UNIT – 4
16 marks
1. Derive the output equation of a three phase induction motor.
2. State and explain factors governing the choice of ampere conductors per metre in the
design of a three phase induction motor.
3.Compute the' main dimensions of a 15 KW, three phase, 400 volts, 50 Hz, 2810 rpm
squirrel cage Im having efficiency of 88 percent and full load power factor of 0.9.Assume
specific magnetic loading equal to 0.5 Wb/m2 and specific electric loading equal to
25,000 A/m. The rotor peripheral speed' may be approximately 20 m/sec at' synchronous
speed.
4. Determined diameter and length of the stator core for a 11 kW, 400 V, 3 phase, 4pole,1425 rpm induction motor. Specific magnetic loading is 0.45 wb/m2 and specific
electric loading is 23000 ac/m. Full load. efficiency is 0.85 and full load power factor is
0.88. The ratio of core length to pole pitch = 1
5.A 90 kW, 500 volts, 50 Hz; three phase, 8 pole slip-ring induction motor has star
connected stator accommodating 6 conductor per slot. The number of stator slots = 63. If
the slip ring voltage on open circuit is to be about 400 volts, find the number of rotor
slots and the number of conductors in each rotor slot.
6. Write notes on: i. Design of rotor bars and slots. ii. Design of end rings.
7. Find the values of diameter and length of stator core of a. 7.5 kW. 220 V, 50 Hz. 4
pole. 3 phase induction motor for best power factor.
8. Find the main dimensions of a 15 kW, three phase, 400 volts, 50 Hz, 2810 rpm squirrel
cage induction. motor having all efficiency of 88 percent and full load power factor of
0.9. Specific magnetic loading is 0.5 Wb/ m2 . Specific electric loading = 25000 A/m.
Take rotor peripheral speed 'as approximately 20 m/sec synchronous speed.
9. A 11 kW, three phase 6 pole, 50 Hz; 220 volts star connected induction motor has 54
stator slots, each containing 9 conductors. Calculate the value of bar and end ring
currents. The number of rotor bars is 64. The machine has an efficiency of 8.6 percent
and a powerfactor of 0.85. The rotor MMF may be assumed to be 85 percent of stator
MM F. Also find the bar 'and the end ring sections if the current density is 5 A/mm2.
UNIT – V
16 marks
1. Compute the main dimensions of a 1000 KVA, 50 Hz, three phase, 375 rpm alternator.
The average air gap flux density is 0.55 Wb/m2 and ampere conductors per metre are
28000. Use rectangular poles. 'Assume the ratio of arc length to pole pitch equal to 2.
Maximum permissible peripheral speed is 50 m/sec. The runaway speed is 1.8 times the
synchronous speed.
2. The field coils of a salient pole alternator are wound with a single layer winding of
bare copper strip 30 mm drop, with separating insulation 0.15 mm thick. Compute
thickness of the conductor, number of turns and height of the winding to develop an mmf
of 12000 ampere turns with a potential difference of 5 volts per coil and a loss of 1200
watts/m2 of coil surface area. Mean length of turn is 1.2 metre. Resistivity of copper is
0.021.Ω/m/mm2 .
3. Calculate the .mmf required for the air gap of a salient pole synchronous machine
having core length of 0.32 metre including 4 ducts of 10 mm each; pole arc = 0.19 metre.
Slot pitch = 65.4 mm, Slot opening = 5 mm. Air gap length = 5 mm. Flux per pole = 52
mwb.
4. Explain the role of digital computers in the design of electrical machine.
5. Describe the construction of Turbo – alternators with sketch.
6. Explain the design of field winding of alternator.
7. State and explain advantages of hydrogen cooling as applied to turbo alternator.
8. Draw cross sectional view of rotor slot and explain the method involved in direct
hydrogen cooling of rotor of turbo alternator.
9. Derive the output equation of a synchronous machine.
10. State and explain the salint features of Computer Aided design of electrical apparatus.
11. Determine the main dimensions of a 75000 KVA, 13.8 kV, 50 Hz, 62.5 rpm, three
phase star connected alternator. The peripheral speed of rotor should be about 40 m/sec.
Assume average gap density equal to 0.65 Wb/ m2 , ampere conductors per metre equal
to 40,000 and current density =4 A/ mm2 . Assume Kw = 0.955.
UNIT IV THREE PHASE INDUCTION MOTORS
Part-A (2 marks)
1. Why does induction motor designed with high specific electric loadings have smaller
over load capacity?
2. Why the harmonic leakage flux in squirrel cage induction rotor is not present?
3. Why the length of air gap in induction motor is kept minimum possible?
4. Why do die-cast rotors is extensively used in making 3 phase cage induction motor?
5. Why do 3 phase squirrel cage induction motor finds wide application in industry?
6. What is hot spot temperature?
7. What is the advantage of having wound rotor construction?
8. What is rotating transformer?
9. What is integral slot winding and fractional slot winding?
10. What types of slots are preferred in induction motor?
11. List the undesirable effects produced by certain combination of rotor and stator slots.
12. What are the advantage and disadvantage of large air gap length in induction motor?
13. What are the factors which influence the power factor of an induction motor?
14. What are the criteria used for the choice for number of slots of an induction machine?
15. What are the factors to be considered for estimating the length of air gap in induction
motor?
16. List out the methods to improve the power factor of an induction motor?
17. Why the air gap of an induction motor is made as small as possible?
18. What happens if the air-gap of an induction motor is doubled?
19. What is the condition for obtaining the maximum torque in case of 3 phase induction
motor?
20. What are the special features of the cage rotor on induction machine?
UNIT V SYNCHRONOUS MACHINES
Part – A (2 MARKS)
1. State three important features of turbo alternator rotors.
2. Why salient pole construction is rejected for high speed alternators.
3. What material user for the construction of turbo alternator rotor.
4. What is run-away speed?
5. What is approximately the run away speed of Kaplan turbine.
6. Write the expression for the output coefficient of synchronous machine.
7. What are the advantages of designing the alternators with higher flux density?.
8. What are the disadvantages of designing the alternators with higher gap flux density?
9. Why semi- closed slots are generally preferred for the stator of induction motors.
10. What is the effect of specific magnetic loading on the size of the machine?
11. What is the effect of specific electric loading on t he copper losses?
12. Write down the main consideration in the selection of specific loadings for the design
of induction motor.
13. What is critical speed of alternator?
14. What are the functions of damper winding?
15. What is Short Circuit Ratio (SCR)?
16 What is the effect of SCR on synchronous machine performance?
17. Why it is necessary to cool an electrical machine?
18. What is limiting factor for the diameter of synchronous machine?
19. Discuss how ventilation and cooling of a large high speed alternator is carried out.
20. Mention the factors to be considered for the design of field system in alternator.