Marine Electro Technology

1.WHAT IS THE FUNCTION OF FUSE?

The function of the fuse is to give short circuit protection and also over load protection and operates in milliseconds.

2.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 eme.

3.WHERE ON THE SHIP IS THE EMERGENCY GENERATOR LOCATED?

The emergency generator is located remotely from the engine room, usually on the Accommodation in deck or at weather deck level or above.

4.WHAT ARE DISADVANTAGES AND ADVANTAGES OF A LEAD ACID BATTERY?

The advantages of a lead acid battery are they require fewer cells to achieve particular voltage an d are reasonably priced The disadvantages are they require regular attention to keep fully charged, always require a trickle charge and they discharge on open circuit

5.WHAT IS THE PURPOSE OF UNDER VOLTAGE TRIP?

It is fitted to all generator breakers. Its main function is to trip the breaker when severe voltage dip (around 50%0. the under voltage trip on a generator circuit breaker also prevents it trip on a generator circuit breaker also prevents it being closed when the generator voltage is very low or absent

6.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 switch board If a prime mover failure occurred the generator would act as a motor. The reverse power relay detects the fault and acts to trip the generator circuit breaker.

7.WHAT SORT OF STARTING CURRENT DO YOU GET WITH A DIRECT ON LINE STARTER?

6 – 8 times full load current

8.WHAT ARE THE DIFFERENT MOTOR SAFETIES?

1. SHORT CIRCUIT PROTECTION In HV motor protection scheme, the back- up fuses are the trigger type. This type of fuses releases a trigger actuated by a spring held in a tension until the element melts. When released, the trigger may be used to indicate a blown fuse or to trip a circuit breaker or contactor. 2. OVER CURRENT & SINGLE PHASING PROTECTION It is provided by inverse time OCR with differential function for single phase fault. 3. UNDER VOLTAGE PROTECTION Under voltage protection is by a separate relay which actuates the LO.

9. WHY IS A MEGGER USED FOR INSULATION TEST AND NOT A MULTI METER?

Ans: With a megger, a voltage of not less than 500 volts DC is used for testing the insulation resistance of windings. With a multi-meter, the voltage used is not more than 3-volts DC. Megger uses high impedance testing and is therefore more accurate than a multi-meter

10. WHAT ARE THE SAFETIES ON MSB?

Ans: Circuit breakers, fuses and over current relays are used. The panels are dead front panel, that is, we cannot open the panel for maintenance until we switch off the power to the panel by circuit breaker.

11. WHEN DOES REVERSE POWER FLOW?

Ans: Situation of a generator is feeding a system through switchgear having several generators connected in parallel with this generator. The flow of current, when the system is running normally, is from the generators to the switchgear. If one generator experiences problems and its terminal voltage falls below the system voltage, the generator will act as a motor, just as a motor can act as a generator, and current will flow from the switchgear to the generator. This is reverse power. The effects can range from minor to extreme in the event of a complete mechanical failure of the generator which fails.

12. WHAT IS THE HARM IF REVERSE POWER FLOWS?

Assume that the generator normally produces an amount of power equal to P, and that when operating as a motor, it will absorb a similar amount, P. Therefore, the net effect on the grid will be the same as if it experienced a step increase in load equal to 2P. Depending on the size of the grid and the strength of the remaining generation, a step increase of 2P could result in a significant frequency change on the grid. There could be prime mover damage in some instances (especially steam turbines).

13. HOW IS THE PROTECTION AGAINST REVERSE POWER GIVEN?

Reverse power protection is used for anti-motoring. This function is used for protection of prime mover not generator. It can cut-off the fuel supply and stop the prime mover.

14. HOW DO YOU TEST REVERSE POWER TRIP?

Ans: When two generators are running in parallel and one generator can carry the load, reverse power trip can be tested by load shifting using governor control. When the load has shifted sufficiently and the off loaded generator is carrying a small percentage of load, its breaker trips and fuel supply to its prime mover cuts off. This means reverse power relay has operated. The relay can be tested by simulation (using the test push button on the relay) to see if it initiates a trip signal.

15. WHAT IS THE FULL FORM OF ACB?

Ans: Air circuit breaker. It is normally used at 400V and higher current applications (generator breakers).

16.IF YOU PRESS THE ACB CLOSE BUTTON ON AN IDLE GENERATOR WHAT WILL HAPPEN?

Normally, the breaker won’t close until you synchronise, so even if you press the breaker close, it will not close. The breaker has under-voltage protection which will not let you close it.

17.WHAT IS UNDER-VOLTAGE PROTECTION?

