300 Important DC Generator MCQ Question and Answer | PDF Free Download
1.In a dc machine, the number of mechanical and electrical degree will be same when the number of poles on the machine is
- A4
- B2
- C8
- D1
2. Air gap at the pole tips of a dc machine is kept more than that at the centre of the pole mainly to reduce
- AReactance voltage
- BEffect of armature reaction
- CLosses of armature core
- DNoise of the machine
3.Commutation in a dc machine may be explained as a process
- AIn which magnetic field is set up
- BBy virtue of which emf is induced I the armature conductors
- CBy which current in ac coil is reveres during the period it is short circuited by the brush
- DHow armature flux affects the main field film
4.Current in a coil undergoing commutation is not able to get reversed completely by the end of commutation period. It is due to
- AReactance voltage induced in the coil
- BCoil resistance
- CArmature emf
- DNone of the above
5.Under commutation results in
- ASparking at the middle of the brush
- BSparking at the leading edge of the brush
- CSparking at the trailing edge of the brush
- DNo sparking
6.In a dc shunt generator working on load, the brushes are moved forward in the direction of rotation, as a result of this commutation will
- AImprove but terminal voltage will fall
- BWorsen and terminal voltage will fall
- CImprove and terminal voltage will rise
- DWorsen and terminal voltage will rise
7.In a dc machine the sparking between brushes and commutator surface may be due to
- AUnder commutation
- BOver commutation
- CToo raid reversal of current
- DAny of the above
8.The self-induced EMF in the coil undergoing commutation is called the
- AReactance voltage
- BStatically induced voltage
- CDynamically induced voltage
- DNone of the above
9.The sparking at the brushes in a dc machine is due to
- AArmature reaction
- BReactance voltage
- CPresence of commutator
- DHigh resistance of carbon brushes
10.The reactance voltage of DC Generator is
- ADirectly proportional to armature current and commutation period
- BDirectly proportional to commutation period and inversely proportional to armature current
- CDirectly proportional to armature current and inversely proportional to commutation period
- DNone of the above
11.If the armature current is increased at double its previous value and the time of commutation is halved, then reactance voltage will be
- AHalved
- BRemain the same
- CDoubled
- Dbecome four times
12.Sparkless commutation can be achieved by employing
- AInterpoles
- BCompensating winding
- CHigh resistance carbon brushes
- DAny one or more of the above methods
13.To have sparkles commutation the armature reaction effect in a dc machine is neutralized by
- ACompensating winding and commutating poles
- BShifting the brush axis from geometrical neutral axis to magnetic neutral axis
- CFixing the brush axis in line with the pole axis
- DIncreasing the filed excitation
14.The interpoles in dc machines have tapering shape in order to
- AReduce the overall weight
- BReduce the saturation in the interpole
- CEconomic on the material required for interpoles and their windings
- DIncrease the acceleration of commutation
15.The compensating windings in a DC machine are placed in the slots in the pole faces to neutralize the
- ACross magnetizing effect
- BDemagnetizing effect
- CBoth of the demagnetizing and cross magnetizing effects
- DArmature reaction mmf under the pole faces and avoid flash over between positive and negative brushes
16.In large sized dc machines, the compensating windings are provided to
- ACompensate for the decrease in speed due to increase of load
- BNeutralize the voltage induced in the coil due to high fluctuation of load
- CReduce eddy current losses by proving local short circuits
- DNone of the above
17.Compensating winding in a dc machine is placed on
- AArmature in slots
- BYoke in the interpolar gap
- CYoke in the pole faces
- DPartly in armature slots and partly in pole faces
18.Compensating windings are employed in dc motors which are likely to operate
- AAt a constant speed over wide range of load
- BWith rapidly charging loads of wide range
- COver wide range of speed by field excitation control
- DBoth (b) and (c)
19.The compensating winding in a dc machine is located
- AIn armature slots for compensation of the armature reaction
- BOn commutating poles for improving commutation
- COn the pole shoes for avoiding for flashover at the commutator surface
- DOn pole shoes to avoid the sparking at the brushes
20.In dc machines the interpoles are provided
- AFor mechanical balancing of machines
- BTo counteract the demagnetizing effect of armature mmf in the commutating zone
- CTo counteract the reactance voltage and the cross-magnetizing effect of armature mmf in the commutation zone
- DTo make commutation sparkless