Term Assessment  I Examination,(28/02/2020) 201920 Question Paper & Model Answer
Monday, March 2, 2020
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Term Assessment  I Examination, 201920
S.Y. B. Tech. [Mechanical]
(Semester II)
Date : 28022020 Max.
Marks: 20
Que.
1

Attempt the following.


A

What is the importance of commutator in DC machine?

2


B

Write a
short note on DC compound motor. Explain any one in detail

4


C

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


OR


Que.
2

Attempt the following.


A

Explain the
speed control of DC series Motor (any one type)

2


B

Explain the construction of DC motor
with neat diagram

4


C

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

4


Que.
3

Attempt the following.


A

Define
Synchronous speed and slip.

2


B

Compare 3 phase squirrel cage induction motor and
slip ring induction motor

4


C

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

4


OR


Que.
4

Attempt the following.


A

2


B

Draw and explain torque speed characteristics
of 3 phase induction motor

4


C

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.

4


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


OR


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

2
2


C

Given:
P=4, V=500,
Z=720, wave connected A=2, Ia=60A,
Ra=0.2Ω,
Vbrush=1V
Eb=0.72NFl
volts…….i
Eb=
VIaRaVbrush
500(60*0.2)2
= 486V ……ii
From eq i & ii
Full load
speed Nfl
0.72Nfl=486=675rpm

1
1
1
1


Que.
3

Attempt the following.


A

Define
Synchronous speed and slip.

2


B

Compare 3 phase squirrel cage induction motor and
slip ring induction motor Min 6 points

4


C

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=NsN/Ns
= 0.05=1500N/1500 = 1425rpm
N
at which max torque = Sm= R2/X2 = 0.2/0.8=0.111
0.111=1500N1/1500

4


OR


Que.
4

Attempt the following.


A

List the different losses that take place in an
induction motor.

2


B

Draw torque speed characteristics of 3 phase induction motor
Explanation

2
2


C

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(1S)=1500(10.05)
=1425 rpm
Power
=T*wSwhr, w=2piNs/60 =149.22 rad/sec
T=Power/wS
=67.01 Nm
Torque
changes with voltage square
Tst= 67.01(375/400)2
=58.8Nm

4

