AC

[1] What Is Electricity?

Nobody knows. It is invisible. Only its effect is measurable. Its main effects are heating, magnetic and chemical effects. When an electric current flows in a material, then that material heats up (as found in lighting and heating). A magnetic field is produced around that material (as used to run motors). It can also alter the chemical composition of the material (as in batteries and anti-corrosion systems).

The current theory is that electricity is the flow of electrons from one atom to another around a loop circuit. If there is a break in the circuit there is no flow of electric current. The breaking of this loop is the method of operation of all electrical safety devices.

Because it cannot be seen, electricity must be treated with care and respect and it must be remembered:

BATTERY

[2] With respect to an emergency standby lead acid battery;

state the safety features incorporated in the design and construction

state, with reasons, two causes of short circuit

state, with reasons, five causes of fall off in rated capacity

a) Safety features that are incorporated in the design of lead acid battery construction are; caps that allow gas to vent off while on charge, cells that are separated, the outside casing of the battery is of a robust material, ie plastic.

b) Two causes of short circuit are a build up of sediment at the bottom of the cells and break down of cells usually due to the battery being over charged, causing the battery to overheat.

c) Five reasons for fall off of rated charge are;

Low specific gravity of electrolyte due to not being charged for a period

Plates/ cells that are not covered with electrolyte

Internal short circuit due to build up of sediment at the bottom of the cells

Cold temperature causing S.G to drop off

Overcharge, causing overheating of the battery, causing breakdown

[3] With reference to shipboard batteries, state the difference between the following;

primary and secondary cell;

acid cell and alkaline cell

a) The difference between primary and secondary cell batteries are that primary cells can only be used once, as where secondary cell batteries can be recharged.

b) The difference between lead acid cell batteries and alkaline cell batteries are that acid cell batteries use sulphuric acid as an electrolyte, as where alkaline cell batteries use potassium hydroxide.

Also the voltage of an acid cell battery is greater than the voltage of an alkaline cell.

The cell of an acid battery consists of a lead peroxide positive plate and a lead negative plate. As where an alkaline cell consists of nickel hydroxide positive plate and a cadmium and iron negative plate.

CABLES

[4] Subjected to over voltage due to seasoned or intermittent fault. with respect to silicon rubber insulated cables briefly discuss :

(a) Its advantages

(b) Its disadvantages.

Advantages of silicon rubber insulated cables:

Silicon rubber has outstanding resistance to high temperature.

It can continuously operate at temperature up to 150ºC.

In case of fire, it burns to form silicon oxide which is an electric insulator. Hence, operation can continue even after a fire, provided conductor is not melted.

It is not mechanically robust, but remains flexible down to very low temperature.

However, modern grade has adequate mechanical strength to withstand rough handling.

Disadvantages:

Not oil resistant. Hence sheathing of glass braiding with heat resisting varnish needed.

It is very expensive and only used for special applications.

ELECTRONICS

[5] How will you check a diode with multi meter?

Remove or disconnect the diode from the circuit. (if the power is turned off instead of disconnecting the diode, discharge all capacitors and remove all fuses in the circuit). Multi meter drives a small dc current (a few ma) through the diode p-n junction, and acts as a Voltmeter to measure the voltage across it. Connect the two probes across the diode. Read the voltage drop, it should be 0.5v – 0.8v (500 mv to 800 mv) for a healthy silicon diode. Reverse the probe connections and the display should indicate ‘over range’. If the display indicates over Range in both directions, means the diode is open circuited. If the display indicates less than 1 v in both directions, diode may be short circuited.

HIGH VOLTAGE

HULL PROTECTION

INSTRUMENTS

INSULATION

[6] What are the common locations of earth faults?

Earth faults are found at lamp fittings and terminals etc., where moisture has found its way in’

Lamp fittings on open deck, exposed to rough weather are prone to ingress of moisture.
In accommodation, washing machines drenched with water area another source of earthly faults.
In galley, the ovens, hot plates etc., are also exposed to earth faults.
Dripping water over electrical machinery.
Motors standing idle for long time – their insulation get affected by condensed moisture.
Overheated insulations of motor and cables cause drop in insulation resistance.
Dirty electrical apparatus with surface tracking (leaking current)
Old electrical apparatus whose insulation has aged has become brittle.
Terminal wire which has become loose from the terminal and is touching the metal casing of electrical machinery.

[7] What are the factors that affect insulation resistance?

