JNTU
JNTU B.Tech II ELECTRICAL MACHINES II Previous Question Paper (April/May - 2023 - R16)
Note: 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 are the demerits of wound rotor induction motor?
Answer
The primary demerits of a wound rotor (slip ring) induction motor are its higher initial cost and increased maintenance requirements due to the presence of slip rings and carbon brushes. Additionally, it has a lower efficiency compared to a squirrel cage induction motor and is more prone to operational issues caused by sparking at the brushes, which makes it unsuitable for hazardous or dusty environments.
What are the differences between 3-phase induction motors and transformers?
Answer
The primary differences between a 3-phase induction motor and a transformer are as follows:
- Energy Conversion: A transformer is a static device that transfers electrical energy from one circuit to another through electromagnetic induction without changing the frequency. An induction motor is a rotating machine that converts electrical energy into mechanical energy.
- Air Gap: A transformer has a closed magnetic core with no air gap, ensuring high permeability and low magnetizing current. An induction motor requires an air gap between the stator and rotor to allow for mechanical rotation, which necessitates a significantly higher magnetizing current.
- Operating Principle: Transformers operate based on mutual induction between stationary windings. Induction motors operate based on the production of a rotating magnetic field that induces currents in the rotor, resulting in torque production through Lorentz force.
What is meant by cogging?
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What is no load test of induction motor? What data can be obtained by the test?
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How are alternators classified?
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What is distribution factor? What is its effect?
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State the characteristic features of a synchronous motor.
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State the requirements for paralleling the alternator.
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What are the applications of single phase induction motors?
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Compare the operating characteristics of a resistance start induction run motor with those of a capacitor start induction rum motor.
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PART – B
Describe the constructional features of slip-ring induction motor. List out its advantages.
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Show that a rotating magnetic field can be produced by the use of 3-phase currents of equal magnitude.
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Discuss the principle of operation of a 3-phase induction motor. Also explain why the rotor is forced to rotate in the direction of rotating magnetic field.
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Explain why a 3-phase induction motor, at no load, operates at a very low power factor.
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Explain the terms air-gap power Pg, internal mechanical power developed Pm and shaft power Psh. How are these terms related with each other? Hence show that Pg: rotor ohmic loss: Pm=1:s: (1-s).
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The stator of a 3-phase, 4 pole slip ring induction motor is connected to 50 Hz supply. At the rotor terminals, a frequency of 30 Hz is required. Find the possible speeds at which the rotor must be driven. What is the ratio of slip ring emfs at these speeds at no load?
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What is represented by the circle diagram of an polyphase induction motor? What information can be obtained from it? Derive the current locus for the rotor circuit of a polyphase induction motor?
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A 20 kW, 400 V, 3-phase induction motor has full load power factor of 0.86 and full load efficiency of 0.88. With stator winding is delta, short circuit line current at 200 V is 70 A. If this motor is fitted with a star-delta starter, find (i) the ratio of starting to full load line current and (ii) the starting torque in terms of full load torque, for a full load slip of 5%.
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Derive an expression for the voltage induced in an alternator. Discuss the role of different factors which appear in the expression.
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Draw the phasor diagram of a round rotor alternator on (i) open-circuit and (ii) short circuit. Discuss why short circuit characteristic is a straight line where as open circuit characteristic is a curve.
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Explain the method of determining the voltage regulation of an alternator by new ASA method.
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Describe how slip test can be conducted in the laboratory, for measuring Xd and Xq.
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Compare the performance of a synchronous generator connected to an infinite bus with that of an isolated alternator operating on its own load.
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Two 750 kW alternators are operate in parallel. The speed regulation of one set is 100% to 103 % from full load to no-load and that of the other is 100 % to 104 %. How will the two alternators share a load of 1000 kW and at what load will one machine cease to supply any portion of the load.
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Describe the method of staring a synchronous motor. Also discuss the effect of varying excitation of synchronous motor under constant load.
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A 400 V, 50 Hz, 3-phase, 37.5 kW star connected synchronous motor has a full load efficiency of 88 %. The synchronous impedance of the motor is (0.2 + j 1.6) ohms per phase. If the excitation of the motor is adjusted to give a leading p.f of 0.9, calculate for full load the induced emf and the total mechanical power developed.
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Describe with the aid of diagrams of connections and phasor diagrams the two methods of producing starting torque in a single phase induction motor.
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What are the demerits of a single phase induction motor when compared with a 3-phase induction motor?
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Explain the double revolving theory for single phase induction motors.
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