MET-Apr-2021 (MUM)

Q1. Sketch a circuit diagram for an automatic voltage regulator illustrating how the AVR utilizes a silicon-controlled rectifier to control the excitation system for an alternator. Describe how AVR monitors the output and controls the excitation system.

Q2. Tank liquid level sensors are an integral part of ships. Describe with aid of suitable sketches the working principle of,

(a) capacitive type level sensor;
(b) Ultrasonic level sensor;
(c) Float.

Q3. The direct online start of the squirrel cage motor is used for most electrical drives on a.c. powered ships.

Describe with sketches as necessary one method of overcoming each of the following Problems:
A. High starting current; B. Low starting torque.

Q4. A. Describe with the aid of a simple sketch the arrangement of the three-phase winding of an alternator showing the neutral point; 

B. Explain why for most ships the neutral point is insulated; 

C. Explain why in some installations the neutral point is Earthed.

Q5. With reference to preferential tripping in a marine electrical distribution system;

A. With the aid of a sketch, describe a typical arrangement to provide three stages of tripping instantaneous protection against short circuits.

B. State why this protection is required;

Q6.A. Electric motors contain a stationary member as well as a rotating member. For each of the following machines, identify in which part of the motor three field winding and the armature winding are located: three-phase induction motor, three-phase synchronous motor, d.c. motor. B. An 18.65 kW,4-pole,50HZ, 3 phase induction motor has friction and windage losses of 2.5 percent of the output. The full load slip is 4% compute for full load (a) the rotor Cu loss (b) the rotor input (c) the shaft torque (d) the gross electromagnetic torque.

Q7. A. Sketch an arrangement showing the principle of proportional plus integral (P+I) control loop.

B. Compare the series and parallel resonance circuits. Find the frequency at which the following circuit resonates.

Q8. A. Compare the effectiveness of a current limiting circuit breaker with that of an HRC fuse.

B. A coil having a resistance of 10 Ohm, and an inductance of 0.15 H is connected in series with a capacitor across a 100V, 50Hz supply. If the current and the voltage are in phase what will be the value of the current in the circuit and the voltage drop across the coil?

 

Q9. A 20kVA, 2000/220V, single-phase transformer has a primary resistance of 2.1ohm and a secondary resistance of 0.026ohm. The corresponding leakage reactance’s are 2.5ohm and 0.03ohm. Estimate the regulation at full load under power-factor conditions of. (a) Unity; (b) 0.5 (lagging) and (c) 0.5 (leading).

Q10. A. Describe the no-load saturation characteristic of a d.c. generator.

B. A 4-pole machine running at 1500 r.p.m. has an armature with 80 slots and 6 conductors per pole.
The flux per pole is 6 x 106 lines. Determine the terminal e.m.f. of d.c. generator if the coils are lap connected. If the current per conductor is 100 A, determine the electrical power.