BATTERY

[1] What are the Advantages and Disadvantages of Alkaline Batteries?

The Advantages of Alkaline Batteries are that they retain charge on open circuit and even if discharged, it can be left for long periods without adverse effect.

The Disadvantages are that they require a greater number of cells to produce a particular Voltage.

They are also more expensive than Lead Acid Batteries.

[2] What are the Disadvantages and Advantages of a Lead Acid Battery?

The Advantages of a Lead Acid Battery are; they require fewer cells to achieve particular voltage and are reasonably priced.

The Disadvantages are that they require regular attention to keep fully charged, always require a trickle charge and they discharge on open circuit.

[3] How is the weak or dead cell in a battery is detected?

By measuring and comparing the specific gravity of all the cells.

In what way the nickel -cadmium batteries are superior to lead-acid Batteries?

At high discharge rates, ni-cd batteries deliver more power and they can be rechalged in a shorter time.

[4] What is the effect of ambient temperature on performance of the Battery?

Increase in ambient temperature increases the local action. However, the capacity increases with temperature, after a certain temperature rise, the sulphation starts, and the battery etliciency decreases. Optimum temperature will be between 40⁰c and 45⁰c.

CABLES

[5] What is armoring in a cable ?

Extra mechanical protection provided by armouring with woven wire braid made of galvanized Steel or tinned phosphor bronze. Steel armouring is not preferred for single core cables carrying Heavy alternating currents because of magnetic properties.

[6] What is meant by current rating of a cable?

The passage of current through a cable conductor results in rise of temperature due to resistance of the conductor. Therefore maximum current rating is based upon maximum operating Temperature of the insulation and sheathing’

[7] Comment on the requirements of conduits in regard to cable installation?

Cable outlets from conduits should be bushed to prevent abrasion at the ends’
Total cross sectional area of cables should be less than 40vo of the ct’oss sectioll ill’ea on Conduit.
Ventilation of conduits should provided, and it should be ensured that water does not collectIn the conduit’
The conduit should be earthed’

CONTROL

[8] Define active and passive transducers.

An active transducer is a self generating transducer which converts energy directly from one state to another without the need for an external power source or excitation.

A passive transducer is one which does not convert energy directly but derives the power required for energy conversion from an external source.

[9] Give some examples of active and passive transducers.

Active transducer: thermocouple, peizo electric crystals, photo voltaic cells

Passive transducer: potentiometer, resistance thermometer, strain gauge, photo conductive cell, thermistor, capacitor transducer, differential transformer and magnetic transducer.

DC

[10] What are the characteristics of dc generator?

Open circuit characteristics or magnetic characteristic

(emf generated under no load Vs field current)

Total or internal characteristics (generated emf Vs armature current)
External characteristics or voltage regulation curve (terminal voltage Vs load current)

ELECTRO-MAGNETISM

[11] What is leakage flux and useful flux?

The flux that follows an undesired path is called leakage flux. To utilize the magnetic flux established by the magnetic material, an air gap is provided. The flux in the air gap is called useful flux, the flux which does not pass through air gap cannot be utilized hence it is considered as leakage flux.

ELECTRONICS

[12] What features can high specification electronic relays provide?

The features that high specification electronic relays can provide are;

Thermal overload protection with adjustable time / current characteristics.

Overload alarm.

High set over current protection.

Zero phase sequence or earth fault protection.

Negative phase sequence or phase unbalanced protection.

Under current protection.

[13] What is a semi conductor?’

A semi conductor is one whose property lies between conductor and insulator, in terms of energy bands, semiconductor is one which has almost an empty conduction band and an almost filled valence band with a very narrow energy gap.

[14] What is meant by doping?

The process of adding impurity to the semiconductor is called doping. If a pentavalent impurity is added we get n type semiconductor. If a trivalent impurity is added we get p type semiconductor.

[15] What is a FET?

FET is a field effect transistor and is a three terminal, unipolar, solid state device. The three terminals are source, drain and gate. In FET, current conduction is either by electrons or holes and controlled by means of an electric field between the gate electrode and conducting channel of the device.

HIGH VOLTAGE

[16] Why are high voltage systems more dangerous than low/medium systems?

High voltage systems are more dangerous than low / medium because some high voltage systems are able to retain a lethal charge even when switched off. In addition, dangerous potentials can exist some distance between / from live high voltage conductors

INSULATION

[17] What is the relationship between the insulation and the temperature?

