GUJARAT TECHNOLOGICAL UNIVERSITY
BE - SEMESTER-VII (OLD) EXAMINATION — SUMMER 2019
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Subject Code: 170902 Date: 14/05/2019
Subject Name:Electrical Machine Design-I
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) Answer the following with respect to transformer design :
(a) Why cores are stepped? (b) Why yoke is designed for low flux density?
(b) Calculate the diameter and length of armature for a 7.5 kW, 4 pole, 1000 rpm, 220 V Shunt motor. Full load efficiency=0.83, maximum gap flux density=0.9 Wb/m?, specific electric loading =30,000 ampere conductors per meter, field form factor=0.7, assume maximum efficiency occurs at full load and field current is 2.5% of rated current. The pole face is square. [07]
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Q.2 (a) Explain how the choice of number of poles in a d.c. machine affects :
(1) Losses in the machine (2) Weight of machine [07]
(b) A 500 kW, 375 rpm, 8 pole D.C. generator has a flux per pole of 0.0885 wb. Determine the armature demagnetizing and cross-magnetizing mmf per pole if the brushes are given a lead of 5% of pole pitch. Assume power developed by armature to be equal to rating of machine. [07]
OR
(b) A350kW, 500V 450 rpm, 6 pole dc generator is built with an armature diameter 0f0.87 m and core length of 0.32 m. The lap wound armature has 660 conductors. Calculate the specific electric and magnetic loadings. [07]
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Q.3 (a) Explain different methods used to improve armature reaction effect in DC machine. [07]
(b) Write a short note on a square and stepped cores in transformer design. [07]
OR
Q.3 (a) Discuss the behavior of current transformer under system short circuit condition. [07]
(b) Derive the expression fot no load current of Transformer. [07]
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Q.4 (a) Explain various factors affecting choice of Average flux density and Ampere conductors per meter.for D.C. machine. [07]
(b) Explain: (a) Significance of mitered joints in transformer. (b) -Design difference between power & distribution transformer. [07]
OR
Q.4 (a) Write a Short Note on : Duty Cycle [07]
(b) Derive output equation of 3 —® Transformer. Write the significance of constant ‘K. [07]
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Q.5 (a) How will the output and losses in transformer change with the linear dimensions? [07]
(b) The ratio of flux to full load mmf in a 400 kVA, 50 Hz single phase core type power transformer is 2.4 x 10°°. Calculate the net iron area and the window area of transformer. Maximum flux density in the core is 1.3 Wb/m? current density 2.7 A/mm? and window space factor 0.26. Also calculate the full load mmf. [07]
OR
Q.5 (a) What is design optimization? Derive necessary condition for designing a transformer with minimum cost. [07]
(b) A 40 Hz transformer is to be used on a 50 Hz system. Assuming the Steinmetz’s coefficient as 1.6 and losses at lower frequency 1.2%, 0.7% and 0.5% for I°R, hysteresis and eddy current respectively. Find losses on 50 Hz for the same supply voltage and current. [07]
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This download link is referred from the post: GTU BE 2019 Summer Question Papers || Gujarat Technological University
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