Anna University Chennai 2005 B.E Electrical and Electronics Engineering Fifth Semester EE336 – Design of Electrical Apparatus - Question Paper

B.E. Electrical and Electronics Engineering
Fifth Semester
EE336 – Design of Electrical Apparatus
Time : three Hours Max Marks : 100
PART – A
ans All ques. (10 x two = 20 marks)
1. Cooling time constant of a rotating machine is normally larger than its heating time
constant Why?
2. discuss the square pole criterion. What is the advantage of it?
3. List the factors that control the length of the air gap in DC Machine.
4. Prove that the maximum ampere turns that can be given in a shunt coil is
independent of exciting voltage.
5. What is the significance of stepping the core of a transformer?
6. In a three phase transformer, the primary coils over the outer limbs draw more
magnetising current that the primary oil over central limb. Why?
7. Comment on the hysteresis loss and eddy loss when voltage and frequency of a
transformer is doubled?
8. Why is it better to have more number of slots for an induction motor?
9. discuss B30 and it is significance in induction motors.
10. Why is it possible to design alternators to generate much higher voltage than AC
generators?
PART – B (5 x 16 = 80 Marks)
11. Determine the main dimensions of the core and the core part of conductors for a
120 kVA 6600 / 460 V, 50 Hz single phase core kind transformer. The maximum flux
density in the core and yoke is 1.25 wb/m2, current density 2.3 amps/m2. presume
cruciform part for the core with a stacking factor of 0.94 and window space factor
of 0.33, height of the window is twice the width of the window. The net cross part
of copper is the window is 0.2 times the net cross part of iron in the core.
12.a) During heat run test of a 100 kVA transformer the temperature rise after one hr and 2
hrs is obtained to be 25° C to 35° C respectively. compute the heating time constant and
final steady temperature rise. If the rate of heat dissipation in increased by 15% by
cooling with an external fan, obtain the new kVA rating for the identical final temperature
rise. presume the maximum efficiency occur at 75% of full load and unity power
factor.
OR
12.b) An introduction motor has a final steady temperature rise of 50° C when running at its
rated output. compute is half hour rating for the identical temperature rise if the copper
losses at rated output are 1.5 times its constant losses. The heating time constant is 90
minutes.
13.a) The commutator of a 10 pole. 1200 kW, 400 volts, 300 rpm dc generator has 450
segments and an external diameter of 100 cm. Assuming suitable values wherever
needed obtain the approximate length of the commutator. Determine the total
commutator losses assuming a drop of 2.2 V due to brush surface resistance and
current density of six amps/cm2. Friction co-efficient of 0.025 and brush pressure of
1250 kg/m2. Also check for the temperature rise.
OR
13.b)i) Derive the output formula of a DC machine related to the main dimensions.
ii) compute the suitable armature core dimensions for a 400 kW, 550 V, 1800 rpm, 16
poles dc generator by assuming an avg. flux density to be 0.55 wb/m2 and the
ampere conductor to be 30,000. presume an efficiency of 92%.
14.a) Determine the approximate diameter and length of the stator core, the stator slots and
the number conductors for a 10 kW, 400 V three phase, four pole, 1425rpm induction motor.
presume Bav = 0.5 wb/m2, q = 4,000 ac/m. presume full load efficiency and power
factor as 0.85. Core length is equal to pole pitch.
OR
14.b) A six pole three phase squirrel cage induction motor has 72 slots with 12 conductors per
slot. There are 55 rotor slots. Determine the current in rotor bars and in end rings if
the equivalent stator current is 30 A.
15.a) A 3000 rpm, 50 Hz, three phase turbo alternator has a core length of 94 cm. The avg.
gap density is 0.40 wb/m2 ampere conductors / metre are 23000. Peripheral speed of
rotor is 100 m/s and length of air gap is two cm. obtain the kVA output of the machine
when the coil is full pitched.
OR
15.b) A 1250 kVA three phase 6600 V salient pole alternator has the subsequent data:
Air gap diameter = 1.55 m,
Length of the core = 45 cm,
No. of poles = 20
Armature ac = 30000
Pole arc/pole pitch = 0.7
Stator slot pitch = 28 mm
Current density in damper bar = three A/mm2
Design suitable damper winding for the machine.
-----

Earning:   Approval pending.