JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III ELECTRICAL MACHINES-III Previous Question Paper (May - 2018 - R13)
Note: This question paper contains two parts A and B. 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 and may have a, b, c as sub questions.
Part A
Why is a rotating field system used in preference to a stationary field?
Answer
A rotating field system is preferred because it allows the high-voltage armature windings to be placed on the stationary stator, making insulation easier and more reliable. It also requires only two slip rings for low-voltage DC excitation, reducing the risk of sparking and mechanical failure compared to handling high-voltage power through brushes.
What is the importance of fractional slot windings in Synchronous machines?
Answer
Fractional slot windings are used to reduce harmonics in the generated EMF waveform, leading to a more sinusoidal output. They also allow for a wider choice of stator slots for a given number of poles, which facilitates better optimization of the machine's physical design and reduces the cogging torque.
Compare voltage regulation methods in brief.
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What do you mean by Synchronous reactance? Explain the term Synchronous Impedance of an Alternator.
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What do you mean by synchronizing torque of power?
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What are causes of transient reactance's in Synchronous Machines?
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Why Synchronous motors are not Self Starting?
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Give the applications of Synchronous Motors.
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Give various types of 1-ϕ Machines. Give Examples.
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Explain the operation of Stepper Motor.
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Part B
Explain the terms coil-span factor and distribution factor in connection with alternator armature windings and deduce the e.m.f equation of an alternator incorporating the effects of these factors?
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Explain the different methods of excitation system of alternators.
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Explain why harmonics are there in the induced EMF of Alternators.
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Give various methods to suppress the Harmonics in induced emf of Alternators.
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Define voltage regulation of an alternator. Explain the various factors which may affect the regulation of an alternator.
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From the following test results, determine the voltage regulation of a 2000v, 1-ϕ alternator delivering a current of 100A at (i) unit pf, (ii) 0.8 leading pf and (iii) 0.71 lagging pf.
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Explain the Poitier- triangle method of determining the voltage regulation of an alternator.
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Describe the slip test method for the measurement of Xd and Xq of synchronous machines.
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What do you mean by Synchronizing of alternator's? Describe any one method of synchronizing.
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What conditions must be fulfilled before an alternator can be connected to an infinite bus?
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Write a short notes on effect of change of excitation in alternators.
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Deduce the expressions for transient, sub-transient and steady state reactance's in alternators.
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Explain the principal of operation of a 3-phase synchronous motor.
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Compare synchronous motors with Induction motors.
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Derive the expression for the power developed in a 3-phase synchronous motors.
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Explain how Synchronous motor operates as a Synchronous Condenser?
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Draw the circuit diagram of a capacitor-start capacitor-run single phase induction motor and explain its working. Where this type of motor is commonly used?
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What are the advantages of 1-ϕ induction motors when compared with 3-ϕ induction motors?
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Explain simply why a universal motor can operate from d.c. as well as a.c supplies?.
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Explain the double revolving theory of 1-ϕ induction motors.
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