GUJARAT TECHNOLOGICAL UNIVERSITY
BE - SEMESTER-VIII (OLD) EXAMINATION — WINTER 2018
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Subject Code: 180904 Date: 03/12/2018Subject Name: Electrical Machine Design -II
Time: 02:30 PM TO 05:00 PM Total Marks: 70
Instructions:
- Attempt all questions.
- Make suitable assumptions wherever necessary.
- Figures to the right indicate full marks.
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Q.1 (a) Explain the factors affecting the selection of Air gap of three phase Induction Motor. 07
(b) Explain factors affecting specific electric loading and specific magnetic loading of 3 phase IM. 07
Q.2 (a) Explain the effect of skewing the rotor slots in a squirrel cage induction motor. 07
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(b) Estimate the stator core dimensions, number of stator slots and number of stator conductors per slot for a 100KW, 3300V, 50 Hz, 12 pole star connected slip ring induction motor. Assume Bav=0.4wb/m?, ac=25000 A/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. 07
OR
(b) Draw and explain briefly the current distribution wave form spreaded over one pole pitch in bars and end rings squirrel cage induction motor. 07
Q.3 (a) What is dispersion coefficient? Show its effect on maximum power factor and overload capacity of three phases Induction Motor. 07
(b) Find the current in the bars and end rings of a 11KW, 6 pole, 50 Hz , 3 phase star connected induction motor having 54 stator slots with 9 conductors in each slot. The number of rotor bars is 64. The.machine has an efficiency of 0.86 and a power factor of 0.85. The rotor mmf'may be assumed as 85 % of stator mmf. Also find the suitable size of the cage bars and end rings if the current density is 5 A/mm?. 07
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OR
Q.3 (a) Explain methods of Improving starting torque of Induction Motor. 07
(b) Draw flow chart and write algorithm steps for design of submersible motors. 07
Q.4 (a) Explain steps of design of field winding of alternator. 07
(b) The field coil of'a salient pole alternator are wound with a single layer winding of bare copper strip 30 mm deep with separating insulation 0f 0.15mm thick. Determine a suitable winding length, number of turns and thickness of conductor to develop mmf of 12000 A with a potential difference of 5 V per coil and with a loss of 1200W/m2 of total coil surface. The mean length of turn is 1.2 m. The resistivity of copper is 0.021ohm/m and mm? 07
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OR
Q.4 (a) Define SCR of a synchronous machine. Discuss the importance of SCR in the design of synchronous machine. 07
(b) For the same h.p. output compare the relative sizes of a 3-phase induction motor to 1-phase induction motor (split phase capacitor start). Assume same loadings, same number of poles, same power factor and efficiency. 07
Q.5 (a) What is the role of damper winding in (i) synchronous generator and (ii) synchronous motor? Derive the equation of MMF of damper winding. 07
(b) Explain the procedure for designing of capacitor for maximum torque in case of capacitor start single phase induction motor. 07
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OR
Q.5 (a) Explain evaluation of Direct and Quadrature axis reactance of Alternator 07
(b) Explain the terms “critical speed” and “run away speed” with reference to synchronous machine. 07
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