Anna University
B.E./B.Tech. DEGREE EXAMINATION, APRIL/MAY 2016
Sixth Semester
Electrical and Electronics Engineering
EE 2355 — DESIGN OF ELECTRICAL MACHINES
(Regulation 2008)
Time : Three hours
Maximum : 100 marks
Answer ALL questions
PART A — (10 × 2 = 20
marks)
1. What is specific
Electric Loading?
2. How materials are
classified according to their degree of magnetism?
3. Name any two
methods to reduce armature reaction.
4. What is slot
loading?
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 aStar-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)
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