JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II ELECTRICAL MACHINES -I Previous Question Paper (February - 2024 - R13)
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
Define Co-energy.
Answer
Co-energy is defined as the area under the magnetization curve (flux linkage vs. current) for a magnetic system. It is mathematically expressed as the integral of flux linkage with respect to current: W' = ∫ λ di.
Why do all practical energy conversion devices make use of the magnetic field as a coupling medium rather than an electric field?
Answer
Magnetic fields are preferred because they can store significantly higher energy densities compared to electric fields within the same volume. Additionally, magnetic fields allow for easier insulation and are more effective at producing the high forces and torques required for practical electromechanical energy conversion.
Define back pitch and front pitch.
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Differentiate Lap winding and Wave Winding of a DC machine.
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Define critical speed of a dc shunt generator.
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What are the requirements of voltage build up in self-excited D.C generator.
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Why starter is necessary for DC motor?
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Draw the torque speed characteristics of all self-excited d.c motor?
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What are the losses that occur in dc machine?
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What are the advantages and disadvantages of swinburne’s test?
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Part B
Draw and explain the general block diagram of an electromechanical energy conversion device.
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Derive the field energy and co-energy for a doubly excited systems.
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Explain the armature reaction and commutation in detail for a DC Machine.
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A 4 pole DC shunt generator, with a shunt field resistance of 100 ohms and an armature resistance of 1 ohm, has 378 wave connected conductors in its armature. The flux per pole is 0.02 Wb. If a load resistance of 10 ohm is connected across the armature terminals and the generator is driven at 1000 rpm. Calculate the power absorbed by the load.
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With neat sketch explain the constructional components of DC Machine.
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A 4 pole DC Shunt Generator with lap connected armature supplies 5 kW at 230 V. The armature and field copper losses are 360 Watts and 200 Watts respectively. Calculate the armature current and generated EMF?
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Explain the different types of DC generators based on the excitation with suitable diagram.
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How the critical field resistance of a d.c self-excited generator is calculated from magnetic characteristics in the laboratory?
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Explain the process of voltage build up in a DC shunt generator.
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Two DC shunt generators running in parallel to supply together 2500A. The machine has armature resistance of 0.04Ω and 0.025Ω, field resistances of 25Ω and 20Ω an induced emf of 440V and 420V respectively. Find the bus bar voltage and output of each machine.
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Derive the torque equation of a DC Motor.
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A DC Series Motor runs at 500 rpm on 220 V supply drawing a current of 50 A. The total resistance of the machine is 0.15Ω, calculate the value of the extra resistance to be connected in series with the motor circuit that will reduce the speed to 300 rpm. The load torque being then half of the previous to the current.
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Explain the following methods for speed control of DC Motor: (i) Armature Control (ii) Field Control
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A 230 volts DC Shunt motor on no-load runs at a speed of 1200RPM and draw a current of 4.5 Amperes. The armature and shunt field resistances are 0.3 ohm and 230 ohms respectively. Calculate the back EMF induced.
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Determine a suitable method for determining the efficiency of d.c series motor.
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Explain brake test for determining the efficiency of a DC machine.
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In Hopkinson’s test on two identical dc shunt machines, the following readings were obtained: Line current: 49.4 A; Line voltage: 460 V; Motor armature current: 300 A; Field currents: 5 A; and 4.4 A. The armature resistance of each machine is 0.05 ohms. Calculate the efficiency of each machine.
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