Insulation is affected by many factors such as:

Moisture – results in a decrease in the measured value of insulation resistance.
Dust and dirt deposits – reduces insulation resistance and failure of insulation due to oxidation.
Oil and grease – prevents heat dissipation and cause dust and dirt to settle on the insulation, Thereby reduces the insulation resistance and leads to failure.

4, Ageing – over a period of time the insulation deteriorates due to it getting affected by Temperature variation, mechanical stresses, vibrations, moisture, dirt deposits, chemical Contacts etc some varnishes tend to become hard with age and crack during the operation.

Temperature – excessive temperature dehydrates and oxidizes the insulation, making it Brittle and disintegrate under vibration and shock.

LIGHTING

MAINTENANCE

[8] Suggest with reasons the problem that may arise if air gap change is not corrected.

If air gap measurement takes at 90° apart indicates that it is due to worn out bearing, then there is a possibility that the rotor will rub against a stator. A very slight rub will generate sufficient heat causing overheating which can destroy the coil insulation.

If the air gap is uneven, it will reduce the life of bearing due to unbalanced torque and vibration occurs as a result of uneven magnetic flux. Excessive heat generate to the bearing may cause seizure, resulting noise sound in the motor.

Directly, it will result in damaging the moving part of the pump which can reduce the efficiency of the pump performance.

MOTORS

[9] What happens if a motor is operated in star instead of delta?

If the motor is designed to be operated in delta on your local supply voltage, then operating it continuously in star can cause damage to the motor. When a delta motor is connected to star, the voltage across the windings is reduced by the square root of three resulting in reduced flux in the iron. This will reduce the magnetizing current, and will also reduce the torque capacity of the motor.

If you operate at light loads, there will be no problem, however if you operate at higher load the slip of the motor will be increased dramatically and it may stall. The increased slip will result dramatic increase in the power dissipated in the rotor. If the motor begins to stall, the stator will also suffer excess heating causing motor failure.

[10] What are the different types of enclosures available for induction motors?

Totally enclosed, non ventilated type: such motors have solid frames and end shields, but no openings for ventilation.

Splash proof type: in the frames of such motors, the ventilating openings are so constructed that the liquid drops or dust particles falling on the motor will not be able to enter the motor when the incident angle is not greater than 100^0.

Totally enclosed, fan cooled type

Protected type: end covers have perforated covers.

Drip proof motors: the frames are so constructed that liquid drops or dust particles, falling on the machine at any angle greater than l50 from the vertical, cannot enter the motor, either directly or by striking and running along a horizontal or inwardly inclined smooth surface.

Self ventilated type: consists of enclosed shields with provision for pipe connection on both the shields. The motor fan circulates sufficient air through pipes which are of ample section.

Separately (forced) ventilated type.

[11] What is meant by ‘single-phasing’ and what are its causes?

Single-phasing is meant the opening of one wire (or leg) of a three-phase circuit there upon the remaining leg at once becomes single-phase. When a three-phase circuit functions normally, there are three distinct currents flowing in the circuit. As is known, any two of these currents use the third wire as the return path i.e. one of the three phase acts as a return path for the other two. Obviously, an open circuit in one leg kills two of the phases and there will be only one current or phase working, even though two wires are left intact. The remaining phase attempts to carry the entire load. The usual cause of single-phasing is, what is generally referred to as running fuse, which is a fuse whose current carrying capacity is equal to the full-load current of the motor connected in the circuit. This fuse will blow-out whenever there is overload (either momentary or sustained on the motor.

[12] Briefly describe the effect on a 3 phase AC motor if the supply voltage was at a lower frequency than the motor design frequency.

At frequency lower than that for which motor is designed, the stator flux increases and magnetizing current increased, so the heating on winding and iron will increase. Motor will run hotter, unless reducing in speed, also bring about reduction in load for example centrifugal pump, fan etc.

An increase in magnetizing current, power factor will be lower which indicating increasing in air gap. Uneven air gap can cause bearing worn out. The possibility rotor and stator rub each other occur causing overheating which can destroy the coil insulation. Life of bearing will reduce due to unbalanced torque and vibration can occur as a result of increase in magnetic flux. Excessive heat generating to the bearing may cause seizure resulting noise sound in the motor. Directly the damage on the moving part of the pump occurs and seizure takes place which reduce efficiency of the pump performance.

[13] Explain why an AC motor which is stopped due to interruption of supply should be disconnected from the supply.

