Important Dc Motors Objective Questions For Competitive’s
1.In a D.C. shunt motor, speed is
independent of armature current
directly proportional to the armature current
proportional to the square of the current
inversely proportional to the armature current
ANSWER : : independent of armature current
2.The type of D.C. motor used for shears and punches is
shunt motor
series motor
differential compoutid D.C. motor
cumulative compound D.C. motor
ANSWER : : cumulative compound D.C. motor
3.If a D.C. motor is connected across the A.C. supply it will
run at normal speed
not run
run at lower speed
burn due to heat produced in the field winding by .eddy currents
ANSWER : : burn due to heat produced in the field winding by .eddy currents
4.In a D.C. shunt motor, under the conditions of maximum power, the current in the armature will be
almost negligible
rated full-load current
less than full-load current
more than full-load current
ANSWER : : more than full-load current
5.Differentially compound D.C. motors can find applications requiring
high starting torque
low starting torque
variable speed
frequent on-off cycles
ANSWER : : low starting torque
6.The speed of a D.C. shunt motor more than its full-load speed can be obtained by
decreasing the field current
increasing the field current
decreasing the armature current
increasing the armature current
ANSWER : : decreasing the field current
7.A direct on line starter is used: for starting motors
iip to 5 H.P.
up to 10 H.P.
up to 15 H.P.
up to 20 H.P.
ANSWER : : iip to 5 H.P.
8.By looking at which part of the motor, it can be easily confirmed that a particular motor is D.C. motor?
Frame
Shaft
Commutator
Stator
ANSWER : : Commutator
9.These days D.C. motors are widely used in
pumping sets
air compressors
electric traction
machine shops
ANSWER : : electric traction
10.To get the speed of D.C, motor below the normal without wastage of electrical energy is used.
Ward Leonard control
rheostatic control
any of the above method
none of the above method
ANSWER : : Ward Leonard control
11.According to Fleming’s left-hand rule, when the forefinger points in the direction of the field or flux, the middle finger will point in the direction of
current in the conductor aovtaat of conductor
resultant force on conductor
none of the above
current in the conductor aovtaat of conductor
ANSWER : If the field of a D.C. shunt motor gets opened while motor is running
12.In case of D.C. shunt motors the speed is dependent on back e.m.f. only because
back e.m.f. is equal to armature drop
armature drop is negligible
flux is proportional to armature current
flux is practically constant in D:C. shunt motors
ANSWER : : flux is practically constant in D:C. shunt motors
13.A D.C. series motor is that which
has its field winding consisting of thick wire and less turns
has a poor torque
can be started easily without load
has almost constant speed
ANSWER : : has its field winding consisting of thick wire and less turns
14.When two D.C. series motors are connected in parallel, the resultant speed is
more than the normal speed
loss than the normal speed
normal speed
zero
ANSWER : : normal speed
15.If a D.C. motor is to be selected for conveyors, which rriotor would be preferred ?
Series motor
Shunt motor
Differentially compound motor
Cumulative compound motor
ANSWER : : Series motor
16.Starters are used with D.C. motors because
these motors have high starting torque
these motors are not self-starting
back e.m.f. of these motors is zero initially
to restrict armature current as there is no back e.m.f. while starting
ANSWER : : to restrict armature current as there is no back e.m.f. while starting
17.Which D.C. motor will be preferred for machine tools ?
Series motor
Shunt motor
Cumulative compound motor
Differential compound motor
ANSWER : : Shunt motor
18.The direction of rotation of a D.C. series motor can be changed by
interchanging supply terminals
interchanging field terminals
either of (a) and (b) above
None of the above
ANSWER : : interchanging field terminals
19.Which of the following application requires high starting torque ?
Lathe machine
Centrifugal pump
Locomotive
Air blower
ANSWER : : Locomotive
20.For starting a D.C. motor a starter is required because
it limits the speed of the motor
it limits the starting current to a safe value
it starts the motor
none of the above
ANSWER : : it limits the starting current to a safe value
21.Which D.C. motor is preferred for elevators ?
Shunt motor
Series motor
Differential compound motor
Cumulative compound motor
ANSWER : : Cumulative compound motor
22.What will happen if the back e.m.f. of a D.C. motor vanishes suddenly?
