Q a) Explain the significance of the root mean square value of an alternating current or voltage waveform. Define the form factor of such type of wave form.
b) A total load of 8000 kW at 0.8 power factor is supplied by 2 alternators in parallel. One alternator supplies 6000 kW at 0.9 power factor. Find the kVA rating of the other alternator & the power factor.
Q a) Describe with help of sketch, an isolator for a 3 phase 440 V, 20 ampere electric supply. List the safety features of the isolator described in the sketch.
b) A moving coil ammeter, a thermal ammeter & a rectifier are connected in series with a resistor across a 110 V sinusoidal a.c. supply. The circuit has a resistance of 50 Ω to current in one direction &, due to the rectifier, an infinite resistance to current in the reverse direction. Calculate:
i. The readings on the ammeters
ii. The form & peak factors of the current wave.
Q a) Explain the preference for a 60 Hz system. Describe the dangers of running a 50 Hz system from a 60 Hz supply.
b) A ring-main, 900 m long, is supplied at a point A at a potential difference of 220V. At a point B, 240 m from A, a load of 45A is drawn from the main, & at a point C, 580m from A, measured in the some direction, a load of 78A is taken from the main. If the resistance of the main (lead & return) is 0.25 ohm per kilometre, calculate the current which will flow in each direction round the main from the supply point A & the potential difference across the main, at the load where it is lowest.
Q What is commutator? Discuss its rectifying action in detail?
Calculate the e.m.f. generated by a 4-pole, wave wound armature having 40 slots with 18 conductors per slot when driven at speed of 1000 r.p.m. The flux per pole is 0.015 wb.
Q a) Describe the materials used as conductors in cables.
b) A section of a supply cable AB 1 km long has a earth fault such that, when end B is disconnected, the resistance measurement form end A to earth is 5 ohm. When end A is disconnected, the resistance reading from end B to earth is 3 ohm. The length of the cable AB has a resistance of 4 ohm when intact. Find the distance of the fault from end A.
Q a) Explain what is meant by the terms wave form, frequency & average value.
b) A moving coil ammeter, a thermal ammeter & a rectifier are connected in series with a resistor across a 110 V sinusoidal a.c. supply. The circuit has a resistance of 50 ohm to current in one direction &, due to the rectifier, an infinite resistance to current in the reverse direction. Calculate:
i. The readings on the ammeters
ii. The form & peak factors of the current wave
Q With reference to alkaline batteries used on board ships:
a) Describe the operation of a battery cell and state the material used
b) Describe how the cells are mounted to form the battery
c) State the advantages & disadvantages compared with lead-acid batteries.
Q a) Sketch a standby battery charging/discharging circuit.
b) Describe the circuit sketched, making special reference to how battery charge is maintained & how it operates upon loss of the main power.
Q Sketch & describe an arrangement for automatic connection of emergency batteries upon loss of main power. Include in your answer:
a) Means of obtaining D.C. charging supply from the a.c. mains.
b) A method of maintaining charge on the lead acid batteries
c) The arrangement to check that a battery operates a loss of the main power.
d) The length of time for which emergency batteries of passenger & cargo ships must provide power.
Q a) By means of a schematic circuit diagram illustrate the peak rectifier. If a supply voltage is v(t) = Vm Sin wt, what is the voltage across the load resistor?
b) A battery-charging circuit is shown below in the figure. The forward resistance of the diode can be considered negligible & the reverse resistance infinite. The internal resistance of the battery is negligible. Calculate the necessary value of the variable resistance R so that the battery charging current is 1.0 A.
Q 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) 3 batteries A, B & C have their negative terminals connected together, between the positive terminals of A & B there is a resistor of 0.5 ohm & between B & C three is a resistor of 0.3 ohm,
Battery A 105 V, Internal resistance 0.25 ohm
Battery B 100 V, Internal resistance 0.2 ohm
Battery C 95 V, Internal resistance 0.25 ohm
Determine the current values in the two resistors & the power dissipated by them.
Q With the aid of sketch describe the main features and principle of operation of a D.C. moving Coil meter. If such a meter is designed to give full scale deflection with the 150 mA, State how it may be adapted:
a) As an ammeter to read up to 150 A.
b) As a voltmeter to read up to 150 V.
No calculations are required.
Q a) i. Describe the characteristics of a D.C. Motor.
ii. Explain the advantages of such a motor for the deck machinery.
b) Describe with the aid of a sketch a control system for the motor in (a).
Q a) What is direct-connected alternator? How is a direct-connected exciter arranged in an alternator?
b) Draw a graph of starting current, & torque against the speed of rotation for a single cage motor.
c) Explain which of the following 3 motors has the poorest speed regulation: shunt motor, series motor or cumulative compound motor?
d) Describe the possible causes & the effect of running a 3 phase motor with one phase open circuited.
e) A 440V shunt motor takes an armature current of 30 A at 700 rev/min. The armature resistance is 0.7 ohm. If the flux is suddenly decreased 20 %, to what value will the armature current rise momentarily? Assuming unchanged resisting torque to the motion, what will be the new steady values of speed & armature current? Sketch graphs showing armature current & speed as functions of time during the transition from initial to final, steady-state conditions?
