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WorksheetsDC Motors
Total questions: 123
Worksheet time: 1hrs 4mins
No-load speed of which of the following motor will be highest ?
Shunt Motor
Series Motor
Cumulative Compound Motor
Differential Compound Motor
The direction of rotation of a D.C. series motor can be changed by
(a) interchanging supply terminals
(b) interchanging field terminals
either a and b
none of the above
Which of the following application requires high starting torque ?
Lathe machine
Centrifugal Pump
Locomotive
Air blower
If a D.C. motor is to be selected for conveyors, which motor would be preferred ?
Series Motor
Shunt Motor
Differential Compound Motor
Cumulative Compound Motor
Which D.C. motor will be preferred for machine tools ?
Series motor
Shunt motor
Cumulative Compound motor
Differential Compound motor
Differentially compound D.C. motors can find applications requiring
high starting torque
low starting torque
variable speed
frequent on-off cycles
Which D.C. motor is preferred for elevators ?
Shunt motor
Series motor
Differential compound motor
Cumulative compound motor
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
Lines of flux
resultant force on conductor
none of the above
Starters are used with D.C. motors because
the 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
If the field of a D.C. shunt motor gets opened while motor is running
the speed of motor will be reduced %
the armature current will reduce
the motor will attain dangerously high speed
the motor will remain constant
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
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
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
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
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 field winding by eddy currents
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
When two D.C. series motors are connected in parallel, the resultant speed is
more than the normal speed
less than the normal speed
normal speed
zero
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
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
A direct on line starter is used: for starting motors
up to 5 HP
up to 10 HP
up to 15 HP
up to 20 HP
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
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
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
These days D.C. motors are widely used in
pumping sets
air compressors
electric traction
machine shops
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
In which of the following applications D.C. series motor is invariably tried?
starter for a car
drive for a water pump
fan motor
motor operation in AC or DC
In D.C. machines fractional pitch winding is used
to improve cooling
to reduce copper loss
to increase the generated emf
to reduce the sparking
A three point starter is considered suitable for
shunt motor
shunt as well as compound motor
shunt, compound and series motor
all dc motors
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%
The ratio of starting torque to full-load torque is least in case of
series motors
shunt motors
compound motors
none of the above
In D.C. motor which of the following can sustain the maximum temperature rise?
slip rings
commutator
field winding
armature winding
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
Which of the following load normally needs starting torque more than the rated torque?
Blowers
Conveyors
Air Compressors
Centrifugal Pumps
The starting resistance of a D.C. motor is generally
low
around 500 Q
1000 Q
infinitely large
the speed of a D.C. series motor is
proportional to the armature current
proportional to the square of the armature current
proportional to the field current
inversely proportional to the armature current
In a D.C. series motor, if the armature current is reduced by 50%, the torque of the motor will be equal
100% of the previous value
50% of the previous value
25% of the previous value
10% of the previous value
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
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
Which D.C. motor has got maximum self loading property?
Series motor
Shunt motor
Cumulatively compounded motor
Differentially compounded motor
Which D.C. motor will be suitable along with flywheel for intermittent light and heavy loads?
Series motor
Shunt motor
Cumulatively compounded motor
Differentially compounded motor
If a D.C. shunt motor is working at no load and if shunt field circuit suddenly opens
nothing will happen to the 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
motor will run at very slow speed
D.C. series motors are used
where load is constant
where load changes frequently
where constant operating speed is needed
in none of the above situations
For the same H.P. rating and full load speed, following motor has poor starting torque
shunt
series
differentially compounded
cumulatively compounded
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
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
Which of the following motor is preferred for operation in highly explosive atmosphere ?
Series motor
Shunt motor
Air motor
Battery operated motor
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
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 uniformity
To reduce the reluctance of the magnetic path
The mechanical power developed by a shunt motor will be maximum when the ratio of back e.m.f. to applied voltage is
4
2
1
.5
The condition for maximum power in case of D.C. motor is
back e.m.f. = 2 x supply voltage
back e.m.f. = 1/2 x supply voltage
supply voltage = 1/2 x back e.m.f.
supply voltage = back e.m.f.
