MET-NOV-2019

Q1. Explain the methods used to control the speed of a 3 Phase induction motors. Draw and Explain a Variable Frequency Drive used for optimization of energy efficiency of auxiliary machineries on board vessels.

 

Q2. 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?

Q3. The direct on line start of 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. Explain the methods used to control the speed of a 3 Phase induction motors. Draw and Explain a Variable Frequency Drive used for optimization of energy efficiency of auxiliary machineries on board vessels.

Q5. With respect to the High Voltage power systems installation, explain the different types of circuit breaker that are used, comparing them on merits and de-merits. Describe the theory of arc phenomenon and the mechanism fitted to mitigate the arc.

Q6. A. Explain what is meant by the terms wave form, frequency and average value.

B. A moving coil ammeter, a thermal ammeter and a rectifier are connected in series with a resistor across a 110 V sinusoidal a.c. supply. The circuit has a resistance of 50Ω to current in one direction and, due to the rectifier, an infinite resistance to current in the reverse direction. Calculate:
(i) The readings on the ammeters; (ii) The form and peak factors of the current wave.

 

Q7. A. Why is it important to maintain high efficiency of operation? And low values of voltages regulation for power transformers?

B. A 100 KVA transformer has 400 turns on the primary and 80 turns on the secondary. The primary and secondary resistances are 0.3 Ω and 0.01 Ω respectively, and the corresponding leakage reactance’s are 1.1 Ω and 0.035 Ω respectively. The supply voltage is 2200 V. Calculate:
(i) The equivalent impedance referred to the primary circuit;
(ii) The voltage regulation and secondary terminal voltage for full load having a power factor of (i) 0.8 lagging and (ii) 0.8 leading.

 

Q8. A. List the factors that determine the starting torque of the three-phase induction motor. How does this torque generally compare with the value of the rated torque?

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. supply 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.

 

Q9. A. With the aid of delta and star connection diagrams, state the basic equation from which delta – star – delta conversion equation can be derived.

B. Three batteries A, B and C have their negative terminals connected together, between the positive terminals of A and B there is a resistor of 0.5 ohm and between B and C three is a resistor of 0.3 ohm
i. Battery A 105 V, Internal resistance 0.25 ohm
ii. Battery B 100 V, Internal resistance 0.2 ohm
iii. Battery C 95 V, Internal resistance 0.25 ohm
C. Determine the current values in the two resistors and the power dissipated by them.

 

Q10. A. Compare the effectiveness of a current limiting circuit breaker with that of a 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?