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. 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)

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)

Q4. With the aid of a block diagram, briefly describe the effect which negative voltage feedback has on an amplifier and state the advantages resulting from the use of negative feedback. (16)

Q8. 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) Compare the use of an insulated neutral system as opposed to use of an earthed neutral system with regards to the risk of electric shock from either system. (8)

Q6. The open-circuit voltage of a cell as measured by a voltmeter of 100 Ω resistance, was 1.5V, and the p.d. when supplying current to a 10 Ω resistance was 1.25V, measured by the same voltmeter. Determine the e.m.f. and internal resistance of the cell. (16)

Q7. A moving-coil instrument has a resistance of 10 Ω and requires a current of 15 mA to give full- scale deflection. Calculate the resistance value of the resistor necessary to enable it to be used to measure

a) currents up 25A

b) voltages up to 500V. (16)

Q8. Find the length of manganese wire required to make a 15.7 Ω resistor, if the diameter is 0.315 mm and the resistivity is 407 μΩ mm (16)

Q9. A 105V, 3kW d.c. shunt motor has a full-load efficiency of 82 percent. The armature and field resistances are 0.25 Ω and 90 Ω respectively. The full-load speed of the motor is 1000 rev/min. Neglecting armature reaction and brush drop, calculate the speed at which the motor will run at no load if the line current at no load is 3.5A. calculate the resistance to be added to the armature circuit, in order to reduce the speed to 800 rev/min, the torque remaining constant at full-load value. (16)