For which of the following applications a D.C. motor is preferred over an A.C. motor ?
low speed regulation
high speed operation
variable speed operation
fixed speed operation
In D.C. machines the residual magnetism is of the order of
2 to 3 percent
10 to 15 percent
20 to 25 percent
50 to 75 percent
Which D.C. motor is generally preferred for cranes and hoists ?
series motor
shunt motor
cumulative compounded motor
differentially compounded motor
Three point starter can be used for
series motor only
shunt motor only
compound motor only
both shunt and compound motor
Sparking, is discouraged in a D.C. motor because
(a) it increases the input power consumption
(b) commutator gets damaged
both (a) and (b)
none of the above
Speed control by Ward Leonard method gives uniform speed variation
in one direction
in both directions
below normal speed only
above normal speed only
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
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
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
Which of the following motor has the poorest speed regulation ?
shunt motor
series motor
differential compound motor
cumulative compound motor
Buses, trains, trolleys, hoists, cranes require high starting torque and therefore make use of
DC series motor
DC shunt motor
induction motor
all of the above
As -the load is increased the speed of D.C. shunt motor will
reduce slightly
increase slightly
increase proportionately
remains unchanged.
The armature torque of the D.C. shunt motor is proportional to
(a) field flux only
(b) armature current only
both a and b
none of the above
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 of the above methods
Usually wide and sensitive speed control is desired in case of
centrifugal pumps
elevators
steel rolling mills
colliery winders
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%
The armature voltage control of D.C. motor provides
constant torque drive
constant voltage drive
constant current drive
none of the above
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
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
If Ia be the armature current, then speed of a D.C. shunt motor is
independent of Ia
proportional to la
varies as Ia
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
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
The speed of a D.C. motor can be varied by varying
field current
applied voltage
resistance in series with the armature
any of the above
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
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
Which motor has the poorest speed control?
Differentially compounded motor
Cumulatively compounded motor
Shunt motor
Series motor
The plugging gives the
zero torque braking
smallest torque braking
highest torque braking
none of the above
The armature voltage control of D.C. motor provides
constant voltage drive
constant current drive
constant torque drive
none of the above
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 recommended by manufacturer and select the next higher H.P. motor
If the terminals of armature of D.C. motor are interchanged, this action will offer following kind of braking
regenerative
plugging
dynamic braking
none of the above
any of the above
Which of the following motors one will choose to drive the rotary compressor ?
DC shunt motor
DC series motor
Universal motor
Synchronous motor
If the speed of a D.C. shunt motor is increased, the back e.m.f. of the motor will
increase
decrease
remain the same
becomes zero
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
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
Synchronous motor
Which of the following motors is most suitable for signaling devices and many kinds of timers ?
DC shunt motor
DC series motor
Induction motor
Reluctance motor
Which motor should not be started on no-load ?
series motor
shunt motor
cumulatively compounded motor
differentially compounded motor
Ward-Leonard control is basically a
voltage control method
field diverter method
field control method
armature resistance control method
For constant torque drive which speed control method is preferred ?
Field control
Armature voltage control
Shunt armature control
Mechanical loading system
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 generator together
all of the above
The main disadvantage of the Ward-Leonard control method is
high initial cost
high maintenance cost
low efficiency at High loads
all of the above
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
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
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
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
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
Torque developed by a D.C. motor depends upon
magnetic field
active length of the conductor
current flow through the conductors
number of conductors
all above factors
D.C. shunt motors are used for driving
trains
cranes
hoists
machine tools
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 parallel 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
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
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
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
The total losses in a well designed D.C. generator of 10 kW will be nearly
100 W
500 W
1000 W
1500 W
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
D.C. generators are normally designed for maximum efficiency around
full-load
rated r.p.m.
rated voltage
all of the above
In a D.C. generator, the iron losses mainly take place in
yoke
commutator
armature conductors
armature rotor
D.C. generators are installed near the load centers to reduce
iron losses
line losses
sparking
corona losses
The purpose of retardation test on D.C. shunt machines is to find out
stray losses
eddy current losses
field copper losses
windage losses
Which of the following tests will be suitable for testing two similar D.C. series motors of large capacity ?
Swinburne’s test
Field test
Hopkinson’s test
Brake test
Hopkinson’s test on D.C. machines is conducted at
no-load
part load
full-load
overload
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
For which types of D.C. motor, dynamic braking is generally used ?
Shunt motors
Series motors
Compound motors
All of the above
Which method of braking is generally used in elevators ?
Plugging
Regenerative braking
Rheostatic braking
None of the above
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
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
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
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
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
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
Which D.C. motor has got maximum self relieving property ?
Series motor
Shunt motor
Cumulatively compounded motor
Differentially compounded motor
In the D.C. motor the iron losses occur in
the field
the armature
the brushes
the commutator
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
none of the above methods
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
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