It prevents closure of the breaker by mistake, or the generator that is coming on load during parallel operation. It also provides protection against loss of voltage while machinery is connected to the switchboard.

18.WHAT IS THE MEANING OF PREFERENTIAL TRIP? WHY IS IT PROVIDED?

Preferential trip is a kind of electrical arrangement on ship which is designed to disconnect the non-essential circuit i.e. non-essential load from the main bus bar in case of partial failure or overload of the main supply. It lets the critical loads run (like steering gear) and trips the non-essential loads (like AC and galley) and is a safety feature.

19.WHAT IS THE PURPOSE OF THE EARTH FAULT INDICATION ON THE SWITCH BOARD?

It detects and indicates phase to earth fault on a circuit.

20.IF YOU GET AN EARTH FAULT ALARM WHAT WILL YOU DO?

The fault can be investigated by first identifying areas of ship that have a high chance of earth fault (example deck lighting or pantry equipment) and then switching off their supplies one at a time to see if the alarm is gone. When a particular load clears the alarm, we know that that circuit is having earth fault and we can go and repair it.

21. WHAT IS DIRECT CURRENT?

Uni-directional flow of current.

22.WHAT IS ALTERNATING CURRENT?

Bi-directional flow of current in a sinusoidal form with a positive and negative peak.

23.WHAT IS MEANT BY PHASE IN AC?

Two or more AC voltages or currents that are out of step with each other. Their peaks and zero points do not match up at the same points in time. This is meant by phase.

24.IS THERE PHASE IN DC?

No

25.WHY IS DC NOT MUCH IN USE NOW?

AC generators are a better technology and voltage transformation (by transformers) is easy is possible in AC and not DC.

26.WHERE ARE YOU LIKELY TO SEE IT USED?

Battery powered DC is generally used in navigation, control and safety systems mainly in navigation, radar, safety systems, antenna and communication systems and emergency lighting.

27.WHAT DOES THREE-PHASE CURRENT MEAN?

Three current sinusoidal waveforms equal in magnitude but with a phase shift of 120 degrees.

28.WHAT IS RMS VALUE?

Root mean square value is the peak value divided by square root of 2.

29.WHAT IS THE MEANING OF POWER FACTOR?

Power factor is the cosine of the angle between voltage and current waveform.

30.WHAT IS THE USUAL VALUE YOU SEE ONBOARD?

Usually 0.8.

31.WHAT IS THE BEST VALUE POSSIBLE?

32. WHAT IS THE BENEFIT OF IMPROVING POWER FACTOR?


Power factor close to 1 means for the same real power in kW, the load current is less and hence the I2R losses are less. The system efficiency is higher as the losses are lower. Improved voltage regulation is achieved.

33. HOW CAN IT BE IMPROVED?


Power factor improvement capacitor banks are used if the loads are mainly resistive.

34. WHAT IS THE MEANING OF SAYING I AND V ARE IN PHASE?


The angle between the I and V waveforms is zero which means the load is purely resistive.

35. WHERE DOES REACTIVE POWER GO?


Reactive power is not ‘lost’. It is delivered to the motor where it sustains the electric field that enables the motor to convert the real power (electrical) into mechanical torque.

36. WHAT IS DOPING?


Doping intentionally introduces impurities into an extremely pure (also referred to as intrinsic) semiconductor for the purpose of modulating its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic. A semiconductor doped to such high levels that it acts more like a conductor than a semiconductor is referred to as degenerate.

37. WHAT HAPPENS TO INSULATION WHEN TEMPERATURE RISES?


For insulators, an increase in temperature will cause their resistance to decrease -which is why excessive temperature is often the main reason why insulation fails

38. WHAT HAPPENS TO RESISTANCE OF CONDUCTORS WHEN TEMPERATURE RISES?


For conductors an increase in temperature will cause their resistance to increase. Q127. WHAT IS AN INDUCTION MOTOR? Ans: An induction motor is an alternating current motor in which the primary winding on one member (usually the stator) is connected to the power source and a secondary winding or a squirrel-cage secondary winding on the other member (usually the rotor) carries the induced current.

39. WHAT IS “SLIP”?


An AC (Amplitude Current) induction motor consists of two assemblies – a stator and a rotor. The interaction of currents flowing in the rotor bars and the stators’ rotating magnetic field generate a torque. In an actual operation, the rotor speed always lags the magnetic field’s speed, allowing the rotor bars to cut magnetic lines of force and produce useful torque. This speed difference is called the slip.