The motor will stop
The motor will continue to run
The armature may burn
The motor will run noisy
ANSWER : : The armature may burn
23.In D.C. shunt motors as load is reduced
the speed will increase abruptly
the speed will increase in proportion to reduction in load
the speed will remain almost/constant
the speed will reduce
ANSWER : : the speed will remain almost/constant
24.The current drawn by the armature of D.C. motor is directly proportional to
the torque required
the speed of the motor
the voltage across the terminals
none of the above
ANSWER : : the torque required
25.The power mentioned on the name plate of an electric motor indicates
the power drawn in kW
the power drawn in kVA
the gross power
the output power available at the shaft
ANSWER : : the output power available at the shaft
26.The speed of a D.C. series motor is
proportional to the armature current
proportional to the square of the armature current
proportional to field current
inversely proportional to the armature current
ANSWER : : inversely proportional to the armature current
27.The ratio of starting torque to full-load torque is least in case of
series motors
shunt motors
compound motors
none of the above
ANSWER : : shunt motors
28.In D.C. motor which of the following can sustain the maximum temperature rise?
Slip rings
Commutator
Field winding
Armature winding
ANSWER : : Field winding
29.Which of the following law/rule can he used to determine the direction of rotation of D.C. motor ?
Lenz’s law
Faraday’s law
Coloumb’s law
Fleming’s left-hand rule
ANSWER : : Fleming’s left-hand rule
30.The mechanical power developed by a shunt motor will be maximum when the ratio of back e.m.f. to applied voltage is
4.0
2.0
1.0
0.5
ANSWER : : 0.5
31.The condition for maximum power in case of D.C. motor is
back e.m.f. = 2 x supply voltage
back e.m.f. = | x supply voltage
supply voltage = | x back e.m.f.
supply voltage = back e.m.f.
ANSWER : : back e.m.f. = | x supply voltage
32.In D.C. machines fractional pitch winding is used
to improve cooling
to reduce copper losses
to increase the generated e.m.f.
to reduce the sparking
ANSWER : : to reduce the sparking
33.For the same H.P. rating and full load speed, following motor has poor starting torque
shunt
series
differentially compounded
cumulativelyc’ompounded
ANSWER : : differentially compounded
34.The starting resistance of a D.C. motor is generally
low
around 500 Q
1000 Q
infinitely large
ANSWER : : low
35.Which D.C. motor will be suitable alongwith flywheel for intermittent light and heavy loads?
Series motor
Shunt motor
Cumulatively compounded motor
Differentially compounded motor
ANSWER : : Cumulatively compounded motor
36.In a D.C. series motor, if the armature current is reduced by 50%, the torque of the motor will be equal to
100% of the previous value
50% of the previous value
25% of the previous value
10% of the previous value
ANSWER : : 25% of the previous value
37.If a D.C. shunt motor is working at no load and if shunt field circuit suddenly opens
nothing will happen to th� motor
this will make armature to take heavy current, possibly burning it
this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses (d) motor will run at very slow speed
this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses (d) motor will run at very slow speed
ANSWER : D.C. series motors are used
38.In case-the conditions for maximum power for a D.C. motor are established, the efficiency of the motor will be
100%
around 90%
anywhere between 75% and 90%
less than 50%
ANSWER : : less than 50%
39.In case of conductively compensated D.C. series motors, the compensating winding is provided
as separately wound unit
in parallel with armature winding
in series with armature winding
in parallel with field winding
ANSWER : : in series with armature winding
40.Which of the following load normally needs starting torque more than the rated torque?
Blowers
Conveyors
Air compressors
Centrifugal pumps
ANSWER : : Conveyors
41.Which D.C. motor has got maximum self loading property?
Series motor
Shunt motor
Cumulatively compounded ‘motor
Differentially compounded motor
ANSWER : : Differentially compounded motor
42.A three point starter is considered suitable for
shunt motors
shunt as well as compound motors
shunt, compound and series motors
all D.C. motors
ANSWER : : shunt as well as compound motors
43.If the supply voltage for a D.C. motor is increased, which of the following will decrease ?
Starting torque
Operating speed
Full-load current
All of the above
ANSWER : : Full-load current
44.Which one of the following is not the function of pole shoes in a D.C. machine ?
To reduce eddy current loss
To support the field coils
To spread out flux for better unifor-mity
To reduce the reluctance of the mag-netic path
ANSWER : : To reduce eddy current loss
45.Which of the following motor is preferred for operation in highly explosive atmosphere ?
