NATIONAL INSTITUTE OF TECHNOLOGY ROURKELA
Department of Electrical Engineering
Mid-Semester Examination, 2019 - 2020 (Spring)
Program: B.Tech
Semester: 4th
Subject Name: Electrical Machines
Subject Code: EE3011
Number of Pages: 1
Full Marks: 30
Figures at the right hand margin indicate marks.
All symbols carry usual meaning. Assume standard value for any missing data.
Duration: 2Hours
Please draw all the diagrams neatly with pencil.
Q.No.
Questions
Marks
1
What are the different types of dc motors classified based on field excitation? Draw the circuit
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diagram for each type and write the corresponding current and voltage equations.
2.
Neglecting the effect of secondary side leakage reactance, sketch the equivalent circuit diagram and
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the phasor diagram of a single-phase transformer connected to pure resistive load at secondary
terminals. Write all the relevant equations.
3.
A 100 kVA, 50Hz single-phase transformer has maximum efficiency of 95% at 90% of rated load
4
and 0.85 power factor lagging. If the leakage impedance drop of transformer is 5%, find the voltage
regulation at rated load and 0.8 power factor leading.
4.
Why open-circuit and short-circuit tests are performed? Narrate the preferred conditions for
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performing both the tests in laboratory and the corresponding reason by taking a suitable numeric
example.
5.
A 100 kVA, 2000/200 V, 50 Hz distribution transformer has iron loss of 500 W at rated voltage and
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copper loss of 1200 W at full load. It supplies to following load profile:
% Load
0%
50%
75%
100%
110%
Power Factor
-
1
0.8 lag
0.85 lag
1
Hours
3
6
8
5
2
Find out the all-day efficiency of the transformer.
6.
A DC machine supplies 20 A at 200 V as a generator. The armature resistance is 0.2 ohms. This
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machine is now operated as a dc motor with same terminal voltage and current but with the flux
decreased by 10%. Find out the motor speed if generator was running at 1000 rpm.
7.
A 200 V DC series motor runs at a speed of 20 revolution per second and draws current of 20 A. if
the armature resistance and series field resistance each is 0.2 Ω and load torque varies proportional
to square of the motor speed, find out the additional series resistance required to decrease the motor
speed by 20%. Assume that there is no flux saturation.
-------------------Best of Luck------------------
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