MET-Jan-2021

Q1. With reference to A.C generators:
A. Give a brief outline of the care maintenance that should be given to the stator and rotor of an A.C. generator;
B. Explain what is likely to occur if the driving power of one A.C. generator suddenly fails when two generators are running in parallel. What safety devices are usually provided for such events?

Q2. A. Sketch a circuit diagram of a push-button direct online contactor starter for a three-phase Incorporating overload and short circuit protection. 

B. Indicate, on a sketch of the typical characteristic curves of current and torque against Speed, disadvantages of a direct online start squirrel cage induction motor.

Q3. 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.

Q4. 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;

Q5. Compare methods of obtaining speed regulation of three-phase induction motor generally used in tankers by means of:
A. Rotor resistance; B. Cascade system; C. Pole-changing.
Give examples where each system may be employed with an advantage.

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 principal 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. Explain the significance of the root mean square value of an alternating current or voltage wave form; Define the form factor of such a wave form.

B. A total load of 8000 kW at 0.8 power factor is supplied by two alternators in parallel. One alternator supplies 6000 kW at 0.9 power factor. Find the KVA rating of the other alternator and the power factor.

 

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 normal criteria used for setting thermal protection relays and its advantage compared to magnetic types. 

B. The low-voltage release of an a.c. motor-starter consists of a solenoid into which an iron plunger is drawn against a spring. The resistance of the solenoid is 35 ohm. When connected to a 220 V, 50 Hz, a.c. 2supply the current taken is at first 2A, and when the plunger is drawn into the “full-in” position the current falls to 0.7 A. Calculate the inductance of the solenoid for both positions of the plunger, and the maximum value of flux-linkages in weber-turns for the “full-in” position of the plunger.