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VTU Electrical and Electronic Engineering (Semester 5)
DC Machines and Synchronous Machines
May 2016
Total marks: --
Total time: --
INSTRUCTIONS
(1) Assume appropriate data and state your reasons
(2) Marks are given to the right of every question
(3) Draw neat diagrams wherever necessary

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1(a) With a neat diagram, list out the functions of all parts of a dc machine.
8 M
1(b) What is meant by critical field resistance? Explain a method of determining it.
8 M
1(c) Explain the process of commutation in a dc machine.
4 M

2(a) What is armature reaction? What are the methods of eliminating it?
8 M
2(b) Derive the torque equation of a dc motor.
4 M
2(c) Draw and explain the characteristics of a dc compound motor.
8 M

3(a) Explain the speed control of a dc series motor.
4 M
3(b) Explain swinburnes test predetermining the 'η' of a dc machine.
8 M
3(c) A retardation test is carried out an a 1000rpm Dc machine. The time taken for the speed to fall form 1030rpm to 970rpm is :
  • 36 seconds with no excitation
  • 15 seconds with full excitation
  • 9 seconds with full excitation and armature supporting on extra load of 10A 219V.

    • Calculate :
  • i) The moment of inertia of the armature in kg-m2
  • Iron loss
  • Mechanical loss at the mean speed of 1000rpm.
    •
    8 M

    4(a) With a neat diagram, explain field test to determine the efficiency of a dc series motor and list out the merits and demerits.
    8 M
    4(b) Write a brief note on permanent magnet motors.
    4 M
    4(c) Rwo identical dc machines when tested by Hopkinson's method gave the following test results. Field currents are 2.5A and 2A, line voltage is 220V, line current including both the field currents is 10A, motor armature current is 73A, the armature resistance of each machine is 0.05Ω. Calculate efficiency of both the machines.
    8 M

    5(a) Explain the advantages of stationary armature over rotating armature in a alternator.
    6 M
    5(b) Derive an expression for emf induced in a distributed, short pitched stator winding of an 3ϕ alternator.
    6 M
    5(c) A 3ϕ, 50Hz, 10 pole alternator has 90 slots. The flux per pole is 0.15wb. If the winding is to be star connected to produce a line voltage of 11000 volts, find the number of armature conductors to be connected in series/phase.
    8 M

    6(a) Discuss the zpf method of determination of voltage regulation of an alternator. Give the merits of this method over emf method in finding voltage regulation.
    10 M
    6(b) The open circuit and short circuit test readings for a 3ϕ star connected 1000KVA, 2000V, 50Hz alternator are
    Field amps 10 20 25 30 40 50
    OC terminal voltage 800 1500 1760 2000 2350 2600
    Sc armature current 'A' - 200 250 300 - -
    The effective armature resistance is 0.2Ω/phase. Draw the characteristics curves and estimate the full load percentage regulation at i) 0.8pf lag ii) 0.8pf lead.
    10 M

    7(a) Derive an expression for synchronizing power and torque when two alternators are connected in parallel.
    10 M
    7(b) Two 3ϕ synchronous generators operate in parallel on the same load. Determine the KW output and power factor of each machine under the following conditions.
    Synchronous impedance of each generator : 0.2+j2Ω/phase
    Equivalent load impedance : 3+j4Ω/phase
    Induced emf per phase 2000+j0 volts for m/c 1 and 2200+j100 for m/c 2.
    10 M

    8(a) Explain the parallel operation of two alternators with
    i) Change in excitation and
    ii) Change in inout power.
    8 M
    8(b) Explain the phenomenon of hunting in synchronous machines.
    4 M
    8(c) With a neat circuit diagram, explain th slip test on salient pole synchronous machines and indicate how xd and xq can be determined from the test.
    8 M


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