DieselShip's Online Content Library

Q1. a) Explain the term single phasing as applied to poly phase induction motors. (4)

b) State the likely causes of single phasing and the consequences if motors are not adequately protected. (6)

c) Describe with the aid of sketches THREE methods for motor protection should single phasing occur. (6)

Q2. With reference to a three-phase shipboard electrical distribution system:

(a) Enumerate the advantages of an insulated neutral system (4)

(b) Enumerate the disadvantages of an insulated neutral system (4)

(c) State why an Earthed neutral system may be earthed through a resistor (4)

(d) Compare the use of an insulated neutral system as opposed to the use of an Earthed neutral system with regard to the risk of electric shock from either system (4)

Q3. (a) Describe the principle of operation of EACH of the following detecting elements: (8)

(i) Bi-metal strips

(ii) Thermistors

(b) Explain, with the aid of sketches, typical applications where the devices described in (a) may be employed in high voltage electrical systems. (8)

Q4. (a) Sketch the following types of electric motor connections: (8)

(i) A star connection

(ii) A delta connection

(b) Explain how and why star and delta connections are combined to produce a Star / Delta starter for an electric motor. (8)

Q5. (a) State the necessary conditions required prior to the synchronizing of electrical alternators. (6)

(b) Describe the type of cumulative damage that may be caused when alternators are incorrectly Synchronized. (6)

(c) Explain how the damage referred to in (b) can be avoided/reduced. (4)

Q6. (a) Explain the principle of conservation of charge and its relationship to Kirchoff's current law. (6)

(b) The open-circuit voltage of a cell as measured by a voltmeter of 100 ohm resistance, was 1.5 V, and the p.d. when supplying current to a 10 ohm resistance was 1.25 V, measured by the same voltmeter. Determine the e.m.f. and internal resistance of the cell. (10)

Q7. The loads of a 4-wire, 3-phase systems are: Red line to neutral current = 50 A, power factor of 0.707 (lagging) Yellow line to neutral current = 40 A, power factor of 0.866 (lagging) Blue line to neutral current = 40 A, power factor 0.707 (leading). Determine the value of the current in the neutral wire. (16)

Q8. (a) Describe the effect of running an induction motor on reduced voltage.

(b) A motor takes a current of 60 amperes at 230 volts, the power input being 12 kW. Calculate the power component and the reactive component of the input current. (10)

Q6. (a) Describe the basic principles of a self-excited generator (6)

(b) The armature resistance of a 200 V, shunt motor is 0.4 ohms and the no-load armature current is 2A. When fully loaded and taking an armature current of 50 A, the speed is 1200 rev/min. Find the no-load speed and state the assumption made in the calculation (10)

Q3. With reference to the protection of electric motors explain EACH of the following in relation to fuse back up protection.

a) How a motor fitted with fuse back up protection may exceed its rated temperature without being tripped by the primary protection. (8)

b) The value of current rating at which the over current relay should be set. (8)

Q7. a) State the conditions, which must be satisfied before an A.C generator can be paralleled with live bus-bars. (4)

b) Sketch a lamp-bright configuration for synchronizing lamps. (8)

c) State the advantages and disadvantages of the lamps-bright system over lamps-darks system. (4)

Q3. (a) Sketch, the circuit diagram of an instrument used for measuring electrical insulation resistance. (8)

(b) Describe the circuit diagram sketched in (a), explaining how it operates when measuring electrical insulation resistance. (8)

Q1. a) Explain the working principle of an alkaline battery. (8)

b) Compare the alkaline battery with Lead-Acid battery. (8)

Q4. a) What is the purpose of AVR in an alternator? (6)

b) With the aid of a simple circuit diagram explain the basic working of a brushless alternator. (10)

Q7. In the following circuit, E1 = 13 V, E2 = 19.5 V, R1 = 5 W, R2 = 7 W, R3 = 9 W. Find the current flowing through each resistor. (16)

Q7. A 24V emergency battery is to be charged from the 110V ship's mains when the e.m.f. per cell has fallen to a minimum value of 1.8V. The battery consists of 12 cells in series, has a capacity of 100 Ahr at a 10 hr rate and the internal resistance is 0.03Ω/cell. If charging continues until the voltage per cell rises to 2.2V, find the value of the variable resistor needed to control the charging. The charging current can be assumed to be equal to the maximum allowable discharge current (16)

Q9. A wooden ring having a mean diameter of 200 mm and a cross-sectional area of 400 mm2 is wound uniformly with a coil of 300 turns. If the current passed through the coil is 5 A calculate the value of flux produced in the coil. (16)

Q6. A coil of resistance 10 ohm and inductance 0.1H is connected in series with a capacitor of capacitance 150pF, across a 200 V, 50 Hz supply.

Calculate:

a) the inductive reactance. (3)

b) the capacitive reactance. (3)

c) the circuit impedance. (2)

d) the circuit current. (2)

e) the circuit power factor. (2)

f) the voltage drop across the coil. (2)

g) the voltage drop across the capacitor. (2)