AC motors which are stopped due to interruption of supply should be disconnected from supply because on restarting of supply it could not be immediately switched on for the following reasons:

Operating personnel may be injured due to motor starting unexpectedly, e.g. engine room overhead cranes, lathe machines, portable electric appliances, tools and cargo winches.

With certain AC motors, a definite sequence of restarting motor operation must be gone through.

If all motors were to restart simultaneously, the generators might be seriously overloaded.

Certain adjustment to the driven machinery may be necessary before restarting.

If the motor is driving some heavy loads, it may be necessary to make adjustments to the driven machinery such as controlling valves, regulating the load so as not to overload the motor if it were to restart immediately, thus preventing damage to motor.

[14] Describe the effect of running a motor with the stator windings connected in star when the motor is designed for running with delta connected windings.

The effect of running a motor with stator windings connected in star when the motor is designed for running with delta connected windings are:

A star delta motor starter gets stuck in star position, malfunction in control timer, contactors, etc.
On full load, each stator winding will be carrying an overload of 1.73 x rated line current.
This will cause overheating, because of I² R effect. The motor will run 3 times hotter.
But the line current will be normal and thermal overload protection will not protect the motor. This will cause overheating and eventually burn out.

[15] Briefly state the points on which attention should be given if a large induction motor is to be shut down for an extended period.

The points on which attention should be given if a large induction motor is to be shut down for an extended period are:

Motor should be cleaned and windings free of oil. This can be done by cleaning with chemical using carbon tetrachloride.

Heavy deposits of dirt will have to be removed by brushing or scraping using a suitable scraper or by compressed air, reusing carbon tetrachloride for final cleaning of surface.

Excess fluid should be wiped off and deposits of fluid in crevices should be avoided.

Unpainted iron and steel shafting, coupling etc. should be given a light of coat to prevent rusting.

Motor should be covered if possible so as to keep the dirt and dust from entering the motors and to reduce dampness in the windings.

If heaters are installed in the motor, it should be kept ‘on’ to prevent accumulation of moisture.

[16] State what investigations would need to be made before starting a large induction motor which had been shut down for an extended period?

Investigation that should be done before starting large induction motor which had been shut down for an extended period are:

Check insulation with megger. If reading is low, dry out winding with heating lamp.

Check heaters, if installed.

If it has brushes, check, clear and be sure they are free in the holders. Check spring pressure.

Check cooling fan if installed.

Check lubrication.

Check temperature recording instrument.

If necessary, remove rotor from stator, clean any dirt accumulated and revarnished if necessary.

Ventilation ducts need to be cleaned for free flow of ventilating air.

Check air gap after assembled.

[17] State the requisite steps of starting up operation mentioning any precautions

Requisite steps of starting up operation are:

Uncoupled motor from load, turn rotor by hand and ensure for free moving and indicating air gaps are correct and bearings are in good condition.

Replace the fuses with correct rating of the motor.

Start motor for short period for direction of rotation and rectified any abnormal noise.

Coupled back motor to the load, check for alignment of the coupling flanges.

Turn the coupling with hand and ensure smooth movement of the motor and pump.

Start motor, observe current reading on the ammeter of motor.

Monitor the temperature of the motor and observed any abnormality for some period after putting the motor into service.

Ø AC Motors

Ø DC Motors

Ø Induction Motors

Ø Single-Phase Motor

Ø Three-Phase Motor

Ø Synchro Motors

POWER GENERATION & DISTRIBUTION

[18] On medium voltage, 440V marine, AC, 3 phase, distribution system, the neutral wire is invariably insulated, whereas in shore practice it is otherwise. Briefly list and give reasons for :

(a) The advantages of insulating the neutral, and

(b) The disadvantages of insulating the neutral.

Advantages of insulating the neutral:

Protect the essential service in case of earth fault occurred.

Fewer fire risks.

Less prone to single phasing.

No danger of triple current (3^rd harmonic current), overheat will activated earth fault relay which may trip off the generator.

Disadvantages:

Earth fault detection harder.

Phase voltage at line voltage (if one phase is earthed, insulation highly stressed due to star connected).

Less suitable in high voltage system.

Required step down transformer, double pole switches/fuses for/and lower voltage sub circuits.

[19] Explain the circumstances under which electrical machinery onboard ships tend to run hotter than usual when supplied from ashore and suggest a remedy.