The insulation resistance falls with temperature rise. Life of insulation will be halved by every 10 deg centigrade above the permitted temperature. For example a machine designed for continuous Operation at 70 deg C will have its useful life cut in half when operating at 80 deg C.

[18] Why The insulation tests are carried out on electrical equipments?

A measurement of the insulation resistance gives the best guide to the state of health of the Electrical equipment. The marine environment is particularly arduous for electrical equipment, Due to damp salt laden atmospheres, extreme temperatures and vibrations. The continuous Operation of equipment on board ship demands high operating efficiency and optimum economy In order to keep down costs.

[19] What happens with one earth fault in an earthed distribution system?

With one earth fault in an earthed distribution system, it would be an equivalent to a short circuit fault across the load via the ship’s hull. The resulting large earth fault blowing the fuses in the line conductors.

[20] Describe three insulating materials suitable for permanent electric wiring.

Three insulating materials suitable for permanent electric wiring are;

PVC; which is a very good insulator and commonly used.

Mica; this is another good insulator and is able to with stand high temperatures.

Rubber; which is another good insulator, it can with stand moderate temperatures.

Silicon rubber; this is also a very good insulator and can with stand high temperatures.

[21] How would you go about finding an Earth Fault in the System?

Finding an Earth Fault would be by the process of elimination i.e. circuit breakers would be opened and closed until earth fault disappeared, taking care which breakers were being opened and closed as they maybe supply essential loads at the time. It would be good practice to start with places such as the Galley and Laundry where faults are common.

[22] Why do you require Earth Lamps on 220V Distribution Board when you have them on the Main Switchboard?

You require Earth Lamps on the 220V Distribution Board as well as the Main Board, due to the air gap in step down transformer, i.e. 440V Earth Lamps can not detect Earth on 220V systems due to the gap.

[23] What is the most preferred condition for measuring insulation resistance?

Insulation resistance becomes leakier at high temperatures. So insulation test should be done while the machine is hot just after it has stopped. This will give the realistic reading at near working temperature.

[24] What should be done if the windings resistance is less than 0.5MΩ?

If winding resistance is less than 0.5 M ohms, they should be given a thorough cleaning and dried out, if the value has recovered to a reasonable value which has become steady during the drying out period, its windings should then be coated with high quality air drying insulating varnish. Should the IR values remain low, this could mean the machine needs to be rewound.

[25] What is the sign of an earth fault using earth lamps?

The sign of an earth fault using earth lamps is as follows; if the system is healthy (no earth faults); the earth lamps all glow at equal half brilliance. If an earth fault occurs on one line, the lamp connected to that line is dim or extinguished and the other two glow.

[26] What consideration is used to choose the value of an earthing resistor?

In choosing an earthing resistor, consideration has to be taken; the ohmic value of each earthing resistor is usually chosen so as to limit the maximum earth fault current to not more than the generator full load current.

[27] What should switchgear insulator be checked for?

A switch gear insulator should be checked for signs of tracking and blistering in the vicinity of exposed live metal. If the material is of the bonded laminated type, the laminate should be checked for signs of spills along it

[28] What indicators does an earth fault instrument give?

See sketch of earth lamps;

The indicators of an earth fault instrument gives is a visual and audible indication in the event of an earth fault. If healthy, insulation resistance is high and greater than 1 M ohm.

If insulation is faulty, insulation resistance is low and less than 5M ohms.

MAINTENANCE

[29] What precautions must be taken when doing work on switchgear?

Precautions to be taken when working on switch gear are; that it is isolated and to ensure that it cannot be made live. Where interlock circuits, pilot lights and control circuits are involved, they may be supplied from a different source, so this must be checked as well.

[30] State with reason the immediate action to be taken upon failure of a fuse in such a system.

Upon failure of a fuse in such a system, it must be replaced by a fuse of same type and same rating. The reason is because loss of one fuse out of three can cause single phasing, resulting the current in the energized winding approximately to double.

[31] What should alternator slip rings be checked for?

Alternator slip rings should be checked for even wear and that carbon brushes have free movement in their boxes. Correct brush pressure can be checked a pull type spring balance (normal pull around 1 – 1.5 kg is usual or according to manufacturer’s instructions). Minimum brush lengths should be no less than 2cm or spring pressure will be too little, which could cause sparking.

[32] What must be done before replacing a fuse?

