Code No: 310205

III B-Tech I-Semester Supplementary Examinations

June - 2003
Set No: 3

ELECTROMECHANICS - III

(Electrical and Electronics Engineering)

[Time:3 hours]
Max. Marks: 80

Answer any FIVE questions

ALL questions carry equal marks

1. Describe how the armature windings are arranged in synchronous generators. What is the effect distribution factor on the performance of the generator?

2. A 3-Phase, 12-pole, 500 r.p.m star connected alternator has 144slots with 10 conductors per slot. The coils are full pitch and the flux per pole is 0.094wb. Determine the phase and line e.m.fs. What harmonics do you expect due to slots and why? What will be the phase voltage if the coils are connected to form a balanced two-phase winding?

3. a ) In brief explain the operation of a3-phase cylindrical rotor alternator under constant load with variable excitation. Draw the Phasor diagram

b) A synchronous generator has a direct axis synchronous reactance of 0.8 p.u. And quadrature axis synchronous reactance of 0.5 p.u. It is supplying full load at rated voltage at rated voltage at 0.8 P.f Lagging. Find the open circuit voltage

4. a) Explain how open-circuit and short-circuit tests are conducted on a synchronous machine

b) What is an air-gap line? In an alternator, explain why short-circuit characteristics is a straight line where as open -circuit characteristics is a curve.

5. a) How do you calculate the time constant in case of an alternator?

b) Show that an alternator running in parallel a constant voltage and frequency bus bars has a natural time period of oscillation. Deduce a formulate for the time of one complete oscillation and calculate the value for a 500KVA, 3-phase, 10000V machine running at 1500rpm and constant 50Hz bus bars.

The moment of Inertia of the whole moving system is 14112 Kg-m²and the steady short circuit current is five times the normal full load value

6. a) What will be the effect of increased load with constant excitation? Explain with phasor diagram.

b) A 75KW, 400V, 4-pole ,3-Φ, star connected synchronous motor has a resistance and synchronous reactance per phase of 0.04Ω and 0.4Ω respectively. Compute for full load 0.8 pf lead the open circuit emf/phase and gross mechanical power developed. Assume an efficiency of 92.5%.

7. a) What is a synchronous condenser? What is the use if synchronous condenser?

b) A 500V, 50Hz, 3-Φ circuit takes 20A at a lagging power factor of 0.8 A synchronous motor is used to raise the power factor to unity. Calculate the KVA input to the motor, and its power factor when driving a mechanical load of 7.5KW.The motor has an efficiency of 85%.

8. a) Explain the constructional details principle of operation of a split phase induction motor. List out its industrial applications.

b) "The centrifugal switch of a single phase motor failed to open". Explain the after effects in the performance.