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### EE2355 DESIGN OF ELECTRICAL MACHINES ANNA UNIVERSITY QUESTION PAPER, IMPORTANT QUESTIONS, 2 MARKS AND 16 MARKS QUESTIONS

Sunday, September 25, 2011 ·

EE 2355 DESIGN OF ELECTRICAL MACHINES ANNA UNIVERSITY QUESTION PAPER, IMPORTANT QUESTIONS, 2 MARKS AND 16 MARKS QUESTIONS

DESIGN OF ELECTRICAL MACHINES ANNA UNIVERSITY QUESTION PAPER, IMPORTANT QUESTIONS, 2 MARKS AND 16 MARKS QUESTIONS

B.E./B.Tech. DEGREE EXAMINATION, APRIL/MAY 2011
Sixth Semester
Electrical and Electronics Engineering
EE 2355 — DESIGN OF ELECTRICAL MACHINES
(Regulation 2008)
Time : Three hours Maximum : 100 marks
PART A — (10 × 2 = 20 marks)
2. How materials are classified according to their degree of magnetism?
3. Name any two methods to reduce armature reaction.
5. Give the relationship between emf per turn and kVA rating in a transformer.
6. What are the factors affecting the choice of flux density of core in a
transformer?
7. How crawling can be prevented by design in an Induction motor?
8. Define dispersion coefficient of an Induction Motor.
9. What is run away speed of Synchronous Machine?
10. Give the need for damper winding in Synchronous Machine.
PART B — (5 × 16 = 80 marks)
11. (a) Discuss about various duties and ratings of Rotating Machines and give
their respective temperature — time curves. (16)
Or
(b) A field coil has a heat dissipating surface of 0.15 m2 and length of mean
turn 1 m. It dissipates loss of 150 W, the emissivity being 34 W/m2_°C.
Estimate the final steady temperature rise of the coil and its time
constant if the cross section of the coil is 100*50 mm2. Specific heat of
copper is 390 J/kg° C. The space factor is 0.56. Copper weighs 8900 kg/m3.
(16)
12. (a) (i) Explain the effects of choice of number of poles in a DC Machine on
(1) Frequency of flux reversal (2) Weight of iron (3) Weight of
copper and (4) Length of commutator. (8)
(ii) A 5 kw, 250 V, 4 pole, 1500 rpm DC Shunt Generator is designed to
have a square pole face. The specific magnetic loading and specific
electric loadings are 0.42 Wb/m2 and 15000 AC/m respectively. Find
the main dimensions of the machine. Assume full load efficiency
= 0.87 and pole arc to pole pitch ratio is 0.66. (8)
Or
(b) (i) Discuss various methods to determine mmf required for teeth of an
Electric Machine. (8)
(ii) Determine the apparent flux density in teeth of a DC machine if the
real flux density in teeth is 2.15 Wb/m2, slot pitch is 28 mm, slot
width is 10 mm, gross core length is 0.35 m, no. of ventilating ducts
is 4 each 10 mm wide. Magnetizing force corresponding to flux
density of 2.15 Wb/m2 is 55000 AT/m and iron stacking factor is 0.9.
(8)
13. (a) (i) Derive the output equation of a three phase transformer. (8)
(ii) The ratio of flux to full load mmf in a 400 kVA, 50 Hz single phase
core type transformer is 2.4*10–6. Calculate the net iron area and
the window area of the transformer if the maximum flux density in
the core is 1.3 Wb/m2, Current density is 2.7 A/mm2 and window
space factor is 0.26. Also calculate the full load mmf. (8)
Or
(b) A 250 kVA, 6600/400V three phase core type transformer has a total loss
of 4800 W at full load. The transformer tank is 1.25 m in height and
1 m × 0.5 m in plan. Design a suitable scheme for tubes if the average
temperature rise is to be limited to 35° C. The diameter of each tube is
50 mm and are spaced 75 mm from each other. The average height of
tubes is 1.05 mm. Specific heat dissipation due to radiation and
convection is respectively 6 and 6.5 W/bm2_°C. Assume that convection is
improved by 35% due to the provision of tubes. (16)
14. (a) Determine the main dimensions, number of radial ventilating ducts,
number of stator slots and turns per phase of a 3.7 kW, three phase,
400 V, 4 Pole, 50 Hz squirrel cage Induction Motor to be started by a
Star-Delta starter. Given that the average flux density in the air
gap = 0.45 Wb/m2; Ampere Conductor per meter of armature periphery =
23000, full load efficiency = 0.85, full load power factor = 0.84 and
kw = 0.955. Take L/ τ = 1.5. (16)
Or
(b) (i) Discuss the factors to be considered in estimating the length of air
gap of an Induction Motor. (8)
(ii) Discuss the step by step procedure to design the rotor of a squirrel
cage Induction Motor. (8)
15. (a) Define Short Circuit Ratio. Explain how it is determined for an
alternator. Also discuss its effects on the performance of alternator. (16)
Or
(b) (i) Derive the output equation of an AC machine. (8)
(ii) Determine the main dimensions of a 100 kVA, 50 Hz, three phase
375 rpm alternator. The average air gap flux density is 0.55 Wb/m2
and ampere conductors per metre is 28000. Given that τ / L must be
between 1 to 5. The maximum permissible peripheral speed is
50 m/sec. The runaway speed is 1.8 times synchronous speed. (8)