40. WHAT IS A SYNCHRONOUS MOTOR? WHERE IS IT USED?

A constant-speed motor, the speed being dependent on the frequency of the ac supply and the number of poles for which it is designed. USES -Synchronous motors find applications in all industrial applications where constant speed is necessary. Improving the power factor as Synchronous condensers. Electrical power plants almost always use synchronous generators because it is important to keep the frequency constant at which the generator is connected. Low power applications include positioning machines, where high precision is required, and robot actuators.

41. WHAT IS AN AVO METER?

The Avometer was a British brand of multimeter, latterly owned by Megger. It is often called simply an AVO and derives its name from the first letter of the words amperes, volts, ohms

42. HOW DO YOU CHECK CONTINUITY?


Set the multimeter in resistance mode and then check with probes the resistance between the points or circuit under check. If the resistance is low or zero, it means continuity. If it is high or infinite, it means open circuit.

43. WHAT LIMITS THE ELECTRICAL LOAD?

The available power is fixed and the voltage is fixed, so the setting of the breaker or fuse is the normal current of the load. If the load exceeds the rated current, then the breaker or fuse will operate and limit the load.
44. WHAT IS THE VOLTAGE USED IN A MEGGAR?

500V DC. Q134. HOW DOES CURRENT FLOW DURING WELDING? Ans: The current flows through the electrode and to the clamp. That’s how it flows in MIG, stick, flux cored. On Tig, the electricity flows through the electrode also except that now that’s the only function of it, the rod is held in your hand and electrode in the other.

45. WHAT IS THE VOLTAGE USED FOR IGNITION IN BOILERS/INCINERATORS ETC.?

Normally 10,000V (10kV).

46. WHY IS IT DIFFERENT FROM THAT FOR WELDING?

It is higher as the ignition requires break down of air gap for park to occur. The air breaks down at this high voltage of 10kV. Therefore, it is higher than normal welding voltage.

47. IF SO WHY DON’T YOU GET A SHOCK?


Because the whole hull is at the same potential, so there is no potential difference to get a shock.

48. WHAT IS SAFE VOLTAGE TO PREVENT SHOCK?

Normally 50V. It depends on the body resistance and how conducting the path is (example use of insulating boots or mat may allow slightly higher voltage to be safe)

49. WHAT IS THE MEANING OF ACB, MCCB, NFB?

Air circuit breaker for 400V higher current ratings. Moulded case circuit breaker for 400V medium current ratings. No Fuse Breaker – 75 A capacity

50. WHAT IS A MAGNETIC CONTACTOR? WHERE IS IT USED?


A contactor is an electrically controlled switch used for switching a power circuit, similar to a relay except with higher current ratings. Contactors are used to control electric motors, lighting, heating, capacitor banks, and other electrical loads.
51. WHAT IS A RELAY? HOW IS IT DIFFERENT FROM A MAGNETIC CONTACTOR?

Contactor is a heavy duty switch whereas the relay is light duty switch. Relays can be as simple as 12 V DC solenoids with 12 V, 0.5 A rated contacts. They are used to detect faults on the power system and signal a local or remote switching device (breaker) to trip the circuit.

52. WHAT IS THE MEANING OF OVERLOAD?


An overload is a current over and above the normal load current (over the load). In other words greater than the original design current. This can be a momentary overload such as the starting current for a motor which is usually ignored or it can be a sustained overload such as plugging in too many appliances at once or a motor that has faulty bearings or gets jammed

53. WHAT IS MEANING OF SHORT CIRCUIT?

A short circuit occurs when conductors are connected (shorted) together by a fault. This is normally phase to phase fault due to failure of insulation by something cutting the wires, fires, etc.

54. HOW IS PROTECTION PROVIDED FOR OL AND SC?

Damage from short circuits and overloads can be reduced or prevented by employing fuses, circuit breakers, or other overload protection, which disconnect the power in reaction to excessive current. The tripping time is much less (in milli seconds) for short circuit due to very high currents but can higher (a few seconds) due to currents say 1.1 to 1.5 times the normal current.

55. WHAT IS THE FUNCTION OF A FUSE?

Fuse places a limit on the amount of current that can be drawn by an electric circuit by opening (blowing or melting) when the current exceeds a pre-set limit. This protects the circuit and the surroundings from fire or damage in the case of an overload or short circuit.

56. WHAT IS THE NORMAL SETTING OF OVERLOAD RELAYS?

Normally 90-100% of rated normal current. Sometimes can be set to 105% of rated current.

