Term Assessment - I Examination,(28/02/2020) 2019-20 Question Paper & Model Answer
Monday, March 2, 2020
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Term Assessment - I Examination, 2019-20
S.Y. B. Tech. [Mechanical]
(Semester II)
Date : 28-02-2020 Max.
Marks: 20
Time : 09:30 am
to 10:30 am
Que.
1
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Attempt the following.
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||
A
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What is the importance of commutator in DC machine?
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2
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B
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Write a
short note on DC compound motor. Explain any one in detail
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4
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C
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5 pole armature has 84 slots with 6 conductor per
slot & driven at 500 rpm the useful flux per pole is 12mWb. Calculate
induced emf in armature winding when it is i). Lap connected ii). Wave
connected
|
4
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OR
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Que.
2
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Attempt the following.
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||
A
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Explain the
speed control of DC series Motor (any one type)
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2
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B
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Explain the construction of DC motor
with neat diagram
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4
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C
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A 4 pole, 500V shunt motor has 720 wave connected
armature conductor. The full load armature current is 60A & flux per pole
is 0.03Wb. The armature winding resistance is 0.2Ω & contact drop is 1V
per brush. Determine full load speed of the motor
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4
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Que.
3
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Attempt the following.
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||
A
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Define
Synchronous speed and slip.
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2
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B
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Compare 3 phase squirrel cage induction motor and
slip ring induction motor
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4
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C
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A
3phase 4 pole 440V, 50Hz induction motor has rotor resistance of 0.2Ω per
phase & stand still reactance of 1.8Ω per phase. The full load slip is
5%. Determine i). The Synchronous speed ii). Motor speed at full load iii).
Speed at which maximum torque is developed
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4
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OR
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Que.
4
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Attempt the following.
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||
A
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2
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||
B
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Draw and explain torque speed characteristics
of 3 phase induction motor
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4
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C
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A 50Hz, 3 phase, 4pole, 400V, 10kW slip ring
induction motor has full load slip of 5% & its starting torque is equal
to full load torque. Determine starting torque if supply voltage is reduced
to 375V.
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4
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Model Answer
Que.
1
|
Attempt the following.
|
||
A
|
Role of commutator in DC machine
(Rectifier)
|
2
|
|
B
|
Short note
on DC compound motor.
Explain any
one in detail
|
2
2
|
|
C
|
Given:
Total no of conductor Z= 84*6=504
For lap winding E= 50.4V, A=P=4
For wave winding E=126V A=2
|
1
1
1
1
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|
OR
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|||
Que.
2
|
Attempt the following.
|
||
A
|
speed
control of DC series Motor (any one type)
with diagram
|
1
1
|
|
B
|
Construction of DC motor with neat
diagram
Explanation
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2
2
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C
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Given:
P=4, V=500,
Z=720, wave connected A=2, Ia=60A,
Ra=0.2Ω,
Vbrush=1V
Eb=0.72NFl
volts…….i
Eb=
V-IaRa-Vbrush
500-(60*0.2)-2
= 486V ……ii
From eq i & ii
Full load
speed Nfl
0.72Nfl=486=675rpm
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1
1
1
1
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Que.
3
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Attempt the following.
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||
A
|
Define
Synchronous speed and slip.
|
2
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B
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Compare 3 phase squirrel cage induction motor and
slip ring induction motor Min 6 points
|
4
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C
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Given:
Vl=440V,P=4, R2=0.2Ω, X2=1.8Ω, S=0.05, F=50Hz
Ns=120f/p
= 120*50/4=1500rpm
N
at full load whr S=0.05
S=Ns-N/Ns
= 0.05=1500-N/1500 = 1425rpm
N
at which max torque = Sm= R2/X2 = 0.2/0.8=0.111
0.111=1500-N1/1500
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4
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OR
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Que.
4
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Attempt the following.
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||
A
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List the different losses that take place in an
induction motor.
|
2
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B
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Draw torque speed characteristics of 3 phase induction motor
Explanation
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2
2
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C
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Given :
F=50Hz,
Vl=400V, P=4, Vs=375, s=0.05, Pin =10kW
Starting
torque at V=375V =?
Ns=120f/P
=120*50/4=1500rpm
N=Ns(1-S)=1500(1-0.05)
=1425 rpm
Power
=T*wSwhr, w=2piNs/60 =149.22 rad/sec
T=Power/wS
=67.01 N-m
Torque
changes with voltage square
Tst= 67.01(375/400)2
=58.8N-m
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4
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