WorksheetsELM1 306
Total questions: 77
Worksheet time: 1hrs 17mins
A DC generator works on the principle of
Electro Magnetic Field
Thermal Electric Field
Electro Magnetic Induction
Magnetic Field
Series Generators are generally NOT used because of their terminal voltage:1. 2. 3.
is very low
varies with load variation
is very high
is very high
In Separately Excited Generator, the field windings are connected in
close circuit
Series with source
open circuit
Shunt with source
The function of pole-shoes in a DC machine is
To increase field winding resistance
To decrease field winding resistance
To increase the armature winding resistance
Uniform distribution of flux
A separately excited generator as compared to a self-excited generator
Easier to control the voltage
Is more stable
His exciting current independent of load current
Has all above features
Series wound Generator, shunt Generator and compound Generator are examples of DC generator
Seif-excited
Permanent Magnet
Separately excited
Configuration
Short - - - - - Compound Generator is a type of DC generator.
Shunt
Negative.
Series
Permanent
The number of parallel paths in an 8 pole, simplex lap wound armature in DC generator is
16
2
8
4
The field coils of a D.C. Generator are usually made of
Mica
Copper
Cast iron
. Carbon
In a D.C. machine stray loss is the sum of
Total copper loss and mechanical loss
Armature copper loss and iron loss
Shunt field copper loss and mechanical loss
Iron loss and mechanical loss
Which of the following generator will have negligible terminal voltage while running on no-load?
Series generator
Shunt generator
Compound generator
Separately excited generator
In a separately excited generator supplying rated load the armature reaction:
Is always present
Is always absent
May be sometimes present
Note serious
When two D.C. series generators are running in parallel, an equalizer bar is used
To increase the speed and hence generated e.m.f.
To increase the series flux
So that two similar machines will pass approximately equal currents to the load
To reduce the combined effect of armature reaction of both machines
A D.C. generator works on the principle of
Lenz's law
Ohm's law
Faraday's law of electromagnetic induction
norton theorem
A D.C. welding generator has
Lap winding
Wave moving
Duplex winding
simplex winding
In D.C. generators, the cause of rapid brush wear may be
Severe sparking
Rough commutator surface
Imperfect contact
Any of the above
The insulating material used between the commutator segments is normally
Graphite
Paper
Mica
Insulating varnish
Satisfactory commutation of D.C. machines requires
Brushes should be of proper grade and size
Brushes should smoothly run in the holders
Smooth, concentric commutator properly undercut
All of the above
Which of the following statement about D.C. generators is false?
Compensating winding in a D.C. machine helps in commutation
In a D. C. generator interpoles winding is connected in series with the armature winding
Back pitch and front pitch are both odd and approximately equal to the pole pitch
Equalizing bus bars are used with parallel running of D.C. shunt generators
The following constitute short-circuit in the armature winding.
Insulation failure between two commutator bars
Insulation failure between two turns of a coil
Two of more turns of the same coil getting grounded
All of the above
Compensating windings are used in D.C. generators
Mainly to reduce the eddy currents by providing local short-circuits
To provide path for the circulation of cooling air
To neutralize the cross-magnetizing effect of the armature reaction
None of the above
In a D.C. generator the critical resistance refers to the resistance of
Brushes
Field
Armature
Load
Lap winding is composed of
Any even number of conductors
Any odd number of conductors
That even number which is exact multiple of poles + 2
That even number which is exact multiple of poles
Which of the following generators will be preferred if they are required to be run in parallel?
Shunt generators
Series generators
Compound generators
None of the above
Laminations of core are generally made of
Case iron
Carbon
Silicon steel
Stainless steel
The direction of the current in a D.C. generator can be determined by
Ohm's law
Fleming's right-hand rule
Newton law
Norton law
In any rotating machine that part which houses the conductors and in which e.m.f. induced is to be utilized is called
Field
Armature
Stator
Rotor
Wave winding is composed of
Any even number of conductors
Any odd number of conductors
That even number which is exact multiple of poles + 2
That even number which is exact multiple of poles
Which of the following could be lamina proximately the thickness of laminations of a D.C. machine?
