the field
b)
the armature
c)
the brushes
d)
the commutator
38.
In Ward-Leonard control the lower limit of speed is imposed by
a)
residual magnetism of the generator
b)
core losses of motor
c)
mechanical losses of motor and generator together
d)
all of the above
39.
No-load speed of which of the following motor will be highest ?
a)
Shunt motor
b)
Series motor
c)
Cumulative compound motor
d)
Differentiate compound motor
40.
Regenerative method of braking is based on that
a)
back e.m.f. is less than the applied voltage
b)
back e.m.f. is equal to the applied voltage
c)
back e.m.f. of rotor is more than the applied voltage
d)
none of the above
41.
Sparking at the commutator of a D.C. motor may result in
a)
damage to commutator segments
b)
damage to commutator insulation
c)
increased power consumption
d)
all of the above
42.
Speed control by Ward Leonard method gives uniform speed variation
a)
in one direction
b)
in both directions
c)
below normal speed only
d)
above normal speed only.
43.
Starters are used with D.C. motors because
a)
these motors have high starting torque
b)
these motors are not self-starting
c)
back e.m.f. of these motors is zero initially
d)
to restrict armature current as there is no back e.m.f. while starting
44.
The armature voltage control of D.C. motor provides
a)
constant torque drive
b)
constant voltage drive
c)
constant current drive
d)
none of the above
45.
The condition for maximum efficiency for a D.C. generator is
a)
eddy current losses = stray losses
b)
hysteresis losses = eddy current losses
c)
copper losses = 0
d)
variable losses = constant losses
46.
The condition for maximum power in case of D.C. motor is
a)
back e.m.f. = 2 x supply voltage
b)
back e.m.f. = | x supply voltage
c)
supply voltage = | x back e.m.f.
d)
supply voltage = back e.m.f.
47.
The current drawn by the armature of D.C. motor is directly proportional to
a)
the torque required
b)
the speed of the motor
c)
the voltage across the terminals
d)
none of the above
48.
The hysteresis loss in a D.C. machine least depends on
a)
Frequency of magnetic reversals
b)
Maximum value of flux density
c)
Volume and grade of iron
d)
Rate of flow of ventilating air
49.
The main disadvantage of the Ward-Leonard control method is
a)
high initial cost
b)
high maintenance cost
c)
low efficiency at Hght loads
d)
all of the above
50.
The mechanical power developed by a shunt motor will be maximum when the ratio of back emf to applied voltage is
a)
4.0
b)
2.0
c)
1.0
d)
0.5
51.
The plugging gives the
a)
zero torque braking
b)
smallest torque braking
c)
highest torque braking
d)
none of the above
52.
The power mentioned on the name plate of an electric motor indicates
a)
the power drawn in kW
b)
the power drawn in kVA
c)
the gross power
d)
the output power available at the shaft
53.
The purpose of retardation test on D.C. shunt machines is to find out
a)
stray losses
b)
eddy current losses
c)
field copper losses
d)
windage losses
54.
The ratio of starting torque to full-load torque is least in case of
a)
series motors
b)
shunt motors
c)
compound motors
d)
none of the above
55.
The speed of a D.C. motor can be varied by varying
a)
field current
b)
applied voltage
c)
resistance in series with armature
d)
any of the above
56.
The speed of a D.C. series motor is
a)
proportional to the armature current
b)
proportional to the square of the armature current
c)
proportional to field current
d)
inversely proportional to the armature current
57.
The speed of a D.C. shunt motor more than its full-load speed can be obtained by
a)
decreasing the field current
b)
increasing the field current
c)
decreasing the armature current
d)
increasing the armature current
58.
The starting resistance of a D.C. motor is generally
a)
low
b)
around 500 Q
c)
1000 Q
d)
infinitely large
59.
The total losses in a well designed D.C. generator of 10 kW will be nearly
a)
100 W
b)
500 W
c)
1000 W
d)
1500 W
60.
The type of D.C. motor used for shears and punches is
a)
shunt motor
b)
series motor
c)
differential compoutid D.C. motor
d)
cumulative compound D.C. motor
61.