Series motor
Shunt motor
Air motor
Battery operated motor
ANSWER : : Air motor
46.Sparking at the commutator of a D.C. motor may result in
damage to commutator segments
damage to commutator insulation
increased power consumption
all of the above
ANSWER : : all of the above
47.Which of the following method of speed control of D.C. machine will offer minimum efficiency ?
Voltage control method
Field control method
Armature control method
All above methods
ANSWER : : Armature control method
48.Sparking, is discouraged in a D.C. motor because
it increases the input power con-sumption
commutator gets damaged
both (a) and (b)
none of the above
ANSWER : : commutator gets damaged
49.The speed of a D.C. motor can be varied by varying
field current
applied voltage
resistance in series with armature
any of the above
ANSWER : : any of the above
50.If I2 be the armature current, then speed of a D.C. shunt motor is
independent of Ia
proportional to la
varies as (Ia)
varies as la
ANSWER : : independent of Ia
51.Which D.C. motor is generally preferred for cranes and hoists ?
Series motor
Shunt motor
Cumulatively compounded motor
Differentially compounded motor
ANSWER : : Series motor
52.Speed control by Ward Leonard method gives uniform speed variation
in one direction
in both directions
below normal speed only
above normal speed only.
ANSWER : : in both directions
53.Which of the following motor has the poorest speed regulation ?
Shunt motor
Series motor
Differential compound motor
Cumulative compound motor
ANSWER : : Series motor
54.Three point starter can be used for
series motor only
shunt motor only
compound motor only
both shunt and compound motor
ANSWER : : both shunt and compound motor
55.Flywheel is used with D.C. compound motor to reduce the peak demand by the motor, compound motor will have to be
level compounded
under compounded
cumulatively compounded
differentially compounded
ANSWER : : cumulatively compounded
56.Which one of the following is not necessarily the advantage of D.C. motors over A.C. motors ?
Low cost
Wide speed range
Stability
High starting torque.
ANSWER : : Low cost
57.Following motor is used where high starting torque and wide speed range control is required.
Single phase capacitor start
Induction motor
Synchronous motor
D.C. motor
ANSWER : : D.C. motor
58.At the instant of starting when a D.C. motor is put on supply, it behaves like
a highly resistive circuit
a low resistance circuit
a capacitive circuit
none of the above
ANSWER : : a low resistance circuit
59.As -the load is increased the speed of D.C. shunt motor will
reduce slightly
increase slightly
increase proportionately
remains unchanged
ANSWER : : reduce slightly
60.In D.C. machines the residual magnetism is of the order of
2 to 3 per cent
10 to 15 per cent
20 to 25 per cent
50 to 75 per cent
ANSWER : : 2 to 3 per cent
61.The armature voltage control of D.C. motor provides
constant torque drive
constant voltage drive
constant current drive
none of the above
ANSWER : : constant torque drive
62.For a D.C. shunt motor if the excitation is changed
torque will remain constant
torque will change but power will remain constant
torque and power both will change
torque, power and speed, all will change
ANSWER : : torque will change but power will remain constant
63.Usually wide and sensitive speed control is desired in case of
centrifugal pumps
elevators
steel rolling mills
colliery winders
ANSWER : : colliery winders
64.As there is no back e.m.f. at the instant of starting a D.C. motor, in order to prevent a heavy current from flowing though the armature circuit
a resistance is connected in series with armature
a resistance is connected parallel to the armature
armature is temporarily open circuited
a high value resistor is connected across the field winding
ANSWER : : a resistance is connected in series with armature
65.In a differentially compounded D.C. motor, if shunt field suddenly opens
the motor will first stop and then run in opposite direction as series motor
the motor will work as series motor and run at slow speed in the same direction
the motor will work as series motor and run at high speed in the same direction
the motor will not work and come to stop
ANSWER : : the motor will first stop and then run in opposite direction as series motor
66.The speed of a D.C. shunt motor can be increased by
increasing the resistance in armature circuit
increasing the resistance in field circuit
reducing the resistance in the field circuit
reducing the resistance in the armature circuit
ANSWER : : increasing the resistance in field circuit
67.The armature torque of the D.C. shunt motor is proportional to
field flux only
armature current only
both (a) and (b)
none of the above
ANSWER : : armature current only
68.In case the back e.m.f. and the speed of a D.C. motor are doubled, the torque developed by the motor will
remain unchanged
reduce to one-fourth value
increase four folds
be doubled
ANSWER : : remain unchanged
69.The speed of a motor falls from 1100 r.p.m. at no-load to 1050 r.p.m. at rated load. The speed regulation of the motor is
2.36%
4.76%
6.77%
8.84%
ANSWER : : 4.76%
70.Buses, trains, trolleys, hoists, cranes require high starting torque and therefore make use of
D.C. series motor
D.C. shunt motor
induction motor
all of above motors
ANSWER : : D.C. series motor
71.The armature voltage control of D.C. motor provides
constant voltage drive
constant current drive
constant torque drive
none of the above
ANSWER : : constant torque drive
72.Which of the following loss in a D.C. generator varies significantly with the load current ?