Q a) What is silicon controlled rectifier (SCR)? How is the break over voltage of the SCR defined?
b) Describe the no load saturation characteristic of the D.C. generator?
c) A d.c. motor takes an armature current of 110 A at 480 V. The resistance of the armature circuit is 0.2 Ω. The machine has 6 poles & the armature is lap connected with 864 conductors. The flux per pole is 0.05 Wb. Calculate
i. The speed.
ii. The gross torque developed by the armature.
Q 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 3 field winding & the armature winding are located: 3 phase induction motor, three phase synchronous motor, d.c. motor.
b) A 220 V, D.C. shunt motor has an armature resistance of 0.5 ohm & an armature current of 40 A on full load. Determine the reduction in flux required for a 50 % reduction in speed. The torque for both conditions can be assumed to remain constant.
Q a) Describe the effects of changes in speed, rotor current and torque as load is applied to an induction motor.
b) How does the motor adjust its stator current with changes in the mechanical load?
c) A shunt motor runs at 900 rev/min when connected to a 440 V supply, the armature current being 60A & armature resistance of 0.4 ohm. At what speed will it run on a 220 V supply with a current of 40 A. Assume 60 % reduction in flux for the 220 V connections.
Q a) Discuss different methods of speed control of a D.C. series motor by adjusting field ampere turns.
b) A 230 V, D.C. shunt motor runs at 1000 r.p.m and takes 5 A. The armature resistance of the motor is 0.025 ohm & shunt field resistance is 230 ohm. Calculate the drop in speed when the motor is loaded & takes the line current of 41 A. Neglect armature reaction.
Q a) What are the factors on which the speed of the motor depends? Discuss them for series and shunt motor?
b) A shunt motor supplied at 230 V runs at 900 rpm. When the armature current is 30 A, the resistance of the armature circuit is 0.4 Ω, calculate the resistance needed in series with the armature circuit to decrease the speed to 500 rpm. Assume that the armature current is 25 Amps.
Q The armature and field resistances of a 220 V shunt motor are 0.25 and 110 ohms respectively and, when running on no load, the motor takes 6 A. Calculate the losses attributable to iron, friction & windage & assuming this value to remain constant on loads, determine the efficiency when the current supplied is 62A.
Q a) Describe the possible causes and the effect of running a three phase motor with one phase open circuit.
b) A 440V shunt motor tenets an armature current of 30A at 700 rev/min. The armature resistance is 0.7 ohm. If the flux is suddenly reduced by 20 %, to what value will the armature current rise momentarily? Assuming unchanged resisting torque to the motion, what will be the new steady values of speed & armature current? Sketch graphs showing armature current & speed as functions of time during the transition from initial to final, steady state conditions.
Q a) What design factor limits the maximum torque of a D.C. motor? Of an a.c. motor?
b) A shunt motor runs on no load at 700 rev/min off a 440 V supply. The resistance of the shunt circuit is 240 Ω. The following table gives the relationship between the flux & the shunt current:
Shunt current (A) 0.5 0.75 1.0 1.25 1.5 1.75 2.0
Flux per pole (m Wb) 6.0 8.0 9.4 10.2 10.8 11.2 11.5
Calculate the additional resistance needed in the shunt circuit to raise the no-load speed to 1000 rev/min.
Q Diesel electric propulsion is now being chosen as the power plant for an increasingly wide variety of vessels.
a) Draw the simple layout of such an installation.
b) Explain the advantages of selecting such a plant.
Q With reference to an emergency source of electrical power in cargo ships:-
a) Describe a typical power source
b) Give a typical list of the essential services, which should be supplied simultaneously.
c) Explain how the emergency installation can be tested periodically.
Q With reference to the provision of a shore electrical supply to a ship:
a) Sketch an arrangement for taking the A.C. shore supply & checks to be carried out prior Taking shore connection?
b) Describe the method of safely connecting the arrangement sketched in (a) to the shore Supply?
Q With reference to the preferential tripping in a marine electrical distribution system:
a) State why this facility is required
b) With the help of a sketch, describe a typical arrangement to provide 3 stages of tripping as an instantaneous protection against short circuit.
Q In the event of failure of the man electrical power sully on a ship, an emergency source of power must be available. State the circuits which should be fed from such a source & discuss the reasons governing the selection of such circuits.
Q With reference to a three phase shipboard electrical distribution system:
a) Enumerate the advantages of an insulated neutral system.
b) Enumerate the disadvantages of an insulated neutral system.
c) Describe how the earthed neutral system is earthed.
d) Compare the use of an insulated neutral system as opposed to the use of an earthed neutral System with regard to risk of the electric shock from either system.
Q What is a marine High voltage System? Sketch and describe a Ship board high voltage switch board and its protective devices.