CUSAT Previous Year Question Papers BE 5th Semester Electrical Machines-II July 2009

CUSAT Previous Year Question Papers BE  5th Semester

Electrical and Electronics Engineering

Electrical Machines-II July 2009




I. a)  Derive emf equation of alternator and explain coil span factor and distribution factor.

b) Find the no load phase and line voltage of a star connected 3-phase, 6 pole alternator tfhich runs at 1200 rpm having flux per pole of 0.1 wb sinusoidally distributed. Its stator has 54 slots having double layer winding. Each coil has 8 turns and coil is chorded by 1 slots. %


II.  a) Enumerate the constructional details of a salient pole synchronous machine.

b) Discuss any three measures to be taken for minimizing harmonics from all waveform.

c)  Deduce the relation between number of poles, the frequency and speed of the alternator?


III.  a) A 3300 V, 3^ star connected alternator has a full load current of 100 A. On short circuit a field current of 5 A was necessary to produce full load current. The emf on open circuit for the same excitation was 900 volts. The armature resistance is 0.8Q /phase. Determine the full load voltage regulation for.

i) 0.8 p.f leading

ii) 0.8 p.flagging                 –

b) Discuss the effect of armature reaction on the terminal voltage of an alternator

i) unity power factor load

ii) zero leading power factor.

Draw the re levant phasor diagram.


IV. a) The following test results are obtained on a 6600 V alternator.

Open circuit (v) Voltage        3100 4900 6600 7500 8300

Field current (A)  :           16 25 37.5 50 70

A field current of 20A is found necessary to circulate full load current on short circuit of the armature. Calculate by ampere-tum method the full load regulation at 0.8 p.f (Kg.). Neglect resistance and leakage reactance.

b) With vector diagram, explain Blonde Is two reaction theory of salient pole synchronous alternator.


V. a) Two single phase alternators. Operate in parallel and supply a load impedance of (3 + j 4) Q. If the impedance of each machine is (0.2 + j2) Ci and emfs are (200 + y‘0) and (220 + j0) volts respectively. Determine for each machine

i) Terminal Voltage

ii) Power factor

iii) Output

b) Derive the expression for power developed by synchronous motor.


VI a) List any four conditions to be fulfilled before synchronizing on alternator to infinite bus. Give any one method of synchronizing.

b) A3 HVA, 6 pole alternator runs at 1000 rpm on 3.3 KV bus bars. The synchronous reactance is 25%. Calculate the synchronizing power and torque per mechanical degree of displacement when the alternator is supplying full load at 0.8 Kg.


VII a) A 3<f> star connected 400 v synchronous motor takes a power input of 5472 W at rated voltage. Its synchronous reactance is 10Q per phase and resistance is negligible. If its excitation voltage is adjusted to the rated voltage of 400V; Compute:

i)  The load angle

ii) Power factor

ii)  Armature current

b) Discuss an experimental method of determining V-curves and inverted V-curves for a synchronous motor.


VIII a) Outline the affect of varying the excitation of synchronous generator connected to infinite bus on the power factor, armature current and load angle.

b) A factory has an average load of 300 kw at a p.f of 0.6 lagging. A synchronous motor with an effect of 88% is used to raise the combined power factor to 0.9 lagging and at the same time supply a mechanical load of 60 Kw. Calculate

i) Total load KVA

ii) KVA capacity of the synchronous motor

iii)  Synchronous motor operating power factor.


IX  a) Enumerate the construction and principle of operation of brush less alternator.

b) Derive an expression for the natural time period of oscillation for a synchronous machine.


X  a) Draw the

i) Short circuit current waveforms in the 3 phase of an alternator.

ii) Symmetrical short circuit current waveform of an alternator.

b) Outline the phenomenon of hunting in a synchronous machine. How is it remedied?

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