# Cochin University Previous Year Question Papers

# BE EE Electrical Machines-II Nov 2010

# 5th Semester

**EE 502 Electrical Machines II**

**(2006 Scheme)**

**PART A**

(Answer all questions)

I. (a) Define breadth factor and derive the expression for it.

(b) Compare the different types of turbo alternators.

(c) Explain the effect of armature reaction in alternators for all power factors.

(d) What are the factors affecting the alternator size?

(e) Write short note on hunting in synchronous motors.

(f) Discuss the methods of starting of synchronous motor.

(g) Write short note on power factor control in synchronous machines.

(h) Derive the condition for maximum power output in alternators.

**PART B**

II. (a) Derive the expression for induced emf in alternators and explain each factor in the expression clearly.

(b) Produce the relation between speed and frequency in alternators.

**OR**

III. (a) what are the causes of harmonics in the voltage waveform of an alternator? How can they be minimized?

(b) A 3 phase, 12 pole star connected alternator has 180 slots with 10 conductors per slot and the conductors of each phase are connected in series. The coil span is 144°(electrical). Determine the phase and line values of emf if the machine runs at 600rpm and the flux per pole in 0.06wb distributed sinusoidally over the pole.

IV. (a) Define synchronous reactance.

(b) A3 phase star connected alternator supplies a load of 10MW at pf 0.85 lagging and at 11KV (terminal voltage). Its resistance is 0. IQ /phase and synchronous reactance 0.66 Q/phase. Calculate the line value of emf generated.

**OR**

V. (a) Explain the steps followed in determining the voltage regulation using synchronous impedance method.

(b) From the following test results, determine the voltage regulation of a 2000V, single phase alternator delivering a current of 100 A at unity pf. Test results: Full load current of 100A is produced on short circuit by a field excitation of 2.5 A. An emf of 500V is produced on open circuit by the same excitation. Armature resistance is 0.8 Q.

VI. (a) Explain the working of synchronous motor.

(b) A3 phase 6600 volts, 50 Hz star connected synchronous motor takes 50A current. The resistance and synchronous reactance per phase are 1 Q and 20 Q respectively. Find the power supplied to the motor and induced emf for a power factor of 0.8 lagging.

**OR**

VII. (a) Write short note on ‘V’ curves and ‘inverted V curves’ of a synchronous motor.

(b) A synchronous motor absorbing 60KW is connected in parallel with a factory load of 240KW having a pf of 0.8 lag. If the combined load has a pf of 0.9, what is the leading KVAR value supplied by the motor and at what pf is it working?

VIII. (a) Discuss the effect of short circuit on alternators.

(b) Define transient and sub transient reactance.

**OR**

IX. (a) Derive the expression for time period of oscillation of a synchronous machine.

(b) A 10,000 KVA 4 pole, 6600V, 50 Hz 3 phase star connected alternator has a synchronous reactance of 25% and operates on constant voltage constant frequency bus bass, if the natural period of oscillation while operating at full load and unity power factor is to be limited to 1.5 second, calculate the moment of inertia of the rotating system.