Before replacing a fuse, the supply must be switched off. The cause of the fault must be located and repaired. The replacement fuse link must be of the correct current rating, grade and type. All three fuses in a three phase supply should be replaced even if one has found to have blown after a fault.

MOTORS

[33] What is the function of inter-poles and how are inter-pole windings connected?

Inter-poles are small poles placed in between the main poles. Their function: commutation by producing the auxiliary or commutating flux. Consequently brush sparking is practically eliminated. Inter-pole windings are connected in series with the armature winding.

[34] What will happen if a shunt motor is directly connected to the supply line?

Small motors up to 1 kw rating may be line-started without any adverse result being produced. High rating motors must be started through a suitable -starter in order to avoid the huge starting current which will (i) damage the motor itself and (ii) badly affect the voltage regulation of the supply line.

[35] What is back emf in a motor?

When the motor rotates, the armature conductors cuts the magnetic flux. According to the Faraday’s laws of electromagnetic induction, an emf is induced in the conductors. This emf acts in a direction opposite to the applied voltage (Lenz’s law). This is-called as back emf. This back

Emf is always less than the applied voltage.

[36] A motor stops after starting i.e. it fails to carry load. What could be the causes?

Any one of the following:

Hot bearings, which increase the load by excessive friction.
Excessive tension on belt, which causes the bearings to heat.
Failure of short cut-out switch.
Single-phasing on the running position of the starter.

Ø Induction Motors

[37] What are the indications of winding faults in an induction motor?

Some of the indications are as under:

Excessive and unbalanced starting currents
Some peculiar noises and (iii) overheating.

Squirrel cage motors sometimes exhibit a tendency stably at speeds as low 1/7^th synchronous speed. This is known as crawling.

[38] How do changes in supply voltage and frequency affect the performance of induction motor?

High voltage decreases both power factor and slip, but increases torque. Low voltage has the opposite effect. Increase in frequency increases the power factor, but decreases the torque. The slip remains unchanged. Decrease in frequency has the opposite effect.

[39] Define the significance of the air gap in AC induction motors.

In an assembled squirrel cage induction motor, the air gap is made as small as possible to ensure that the strongest possible electromagnetic induction will takes place.

The size of the air gap affects the power factor of the motor because if the air gap width increase, the magnetizing current will be increased and the power factor is lowered.

[40] What modifications would be necessary if a motor is required to operate on voltage different from that for which it was originally designed?

The number of conductors per slot will have to be changed in the same ratio as the change in voltage. If the voltage is doubled, the number of conductors per slot will have to be doubled.

Ø Three-Phase Motor

[41] Enumerate the possible reasons if a 3-phase motor fails to start.

Any one of the following reasons could be responsible:

One or more fuses may be blown.
Voltage may be too low.
The starting load may be too heavy.
Worn bearings due to which the armature may be touching field laminae, thus introducing excessive friction.

[42] What is Single Phasing of a Motor?

Single Phasing is where one of the 3-phase’s supplying the Motor becomes disconnected. The Motor will continue to run if this happens and can result in Motor Burnout. The effect of Single Phasing is to increase the Current in the two remaining Lines and cause the Motor to become very noisy due to uneven Torque produced.

[43] What are the advantages and disadvantages of 3 phase AC motors with wound rotor (slip ring rotors)?

Advantages wound rotor (slip ring rotor):

This has the benefit of reducing the starting current surge while providing a high starting torque. Speed can be regulated, except that rotor external resistance brush is continuously rated.

Disadvantages wound rotor (slip ring rotor):-

It is inefficient because of power wasted in the resistance for speed control purposes.

The external resistance bank may require additional cooling facility.

Ø Synchro Motors

[44] What could be the reasons if a synchronous motor fails to start?

It is usually due to the following reasons;

Voltage may be too low
Some faulty connection in auxiliary apparatus
Too much starting load
Open-circuit in one phase or short-circuit
Field excitation may be excessive.

POWER GENERATION & DISTRIBUTION

[45] What are the reasons for installing fault protection in electrical distribution system?

To disconnect and isolate faulty equipment in order to maintain the power supply to the remaining healthy circuits in the system.
To prevent damage to equipment from thermal and magnetic forces that occur during short circuit and overload faults.
To protect personnel from electric shock.

[46] What is meant by Preference Tripping?

Preference Tripping is the tripping of non-essential loads when an overload condition occurs on a Generator.

