JNTU
JNTU B.Tech III ELECTRICAL MACHINES-III Previous Question Paper (March - 2024 - R13)
i) Question paper consists of Part A, Part B.
ii) Part A is compulsory, which carries 25 marks. In Part A, Answer all questions.
iii) In Part B, Answer any one question from each unit. Each question carries 10 marks and may have a, b as sub questions.
Part A
What happens to the value of synchronous reactance if air gap is increased?
Answer
If the air gap of a synchronous machine is increased, the synchronous reactance decreases. This occurs because the air gap provides the primary reluctance in the magnetic circuit; increasing the gap length increases the reluctance, which reduces the armature reaction flux for a given armature current. Since synchronous reactance is the sum of leakage reactance and armature reaction reactance, the reduction in the armature reaction component leads to an overall decrease in the synchronous reactance.
What is the importance of fractional slot windings in Synchronous machines?
Answer
Fractional slot windings, where the number of slots per pole per phase is not an integer, are employed in synchronous machines primarily to improve the quality of the generated voltage waveform. The key benefits include:
- Reduction of Harmonics: They significantly reduce the magnitude of slot harmonics and other high-frequency space harmonics in the air-gap MMF, resulting in a more sinusoidal EMF waveform.
- Improved Winding Distribution: They allow for a more flexible choice of the number of slots, which helps in optimizing the machine design for a specific frame size.
- Noise and Vibration Reduction: By suppressing harmonic content, they minimize parasitic torques, thereby reducing magnetic noise and vibrations during operation.
Differentiate between MMF and EMF methods of finding regulation of an alternator.
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What do you mean by Synchronous reactance? Explain the term Synchronous Impedance of an Alternator.
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Write a short note on synchronizing power.
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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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How are ‘V’ and ‘Ʌ’ curves of synchronous motor are defined?
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In which direction does a shaded pole motor run? Can such a motor can be reversed?
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The full load slip of a single phase induction motor is higher than of corresponding 3 phase induction motor. Why?
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Part B
Give the constructional details of a synchronous machines.
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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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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 0.04 wb is distributed sinusoidally. All the conductors in phase are connected in series.
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What are the different methods for calculating voltage regulation? Discuss the MMF method of calculating voltage regulation.
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Explain the two-reaction theory as applied to salient pole synchronous machine.
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A 3- phase generator rated at 25 MVA, 13.8 kV is operating at normal terminal voltage and rated load at 0.8 pf lag. The direct axis synchronous reactance is 7.62Ω, Quadrature axis synchronous reactance is 4.57Ω and the armature resistance is 0.15Ω/ph. Determine the direct axis and quadrature axis components of armature current and internal induced voltage. Also find the regulation.
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Describe synchronizing alternator with infinite bus bars.
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Two similar three phase ‘Y’ connected alternators are in parallel connection. Each machine has a synchronous reactance of 4.5 ohm per phase and very small resistance. The alternator is excited to generate 1910 V per phase. The machine has a phase displacement of 300 (electrical) relative to each other. Calculate (i) the circulating current (ii) the terminal voltage per phase (iii) the power supplied from one machine to other.
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Explain the effect of change in excitation and mechanical power input on the alternator performance.
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Draw phasor diagram of synchronous motor at different power factors.
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Derive the expression for the maximum power developed by a synchronous motor.
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Explain in detail about hunting and its suppression in synchronous motor.
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A salient-pole synchronous motor has Xd = 0.85pu and Xq = 0.55 pu. It is connected to bus bars of 1.0 pu voltage, while its excitation is adjusted to 1.2 pu. Calculate the maximum power output that the motor can supply without loss of synchronism. Compute the minimum pu excitation that is necessary for the machine to stay in synchronism while supplying the full-load torque.
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Discuss in detail about split phase motors.
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Discuss about torque – speed curve of single-phase induction motor.
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Explain about double revolving field theory.
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Explain the principle of operation of stepper motor.
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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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