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MCQ
121. The constants losses in transformer are-
copper loss
hysteresis loss
eddy current loss
both b & c
ব্যাখ্যা: Eddy current loss and hysteresis loss are almost independent of load, significantly depending on supply voltage and frequency. As the flux density or flux is constant for a given voltage and frequency, eddy current loss and hysteresis loss remain constant at any load. Therefore, these losses are called constant losses. Copper loss varies as the square of load current and called variable loss.
122. Which of the following ac motors are widely used?
squirrel caged induction motor
slip-ring induction motor
both a & b
none
ব্যাখ্যা: In slip-ring induction motor brushes are present due to this frequent maintenance is required. Their design is complicated due to winding, slip-rings and brushes and also expensive. Slip ring induction motor is comparatively having more weight than squirrel case induction motor. The construction of squirrel case induction motor is simple and robust and it is cheap as compared to slip ring induction motor. Therefore squirrel cage induction motor is widely used.
123. Which of the following are starting methods for squirrel cage induction motor?
auto transformer starting
stator resistance starting
star-delta starting
all of these
ব্যাখ্যা: As the rotor of induction motor is under short circuit, a large voltage induced in it during starting produces a high rotor current which comes through the stator. Therefore the motor draws large current at starting To control the rotor current in squirrel cage motor only option is to go for reduced voltage starting. By using all the above methods we can get the reduced voltage ar starting. These above methods are used as there is no rotor conductor but rotor bars only
124. লুপ টেস্টের সময় টেস্টিং প্রান্তে সংযোগ করা হয়-
একটি অ্যামিটার এবং একটি ভোল্টমিটার
একটি কেলভিন ডবল ব্রিজ
একটি হুইটস্টোন ব্রিজ
একটি ম্যাক্সওয়েল ব্রিজ
125. Considering the voltage regulation of transformer, which one is the best?
10%
5%
20%
0%
126. একটি ক্যাপাসিটর স্টার্ট মোটরের ক্যাপাসিট্যান্স হ্রাস করার ফলে কোনটি হ্রাস পায়?
শব্দ
স্পিড
স্টার্টিং টর্ক
আর্মেচার রিয়্যাকশন
127. The output power indicated on the nameplate of the following motor is suitable for frequent starting and stopping-
kW
KVA
HP
gross power
128. The point, through which the whole weight of the body acts is known as-
moment of inertia
center of gravity
center of percussion
center of mass
ব্যাখ্যা: Centre of gravity: The point, through which the whole weight of the body acts, irrespective of its position, is known as centre of gravity. Moment of inertia: The moment of inertia of the cross- section of a body is its resistance to changes in its rotation. The moment of inertia is a measure of how resistant an object is to changes in its rotational motion.
129. The unit of force in Sl system is.--
dyne
kg-m
N
Watt
130. Total core loss is also called as-
AD current loss
hysteresis loss
magnetic loss
copper loss
ব্যাখ্যা: As iron loss is proportional to flux density or flux, it is also called magnetic loss. The total core loss or magnetic loss consists of eddy current loss and hysteresis loss.
131. Find the number of poles required when the frequency is 50 Hz and the speed of the motor is 500 RPM-
5
12
10
24
132. লুপ টেস্টের সময় একটি ত্রুটিযুক্ত এবং একটি ভালো ক্যাবলের দূরপ্রাপ্ত সংযোগ করা হয়-
একটি হাই-রেজিস্ট্যান্স দ্বারা
একটি লো-রেজিস্ট্যান্স দ্বারা
একটি অপরিবাহী পদার্থ দ্বারা
ওপেন সার্কিট দ্বারা
133. The unit of energy is SI I system is—
joule
watt
joule meter
joule/meter
ব্যাখ্যা: The Sl unit of energy/work is the joule (J), named for English physicist James Prescott Joule.
134. The rate of charge of displacement of a body is called--
velocity
acceleration
momentum
none
ব্যাখ্যা: Velocity: The rate of change of displacement of an object (displacement over elapsed time) is velocity. Velocity is a vector since it has both magnitude (called speed) and direction. Like velocity, acceleration is a vector and has both magnitude and direction.
135. Maximum efficiency will occur when copper loss is---to iron loss.
greater than
equal to
less than
none
ব্যাখ্যা: Condition for maximum efficiency is, copper loss iron loss or variable losses is equal to fixed losses.
136. Which of the following is the extensive property of a thermodynamic system?
pressure
temperature
volume
density
ব্যাখ্যা: By contrast, an extensive property of a systent does depend on the system size or the amount of material in the system According to the definitions, density, pressure and temperature are intensive porperties and volume, internal energy are extensive properties.
137. All day efficiency of a distribution transformer is the efficiency.
the same as
less than
more than
none of the above
138. A motor takes huge starting current because-
Armature resistance high
Back e.m.f. low
Shunt field is weak
None of the above
139. What is the frequency (Hz) for a 4-pole generator of speed 1200 rpm?
50
60
40
None
140. Transformer oil is used as--
an insulator
a cootant
both a & b
inert medium
ব্যাখ্যা: Small transformers less than 25 kVA are naturally air cooled and insulation type. But in large transformers, transformer oil is used. It has two functions-1. Cooling and 2. Insulating It should contain high dielectric strength