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Worksheets21EE5201-DEM U2 DC MACHINES
Total questions: 25
Worksheet time: 13mins
__________ limits the output of a DC machine.
Commutator diameter
Peripheral velocity
Frequency of flux reversals
Temperature rise
Which one does not influence specific electric loading in DC Machines?
Magnetizing current
Machine size
Heating or temperature rise
Speed of machine
Which one does not influence specific magnetic loading in DC machine?
Maximum flux density in the iron parts of the machine
Iron losses
Armature reaction and commutation
all the options are correct
The limiting factor for higher specific electric loading in DC machine is due to
armature reaction
heat dissipation
commutation
all the options are correct
The diameter of armature is chosen based on _________ in a DC machine
peripheral speed
pole pitch
core length
both peripheral speed and pole pitch
DC machines designed with high current density causes
efficiency
temperature rise
copper losses
both temperature rise and copper losses
The width of ventilating ducts usually varies from
2 to 3 mm
4 to 5 mm
8 to 10 mm
12 to 15 mm
Air gap in a DC machine is ____________ compared to induction machine
very small
small
large
equal
In a DC machine, air gap at the pole tips is kept more than that of the center of the pole in order to reduce
effect of armature reaction
losses in armature core
reactance voltage
all the options are correct
Choice of large air gap in DC machine
reduces distortion effect
reduces pulsation losses
provides better ventilation
all the options are correct
Speed of DC Machine in rps is
N / 60
N * 60
60 / N
60 * n
Pole Pitch of a DC Machine is
πD/p
πDr/p
πD/P
πD/2p
mmf required for air-gap with Slotted armature structure
ATg= 800000 Bg Kg lg
ATg= 800000 Bg lg
ATg= 800000 Bg Kg 2lg
ATg= 800000 Bg Kg lg x 10-3
The number of parallel paths for a four-pole duplex lap winding is
2
4
8
16
Brush friction loss depends on
brush pressure
peripheral speed of commutator
coefficient of friction between brush and commutator
all the options are correct
Stator slot pitch, Yss is
Yss = πD/Ss
Yss = πD/Sr
Yss = πDr/Ss
Yss = πDL/Ss
Real Flux density is
Due to the actual flux through a tooth
Due to the actual flux through a slot
Due to the actual flux through a air-gap
Due to the actual flux through yoke
Copper space factor is
Conductor area / Area of cross section of a core
Conductor area / Area of cross section of a coil
Airgap area / Area of cross section of a coil
Conductor area / Air gap Area of cross section of a core
Peripheral Speed of DC Machine is
πDn
2πDn
πDLn
πDN
Design of armature with higher air gap flux density will lead to
reduction in iron losses
reduction in weight
improved commutation
improved efficiency
The peripheral speed of armature should not normally exceed
12 m/s
20 m/s
30 m/s
36 m/s
__________ does not vary with load and flux density in a DC machine
Hysteresis loss
Windage loss
Eddy current loss
Copper loss
The commutator pitch in DC machine should not be less than
0.4 mm
4.0 mm
5.4 mm
4.5 mm
Poles of DC machines are usually laminated in order to
reduce iron weight
reduce armature reaction
dissipate more heat
reduce pulsation loss
Field form factor is
Bg/ Bav
Bav / Bg
2Bg/ Bav
Bg/ 2Bav