Electrical machinery onboard ship tend to run hotter than normal when supplied from ashore :

The shore supply may have a different frequency and / or voltage to that ship system. A higher frequency will cause motors to run faster thus overload and overheat while with a higher voltage will generally cause equipment to take excess current and overheat. It will also cause motor to accelerate more rapidly and this may overstress driven load.

With a lower voltage is generally not so serious but may cause motor to run slower and overheat and may cause motor to stall which is more dangerous. In order that problems to overcome, its best that to supply the voltage and frequency at the ship operating voltage and frequency.

[20] Briefly explain the statement, ‘three phase, three wire distributions with insulated neutral is favored’.

3 phase three wire distribution with insulated neutral is favored because:

one earth fault occurred on an equipment does not interrupt in supply but only raise a warning on the leakage system, thereby avoiding loss of essential services.

This allows the operator to search for and clear the fault at a convenient time.

Further advantage is that the electric shock risk is reduced, since (in 440V system) the capacitive return current is usually only a few milliamps.

Thus with such values of earth fault current, the risk of fire due to such fault is reduced.

Thus in insulated systems at 440V, the risk of failure due to over voltage are not regarded as serious.

[21] State what are the main considerations when designing a ship’s main electrical power supply system, with respect to the type of services supplied, and state the minimum number of generators that may be utilized?

Main consideration for electrical circuit design with respect to type of sources supplied:

Electrical load is divided into essential and non essential sources for safety of personnel and safe navigation. Essential services are necessary for loads which required handling in emergency situation.

In case of fire in ER or blackout, essential services will receive power from emergency sources.

The distribution system provides power to essential services which are duplicated from separate section of switchboard.

Lighting in machinery spaces:

2 sets to prevent darkness.

Switches of double pole type.

Emergency lighting.

Undesired restarting after trip due to low voltage except for essential services e.g. steering gear.

Remote stop of vent fan and pumps delivery oil to boilers and main engine.

Electrical power to starting fire pump not passing thru engine room.

Heating and cooking equipment is controlled by multiple switch mounted reset to equipment.

Fire detection system from main and emergency sources.

[22] Explain how the correct order of phase connection from three phase supply lines to motor terminals can be deduced from standard markings, and state a necessary precaution in guarding against incorrectly marked terminals.

Standard markings with British Standard, if generator is driven with clockwise rotation when looking at its driven end, the terminal phase sequence will be in the order A, B, C. (For colour marking, the sequence in Red, Yellow, Blue). If the motor is connected to a supply terminal hase sequence in the order of A, B, C, and then the motor would rotate in clockwise direction.

A necessary precaution is guarding against incorrectly marked terminal is by run the motor with coupling to mechanical equipment disengaged. Ensure correct running direction. In case of air compressor if wrong direction, compressed air still can be produced but LUB oil pump engaged to compressor will not working, no lubrication cause bearing damage.

Ø Emergency Power

[23] 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.

They are used for supplying essential power to: Radio Equipment, Telephone Exchange, Fire Detection, General Alarm Circuits, etc. These Systems are often supplied from two sets of batteries worked on a regular charge/discharge cycle.

They are also used as Emergency Supplies, i.e. for Emergency Generator Start-up and Emergency Lighting. Being used in this case in a “stand-by” role to give power when Main Supplies fail.

Ø Generators

[24] In large DC generators where the current is in excess of 1000A, it is necessary to take steps to avoid magnetizing the shaft :

(a) Explain why this is so, and

(b) Briefly describe how this is accomplished.

In large generators where the current exceed 1000A, it is necessary to take steps to avoid magnetizing the shaft as this could lead to the penetration of stray current accumulated at the bearing, sufficient to puncture the oil film with consequent pitting on the bearing and journal faces. This can lead to bearing failure in due course and possibility of sludging of lubricating oil.

In DC generator, the compounding coils are connected in series and in effect constitute one complete turn around the shaft.

The commutating poles are similarly connected in series and act as if it is one complete turn around the shaft. By arranging for the current in the two sets of connection to in opposite directions to each other around the shaft, the current neutralize each of them, avoiding magnetizing of the shaft.

[25] What are the considerations under which minimum number of generators may be designed for? Discuss if the generators should be design for equal load sharing, or if not the design considerations you would deem essential. List the main loads you would include in your design appraisal.

Number of rating of generator sets:

Power rating of generators determined by power demand of the electrical load.

