Reg. No.
Question Paper Code : 19676
B.E. / B.TECH. DEGREE EXAMINATIONS, MAY / JUNE 2023
Sixth Semester
B.E. – Electrical and Electronics Engineering
19EE601 – DESIGN OF ELECTRICAL MACHINES
(Regulations: Mepco – R2019)
Duration: 3 Hours Max. : 100 Marks
Answer ALL Questions
BTL, CO PART A – (10 2 = 20 Marks)
R, CO1 1. Define specific magnetic loading.
U, CO1 2. Ferromagnetic material has the relative permittivity µr is _________. Justify your
answer.
A) Less than 1 B) Greater than 1 C) Equal to 1 D) Equal to 0
R, CO2 3. List the methods used for estimating the mmf for the tapered teeth.
A, CO2 4. A 300 W DC motor has a full load efficiency of 60 %. Calculate the approximate
power developed by the armature at full load if sum of iron and mechanical
losses is one third of total loss. Justify your answer.
A) 433 W B) 300 W C) 366 W D) 500 W
R, CO3 5. What is meant by window space factor?
U, CO3 6. What is the net core area for three stepped transformers? Justify your answer.
A) 0.45 B) 0.56 C) 0.6 D) 0.62
U, CO4 7. Why squirrel cage induction motor is mostly employed in industries?
U, CO4 8. In case of induction motors the ratio of length to pole pitch for minimum cost, is
taken as___________. Justify your answer.
A) 1.0 B) 1.5 to 2 C) 2 to 3 D) 3 to 5
R, CO5 9. Define run away speed in a synchronous machine.
U, CO5 10. What is the formula for stator turns per phase? Justify your answer.
A) Stator turns per phase = Stator voltage per phase / (4.44 * f * maximum flux /
stator winding factor)
B) Stator turns per phase = Stator voltage per phase * 4.44 * f * maximum flux *
stator winding factor
C) Stator turns per phase = Stator voltage per phase / (4.44 * f * maximum flux *
stator winding factor)
D) Stator turns per phase = Stator voltage per phase * 4.44 * f * maximum flux /
(stator winding factor)
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PART B – (5 16 = 80 Marks)
U, CO1 11. a) i. Discuss the factors that influence the choice of specific electric
loading for an electromagnetic type rotating machine. (8 Marks)
A, CO1 11. a) ii. An electric motor has load variation as given below: Torque
240 Nm for 20 minutes, 140 Nm for 10 minutes, 300 Nm for
10 minutes and 200 Nm for 20 minutes. If the speed of the motor
is 750 rpm, find the power rating of motor. (8 Marks)
OR
U, CO1 11. b) i. What are the main groups of electrical conducting materials?
Explain the properties and applications of those materials. (8 Marks)
A, CO1 11. b) ii. Calculate specific electric and magnetic loadings of a 350 kW,
500 V, 450 rpm, 6 pole DC generator is built with armature
diameter of 0.87 m and core length 0.32 m. The lap wound has
660 conductors. (8 Marks)
U, CO2 12. a) i. Derive the output equation of a DC Machine in terms of specific
electric and magnetic loadings and its main dimensions. (8 Marks)
A, CO2 12. a) ii. A DC shunt generator is rated for 50 kW. It has 4 poles and runs
at 600 rpm. The full load terminal voltage is 220 V. If the
maximum density is 0.83 wb/m2 and the armature ampere
conductors per metre are 30,000. Calculate suitable dimensions of
the armature core to give a square pole face. Assume that the full
load voltage drop is 3 % of the rated terminal voltage and that the
field current is 1 % of rated full load current. Ratio of pole arc to
pole pitch is 0.67. (8 Marks)
OR
U, CO2 12. b) i. Derive the relationship between real and apparent flux densities (8 Marks)
A, CO2 12. b) ii. Determine the air gap length of a DC machine for the following
data: gross length of core is 0.12 m, one ventilating duct of 10 mm
width, slot pitch is 25 mm, slot opening is 10 mm, carter’s
coefficient is 0.32 and gap density at pole centre is 0.7 Tesla.
Field MMF per pole is 3900 AT and MMF required for iron
portion is 900 AT. (8 Marks)
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U, CO3 13. a) i. Deduce the relationship between Ai/ACC and Agi/ACC for the
square core section and cruciform core section. (8 Marks)
A, CO3 13. a) ii. Calculate kVA output of a 1 transformer from the following
data: Core height / Distance between core centre = 2.7; Diameter
of circumscribing circle / Distance between core centre = 0.56;
Net iron area / Area of circumscribing circle = 0.7; Current
density = 2.3 A /mm2; Supply frequency = 50 Hz; Window space
factor is 0.27; Distance between core centres = 0.4 m; flux
density in core is 1.2 wb/m2 (8 Marks)
OR
U, CO3 13. b) i. Derive the output equation of a transformer having two windows. (8 Marks)
A, CO3 13. b) ii. A 125 kVA transformer has tank dimensions of
0.65 × 1.55 × 1.85 m. Full load losses = 13.1 kW. Temperature
rise = 40°C. Mean length of cooling tubes is 1 m. Find the
number of cooling tubes required. Assume convection is
improved by 40 % due to provision of cooling tubes. (8 Marks)
U, CO4 14. a) i. Derive the expression of equivalent rotor resistance referred to
stator of three phase squirrel cage induction motor. (8 Marks)
A, CO4 14. a) ii. A 11 kW, 3 Φ, 6 pole, 50 Hz, 220 V, star connected induction
motor has 54 slots, each containing 9 conductors. Calculate the
values of bar and end ring currents. The number of rotor bars is
64. The machine has an efficiency of 86 % and a power factor of
0.85. Also, find the cross sectional area of the bar and end ring, if
the current density is 5 A/mm2 and Kws = 0.955. (8 Marks)
OR
U, CO4 14. b) i. Derive an expression for rotor bar current and end ring current of
a squirrel cage induction motor. (8 Marks)
A, CO4 14. b) ii. Estimate the main dimensions to give best power factor of a
100 kW, 3300 V, 50 Hz, 12 pole Star connected slip ring
induction motor. Specific magnetic loading is 0.4T and specific
electric loading is 25000 ampere conductor per meter. Efficiency
is 0.9 at 0.9 pf. (8 Marks)
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A, CO5 15. a) i. Estimate for a 250 kVA, 1100 V, 12 pole, 500 rpm, 3-phase star
connected alternator A) Diameter B) Core length C) Number
of stator conductors. Assume average gap density as 0.6 Wb/m2,
specific electric loading of 30000 amp cond/m, L/τ = 1.5, winding
factor = 0.955. (8 Marks)
U, CO5 15. a) ii. Explain the design of field winding of turbo alternators. (8 Marks)
OR
A, CO5 15. b) i. Find the main dimensions of a 2500 kVA, 187.5 rpm, 50 Hz, 3Φ,
3 kV salient pole synchronous generator. The generator is to be a
vertical, water wheel type. The average gap density is 0.6 Wb/m2,
specific electric loading = 34,000 amp. conductors / metre. Use
circular poles with ratio of core length to pole pitch as 0.65.
Specify the type of pole construction used, if the run-away speed
is about 2 times the normal speed. (8 Marks)
U, CO5 15. b) ii. Derive from first principles, the output equation for an
AC machine in terms of specific electric and specific magnetic
loading. (8 Marks)
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