Tripura University 2010-4th Sem Diploma Electrical Engineering (DEE) Engineering (ester) .,Sub.:-Electrical Machine-II.,: DEE-401, Full ks:70, Time: 3 hours,This belongs to Tipura Instituteof Techn

DE/DEE-401/10
Diploma Engineering (4th Semester)
examination., 2010

ELECTRICAL MACHINES-II

Paper: DEE-401

Full Marks:70

Time:3 hours

The figures in the margin indicate full marks
for the ques.
Write the answers to every Half in a separate book

1st HALF

Answer ques. No.1 and 3 from the rest

1. (a) What kinds of alternator will be used for (i) hydraulic turbines
and (ii) steam turbines, as prime movers?
(b) Name essential elements for generating e.m.f. in alternators.
(c) Why will a 3-phase synchronous motor always run at synchronous speed?
(d) How hunting of synchronous motor be minimized?
[Marks:2+2+2+2=8]

2. (a) What is meant by synchronous condenser?
(b) How is the synchronous motor made self-starting?
(c) Draw the phaser diagram of a synchronous motor operating at
overexcited and underexcited conditions.
[Marks: 3+2+4=9]

3. (a) discuss 'V' and inverted 'V' curves.
(b) A 20-pole, 693-volt, 50-Hz, 3-phase, delta-connected synchronous
motor is operating at no load with normal excitation. It has armature
resistance per phase of zero and synchronous reactance of 10 ?. If the
rotor is retarded by 0.5®(mechanical) from its synchronous position,
compute--
(i) the rotor displacement in electrical degrees; (b) the armature e.m.f. per phase.
[Marks:5+4=9]

4. (a) Write the advantages of stationary armature and rotating field in an alternator.
(b) obtain the no-load phase and the line voltage of a star-connected 3-phase,
8-pole alternator which runs at 1200 r.p.m, having flux per pole of 100 mWb
sinusoidally distributed. Its stator has 72 slots having double-layer
winding. every coil has eight turns and the coil is chorded by one slot.
[Marks:4+5=9]

5. (a) Derive e.m.f. formula of alternator and explaine the coil-span factor
and distribution factor.
(b) When the load on an alternator is varied, its terminal voltage is
also obtained to be varied. Why?
[Marks:6+3=9]

6. (a) elaborate the effects on voltage regulation of an alternator of power
factor (i) at lagging power factor and (ii) at leading power factor?
(b) A 100-KVA, 3000-V. 50Hz, 3-phase, star-connected alternator has
effective armature resistance of 0.2?. The field current of 40A produces
short circuit current of 200A and an open circuit e.m.f. of 1040V (line value).
compute the full-load voltage regulation at 0.8 power factor lagging.
[Marks:4+5=9]



SECOND HALF
Answer ques. No. seven and any 3 from the rest

7. (a) Why is induction motor called 'asynchronous'?
(b) How can the direction of rotation of the 3-phase induction motor be reversed?
(c) What is slip of an induction motor?
(d) elaborate the losses in a 3-phase induction motor?
[Marks:2+2+2+2=8]

8. (a) discuss why it is not possible for the rotor speed to be equal to
that of the revolving field.
(b) A 4-pole, 3-phase induction motor operates from a supply
whose frequency is 50 Hz. compute (i) the speed at which
the magnetic field of the stator
is rotating, (ii) the speed of the rotor when the slip is 4%,
(iii) the frequency of the rotor currents when n slip is 0.03,
and (iv) the frequency of the currents at standstill.
[Marks:3+6=9]

9. (a) How will you improve the starting torque of a 3-phase induction motor?
(b) Why are the slots on the rotor of squirrel cage induction motors usually skewed?
(c) A 3-phase, 50-Hz, 8-pole induction motor has a full-load slip of 4%.
The rotor resistance is 0.001?/phase. obtain the ratio of maximum to full-load
torque and the speed at which the maximum torque occurs.
[Marks:1+2+6=9]

10. (a) Why do we require starters for starting 3-phase induction motor?
(b) Name different kinds of starter used for starting squirrel cage motor.
(c) An 8-pole, 3-phase, 50-Hz induction motor is running at a speed of 710 r.p.m.
with an input power of 35 kW. The stator losses at this operating
condition are known to be 1200 watt, while the rotational losses are 600 watt.
obtain (i) the rotor copper loss, (ii) the gross torque developed, and
(iii) the mechanical power output.
[Marks:1+2+6=9]

11. (a) discuss why a single-phase motor is not self-starting.
(b) Classify single-phase motor depending on their constructions and methods of starting.
(c) Draw the circuit and vector diagrams of a capacitor-start induction-run motor.
[Marks:4+2+3=9]

12. (a) discuss the terms 'Crawling' and 'Cogging'.
(b) Name different methods by which speed control of induction motors is achieved.
[Marks:6+3=9]

Earning:   Approval pending.