Visvesvaraya Technological University (VTU) 2010-5th Sem B.E Electrical and Electronics Engineering , EEE,DC Machines and Synchronous Machines. - Question Paper

06EE54

5th Semester B.E. Degree Examination, May/June 2010

three hrs. Max. Marks: 100

Note: ans any FIVEfull questions, selecting
at lowest 2 questionsfrom every part.

FART-A


1 a) What is armature reaction? discuss the identical for a D.C. generator. (08 Marks)

b) discuss what is meant by critical field resistance in a D.C. shunt generator and discuss a
method of determining it. (06 Marks)

c) The brushes of a lap connected 400 kW, six pole D.C. generator are provided a lead of 21
electrical degrees. From the data given, compute the demagnetizing and cross magnetizing
ampere turns. The oil load current is 750 amperes and total number of conductors in the
armature is 900. (06 Marks)


2 a) discuss the various characteristics of D.C. series motor. (06 Marks)

b) With a neat sketch, discuss the Ward-Leonard method of speed control of a D.C. shunt
motor. elaborate the advantages and disadvantages of this method? (08 Marks)

c) A 230 volts DC shunt motor runs at 800 rpm and takes an armature current of 50 Amperes.
obtain the resistance to be added to the field circuit to increase the speed from 800 rpm to
1000 rpm at an armature current of 80 Amperes. presume that fulx is proportional to the field
current. Take Ra =0.150Q, Rsh =250 ohms. (06 Marks)

3 a) elaborate the various losses in D.C, shunt motor? How do they vary with load? (04 Marks)

b) discuss the process of commutation in D.C. generator and define any 1 method of
improving commutation. (08 Marks)

c) A series motor having a resistance of one ohm ranging from its terminals, drives a fan, the torque of
which is proportional to the square of the speed. At 230 volts, its speed is 300 rpm and takes
15 amperes current. The speed of the fan is to be raised to 375 rpm by applying voltage
control. Estimate the supply voltage needed. (08 Marks)


4 a) define the Hopkinson's test for 2 identical shunt motors indicating how the efficiency
of every machine on full load is found. (08 Marks)

b) A retardation test is performed on a separately excited D.C. machine as a motor. The
induced voltage falls from 240 volts to 220 volts in 25 seconds, on opening the armature
circuit and in six seconds on suddenly changing the armature connection from supply to a load
resistance, which takes an avg. current of 10 Amperes. obtain the efficiency of the
machine running as a motor taking a current of 25 ampere's from 250 volts supply. Take :
Ra = 0.30Q and Rsh = 200Q. (08 Marks)

c) With a neat diagram, discuss the construction of a permanent magnet D.C. motor. (04 Marks)



PART-B

5 a) describe the terms pitch factor and distribution factor in case of an altenator. Derive an
expression for distribution factor. (10 Marks)

b) A three phase, 50 Hz, 16 pole synchronous generator has a star connected winding with 144
slots and 10 conductors per slot. The lux per pole is 0.03 Webers, sinusoidally distributed
and the speed is 375 rpm. compute the line induced emf. Take pitch factor as 1. (06 Marks)

c) discuss the advantages of having chorded coils in synchronous generator. (04 Marks)



6 a) Name the different methods for determining the voltage regulation for a three phase alternator
and define any 1 method in detail. (09 Marks)

b) A3 phase, star connected alternator is rated at 1600 KVA, 13500 volts. The armature
resistance and synchronous reactances are 1.5 Q and 30 Q. respectively per phase. compute
the percentage voltage regulation for a load of 1280 kW at 0.8 power factor leading.
(08 Marks)

c) List the conditions to be fulfilled to connect 2 alternators in parallel (03 Marks)


7 a) Derive the expression for the output power of cylindrical rotor alternator connected to
infinite bus in terms of excitation voltages bus bar voltage and load angle. (08 Marks)

b) discuss why a synchronous motor is not self starting and define any 1 method of
starting. (08 Marks)

c) discuss the phenomenon of hunting in synchronous motors. (04 Marks)


8 a) With a phasor diagram, discuss the concept of two-reaction theory in a salient pole
synchronous machine. (07 Marks)

b) With a neat circuit diagram, discuss the slip test on salient pole synchronous machines and
indicate how Xd and Xq can be determined from the test. (07 Marks)

c) A3 phase, 50 Hz, two pole alternator is excited to generate the bus bar voltages of 11 kV at no
load. compute the synchronizing power per degree of mechanical displacement of the rotor.
The machine is star connected and the short circuit current for this excitation is 1200
amperes. Neglect armature winding resistance. (06 Marks)

Earning:   Approval pending.