Cochin University Previous Year Exam Papers Electrical Machines Nov 09
EE 504 Electrical Machines
I. (a) Compare salient pole and non-salient pole synchronous machine.
(b) Derive emf equation of alternator and explain coil span factor and distribution factor.
II. (a) State the advantages and disadvantages of using short-pitched winding and distributed winding.
(b) An 8-pole, 3-phase, 60° spread, double layer winding has 72 coils in 72 slots. The coils are short pitched by two slots. Calculate the winding factor for the fundamental and third harmonic.
III. (a) What is armature reaction? Explain the effect of armature reaction on the terminal voltage of an alternator at (i) unity power factor load and (ii) zero leading power factor load. Draw the relevant phasor diagram.
(b) A 3-phase, star connected, 1000 KVA, 11000V alternator has rated current of52.5A. The ac resistance of the winding per phase is 0.45 Q. The test results are given below:
OCTest: field current-12.5 A,
voltage between lines = A22V.
SC Test: field current – 12.5A, line current = 52.5 A.
Determine the full-load voltage regulation of the alternator (a) 0.8 pf lagging and (b) 0.8 pf leading.
IV. (a) Describe the slip test method for the measurement of Xd and X, of synchronous machine.
(b) A 10KVA, 380V, 50 Hz, 3-phase, star-connected salinet pole alternator has direct axis and quadiature axis reactances of 12 Q and8Q respectively. The armature has a resistance of 10 per phase. The generator delivers rated load at 0.8 pf lagging with the terminal voltage being.maintained at rated value. If the load angle is 16.15°, determine (i) the direct axis and quadiature axis components of armature current (ii) excitation voltage of the generator.
V. (a) What are the conditions to be fulfilled for parallel operation of two synchronous machines? Give any one method of synchronizing.
(b) 5000 KVA, 10000 V, 1500 rpm, 50 Hz alternator runs in parallel with other machines. Its synchronous reactance is 20%. Find for (a) no load and (b) full load at power factor 0.8 lagging synchronizing power per unit mechanical angle of phase displacement and calculate the synchronising
VI. (a) Draw a phasor diagram of a synchronous motor. Explain the effect of
(i) change in excitation if load is constant (ii) change in load if excitation is constant.
(b) The excitation of a 415 V, 3-phase, mesh connected synchronous motor is such that the induced emf is 520 V. The impedance per phase is (0.5+J4.0) ohms. If the friction and iron losses are constant at 1000 watts, calculate Hie power output, line current, power factor and efficiency for maximum power output.
VII. (a) Explain the effect of varying the excitation of a synchronous generator connected to infinite bus on the power factors armature current and load angle.
(b) A 50 Hz, 4-pole 3 phase star-connected synchronous motor has a synchronous reactance of 12.0 Q /phase and negligible armature resistance. The excitation is such as to give an open-circuit voltage of 13.2 kV. The motor is connected to 11.5 kV, 50 Hz supply. What maximum load can the motor supply before losing synchronism? What is the corresponding motor torque, line current and power factor?
VII (a) Explain V and inverted V-curves.
(b) A salient pole synchronous motor has Xj = 0.85 pu and Xq= 0.55 p.u. It is connected to bus-bar of 1.0 pu voltage while its excitation is adjusted to 1.2 p.u. Calculate the maximum power the motor can deliver without loss of synchronism. What minimum p.u. excitat on will be required for the machine to remain in synchronism while delivering full-load torque.
IX. (a) Briefly describe the phenomenon of‘hunting’ in a synchronous machine. What is it remedial?
(b) Explain two important functions served by damper winding in a synchronous motor. State applications of synchronous motor.
(c) Explain why is it necessary to short the field winding through a suitable resistance while starting a synchronous motor.
X. (a)Define transient and sub transient reactance’s.
(b)Briefly explain the sequence of events when an alternator is suddenly short circuited. What factors affect the maximum short circuit current? Draw and explain the current waveform for symmetrical and asymmetrical short circuits.