DieselShip's Online Content Library

Q1. (a) Sketch a circuit diagram for an automatic voltage regulator illustrating how the A.V.R. utilizes a silicon-controlled rectifier to control the excitation system for an alternator. (10)

(b) Describe how the A.V.R. monitors output and controls the excitation system. (6)

Q2. Overcurrent protection relays are built into main alternator breakers to safeguard the individual alternators and the distribution system against certain faults.

(a) Sketch a typical relay. (8)

(b) Describe the operation of the relay sketched in (a) (8)

Q3. (a) Sketch and describe a power (watt) meter for an A.C. switchboard. (8)

(b) State why type of load governs power factor and give examples of power factor for a resistance load and for normal marine operation. (8)

Q2. Explain clearly why, in D.C. installation, a compound-wound electric generator is usually adopted for ship lighting purposes. Compare it's performance with that of shunt and series wound machines. What attention does such a machine require when working and what care is necessary for its maintenance in a satisfactory condition? (16)

Q1. With reference to an emergency source of electrical power in cargo ships:

a) Describe a typical power source (6)

b) Give a typical list of essential services, which must be supplied simultaneously. (5)

c) Explain how the emergency installation can be periodically tested. (5)

Q6. (a) What are the different types of DC motors? (6)

(b) A 10 H.P. 230 V shunt motor takes an armature current of 6A from 230 V mains at no load runs at 1200 r.p.m. The armature resistance is 0.25Ω. Determine speed and electromagnetic torque when the armature takes 36 amps with the same flux. (10)

Q7. An amplifier has an open-circuit voltage gain of 1000, an input resistance of 2000 Ω and an output resistance of 1.0 Ω. Determine the input signal voltage required to produce an output signal current of 0.5A in a 4.0Ω resistor connected across the output terminals. If the amplifier is then used with negative series voltage feedback so that one tenth of the output signal is fed back to the input, determine the input signal voltage to supply the same output signal current. (16)

Q8. (a) Explain how fluorescent tubes power factor is improved. (6)

(b) A fluorescent lamp taking 80W at 0.7 power factor lagging from a 230V, 50-Hz supply is to be connected to unity power factor. Determine the value of the correcting approach required. (10)

Q9. (a) Explain about non-linear resistors with some examples and illustration on how they differ from linear resistor. (6)

(b) A half-wave rectifier is used to supply 50V d.c. to a resistive load of 800Ω. The diode has a resistance of 25 Ω. Calculate a.c. voltage required. (10)

Q1. (a) Sketch a diesel electric propulsion arrangement for a ship (8)

(b) Describe the operation of the propulsion arrangement sketched in (a), including in your description how reversal of the propulsion motor is achieved (8)

Q2. Differentiate between squirrel cage and wound rotor motors, of the three phase a.c. induction type, in respect of the following: (16)

(a) Rotor construction

(b) Torque characteristics

(c) Speed variation

Q3. (a) Explain open loop control system and closed loop control system with suitable examples (8)

(b) What are the merits and demerits of the two systems? (8)

Q4. (a) Explain why it is necessary to have reverse power protection for alternators intended for parallel operation (6)

(b) (i) Sketch a reverse power trip (5)

(ii) Explain briefly the principle on which the operation of this power trip is based and how tripping is activated (5)

Q5. With reference to the condition monitoring of electrical machinery:

(a) State TWO important parameters that may be recorded (8)

(b) Explain how the parameters are measured and what defects may be revealed (8)

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)

Q7. A 4 pole, lap wound shunt generator delivers 200 A at terminal voltage of 250 V. It has a field and armature resistance of 50 Ω and 0.05 Ω respectively. Determine: (16)

(a) Armature current

(b) Generated e.m.f

(c) Current per armature parallel paths

(d) Power developed

Q8. (a) State the relationship between impedance, voltage and current (6)

(b) The filament of a 230V lamp takes a current of 0.261A when working at its normal temperature of 2000° C. The temperature coefficient of the tungsten filament material can be taken as 0.005 ohms/ohms at 0ºC/ºC. Find the approximate current which flows at the instant of switching on the supply to the cold lamp, which can be considered to be at a room temperature of 20° C (10)

Q9. (a) Describe the effect of running an induction motor on reduced voltage (6)

(b) A 90V D.C. generator is used to charge a battery of 40 cells in series, each cell having an average e.m.f. of 1.9 V and an internal resistance of 0.0025 Ω. If the total resistance of the connecting cells is 1Ω, calculate the value of the charging current (10)