0.005 mm
0.05 mm
0.5 m
5 m
Brushes of D.C. machines are made of
Carbon
Soft copper
Hard copper
All of above
Two generators are running in parallel. One of the generators may run as motor for which of the following reasons?
The direction of that generator is reversed
The speed of that generator is increased
The field of that generator is weakened
That generator takes large share of loads
In a D.C. generator in order to reduce sparking at brushes, the self-induced e.m.f. in the coil is neutralized by all of the following except
Interpoles
Dummy coils
Compensating winding
Shifting of axis of brushes
In D.C. generators, current to the external circuit from armature is given through
Commutator
Solid connection
loose connection
(D) None of above
Welding generator will have
Lap winding
Wave winding
Delta winding
Duplex wave winding
D.C. series generator is used
To supply traction load
To supply industrial load at constant voltage
Voltage at the toad end of the feeder
For none of the above purpose
The purpose of providing dummy coils in a generator is
To reduce eddy current losses
To enhance flux density
To amplify voltage
To provide mechanical balance for the rotor
In a D.C. generator in case the resistance of the field winding is increased, then output voltage will
Increase
Decrease
Remain unaffected
Fluctuate heavily
The commutator segments are connected to the armature conductors by means of
Copper lugs
Resistance wires
Insulation pads
Brazing
In D.C. generators, lap winding is used for
High voltage, high current
Low voltage, high current
High voltage, low current
Low voltage, low current
An exciter for a turbo generator is a
Separately excited generator
Shunt generator
Series generator
Compound generator
With a D.C. generator which of the following regulation is preferred?
100% regulation
Infinite regulation
50% regulation
1% regulation
Iron losses in a D.C. machine are independent of variations in
Speed
Load
Voltage
Speed and voltage
In case of D.C. machines, mechanical losses are primary function of
Current
Voltage
Speed
load
Following D.C. generator will be in a position to build up without any residual magnetism in the poles
Series generator
Shunt generator
Compound generator
Self-excited generator
If residual magnetism is present in a D.C. generator, the induced e.m.f. at zero speed will be
Zero
Small
The same as rated voltage
High
The number of brushes in a commutator depends on
Speed of armature
Type of winding
Voltage
Amount of current to be collected
The bearings used to support the rotor shafts are generally
Bush bearings
Magnetic bearings
Needle bearings
Ball bearings
Which of the following components of a D.C, generator plays vital role for providing direct current of a D.C. generator?
Dummy coils
Commutator
Eye bolt
Equalizer rings
If B is the flux density, I the length of conductor and v the velocity of conductor, then induced e.m.f. is given by
Blv
Blv2
Bl2v
Bl2v2
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
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 de-rated by a factor recommended by manufacturer and select the next higher H.P. motor
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
For which types of D.C. motor, dynamic braking is generally used?
Shunt motors
Series motors
Compound motors
All of the above
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
None of these
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
Which of the following load normally needs starting torque more than the rated torque
Blowers
Conveyors
Air compressors
Centrifugal pumps
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
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
Torque developed by a D.C. motor depends upon
Magnetic field
Active length of the conductor
Current flow through the conductors
All above factors
D.C. generators are installed near the load centers to reduce
Iron losses
Line losses
Corona losses
Sparking
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
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 starting resistance of a D.C. motor is generally
Low
Around 500 Q
1000 Q
Infinitely large
For constant torque drive which speed control method is preferred?
Field control
Armature voltage control
Shunt armature control
Mechanical loading system
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
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 nonmagnetic material for frame
Using material of low hysteresis coefficient for armature core material
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 of the following motor has the poorest speed regulation?
Shunt motor
Series motor
Differential compound motor
Cumulative compound moto
Which of the following motors is usually used in household refrigerators?
D.C. shunt motor
D.C. series motor
Single phase induction motor (split phase start or induction run motor)
Reluctance motor
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
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
Which method of braking is generally used in elevators?
Plugging
Regenerative braking
Rheostat braking
None of the above
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
These days D.C. motors are widely used in
Pumping sets
Air compressors
Electric traction
Machine shops
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
Coulomb’s law
Fleming's left-hand rule
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
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