Field copper loss
Windage loss
Armature copper loss
None of the above
ANSWER : : Armature copper loss
73.Which of the following motors is most suitable for signalling devices and many kinds of timers ?
D.C. shunt motor
D.C. series motor
Induction motor
Reluctance motor
ANSWER : : Reluctance motor
74.If the speed of a D.C. shunt motor is increased, the back e.m.f. of the motor will
increase
decrease
remain same
become zero
ANSWER : : increase
75.If a D.C. motor designed for 40�C ambient temperature is to be used for 50�C ambient temperature, then the motor
of lower H.P. should be selected
of higher H.P. should be selected
can be used for 50�C ambient temperature also
is to be derated by a factor recom-mended by manufacturer and select the next higher H.P. motor
ANSWER : : is to be derated by a factor recom-mended by manufacturer and select the next higher H.P. motor
76.D.C. shunt motors are used for driving
trains
cranes
hoists
machine tools
ANSWER : : machine tools
77.The main disadvantage of the Ward-Leonard control method is
high initial cost
high maintenance cost
low efficiency at Hght loads
all of the above
ANSWER : : all of the above
78.Which of the following motors is usually used in house-hold refrigerators ?
D.C. shunt motor
D.C. series motor
Single phase induction motor (split phase start or induction run motor)
Reluctance motor
ANSWER : : Single phase induction motor (split phase start or induction run motor)
79.Regenerative method of braking is based on that
back e.m.f. is less than the applied voltage
back e.m.f. is equal to the applied voltage
back e.m.f. of rotor is more than the applied voltage
none of the above
ANSWER : : back e.m.f. is equal to the applied voltage
80.In Ward-Leonard control the lower limit of speed is imposed by
residual magnetism of the generator
core losses of motor
mechanical losses of motor and gen�erator together
all of the above
ANSWER : : residual magnetism of the generator
81.Ward-Leonard control is basically a
voltage control method
field divertor method
field control method
armature resistance control method
ANSWER : : voltage control method
82.Which of the following motors one will choose to drive the rotary compressor ?
D.C. shunt motor
D.C. series motor
Universal motor
Synchronous motor
ANSWER : : Synchronous motor
83.Which of the following steps is likely to result in reduction of hysteresis loss in a D.C. generator ?
Providing laminations in armature core
Providing laminations in stator
Using non-magnetic material for frame
Using material of low hysteresis co-efficient for armature core material
ANSWER : : Using material of low hysteresis co-efficient for armature core material
84.Which of the following loss in a D.C. generator is dissipated in the form of heat?
Mechanical loss
Core loss
Copper loss
All of the above
ANSWER : : All of the above
85.In a D.C. generator all of the following could be the effects of iron losses except
Loss of efficiency
Excessive heating of core
Increase in terminal voltage
Rise in temperature of ventilating air
ANSWER : : Increase in terminal voltage
86.For constant torque drive which speed control method is preferred ?
Field control
Armature voltage control
Shunt armature control
Mechanical loading system
ANSWER : : Armature voltage control
87.The losses occurring in a D.C. generator are given below. Which loss is likely to have highest proportion at rated load of the generator ?
hysteresis loss
field copper loss
armature copper loss
eddy current loss
ANSWER : : armature copper loss
88.The hysteresis loss in a D.C. machine least depends on
Frequency of magnetic reversals
Maximum value of flux density
Volume and grade of iron
Rate of flow of ventilating air
ANSWER : : Rate of flow of ventilating air
89.Which motor should not be started on no-load ?
Series motor
Shunt motor
Cumulatively compounded motor
Differentially compounded motor.
