# Cochin University Previous Year Question Papers BE EE Electrical Machines Nov 2007

# Cochin University Previous Year Question Papers

# BE EE Electrical Machines Nov 2007

# 5th Semester

**EE 504 Electrical Machines II**

**(2002 to 2005 Admissions)**

I. a) What are alternators? Explain its construction.

b) Give any 4 features of salient pole and cylindrical rotor machines.

c) A3 phase, 4 pole, 50 Hz Y connected alternator has 60 slots with 2 conductors per slot and having armature winding of the double layer type Coils are short pitched with one coil side in slot 1 and other in slot 13, Find the useful flux per pole required to induce a line voltage of 6.6 KV.

**OR**

II. a) Derive the emf equation of the alternator.

b) Calculate the distribution factor for a 36 slot, 4 pole, single layer, 3 phase winding.

c) Calculate the rms value of the induced emf per phase of a 10 pole. 3 <p, 50 Hz alternator with 2 slots/pole/phase and 4 conductors/slot in 2 layers. The coil span is 150°. The flux/pole has a fundamental component of 0.12Wb and a20% 3^{rd} harmonic component.

III. a) Explain the effect of armature reaction on an alternator’s terminal voltage at lagging, leading and unity power factor.

b) In a 50 – KVA, star connected, 440V, 3 <f>, 50 Hz alternator, the effective armature resistance is 0.25 /phase. The synchronous reactance is 3.2 Q /phase and leakage reactance is 0.5 Q /phase. Determine at rated load and unity power factor: ‘

i) Internal emf E_{a}

ii) No load emf E_{0}

i) Percentage regulation on full-load

IV. a) Explain the Potier method for finding the voltage regulation of an alternator.

b) An 11 KV, 1000 KVA. 3 ^, Y connected alternator has a resistancebf 2 Q per phase.The open circuit and full load zero power factor characteristics are given below. Find the voltage regulation of the alternator for full load current at 0.8pf lagging by Potier method.

Field current – 40 50 110 140 180

OCC line voltage 5,800 7,000 12,500 13,750 15,000

Line Volts Zero pf. 0 1,500 8,500 10,500 12,500

V. a) What are the conditions to be fulfilled for parallel operation of two synchronous machines? Explain the procedure for synchronizing 3- phase alternators.

b) Derive the expression for synchronizing power.

**OR**

VI. a) Derive the expression for power developed by a synchronous motor.

b) A 3300V, 1.5MW, 3- (f>, Y-connected synchronous motor has X /phase and Xq-3 Q /phase. Neglecting losses, calculate the excitation emf when motor supplies rated load at unity pf. Calculate the maximum mechanical power which the motor would develop for this field excitation.

VII. a) Explain an experimental method for determining V curves for a synchronous motor.

b) What are synchronous condensers? Explain with phasor diagrams.

**OR**

VIII. a) Explain the effect of varying the excitation of a synchronous machine connected to infinite bus bar.

b) A3 phase, 11,000 V, star connected turbo alternator having a synchronous reactance of 6 C2 per phase and negligible resistance has an armature current of 200A at unity power factor when operating on constant frequency, constant voltage bus bars. If the steam admission remain the same and the emf is raised by 25% determine the new values of current and power factor.

IX. a) Explain the following terms:

i) Direct Axis Transient Reactance

ii) Direct Axis Sub-Transient Reactance

iii) Synchronous Machine Time Constants

b) A salient pole alternator is operated at 0.8 pf lagging. It is delivering rated current at rated voltage.

Ra = 0.02 pu, X<j = 1.2 pu and Xq = 0.8pu. Find direct and quadrature axis currents and load angle.

**OR**

X. Explain:

i) Hunting in synchronous machines

ii) Symmetrical short circuits

b) What are brush less alternators? Explain its construction and excitation methods.