If a Generator Overload develops the Preference Trip Relay operates an Alarm and acts to trip selected non-essential loads. These loads may trip at set intervals depending on how essential the load is i.e. 1st trip 5 seconds, 2nd trip 10 seconds.

[47] What is the Function of the A V R?

The function of the A V R is to regulate the Exciter Field Current automatically, comparing the Generator Terminal Voltage with the Standard Set Voltage and thus maintaining the Generator Terminal Voltage to a Constant Value, regardless of Load Changes.

[48] Explain what would happen if you were to lose a Generator.

First the Standby Generator set would start up and automatically put itself on the Board. If this did not happen, the Ship would “Black Out” and after a time delay, usually 30 seconds, the Emergency Generator would start up and supply the Emergency Switchboard, which supplies essential equipment.

[49] State where an AC shores supply is connected to a ship’s distribution system.

An AC shore supply s connected to a ship’s distribution system at the entrance to accommodation or in the emergency generator room to accept shore supply power. Ashore power supply is required so that the ship generators can be shut down during dry docking for maintenance.

[50] State what is likely to occur if the driving power of an AC generator suddenly fails when two generators are operating in parallel.

When prime mover of a generator fails while it is running parallel with another, the generator of the failed prim mover acts as synchronous motor placing additional load on the other generator.

At the preset value, reverse power protection detects this fault and trips failed generator circuit breakers.

Ø Emergency Power

[51] What is done to ensure the Emergency Generator is Always Available and Will Start?

The System should be checked regularly and operated weekly to ensure it’s availability if required. Fuel tanks should be kept full, ample cooling water in radiator cooling system and starting equipment should be functional, i.e. batteries should be fully charged or air receivers full

[52] Why emergency source of electrical power is provided?

Emergency source of power supply is provided so that in the event of an total power failure, A supply will still be available for emergency lighting, alarms, communications, water tight Doors and services necessary to maintain safety.

[53] Why must an emergency electrical power system be proved?

An emergency electrical power supply must be provided so that in the event of an emergency involving total power failure, a supply will be available for emergency lighting, alarms, communications, watertight doors and other necessary services to maintain safety and to permit safe evacuation of the vessel to lifeboats.

[54] What is done to ensure the Emergency Generator is Always Available and Will Start?

[55] What are the Essentials supplied by the Emergency Switchboard?

Essentials are: Emergency Lighting, Alarms, Communications, Watertight Doors and other services necessary to maintain safety and to permit safe evacuation of the Ship by Lifeboats. Also you will find that a Steering Gear Motor, Emergency Air Start Compressor and Emergency Fire Pump Motor are supplied from the Emergency Switchboard.

[56] State the danger arising from reducing the excitation of the outgoing generator before it is off loaded.

When the excitation of the outgoing generator is reduced, the current taken by it is reduced. The current (load) of the other generator is thus increased and if exceed the rated current, the overload relay will trip the generator and a blackout would occur.

[57] Briefly describe how a running AC generator would be taken off load and another generator put on load without a blackout.

The incoming generator is synchronized with the running generator after having ensured that the frequency and voltage of both generators are identical, both having the same phase sequence and the phase angle difference between the voltages is zero. To minimize the possibility of blackout, both generator should be allow to run in parallel for a short time before taking the outgoing are off load.

Ø Generators

[58] How the shaft current is prevented?

To eliminate shaft currents either (a) outboard bearing shell is insulated in the housing, or (b) outboard bearing pedestal is insulated from the foundation. All lub oil pipe connections that make connection with the shaft or the particular bearing must also be insulated.

[59] What is critical resistance?

It is the value of resistance in the field circuit where the generator will just excite. Above this resistance value, there will be no voltage generated. This is the tangent drawn to the linear portion of the open circuit characteristic.

[60] What is the difference between direct-connected and direct coupled units?

In the former, alternator and driving engine are directly and permanently connected. In the latter case, engine and alternator are each complete in itself and are connected by some device such as friction clutch, jaw clutch or shaft coupling.

[61] How is a direct connected exciter arranged in an alternator?

The armature of the exciter is mounted on the shaft of the alternator close to the spider hub. In some cases, it is mounted at a distance sufficient to permit pedestal bearing to be placed between the exciter and the hub.

[62] What are the advantages and disadvantages of direct connected exciter’?

