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41. A de motor can be looked upon as de generator with the power flow-
Reduced
reversed
increased
modified
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ব্যাখ্যা: A de motor can be looked upon as de generator with the power flow reversed.
42. The ideal de generator is one that has ---voltage regulation
low
zero
negative
positive
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ব্যাখ্যা: Ideally DC generators should have zero voltage regulation Any increase in load current caveses an increase in the field and hence the terminal voltage rise. Hence it has negative voltage regulation and it has the poorest voltage regulation.
43. The back emf in a de motor-
opposes the applied voltage
aids the applied voltage
aids the armature current
none of the above
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ব্যাখ্যা: When DC Motor routes on the action of driving torque, the conductors move through the magnetic field and hence emf induce the direction of this emf oppose to applied voltage. This is called as back emf.
44. The direction of a rotation of a de motor can be determined by
Lenz's law
Fleming's left-hand rule
Fleming's right-hand rule
Ampere's law
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ব্যাখ্যা: Fleming's left hand rule is mainly applicable to electric motors and Fleming's right hand rule is mainly applicable to electric generator.
45. pole de machines are most common.
2
4
6
8
46. The greatest heat loss in a de machine is due to loss.
eddy current
hysteresis
copper
friction
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ব্যাখ্যা: Copper loss of de machine is PR. If current flow is increase then copper loss will increase, simultanensly more heat will be produce.
47. The induction motor is preferred to DC motor because it-
provides high-starting torque
provide fine speed control
has simple and rugged construction
none of the above
48. What is a battery?
Current source
Voltage source
Power source
None
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ব্যাখ্যা: A battery is a device that stores electric power in the form of chemical energy. When necessary the energy is again released as electric power for DC consumers such as lighting and starter motors.
49. A de motor is still used in industrial application because it
is cheap
is simple in construction
provides tine speed control
none of the above
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ব্যাখ্যা: DC motor is populer in industrial application because it provides fine speed control with adjustable speed and constant or low speed torque.
50. Copper when alloyed with zinc is known as-
Brass
Bronze
Babbits
All of the above
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ব্যাখ্যা: Brass is an alloy of zinc. The composition of brass, generally 66% copper and 34% zinc, makes it a favourable sustitute for copper based jewellery as it exhibits greater resistance to corrosion.
51. Motors used in high-speed applications have-
high torque with high speed
low torque with high speed
high torque with low speed
none of the above
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ব্যাখ্যা: DC Differentially compound motor has low starting torque and has a constunt speed, but the dangerously high speed at no load under the circumstunce it is seldom used.
52. The shunt generator gives its greatest voltage at
no load
full load
half-load
quarter load
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ব্যাখ্যা: With the increase of load the armature voltage drop, as a result the output voltage of shunt generator drops slightly. So at no load condition there is no armature voltage drops and greatest output voltage is found.
53. The effect of a armature reaction is to-
decrease the total flux
increase the total flux
make air gape flux uniform
none of the above
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ব্যাখ্যা: The effect of a armature reaction is to-
- Reduction of generated emf due to decrease in value of flux per pole.
-The maximum value of gap flux density increases.
-The armature reaction shifts brush axis from GNA
54. The current in armature conductors of a de machine is
pure de
pulsating de
ac
pure de + pulsating de
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ব্যাখ্যা: In DC generatora, varying conduction cuts the fixed magnetic field then emf will be induced that will be in the form of AC but not DC
55. The demand for a large increase in torque of a de series motor is met by a-
large decrease in current
large decrease in speed
large increase in speed
large increase in speed
56. The armature of a de machine is laminated in order to reduce-
eddy current loss.
hysteresis loss
copper loss
friction loss
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ব্যাখ্যা: In electric machine to reduce eddy current loss we may use laminated core or selecting magnetic core materials that have low electrical conductivity.
57. Shunt are generally made of-
copper
aluminium
silver
manganin
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ব্যাখ্যা: Most shunts are made from maganin, an alloy that is 84% copper, 12% magnesium and 4% nickel .
58. The emf of a cell does not depend upon
nature of electrolyte
concentration of electrolyte
nature of material of electrodes
size and spacing of electrode
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ব্যাখ্যা: EMF of cell is-
- Directly proportional to the distance of separation between the two electrodes.
-Inversly proportional to the area of electraodes dipped in electrolyte.
-
-Inversly proportional to the temperature of electrodes.
-Inversaly proportional to the concentration of electrolyte
59. The term ampere-hour (Ah) is associated with-
rectifiers
transformer
electromagnets
storage cells
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ব্যাখ্যা: An ampere-hour (Ah) or sometimes amp-hour is the amount of energy charge in storage cells or battery that will allow one ampere of current to flow for one hour.
60. Which of the following quantity maintains the same direction whether a de machine runs as a generator or as a motor?
Induced emf
Armature current
Field current
Supply current