Reg. No.

Question Paper Code : 16283

B.E. / B.TECH. DEGREE EXAMINATION, NOVEMBER / DECEMBER 2020

Fifth Semester
B.E. – Electrical and Electronics Engineering
15EE501 – INDUCTION AND SYNCHRONOUS MACHINES
(Regulations: Mepco – R2015)
Duration: 3 Hours Maximum Marks:100
Max. : 100 Marks
Answer ALL Questions
PART A – (10  2 = 20 Marks)
CO,BTL

CO1, R 1. List out the different methods of determining regulation of alternators.

CO1, U 2. The main drawback of using short-pitch winding in alternators is that it
_________. Justify your answer.
A) reduces harmonics in the generated voltage
B) reduces the total voltage around the armature coils
C) produces asymmetry in the three phase windings
D) increases copper of end connections
CO2, R 3. Define pull out Torque of synchronous motor.
CO2, U 4. Synchronous capacitor is _______. Justify your answer.
A) an ordinary static capacitor bank
B) an over-excited synchronous motor driving mechanical load
C) an over-excited synchronous motor running without mechanical load
D) none of the above
CO3, R 5. What is Crawling?
CO3, A 6. The number of stator poles produced in the rotating magnetic field of a three
phaseinduction motor having 54 slots 3 slots per pole per phase is
__________. Justify your answer.
A) 3 B) 6 C) 2 D) 12
CO4, R 7. List the methods of speed control of three phase induction motor which can
be applied from stator side.

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CO4, U 8. For the purpose of starting an induction motor, a star-delta starter is
equivalent to an auto-transformer starter, if transformation ratio is
__________ per cent. Justify your answer.
A) 33.3 B) 58 C) 73.2 D) 60
CO5, R 9. State the advantages of capacitor start capacitor run single phase induction
motor.
CO5, U 10. If starting winding of a single-phase induction motor is left in the circuit, it
will __________. Justify your answer.
A) draw excessive current and overheat B) run slower
C) run faster D) spark at light loads

PART B – (5  16 = 80 Marks)
CO1 ,U 11.a) i. Derive the equation of RMS value of induced EMF per phase
of an alternator. Also write the expression for actually
available voltage per phase. (8 Marks)
CO1, A 11.a) ii. A 125 KVA, 3300 volts, 50 Hz, 3 phase, star connected
synchronous generator has an effective armature resistance as
0.25 ohms. Field current of 45 amps produces short circuit
current of 225 amps and an open circuit voltage of 1120 volts
(line value). Determine the full load percentage regulation at a
power factor of 0.85 lagging. How will the regulation gets
affected if the alternator delivers its full load output at a power
factor of 0.85 leading? (8 Marks)
OR
CO1, U 11.b) i. Explain in detail the principle of operation of salient pole
synchronous generator with neat sketch. (8 Marks)
CO1, A 11. b) ii. A 2875 KVA, 4 pole alternator runs at 1500 RPM in parallel
with other machines on 3300 Volts bus bars. The synchronous
reactance of the machine is 20 %. Calculate the Synchronizing
power for one mechanical degree of displacement when the
alternator is supplying full load at 0.8 lag and also calculate
the synchronizing torque. (8 Marks)

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CO2, U 12. a) i. Explain the principle of operation of Synchronous motor with
necessary sketches. (8 Marks)
CO2, A 12. a) ii. Determine mechanical power developed, armature current,
induced EMF, coupling angle and pull out torque of the
cylindrical rotor synchronous motor with the following
specifications: 60 kW, 3 phase, 4 pole, 50 Hz, star connected,
415 volts, 0.85 power factor leading, synchronous reactance is
3.0 ohms and motor efficiency excluding field and stator
losses is 92 %. (8 Marks)
OR
CO2, U 12. b) i. Discuss clearly the effect of varying excitation on armature
current and power factor in synchronous motor. (8 Marks)
CO2, A 12. b) ii. A 950 kVA, 11 kV, star connected, 3 phase synchronous
motor has an armature resistance and reactance per phase of
3.0 ohms and 35.0 ohms respectively. Determine the EMF
induced and load angle (or) power angle of the rotor when
fully loaded at 0.85 power factor lagging and 0.90 power
factor leading. (8 Marks)

CO3, R 13. a) i. With explanatory diagrams, enumerate the construction of slip

ring induction motor clearly specifying the provisions for
improving starting torque. (8 Marks)
CO3, A 13. a) ii. A 400 V, 4 pole, 50 Hz squirrel cage induction motor runs at
1450 RPM. The motor takes 50 kW as input from the supply
mains. Regarding the losses, stator losses are 1.5 kW and the
friction & windage losses are 2.5 kW. Determine slip, rotor
copper losses, shaft power and efficiency. (8 Marks)
OR
CO3, R 13. b) i. Write the expression for Torque under running conditions.
Also, draw and explain the torque – slip characteristics of
three phase induction motor. (8 Marks)

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CO3, A 13. b) ii. A 10 kW, 3 phase, 50 Hz, 12 pole induction motor has a rotor
resistance and rotor reactance of 0.05 ohms and 0.25 ohms at
standstill condition. The motor provides full load torque when
running at 480 RPM. Determine, (A) ratio of maximum to full
load torque (B) speed of maximum torque (C) the value of
rotor resistance to be added to obtain maximum torque. (8 Marks)

CO4, U 14. a) i. With neat sketch, explain the working of auto-transformer

starter. Also derive the necessary torque equation. (8 Marks)
CO4, A 14. a) ii. Two three phase induction motors, first motor speed being
1000 RPM and second motor speed being 1500 RPM are
connected in cumulative cascade. The 1000 RPM motor is
provided with supply. Estimate the frequency of rotor currents
and the slip referred to each motor stator field, if the combined
set has a slip of 3 %. (8 Marks)
OR
CO4, U 14. b) i. Describe the concept of speed control by injecting slip
frequency EMF into the rotor circuit of an induction motor.
Also, with diagram, explain the Krammer system of speed
control. (8 Marks)
CO4, A 14. b) ii. Determine the auto-transformer ratio for quick starting of a
three phase squirrel cage induction motor with line current not
exceeding 4 times the full load current and the slip at full load
is 4 %. The short circuit current is 4 times the full load
current. Estimate also the starting torque in terms of full load
torque. (8 Marks)

CO5, U 15. a) i. Explain clearly, how to make single phase induction motors
self starting and also describe the working of split phase
induction motor. (8 Marks)
CO5, R 15. a) ii. Discuss the principle of working of Reluctance motor. (8 Marks)
OR
CO5, U 15. b) i. Explain the principle of operation of Hysteresis motor and list
the special features of the same. (8 Marks)
CO5, R 15. b) ii. Discuss the principle of working of linear induction motor.
Also specify the important applications of it. (8 Marks)

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