JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III ELECTRICAL MACHINES-III Previous Question Paper (May/June - 2019 - 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
How can a DC generator be converted into an alternator?
Answer
A DC generator cannot be directly converted into an alternator because a DC generator requires a commutator for rectification, whereas an alternator requires slip rings to provide an AC output. Converting one to the other would require complete mechanical reconstruction of the rotor assembly and the removal of the commutator.
Define the following i) Short pitch winding ii) Full pitch winding.
Answer
i) Short pitch winding: A winding where the coil span is less than the pole pitch (180 electrical degrees). It is used to reduce harmonics and save copper.
ii) Full pitch winding: A winding where the coil span is exactly equal to the pole pitch (180 electrical degrees).
Define Xd and Xq.
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What are the advantages of ASA method?
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A machine with large air-gap has a higher synchronizing power, why?
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At what power angle a synchronous generator will develop maximum power?
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What are the different methods of starting synchronous motor?
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Why it is necessary to increase the excitation of synchronous motor to obtain minimum current with application of load?
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What is meant by split phase motor?
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What is the function of capacitor in a single phase induction motor?
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PART - B
Discuss the differences between distributed and concentrated windings of synchronous machine.
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A 3-phase, star connected, 8 pole, 750 rpm alternator has 72 slots on its periphery. Each slot has 12 conductors and the winding is short pitched by 2 slots. Find the pitch factor and distribution factor. Also, calculate the induced e.m.f between lines if the flux of 0.04wb is distributed sinusoidally. All the conductors in phase are connected in series.
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Explain the load characteristics of an alternator.
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Determine the frequency of a 12 pole alternator rotating at 600 R.P.M. If the no. of poles is tripled, then what will be its new frequency?
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Describe why, synchronous impedance method of computing the voltage regulation, leads to a pessimistic value at lagging power factor loads.
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A synchronous generator has Xd=0.7 pu and Xq=0.4 pu. It is supplying full-load at rated voltage at 0.85 lagging power factor. Draw the phasor diagram and compute the excitation emf.
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A 3-phase star-connected, 1000KVA, 2000V, 50hz alternator gave the following open-circuit and short circuit test readings: Field current (amp): 10 20 25 30 40 50 Open-circuit voltage (v): 800 1500 1760 2000 2350 2600 Short-circuit current(amp):- 200 250 300 - - Draw the characteristic curves and estimate the full-load percentage regulation at (a) 0.8 p.f. lagging and (b) 0.8 p.f. leading. The armature resistance per phase may be taken as 0.2Ω. Use mmf method.
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What are the conditions to be fulfilled for running two generators in parallel? Describe methods of synchronizing two 3-phase alternators.
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Two identical 3-phase star connected generators operating in parallel, share equally a total load of 750KW at 6000V and power factor 0.8. The synchronous reactance and resistance of each machine are respectively 50 Ohm and 2.5 Ohm respectively per phase. The field of the first generator is excited so that the armature current is 40A (lagging). Find i) armature current of second alternator, ii) power factor of each machine.
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A synchronous generator is connected to an infinite bus. Discuss the effect of changing the input at constant excitation with the help of phasor diagrams.
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Illustrate the determination of sub-transient, transient and steady state reactances.
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What is a synchronous condenser? Explain its operation.
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The input to an 11000V, 3-phase star connected synchronous motor is 60A. The effective resistance and synchronous reactance per phase are respectively 1 Ω and 30Ω. Find the power supplied to the motor, and the induced electromotive force for a power factor of 0.8 i) lagging ii) leading
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Explain the variation of current and power factor of a synchronous motor with excitation.
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A 3-phase, 400V, 50 Hz, 27.5 kW synchronous motor has 88% efficiency at full load. Za=(0.2+j1.6) ohms. If the excitation is adjusted to give a leading p.f. of 0.9. Calculate i) the excitation emf ii) total mechanical power developed.
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Why 1-ϕ induction motor is not a self starting? Explain the starting methods of synchronous motors?
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Obtain the equivalent circuit of a 1- ϕ Induction motor.
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Explain the principle of operation and applications of A.C. series motors.
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Discuss the principle of operation of a stepper motor. Also mention its advantages.
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