57. HOW DOES A THERMAL OVERLOAD RELAY WORK?


The bi-metallic strip in the relay heats up by the over load current and the movement in the bi-metallic strip causes the breaker to trip. The breaker has to be reset (turn off) before turning on again. Sometimes it takes a time delay for the bi-metallic strip to cool down after which the breaker can be reset and switched on.

58. WHAT IS SINGLE PHASING?


Single phasing is a condition in three phase motors and transformers wherein the supply to one of the phases is cut off.

59. HOW CAN IT HAPPEN?

Single phasing is caused by the use of single-phase protection devices such as fuses. Three phase loads should be protected by devices which cause the interruption of power to all three phases simultaneously when a fault occurs. Defective contacts or loose connections in three phase breakers can also cause single phasing.

60. WHAT IS THE HARM CAUSED BY THIS?

Single phasing causes higher than rated currents in the healthy phases of loads such as motors causing over heating of the motor and consequently motor failure. Single phasing can sometimes cause excessive noise and vibration in motors.

61. WHAT PROTECTION IS PROVIDED AGAINST THIS?


Single phasing can be identified by special protective relays which can identify and isolate the connected loads. Smaller motors rely on over current and negative phase sequence relays. Motor protection relays for larger motors come readily fitted with protection against single phasing.

62. WHY IS IT NECESSARY THAT INCOMING ALTERNATOR FREQUENCY IS MORE THAN BUS BAR?


Prior to paralleling alternators together, the frequency of the incoming alternator is made slightly higher. This is to ensure that the incoming generator will take some of the bus load. If the incoming generator is slow it will become motorized and will add load onto the bus. If this happens it is possible to cause other generators to trip off line.

63. APART FROM RENEWING BEARINGS IS THERE ANY REASON FOR OPENING UP MOTORS FOR MAINTENANCE?


Insulation checks and renewal (example using insulation spray on stator winding insulation).

64. WHY IS THIS REVERSE POWER USED INSTEAD OF REVERSE CURRENT IN ALTERNATORS?

It is extremely difficult to detect reverse current with an alternating current system, reverse power can be detected and protection can be provided by reverse power relay.

65. WHAT IS THE MEANING OF EXCITATION IN AN ALTERNATOR?


An electric generator or electric motor consists of a rotor spinning in a magnetic field. The magnetic field may be produced by permanent magnets or by field coils. In the case of a machine with field coils, a current must flow in the coils to generate the field, otherwise no power is transferred to or from the rotor. The process of generating a magnetic field by means of an electric current is called excitation.

66. HOW IS IT SUPPLIED?


The rotor’s magnetic field is supplied by a rotor winding energized with direct current through slip rings and brushes.

67. WHAT IS A BRUSH LESS ALTERNATOR?


A brushless alternator is composed of two sections: main alternator and the smaller exciter. The exciter has stationary field coils and a rotating armature (power coils). The main alternator uses the opposite configuration with a rotating field and stationary armature. A bridge rectifier, called the rotating rectifier assembly, is mounted on a plate attached to the rotor. Neither brushes nor slip rings are used, which reduces the number of wearing parts. The main alternator has a rotating field as described above and a stationary armature (power generation windings).

68. HOW IS THE EXCITATION ACHIEVED IN THIS?

Varying the amount of current through the stationary exciter field coils varies the 3-phase output from the exciter. This output is rectified by a rotating rectifier assembly, mounted on the rotor, and the resultant DC supplies the rotating field of the main alternator and hence alternator output. The result of all this is that a small DC exciter current indirectly controls the output of the main alternator.

69. WHAT IS THE MEANING OF RESIDUAL MAGNETISM?

Residual magnetism is a property in which certain amount of excitation remains back in the conductor even after the removal of the magnets.

70. WHY AN AIR COMPRESSOR IS STARTED UNLOADED?


The air pressure inside the compressor cylinder offers resistance to the movement of the piston. Hence started unloaded. Ask 4/E or 2/E in ship.

71. HOW IS THE SPEED OF A 3-PHASE INDUCTION MOTOR VARIED?


The speed of a normal 3-phase induction motor is a function of the frequency of the supply voltage. Changing the speed of such a motor hence requires building a 3-phase power frequency convertor. This can be realised by using power MOSFETs (or IGBTs) capable of handling high voltages and fast switching speeds.

72. HOW IS THE DIRECTION OF ROTATION VARIED?


By changing the phase sequence (from say R-Y-B to R-B-Y) Q164. HOW DOES THIS CAUSE A ROTATION TO CHANGE? Ans: Due to the change in phase sequence, the rotating magnetic field changes direction and the rotor rotates in the opposite direction. Q165. HOW IS THE SPEED OF A DC MOTOR VARIED? Ans: To vary the speed of a DC motor we need a constant voltage power supply with a series POT resistor of higher watt rating. A Pulse Width Modulator (PWM) can be used.

