JNTU
JNTU B.Tech III ELECTRICAL MACHINES-III Previous Question Paper (August - 2022 - R13)
Answer any five questions
All questions carry equal marks
Main
Explain in detail about the constructional features of round rotor synchronous machines.
Answer
A round rotor (or non-salient pole) synchronous machine is designed for high-speed operation, typically driven by steam turbines. Its construction features include:
- Stator: Similar to an induction motor, the stator consists of a cylindrical frame made of cast iron or steel, housing a core built from silicon steel laminations to reduce eddy current losses. The inner periphery contains slots to accommodate the three-phase armature winding.
- Rotor: The rotor is a solid steel cylinder with slots cut along its outer periphery, extending over about two-thirds of the pole pitch. These slots house the field windings, which are secured by heavy steel wedges to withstand high centrifugal forces.
- Air Gap: The air gap is uniform throughout the circumference, which minimizes flux harmonics and ensures a sinusoidal induced EMF.
- Cooling: Due to high rotational speeds and large physical size, these machines require sophisticated forced-air or hydrogen cooling systems to dissipate heat generated in the windings and core.
A three phase star connected alternator has an open circuit voltage of 7000V. The armature resistance and synchronous resistance are 0.5 Ω and 5 Ω per phase respectively. Find the terminal voltage and the phase difference between terminal voltage and open circuit EMF at a power factor of 0.85 lagging. Given load current is 170A.
Answer
Given: Phase voltage E = 7000 / √3 = 4041.45 V, R = 0.5 Ω, Xs = 5 Ω, I = 170 A, cos φ = 0.85 (lagging), sin φ = 0.5268.
Terminal phase voltage V = √(3546.22² + 677.72²) = 3610.4 V. The line-to-line terminal voltage is 3610.4 × √3 = 6254.5 V. The phase difference δ is:
Explain in detail about pitch and winding factors in synchronous machines.
Answer
Sign in to view the complete answer paper.
A 100 kVA, 400V, 3 phase star connected alternator has the following test results: OCC: Line to line voltage is 400V, Field current is 3.5 A; Air-gap line: Line to line voltage is 300V, Field current is 2.5A; SCC: Field current is 2.5A, Armature current is 144.34 A. The armature resistance is negligible. Calculate the rated phase voltage, rated phase current and base impedance.
Answer
Sign in to view the complete answer paper.
Explain in detail about the experimental determination of Xd and Xq.
Answer
Sign in to view the complete answer paper.
The OCC of a 6-pole, 440V, 50Hz, three phase star connected alternator is given. A field current of 7.5 A is needed to circulate the full load rated current of 40A under short circuit conditions. The field current for rated terminal voltage under full load zero power conditions is 15A. The armature resistance is 0.25 Ω per phase. Find the regulation at full load current of 40A at 0.85 lagging power factor using synchronous impedance method.
Answer
Sign in to view the complete answer paper.
Explain in detail about finding the regulation of synchronous generators using ZPF method.
Answer
Sign in to view the complete answer paper.
A three phase 11 kV star connected alternator has armature resistance of 1 Ω and synchronous reactance of 20 Ω per phase. Find regulation for a load of 1000 kW at power factor of 0.9 leading.
Answer
Sign in to view the complete answer paper.
Explain the effect of change of excitation on load sharing in synchronous generators.
Answer
Sign in to view the complete answer paper.
A 1 MVA, three phase star connected, 8 pole, 1500 rpm alternator is operating on 6 kV bus bars. Synchronous reactance is 5 Ω per phase. Find synchronizing power and torque per mechanical degree of displacement for full load 0.9 power factor lagging.
Answer
Sign in to view the complete answer paper.
Give the detailed analysis of short circuit current wave form in synchronous generators.
Answer
Sign in to view the complete answer paper.
Two three phase star connected alternators supply a load of 2000 kW at 0.9 power factor lagging and share the load equally. The excitation of the second machine is so adjusted that it supplies 100A at a lagging power factor. The synchronous impedances of alternators are (0.3+j9) Ω and (0.4+j7) Ω respectively. Find current and power factor of each machine. The terminal voltage is 5 kV.
Answer
Sign in to view the complete answer paper.
Discuss hunting and its suppression in case of synchronous motor.
Answer
Sign in to view the complete answer paper.
A 5 MVA, three phase star connected, 6 kV, 16-pole, 50 Hz, salient pole synchronous motor has xd = 5 Ω and xq = 3 Ω per phase. The motor is working at full load unity power factor and rated voltage. Find (i) Current (ii) Back EMF.
Answer
Sign in to view the complete answer paper.
Draw the schematic diagram of a shaded pole motor and explain the operation.
Answer
Sign in to view the complete answer paper.
Explain in detail about the constructional features of single phase induction motors.
Answer
Sign in to view the complete answer paper.