DESIGN OF ELECTRICAL MACHINES
(2015 Pattern) (Semester – II)
Time : 2½ Hours Max. Marks : 70
Instructions to the candidates:
1) Solve Q1 or Q2, Q3 or Q4, Q5 or Q6, Q7 or Q8.
2) Neat diagrams must be drawn wherever necessary.
3) Figures to the right indicates full marks.
4) Use of Calculator is allowed.
5) Assume Suitable data if necessary.
Q1) a) Derive the output equation for a three phase core type transformer. [6M]
b) Explain the different modes of heat dissipation. [6M]
c) Explain the mechanical forces developed under short circuit conditions. Also state the measures to overcome this effect. [8M]
Q2) a) Explain any three types of windings used in a transformer. [6M]
b) Explain the procedure to calculate the no load current in case of a three phase transformer. [6M]
c) Explain the procedure for the design of tank with tubes and derive the relation for the number of tubes. [8M]
Q3) a) Define specific electrical and magnetic loadings. Explain the factors to be considered for the choice of specific electrical loading and specific magnetic loading. [8M]
b) Derive the Output equation for three phase induction motor with usual notations. [8M]
Q4) a) Explain the design of any two types of AC windings. [6M]
b) Explain the various factors in detail which play a major role while deciding the number of stator slots. [8M]
Q5) a) What are the suitable combinations of designing stator and rotor slots? [8M]
b) Derive the equation for end ring current for the rotor of squirrel cage
induction motor. [8M]
Q6) a) Explain the various factors which affect the length of air gap in an induction motor. [8M]
b) A 11 KW, 220 v ,3phase ,6 pole , star connected squirrel cage induction motor has the following data : number of stator slots = 54,number of conductor in each stator slot = 9, number of rotor bars = 64, efficiency = 0.86, power factor = 0.85,current density = 5A/mm2 . Find bar current , end ring current , area of bar , area of end ring . Assume Rotor mmf as 85% of Stator mmf. Assume suitable data if required. [8M]
Q7) a) Explain [6M]
i) Slot leakage
ii) Tooth top leakage
iii) Zig-zag leakage
iv) Overhang leakage with the help of necessary diagrams.
b) Explain the effects of ducts on calculation of magnetizing current. [6M]
c) A 75KW, 3300v, 50 Hz, 8 pole, 3 phase , star connected induction motor has a magnetizing current which is 40% of full load current. Calculate the value of stator turns per phase if the mmf required for flux density at 60° from the pole axis is 500A. winding factor 0.95 , efficiency = 0.94 and
power factor =0.86. Assume suitable data if required. [6M]
Q8) a) Explain the MMF Calculation for air gap, stator teeth, stator core, rotor teeth and rotor core. [6M]
b) Explain how the no load current is calculated in case of induction motor.[6M]
c) Calculate the magnetizing current for 415 V, 3phase, 50 Hz, 4 pole induction motor which has the following dimensions: air gap length is 0.5mm; flux density at 60° is 0.478 wb/m2. The stator winding is delta connected with 4 slots / pole / phase and 28 conductors / slot. The ampere turns for the iron path is equal to 45% of the air gap ampere turns. Assume the gap
contraction factor as 1.2, stator winding factor =0.955. Assume suitable data if required. [6M]