ANSWER : : Series motor
90.If the terminals of armature of D.C. motor are interchanged, this action will offer following kind of braking (o) regenerative
plugging
dynamic braking
none of the above
any of the above
ANSWER : : plugging
91.In a manual shunt motor starter
over load relay is connected in series and no volt relay in parallel with the load
over load relay is connected in paral�lel and no volt relay in series with the load
over load relay and no volt relay are both connected in series with the load
over load relay and no volt relay are both connected in parallel with the load
ANSWER : : over load relay is connected in series and no volt relay in parallel with the load
92.Why are the D.C. motors preferred for traction applications ?
Torque and speed are inversely proportional to armature current
Torque is proportional to armature current
Torque is proportional to square root of armature current
The speed is inversely proportional to the torque and the torque is proportional to square of armature current
ANSWER : : The speed is inversely proportional to the torque and the torque is proportional to square of armature current
93.The plugging gives the
zero torque braking
smallest torque braking
highest torque braking
none of the above
ANSWER : : highest torque braking
94.Which of the following losses are significantly reduced by laminating the core of a D.C. generator ?
Hysteresis losses
Eddy current losses
Copper losses
Windage losses
ANSWER : : Eddy current losses
95.D.C. motor is to a drive a load which is almost nil for certain part of the load cycle and peak value for short duration. We will select this
series motor
shunt motor
compound motor
any of the above
ANSWER : : compound motor
96.Where D.C. motor of H.P. 12 or more requires frequent starting, stopping, reversing and speed control
drum type controller is used
three point starter is used
four point starter is used
all above can be used
ANSWER : : drum type controller is used
97.D.C. generators are normally designed for maximum efficiency around
full-load
rated r.p.m.
rated voltage
all of the above
ANSWER : : full-load
98.The purpose of retardation test on D.C. shunt machines is to find out
stray losses
eddy current losses
field copper losses
windage losses
ANSWER : : stray losses
99.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
series motor
shunt motor
compound motor
any of the above
ANSWER : : series motor
100.In variable speed motor
a stronger commutating field is needed at low speed than at high speed
a weaker commutating field is needed at low speed than at high speed
same commutating field is needed at low speed than at high speed
none of the above is correct
ANSWER : : a weaker commutating field is needed at low speed than at high speed
101.In the D.C. motor the iron losses occur in
the field
the armature
the brushes
the commutator
ANSWER : : the armature
102.One D.C. motor drives another D.C. motor. The second D.C. motor when excited and driven
runs as a generator
does not run as a generator
also runs as a motor comes to stop after sometime
also runs as a motor comes to stop after sometime
ANSWER : : runs as a generator
103.Which of the following tests will be suitable for testing two similar D.C. series motors of large capacity ?
Swinburne’s test
Hopkinson’s test
Field test
Brake test
ANSWER : : Field test
104.The speed of a D.C. shunt motor is required to be more than full load speed. This is possible by
reducing the field current
decreasing the armature current
increasing the armature current
increasing the excitation current
ANSWER : : reducing the field current
105.If a D.C. shunt motor is working at full load and if shunt field circuit suddenly opens
this will make armature to take heavy current, possibly burning it
this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses
nothing will happen to motor
motor will come to stop
ANSWER : : this will make armature to take heavy current, possibly burning it
106.During rheostat braking of D.C. series motors
motor is run as a generator
motor is reversed in direction
motor is run at reduced speed
motor is run as a generator
ANSWER : For which types of D.C. motor, dynamic braking is generally used ?
107.Hopkinson’s test on D.C. machines is conducted at
no-load
part load
full-load
overload
ANSWER : : full-load
108.The condition for maximum efficiency for a D.C. generator is
eddy current losses = stray losses
hysteresis losses = eddy current losses
copper losses = 0
variable losses = constant losses
ANSWER : : variable losses = constant losses
109.D.C. generators are installed near the load centres to reduce
iron losses
line losses
sparking
corona losses
ANSWER : : line losses
110.When the armature of a D.C. motor rotates, e.m.f. induced is
self-induced e.m.f.
mutually induced e.m.f.
back e.m.f.
none of the above
ANSWER : : back e.m.f.
111.Which method of braking is generally used in elevators ?
Plugging
Regenerative braking
Rheostatic braking
None of the above
ANSWER : : Plugging
112.Which D.C. motor has got maximum self relieving property ?
Series motor
Shunt motor
Cumulatively compounded motor
Differentially compounded motor
ANSWER : : Series motor
113.In a D.C. generator, the iron losses mainly take place in
yoke
commutator
armature conductors
armature rotor
ANSWER : : armature rotor