The designer estimates the number and power rating of the required generator by assessing power demand at sea, port and all other situations.

With one generator out of services:

The remaining set or sets must ensure power to essential services.

The remaining set or sets must have reserve power to permit starting of largest motor without excessive voltage dip.

4) For UMS ship:

Where one generator supplies power, stand by generator should start automatically in case of power failure.

Where 2 generators operate in parallel, any loss of one generator, and the remaining set should be able to supply power.

PROTECTION

[26] How the overload protection device is tested and set?

Overload relays and circuit breakers should be tested periodically. Bimetal elements in thermal overload relays become insensitive to heat from metal fatigue and fail to operate under overload condition. These relays should be checked for corrosion, accumulation of dirt and other foreign matter that block free movement of tripping element. Other current relays and circuit breakers may be tested by passing a specified over current through the heater or overload coil and observing the time required for it to trip. The actual tripping time should then be compared with the manufacturer’s current time characteristics. To test an overload rely, it should be disconnected from the power line and connected to low Voltage, high current power source like variable autotransformer. Current is adjusted to test Value by connecting to one of the bimetal heaters via heavy test leads. A stop watch is used to determine tripping time. Magnetic overload trips and circuit breakers may be tested in similar manner.

[27] What is under voltage protection for generator and motor?

An under voltage release is fitted to all generator’s circuit breaker. Its main function is to trip the circuit breaker when severe voltage dip occurs. The under voltage release on a generator Circuit breaker also prevents it being closed when the generator voltage is very low or absent. The under voltage relay prevents the closure of the circuit breaker of the dead generator. Under voltage protection is also required for motor starters. The starter contactor normally provides this protection as it drops out when the supply voltage is lost or is drastically reduced. The starter circuit will not normally allow the motor to restart when the voltage supply is restored, except when special automatic restarting facilities are provided. This ensures that when the supply returns after a black out, all the motors do not start together, there by tripping the generator circuit breaker again. Essential motors may be arranged to start automatically by Sequential start with staggered time delays.

Ø Fuses

[28] Explain why it is not good practice to replace only single fuse in a 3 phase motor and why fuse clips and connections need examination during fuse renewal.

It is not a good practice to replace only a single fuse in a three phase motor because:

Due to aging of the fuses and cause dampness of the fuse.

Material properties f the fuse will be weakened due to expose to the increased current.

Rating will be drop and could operate at a much lower operating value than it was originally designed.

Thus the loss of machinery during normal operation could occur and it could be dangerous if it were on essential service equipment. As we know, burnt of one fuse can cause single phasing resulting the current in the energized winding approx. to double. Frequent single phasing can damage the respective machinery which is unsafe while operating.

Fuse clips and connections should be examined when renewing fuses as a scored contact may cause, operation below the correct value due to being oxidized, melted or weakened by the high current.

SAFETY

STARTERS

SWITCH GEARS

Ø Circuit Breaker

Ø Contactor

SWITCHBOARDS

TRANSFORMERS

[29] How does change in frequency affect the operation of a given transformer ?

Iron loss – increases with a decrease in frequency. A 60-hz transformer will have nearly 11% higher losses when worked on 50 hz instead of 60 hz. However, when t 25 hz transformer is worked on 60 hz, iron losses are reduced by 25%.

Cu loss – in distribution transformers, it is independent of frequency.

Efficiency – since cu loss is unaffected by change in frequency, a given transformer efficiency is less at a lower frequency than at a higher one.

Regulation – regulation at unity power factor is not affected because IR drop is independent of frequency. Since reactive drop is affected, regulation at low power factor decreases with a decrease in frequency and vice versa. For example, the regulation of a 25Hz transformer when operated at 50-hz and low power factor is much poorer.

Heating – since total loss is greater at a lower frequency, the temperature is increased with decrease in frequency.

[30] State where on board ship transformer may be used with advantage.

Onboard ship transformer used :

Electric generation is typically 3 phase 440V, 60Hz, while fixed lighting and other low power loads are applied with 220V or 110V single phase from transformer units.

Auto transformer starter consists of a three coil which normally use 65% large motor drives.

Advantages :

3 phase lighting transformer, if transformer fault develops, the faulty unit can be disconnected. Power supply will still be available at a reduced rating from other transformers.

By auto transformer starter, it will reduce starting voltage thus limit the starting surge current by step it down such as electric cargo pumps, ballast pumps, FD fan etc.

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