B. Techl20 15IESIEE-304/21 A
NATIONAL INSTITUTE OF TECHNOLOGY
(Established by Ministry of Human Resource Development, Govt. of
India)
Yupia, District Papum Pare, Arunachal Pradesh, Pin-791112
End Semester Examination (July - Dee 2015)
EE- 304 (Electrical Machines-I)
Time: 3:00 Hrs
Full Marks: 100
INSTRUCTIONS:
i) The question paper consists of four units each having 35 marks.
Answer as many questions as you can. Evaluation will be done for 25
marks.
ii) Borrowing of calculator, pen, pencil, etc. is not allowed.
1.1
1.2.
1.3.
1.4.
2.1.
2.2.
2.3.
2.4.
Unit - I
Explain different kinds of starters for DC machines.
With is armature reaction. Derive expressions for demagnetizing and
cross magnetizing AT/pole for DC machines.
Explain principle of generation of torque and back EMF in DC
motors. Explain significance of back EMF.
Derive EMF equation of DC generators. Derive torque equation of DC
Motors.
Unit - II
With winding diagrams explain on wave winding in armature of DC
machine.
A 6 pole, lap wound generator rotating with a speed of 2000 RPM
have 100 conductors per pole generates an EMF of 500 volts. Find
flux per pole.
A 4 pole, lap wound DC machine rotating with a speed of 2000 RPM
have 120 conductors per pole inside a field of flux 1.4 Wb. If the
current flowing through the armature of the machine in motor mode
is 5A. Calculate EMF generated in generator mode and torque
generated in motor mode.
With neat diagram explain a three point starter.
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3.1.
3.2.
3.3.
3.4.
4.1.
4.2.
4.3.
4.4.
Unit - III
What is the need of a starter for DC motors.
A shunt connected DC motor is supplied with a voltage source of
50KV is with 6 pole lap winding armature. The motor works with a
flux of 1.5 Wb/pole having 200 conductors per pole and rotates with
2000 RPM. If armature resistance is 5 ohms calculate armature
current. If field resistance is 200 ohms calculate field current and
current supplied by the source.
Explain the principle of generation of torque in induction motors.
Explain the principle of generation of rotating magnetic field
Unit - IV
Explain the tests on Alternator.
With neat diagrams explain constructional features of Alternator.
A 3-phase alternator produces an open circuit line voltage of 692SV
when the exciting current is 50A. The AC terminals are then short
circuited and three line currents are found to be SOOA.Calculate the
synchronous reactance per phase. Also calculate the terminal Voltage
if three 12 ohm resistors are connected in star across the terminals.
A 3-phase, 1500kVA, Y connected, 50Hz, 2300V alternator has an
effective resistance per phase is 0.12 ohms. A field current of 70 A
produces a current of 376A in each line on short circuit and a voltage
of700V on open circuit. Determine Synchronous impedance and full
load Voltage regulation at O.Spower factor lagging.
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