These days D.C. motors are widely used in
a)
pumping sets
b)
air compressors
c)
electric traction
d)
machine shops
62.
Three point starter can be used for
a)
series motor only
b)
shunt motor only
c)
compound motor only
d)
both shunt and compound motor
63.
To get the speed of D.C., motor below the normal without wastage of electrical energy is used
a)
Ward Leonard control
b)
rheostatic control
c)
any of the above method
d)
none of the above method
64.
What will happen if the back emf of a D.C. motor vanishes suddenly?
a)
The motor will stop
b)
The motor will continue to run
c)
The armature may burn
d)
The motor will run noisy
65.
When the armature of a D.C. motor rotates, emf induced is
a)
self-induced e.m.f.
b)
mutually induced e.m.f.
c)
back e.m.f.
d)
none of the above
66.
When two D.C. series motors are connected in parallel, the resultant speed is
a)
more than the normal speed
b)
loss than the normal speed
c)
normal speed
d)
zero
67.
Where D.C. motor of 12 HP or more requires frequent starting, stopping, reversing and speed control
a)
drum type controller is used
b)
three point starter is used
c)
four point starter is used
d)
all above can be used
68.
Which D.C. motor has got maximum self loading property?
a)
Series motor
b)
Shunt motor
c)
Cumulatively compounded ‘motor
d)
Differentially compounded motor
69.
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
70.
Which D.C. motor will be preferred for machine tools ?
a)
Series motor
b)
Shunt motor
c)
Cumulative compound motor
d)
Differential compound motor
71.
Which D.C. motor will be suitable along with flywheel for intermittent light and heavy loads?
a)
Series motor
b)
Shunt motor
c)
Cumulatively compounded motor
d)
Differentially compounded motor
72.
Which method of braking is generally used in elevators ?
a)
Plugging
b)
Regenerative braking
c)
Rheostatic braking
d)
None of the above
73.
Which motor has the poorest speed control?
a)
Differentially compounded motor
b)
Cumulatively compounded motor
c)
Shunt motor
d)
Series motor
74.
Which motor should not be started on no-load ?
a)
Series motor
b)
Shunt motor
c)
Cumulatively compounded motor
d)
Differentially compounded motor.
75.
Which of the following application requires high starting torque ?
a)
Lathe machine
b)
Centrifugal pump
c)
Locomotive
d)
Air blower
76.
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
77.
Which of the following load normally needs starting torque more than the rated torque?
a)
Blowers
b)
Conveyors
c)
Air compressors
d)
Centrifugal pumps
78.
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
79.
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
80.
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
81.
Which of the following motor has the poorest speed regulation ?
a)
Shunt motor
b)
Series motor
c)
Differential compound motor
d)
Cumulative compound motor
82.
Which of the following motor is preferred for operation in highly explosive atmosphere ?
a)
Series motor
b)
Shunt motor
c)
Air motor
d)
Battery operated motor
83.
Which of the following motors is most suitable for signalling devices and many kinds of timers ?
a)
D.C. shunt motor
b)
D.C. series motor
c)
Induction motor
d)
Reluctance motor
84.
Which of the following motors one will choose to drive the rotary compressor ?
a)
D.C. shunt motor
b)
D.C. series motor
c)
Universal motor
d)
Synchronous motor
85.
Which of the following steps is likely to result in reduction of hysteresis loss in a D.C. generator ?
a)
Providing laminations in armature core
b)
Providing laminations in stator
c)
Using non-magnetic material for frame
d)
Using material of low hysteresis co-efficient for armature core material
86.
Which of the following tests will be suitable for testing two similar D.C. series motors of large capacity ?
a)
Swinburne’s test
b)
Hopkinson’s test
c)
Field test
d)
Brake test
87.
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.
88.
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
89.
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
90.
Commutator performs rectification so that output of the machine is unidirectional.
a)
true
b)
false
91.
For which of the following operations a DC motor is preferred over an AC motor?
a)
a) Low speed operation
b)
b) High speed operation
c)
c) Variable speed operation
d)
d) Fixed speed operation
92.
In a DC machine fractional pitch winding is used to _______________
a)
a) To improve cooling
b)
b) To reduce sparking
c)
c) To reduce copper losses
d)
d) To increase generated EMF
93.
