JNTU
JNTU B.Tech II ELECTRICAL MACHINES I Previous Question Paper (April/May - 2018 - R15)
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.
Part A
Explain how is torque produced in a doubly excited system?
Answer
Torque is produced in a doubly excited system due to the interaction of two independent magnetic fields, one produced by the stator winding and the other by the rotor winding. The torque is generated as the system attempts to align these magnetic fields to minimize the energy stored in the magnetic field, expressed as the partial derivative of the co-energy with respect to the mechanical displacement.
In a rectangular electromagnetic relay, the exciting coil has 1000 turns. The cross-section area of the core is A=5cm × 5cm. Neglect the reluctance of the magnetic circuit and fringing effects, calculate the maximum force on armature if saturation flux density in the iron part is 1.8T.
Answer
The force on the armature is given by the formula:
Given: B = 1.8 T, A = 5 cm × 5 cm = 0.0025 m2, μ0 = 4π × 10-7 H/m. Substituting the values:
State the uses of lap and wave windings?
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What are the factors on which the emf of a DC generator depends?
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Under What circumstances a dc shunt generator fails to generate?
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What is the purpose of equalizer bar and cross connection of field windings in a DC generator?
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What are the constructional differences between a DC generator and a DC motor?
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State the applications of d.c series motor and d.c shunt motor.
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Draw the power flow diagram of DC motor.
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Write the expressions for core losses and remedial measures to reduce them in a dc machine.
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Part B
Derive the expressions for the force developed in singly excited system in terms of L and S.
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Explain the i-λ characteristics of a magnetic system and also derive expression for co-energy density, assume that the i-λ relationship of the magnetic circuit is linear.
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Define armature reaction in a DC machine and explain its effects on machine performance.
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A short shunt compound DC generator supplied 7.5kW at 230V. The shunt, series field and armature resistances are 100Ω, 0.3Ω and 0.4Ω respectively. Calculate the induced emf and the load resistance.
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Explain the role of commutator in DC machines also describe two ways of achieving good commutation and compare them.
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Draw and explain the no-load and load characteristics of DC shunt, series and compound generators. State the applications of various types of generators.
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Explain the significance of critical field resistance and critical speed of DC shunt generator.
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Two DC shunt generators are connected in parallel to supply a load of 5000 A. Each machine has an armature resistance of 0.03 Ω and field resistance of 60 Ω but the emf of one machine is 600V and that of the other machine is 640 V. What power does each machine supply?
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What is the necessity of starter in dc motors? Explain the purpose of starter along with necessary equations.
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A 500V shunt motor has an armature resistance of 0.2 ohm. It takes as armature current of 60A at 750rpm. Find the speed and armature current if the flux is increased to 150% of its initial value. Assume the torque to be constant throughout. Contact drop/brush = 1V.
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Enumerate the factors on which speed of a D.C motor depends. Explain briefly about speed control methods of a DC motor along with neat circuit diagram.
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Two identical DC shunt machines when tested by Hopkinson’s method, gave the following results: Line voltage – 230V; Line current excluding both the field currents-30A; motor armature current – 230A; field currents 5A and 4A. If the armature resistance of each machine including brushes is 0.025ohm, calculate efficiency of both the machines as motor.
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What are the advantages and disadvantages of brake test?
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With neat circuit diagram explain the conduction of Swinburne’s test. Mention its limitations.
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