Code No: NR-310205

III B-Tech I-Semester supplementary Examinations

May - 2004
Set No: 1

ELECTROMECHANICS - III

(Electrical and Electronic Engineering)

[Time:3 hours]
Max. Marks: 80

Answer any FIVE questions

ALL questions carry equal marks

1. Derive an expression for and inducted e.m.f in a synchronous generator. Also explain how the e.m.f is having sinusoidal wave form.

2. a) Describe the method of finding synchronous impedance of a given alternator.

b) The following data is obtained for 100kVA,1100V, 3-phase alternator:
D.C resistance test between lines=10Vd.c,
current in line=10 A.d.c
Line circuit test; field current I_f = 12 A.d.c
Line voltage=420v;
Short circuit test; I_sc=rated value
Calculate synchronous impedance.

3. How do you find X_d and X_q of a given salient-pole alternator experimentally

4. A 6000KVA, 5000VM, 50Hz, 3 phase alternator with 4 poles and a synchronous reactance of 25% operates an constant voltage and constant frequency bus bars. The moment of inertia of the whole rotating system is 16800kg-m². Calculate the time of one complete oscillation for full load and unity power factor.

5. a) What is a synchronous condenser? What is the use of synchronous condenser ?

b) A 500V, 50Hz, 3-? 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 7.5 K.W. The motor has an efficiency of 85%.

6. A synchronous motor runs at a load angle of 20⁰ at rated voltage and at rated frequency. Armature circuit resistance is neglected. For constant field current, compute the value of load angle with the following changes in its operating conditions:

a) Frequency increased by 10%, load power and applied voltage constant.
b) Frequency reduced by 10%, load torque an applied voltage constant.
c) Both applied voltage and frequency reduced by 10%, load power constant.
d) Both applied voltage and frequency reduced by 10%load torque constant.

7. a) Explain how the performance of a single phase induction motor is estimated from the equivalent circuit ?

b) A2 pole 50Hz single phase induction motor has an effective rotor resistance and stand still leakage reactance of 0.5 ohm and 5.0 ohms respectively. If the motor is running at 2600 rpm determine the frequencies of the motor current components and the relative magnitudes of the forward and backward fluxes. Neglect magnetisation and stator impedance drop.

8. a) A capacitor connected in series with the starting winding of a resistance start split phase induction motor. Explain the changes in the performance characteristics.

b) A 220V, 50Hz 4 pole single phase induction motor has the following equivalent circuit parameters.

R1m=3.6 ohms        (X1m + X2) = 15.6 ohms
R2 = 6.8 ohms           X = 96 ohms

The rotational losses of the motor are estimated to be 80 watts. Calculate the current, power factor and efficiency when the motor is running 1410 rpm.