Download Unit_4_QA

UNIT 4
INDUCTION MACHINES
1. State the main constructional difference between cage and slip ring I.M.
2. What are the different losses in an induction motor?
3. What is dispersion coefficient in an induction motor?
4. Give the methods to reduce harmonic torques in an induction motor.
5. What are the effects of change of air gap length in an Induction motor.
6. What is the effect of change of air gap length in an induction motor?
7. List the main parts of a Slip ring induction motor.
8. What is the function of end rings in the rotor of cage induction motor?
9. What are the merits of slip ring induction motor over squirrel cage induction motor?
10. How does the external resistance of slip ring I.M influence the motor performance?
11. What is the overload capacity in 3 phase induction motor?
12. What are the materials used for slip rings and brushes in induction motor?
13. Name the materials used to insulate the laminations of the core of induction motor.
14. What are the factors to be considered for the choice of specific magnetic loading?
15. What are the factors to be considered for the choice of specific electric loading?
16. How the induction motor can be designed for best power factor?
17. What are the types of slots preferred in induction motor?
18. What are the advantages and disadvantages of large air gap length in I.M?
19. List out the methods to improve the power factor of induction motor.
20. What is crawling and cogging?
PART B
1)
2)
Estimate the stator core dimensions, number of stator slots and number of stator conductors per
slot for a 100kW, 3300V, 50Hz, 12 pole star connected slip ring induction motor. Assume
average gap density =0.4wb/m2; Conductors per metre =25000A/m; Efficiency =0.9; Power
factor =0.9 and winding factor =0.96. Choose main dimensions to give best power factor. The
slot loading should not exceed 500-ampere conductors. Explain the design of rotor bars and slots.
Estimate the main dimension, air gap length, stator slots, slots/phase and cross sectional area of
stator and rotor conductors and end ring for three phase, 15HP, 400V, 6 pole, 50Hz, 975 rpm
induction motor. The motor is suitable for star – delta starting. Bav = 0.45 wb/m2. ac = 20000
ampere conductors/m. L /  =0.85. Efficiency = 90%, powerfactor = 0.85.
3)
Write short notes on i) design of rotor bars and slots ii) design of end rings.
4)
Find the values of diameter and length of stator core of a 7.5kW, 220V, 50Hz, 4 pole, Three
phase induction motor for best power factor.
Given : specific magnetic loading = 0.4 wb/m2; Specific electric loading – 22000 ac/m;
Efficiency = 0.86; Power factor = 0.87. Also find the main dimensions if the ratio of core length
to pole pitch is unity.
5)
6)
Determine the diameter and length of the start core for 11kW, 400V, three phase, 4 pole, 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.
A 90 kW, 500V, 50Hz, three phase 8 pole slip ring induction motor has star connected stator
accommodating 6 conductors 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.
7)
Derive the output equation of a three phase induction motor (8)
8)
State and explain factors governing the choice of ampere conductors per metre in the design of
three phase induction motor.(8)
9)
Compute the main dimensions of a 15kW, three phase, 400V, 50Hz, 2810 rpm squirrel cage
induction motor having efficiency of 88 % and full load power factor of 0.9. Assume specific
magnetic loading = 0.5 wb/m2 and Specific electric loading = 25000 ac/m. The rotor peripheral
speed may be approximately 20 m/sec at synchronous speed.
10) A 11kW, three phase, 6 pole, 50Hz, 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 and efficiency of 86% and a power factor of 0.85 the rotor
mmf may be assumed to be 85% of stator mmf. Also find the bar and the end ring sections if the
current density is 5A/mm2.
11) Estimate the main dimensions, air gap length, stator slots, stator turns per phase and cross
sectional area of stator and rotor conductors for a 3 phase, 15HP, 400V, 6 pole, 50Hz, 975 rpm
induction motor. The motor is suitable for star delta starting. Average flux density =0.45wb/m2,
specific electric loading =20000 amp. conductors/m. The ratio of core length to pole arc =0.85;
full load efficiency =0.9; power factor =0.85.
12) A 15 kW, three phase, 6 pole, 50 Hz, squirrel cage induction motor has the following data, stator
bore diameter = 0.32m, axial length of stator core = 0.125 m, number of stator slots = 54,
number of conductor / stator slot = 24, current in each stator conductor = 17.5 A, full load P.F
= 0.85 lag. Design a suitable cage rotor giving number of rotor slots section of each bar and
section of each ring. The full speed is to be 950 rpm, use copper for rotor bar and endring
conductor. Resistivity of copper is 0.02 ohm-m.
-------- %%%% --------