# Cochin University Previous Year Question Papers

# BE EE Electrical Machines Nov 2009

# 5th Semester

**EE 502 Electrical Machines**

**(2006 Scheme)**

**PART A**

(Answer ALL questions^

1. (a) What are the causes of harmonics in the voltage wave form of an alternator? How can these be minimized?

(b) What are the advantages of rotating field system in an alternator?

(c) Which method of voltage regulation is known as pessimistic method? Why it is called pessimistic method?

(d) What do you mean by synchronization of alternators? Give the conditions for synchronization.

(e) Explain, synchronous condensor.

(f) What are the different losses in a synchronous machine?

(g) Explain transient and sub transient synchronous reactance.

(h) When alternators working in infinite bus bar prove that voltage remains constant and also frequency remains constant?

**PART B**

2. (a) Derive emf equation of an alternator.

(b) A 3 phase star connected alternator has the following data. Voltage required to be generated on open circuit is 4000V (line value) at 50Hz. Speed = 500 rpm, stator slots/pole/phase = 3, conductors/slot =12, calculate

(i) number of poles (ii) useful flux per pole Assume full pitched coil and all conductors per phase to be connected in series.

**OR**

3. (a) Explain the constructional details of an alternator.

(b) A 3 if), 10 pole alternator has 2 slots/pole/phase on its stator with 10 conductors/slot.The air gap flux is sinusoidally distributed and equals to 0.05 Wb. The stator has double layer winding with a coil span of 150° electrical. If the alternator is running at 600 rpm, calculate the emf generated per phase at no load?

(a) What is armature reaction? Explain the effect of armature reaction on the terminal voltage of an alternator at

(i) Upf

(ii) Zero leading Pf

(iii) Zero lagging Pf

(b) What is short circuit ratio (SCR) of an alternator? Explain the significance of SCR.

**OR**

4. (a) Explain the procedural steps of finding voltage regulation by Potier method?

(b) Determine voltage regulation of a 2000V single phase alternator giving a current of 100A at 0.8 p.f leading. Full load current of 100A is produced on short circuit by a field excitation of ?.5A. An emf of 500V produced on open circuit by the same excitation. The armature resistance is 0.8 Q. Draw vector diagram.

5. (a) Explain V curves and inverted V curves of synchronous motor.

(b) Explain different torques in a synchronous motor.

**OR**

6. (a) Explain the starting methods of a synchronous motor.

(b) By neglecting Ra, Draw the phasor diagram of synchronous motors at

(i) lagging power factor

(ii) leading power factor

(iii) unity power factor

7. (a) Derive an expression for power developed in a salient pole synchronous machine. Give expression for reluctance power. Draw power versus power angle diagram.

(b) What is hunting in synchronous machines?

**OR**

8. (a) Derive an expression for natural frequency of oscillations.

(b) A 12 MVA, 5000V, 3 phase, 4 pole, 50 Hz alternator is connected to infinite busbars. The short circuit current is 4 time the normal full load current and the moment of inertia of the rotating system is 22000 Kgm^{2}. Determine the normal period of oscillation.