Jawaharlal Nehru Technological University Hyderabad
Jawaharlal Nehru Technological University Hyderabad B.Tech III ELECTRICAL MACHINES-III Previous Question Paper (November/December - 2018 - R13)
Part A is compulsory which carries 25 marks. Answer all questions in Part A. Part B consists of 5 Units. Answer any one full question from each unit. Each question carries 10 marks.
Part A
Differentiate between short pitch and full pitch winding.
Answer
In full pitch winding, the coil span is exactly equal to the pole pitch (180 electrical degrees). In short pitch winding, the coil span is less than the pole pitch, which is used to eliminate specific harmonics and reduce the amount of copper required.
Give an expression for pitch and distribution factors in an alternator.
Answer
The pitch factor (kp) is given by:
where α is the chording angle. The distribution factor (kd) is given by:
where m is the number of slots per pole per phase and β is the slot angle in electrical degrees.
Differentiate between MMF and EMF methods of finding regulation of an alternator.
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Define Xs and Xa of a synchronous machine.
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What is synchronization of alternators?
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What will be effect if input to one alternator is changed in parallel operation?
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What is a synchronous condenser?
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Give any three applications of synchronous motors.
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Give classification of capacitor motors.
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Give applications of shaded pole induction motors.
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Unit I
Derive an emf equation of alternator considering various factors.
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Explain merits and demerits of rotating magnetic field over stationary.
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With a neat experimental set up explain how to find synchronous impedance of an alternator.
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Explain the effect of armature reaction at different power factors in an alternator.
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Unit II
An alternator is rated as 400 V and 20A on full load. It has armature and synchronous impedance of 2.5 ohms and 18 ohms respectively. Find the induced emf at:
0.9 pf lead
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0.8 pf lag
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at upf.
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What is direct axis and quadrature axis reactance? How can we determine experimentally?
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Explain the method of finding regulation of a salient pole alternator.
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Unit III
Explain the necessity of parallel operation of alternators.
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What are the conditions to be full filled for synchronization of alternators.
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Derive an expression for load shared between two alternators with unequal KVA ratings.
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Derive an expression for synchronizing power in an alternator.
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Unit IV
Why are synchronous motors not self starting. Explain various methods of making self starting.
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Derive an expression for power developed by a synchronous motor.
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What is hunting in a synchronous motor. What are its effects?
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Explain about V-Curves and Inverted V-Curves.
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Unit V
Why are single phase motors not self starting. Explain.
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Explain double field revolving theory.
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Explain the working of a stepper motor with a neat diagram.
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What is an universal motor explain its operation.
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