Maximum torque in a DC machine is limited by ______________
a)
a) Commutation
b)
b) Heating
c)
c) Losses other than heating
d)
d) Stability
94.
Wave winding machines are used in ______ currents applications.
a)
a) High
b)
b) Moderate
c)
c) Low
d)
d) Can be used anywhere
95.
What is the value of Yb for a lap winding with a 4-pole, 12-slot armature with two coil sides/slot. Assume single-turns coils.
a)
a) 3
b)
b) 5
c)
c) 7
d)
d) 9
96.
A series DC motor may be connected as a shunt motor, and a shunt motor may be connected as a series motor, if necessary.
a)
true
b)
false
97.
A shunt motor, started with resistance inserted in its field, will develop higher starting torque than without resistance.
a)
true
b)
false
98.
A two pole motor has its field windings series connected, and then paralleled with the armature. This indicates a series motor.
a)
true
b)
false
99.
An under compounded generator has better voltage regulation than a straight shunt generator (of equal rating).
a)
true
b)
false
100.
As a general rule, to change rotation for a compound motor, it is best to reverse current flow through the armature and reverse both field windings.
a)
true
b)
false
101.
DC shunt motor is still used instead of synchronous motor in _____________
a)
a) High speed applications
b)
b) Low speed applications
c)
c) Medium and high-speed applications
d)
d) Everywhere
102.
Differential connected motors have lower starting torque than cumulative connected compound motors.
a)
true
b)
false
103.
Emf and torque produced in a DC machine are proportional to ________ and _________ respectively.
a)
a) Armature speed and armature emf
b)
b) Armature emf and armature speed
c)
c) Armature current and armature emf
d)
d) Armature speed and armature current
104.
Equalizer rings are needed in lap winding.
a)
True
b)
false
105.
For a progressive wave winding Yc = ______
a)
a) 2C/P
b)
b) 2(C+1)/P
c)
c) 2(C-1)/P
d)
d) 2C/(P+1)
106.
For a wave winding when a machine is 6-pole DC armature with 16 slots having 2-coil sides per slot and single-turn coil, Yf value is ____
a)
a) 5
b)
b) 3
c)
c) 2
d)
d) 7
107.
Function of yoke is to provide the return path for magnetic flux.
a)
true
b)
false
108.
If the supply lines to a series DC motor are reversed, the armature will rotate in the opposite direction.
a)
true
b)
false
109.
In a cumulative compound motor, reversing the series field connections will change the direction of rotation
a)
true
b)
false
110.
in lap winding What is the relation between number of parallel paths(A) and number of poles(P)?<br />
a)
a) A = P
b)
b) A < P
c)
c) A > P
d)
d) No relation exists
111.
In normal dc machines operating at full-load conditions, the most powerful electromagnet is _________
a)
a) Field winding
b)
b) Interpole Winding
c)
c) Interpole and compensating winding together
d)
d) Armature winding
112.
In which mode machine is operating, given that conductor current is in the same direction of conductor emf?
a)
a) Motoring
b)
b) Generating
c)
c) Can’t be determined using directions
d)
d) In both modes for different cycles
113.
It is best to reverse the shunt field leads of a cumulative compound motor to reverse rotation.
a)
true
b)
false
114.
Less current through the field of a shunt motor will cause a speed increase.
a)
true
b)
false
115.
No-load speed of which of the following motor will be highest ?
a)
(A)Shunt motor
b)
(b) Series motor
c)
(c) Cumulative compound motor
d)
(d) Differentiate compound motor
116.
Number of parallel paths in wave winding are ______
a)
a) Equal to P
b)
b) Equal to P/2
c)
c) 2
d)
d) Depends on other parameters
117.
Resultant pitch in the lap winding is__________
a)
a) Depends on Yb value
b)
b) Depends on Yf value
c)
c) Depends on Yb and Yf value
d)
d) Always equal to 2
118.
Separately excited DC generators are still used in _________________
a)
a) Thermal power plants
b)
b) Ward Leonard speed control system
c)
c) Hydro power plant
d)
d) In all fields
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