# JNTU Exam Papers II B.Tech. I Semester Regular Examinations, ELECTRICAL TECHNOLOGY,November 2008

## SET 3

1.(a) Define the terms ‘critical resistance’ and ‘critical speed’ and bring out their role in the process of self excitation of a dc machine.
(b) Distinguish between external and internal characteristics of dc generator. How internal characteristics are derived from external characteristics of shunt generator?
2. (a) Explain the different protective devices used in 3 – point starters.
(b) Explain the method of armature voltage speed control using neat diagram.

3. (a) Explain the principle of operation of a transformer.
(b) The maximum flux density in the core of 240/2400V, 50Hz, 1- φ transformer is 1.0Wb/sq.m. If the EMF per turn is 16 volts, determine:
i. the primary & secondary turns and
ii. area of the core.

4. (a) A 20 kVA transformer has its maximum efficiency of 0.98 at 15 kVA at upf. The iron loss is 350W. Calculate the efficiency at full load, for 0.8 p.f lag and upf.
(b) With neat circuit diagrams, explain the procedure for conducting OC & SC tests on a given 1- φ transformer to predetermine its regulation & efficiency.

5. (a) Draw the diagram of connections of a plain auto transformer starter for an induction motor and explain its action.
(b) A 25hp, 3-phase, 50 Hz, 6 pole slip ring induction motor has a full load speed of 960rpm. The stator loss is 5% of the input and mechanical losses are 4% of the output. The current in each rotor phase is 30 A. Find the resistance per phase of the rotor and the efficiency of the motor. If the rotor standstill resistance is five times the rotor reactance, find the speed at which the maximum torque
occurs.

6. (a) Explain the alternator armature winding types with neat sketches.

(b) An alternator on open circuit generates 360V at 60Hz when the field current is 3.6A. Neglecting saturation, determine the open circuit e.m.f. When the frequency is 40Hz and the field current is 2.4A.

7. (a) What are the different types of capacitor start motors? Explain in detail any one type.
(b) A 250W, 230V, 50Hz capacitor start motor has the following constants for the main and auxiliary windings: Zm=(4.5+j3.7)ohms. Za=(9.5+j3.5)ohms. Determine the value of the starting capacitor that will place the main and auxiliary winding currents in quardrature at starting.

8. The coil of a moving coil voltmeter is 40mm long and 30mm wide and has 100 turns on it. The control spring exerts a torque of 240×10−6 N-m. When the deflection is 100 divisions on full scale and the flux density of the magnetic field in the air gap is 1.0 Wb/m2, estimate the resistance that must be put in series with the coil to give one volt per division. The resistance of the voltmeter coil may be neglected.