The exciter has to run at the same speed as the alternator which is slower than desirable. Hence it must be larger for a given output than the gear driven type, because it can be run at high speed and so made proportionally smaller.

PROTECTION

[63] Give reasons why Protection Equipment is essential in an Electrical Distribution System.

To disconnect and isolate faulty equipment in order to maintain the power supply to the remaining healthy circuits in the system.

To prevent damage to equipment from the thermal and magnetic forces that occur during short circuit and overload faults.

To protect personnel from electric shock.

[64] How is the over current protection tested?

Over current protection is tested by injecting test currents into the breaker to check their current trip levels and time lags. The test is essentially a transformer and controller rather like a welding set.

[65] What problem can occur if the full load current is greater than the reset current?

The problem that can occur if the full load current is greater than the reset current is that the over current device may remain in a partially operated state, from this position it may creep even with normal load and trip the breaker.

[66] How does an electronic over current relay operate?

The electronic over current relay operates on the principal that it converts the current into a proportional voltage; this is compared with a set voltage level within a monitoring unit. The time delay is taken by the time to charge the capacitor. This sort of relay usually has separate adjustments for current trip level and for time trip.

[67] What is the purpose of the Under Voltage Trip?

An Under Voltage Trip is fitted to all Generator Breakers. Its main function is to Trip the Breaker when severe Voltage Dip (around 50%) occurs. The Under Voltage Trip on a Generator Circuit Breaker also prevents it being closed when the Generator Voltage is very low or absent.

[68] What is Reverse Power Protection for?

Generators intended to operate in Parallel must have a Reverse Power Protection Trip. A Reverse Power Relay monitors the direction of power flowing between the Generator and the Switchboard.

If a Prime Mover Failure occurred the Generator would act as a Motor. The Reverse Power Relay detects this fault and acts to trip the Generator Circuit Breaker.

[69] What is the purpose of the Over Current Protection Trip?

The purpose of the Over Current Protection is to Trip the Generator in Overload situations. The Trip is usually set at 150% Operating Capacity and has a time delay of usually 20 seconds. This allows for short periods of Overload Currents. It also protects against a Short Circuit in the Generator.

Ø Fuses

[70] Why HRC fuse is normally preferred for electrical installation onboard?

Time/current fusing characteristics are very consistent.
HRC fuses are reliable and non-deteriorating in service
Fuses have very high breaking capacity.
They also have very high speed of operation at high short circuit fault, faster than a circuit Breaker.

SAFETY

[71] What is meant by the Term, Type Ex’d’?

Ex’d’ = Flame Proof Enclosure.

This is an enclosure that will withstand an Internal Explosion of Flammable Gas within itself and dissipate heat from that explosion before reaching outside atmosphere, which could ignite the gas in surrounding atmosphere, i.e. a Ship’s Battery Locker could be classed as Ex’d’.

STARTERS

[72] With a manual change-over in a star delta starter which is the interlock for?

With a manual change over in a star – delta starter, the interlock is to prevent and lock the handle in place if the operator has left it too late to change over from star to delta, as it would cause current surge as speed would have dropped off. This surge could also cause voltage dip

[73] Why are motor starters fitted with under voltage trips?

Motor starters are fitted with under voltage trips to prevent the generators from tripping when power is restored owing to the total starting current of all connected motors. Some motors have special automatic restarting facilities.

SWITCH GEARS

Ø Circuit Breaker

[74] What is the Function of a Circuit Breaker?

The Circuit Breaker is an Isolating Switch that also acts as a Fuse. It has two designed ratings; one of normal safe working current, and the other against overload, which may also have a time delay.

[75] What factors determine the current rating of a circuit breaker?

The factors that determine the current rating of a breaker are; the maximum continuous permissible operating temperature of the circuit breaker copper work and contacts, the ambient temperature, the temperature rise of copperwork due to the load current.

[76] State why a breaker may not open upon severe and prolonged voltage dip.

Most breakers uses magnetic contact spring forces tends to open sufficient voltage to magnetize are for long time, contact overheated, melts and welded together, thus it cannot be opened after a prolonged voltage dip.

TRANSFORMERS

[77] State the normal range of efficiency of a transformer.

Normal range of efficiency of a transformer is nearly 98%. Primary winding power is aprox. equal to secondary winding power.

IiVi x primary p.f. » I2V2 x secondary p.f.

Therefore I1 » V2 » N2

I2 V1 N1

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