DieselShip's Online Content Library

Q1. With reference to the protection of electric motors explain EACH of the following:

(a) Fuse back up protection (6)

(b) How a motor fitted with fuse back up protection may exceed its rated temperature without being tripped by the primary protection (6)

(c) The value of current rating at which the over current relay should be set (4)

Q2. 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)

Q4. a) Explain the working principle of an alkaline battery. (8)

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

Q5. 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)

Q6. A transformer steps down 11,000 V to 220 V and supplies a load of 10 kW at 0.8 power factor (lagging). The primary winding has resistances 50 Ω and leakage reactance 200 Ω. Find: (a) turns ratio, (b) secondary current, (c) primary current (referred to primary), (d) voltage regulation. (16)

Q7. Two batteries are connected in parallel: Battery A has EMF 10 V and internal resistance 1 Ω; Battery B has EMF 8 V and internal resistance 2 Ω. An external resistance of 5 Ω is connected across the combination. Find the current through the external resistance and through each battery. (16)

Q8. (a) List three main reasons why varnish is applied to transformer windings. (6)

(b) A wooden ring having a mean diameter of 200 mm and a cross-sectional area of 400mm2 is wound uniformly with a coil of 300 turns. If the current passed through the coil is 5A calculate the value of flux produced in the coil. (10)

Q9. 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)

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. (4)

c) Describe with the aid of sketches THREE methods for motor protection should single phasing occur. (8)

Q2. a) What is the purpose of Preferential Tripping system on ship's electrical network? (6)

b) Explain the various stages of preferential trips with the loads connected to those stages. (10)

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

(i) Bi-metal strips.

(ii) Thermistors. (8)

(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) A 5kVA, 200/400 V transformer has iron losses of 40 W and full-load copper losses of 100 W. Find (a) efficiency at full load, unity pf. (b) efficiency at half load, 0.8 pf. (c) the load at which maximum efficiency occurs. (10)

Q7. The no-load current of a transformer is 4.0 A at 0.25 power factor when supplied at 250 V, 50 Hz. The number of turns on the primary winding is 200. Calculate: (i) Flux in the core. (ii) Core Loss. (iii) Magnetising current. (16)

Q8. a) Explain Fleming's Right hand rule. (6)

(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)

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

(b) A single phase transformer has 100 turns on the primary and 200 turns on the secondary. The no load current is 3 Ampere at a power factor of 0.2 lagging. Calculate the primary current and power factor when the secondary current is 280 A at a power factor of 0.8 lagging. (10)