73. HOW IS THE DIRECTION VARIED?


A general purpose DC motor can be reversed by changing the polarity of either the armature or the field but not both.

74. WHAT IS A DIODE?


In electronics, a diode is a two-terminal electronic component with asymmetric transfer characteristic, with low (ideally zero) resistance to current flow in one direction, and high (ideally infinite) resistance in the other. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material with a p-n junction connected to two electrical terminals

75. WHAT IS A ZENER DIODE?


A zener diode is a special kind of diode which allows current to flow in the forward direction in the same manner as an ideal diode, but will also permit it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage, “zener knee voltage” or “zener voltage.”

76. WHAT IS A THYRISTOR? WHAT IS AN SCR?


A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current trigger, and continue to conduct while they are forward biased (that is, while the voltage across the device is not reversed). A silicon-controlled rectifier (or semiconductor-controlled rectifier) is a four-layer solid state device that controls current. It is a type of thyristor.

77. HOW DOES AN INVERTER WORK?


An inverter converts DC supply into AC by use of electronic devices such as diodes and thyristors.

78. WHAT IS THE FUNCTION OF THE CHOKE?


The choke has two main functions. In conjunction with the starterit causes the tube to ignite by using the back emf to create a plasma in the tube and it controls the current through the tube when it is ignited.

79. WHERE IS A SLIP RING USED AND WHERE IS A COMMUTATOR USED?


Slip rings are commonly found in slip ring motors, electrical generators for alternating current systems and alternators. They can be used on any rotating object to transfer power. A commutator is a common feature of direct current rotating machines. By reversing the current direction in the moving coil of a motor’s armature, a steady rotating force (torque) is produced.

80. WHAT IS CAPACITANCE?


Capacitance is the ability of a body to store an electrical charge. Anybody or structure that is capable of being charged, either with static electricity or by an electric current exhibits capacitance. C = Q/V where Q is the charge and V is the voltage

81. DOES CURRENT FLOW THOUGH A CAPACITOR?

No. Current (or more specifically, charge) flows into one plate, and an opposing current (charge) flows out of the other plate, but the current (except for leakage current) does not flow across the dielectric. The result is that there is a charge differential between the plates.

82. WHAT IS THE UNIT FOR CAPACITANCE?

Farad

83. WHAT IS A NEUTRAL?

In multiphase circuits, the conductor used to carry unbalanced current. In single-phase systems, the conductor used for a return current path.

84. WHAT IS THE DIFFERENCE BETWEEN NEUTRAL AND EARTH?

The ground or “earth” wire is a circuit’s safety protective wire that normally carries no current.

85. WHAT IS A SYNCHRONOUS CONDENSER?

A synchronous condenser is a device identical to a synchronous motor, whose shaft is not connected to anything but spins freely. Its purpose is not to convert electric power to mechanical power or vice versa, but to adjust conditions on the electric power transmission grid. Its field is controlled by a voltage regulator to either generate or absorb reactive power as needed to adjust the grid’s voltage, or to improve power factor.

86. WHERE AND WHY IS IT USED?

Used for power factor correction as a synchronous capacitor.

87.IF A MOTOR IS WOUND STAR AND YOU WANT TO CHANGE TO DELTA WHAT SHOULD YOU DO?

Open the terminal connection box. Remove the shorting link connecting the common connection (star) point of A2, B2 and C2. With the 3 phase windings marked A

88.WHAT IS RESIDUAL MAGNETISM? WHERE IS IT IMPORTANT?

Residual magnetism is a property in which certain amount of excitation remains back in the conductor even after the removal of the magnets. It is required as we need some magnetism to start off the generation before the field winding has power to produce the full field. Sometimes, when you change the running direction of a E/R blower (from supply to exhaust) it trips. Why? If the flaps of the blower are not set or opened properly or the filters are clogged, the air supply is affected and the blower may be overloaded causing it to trip. Ask E/O for more information.

89. WHAT IS THE REGULAR MAINTENANCE DONE ON BATTERIES?

Every week the cell voltage is checked. The specific gravity is monitored using the battery hydrometer. The terminals are lubricated to prevent corrosion. Sometimes load tests are done on the batteries.

90. WHAT SAFETY PRECAUTIONS NEED TO BE TAKEN DURING MAINTENANCE ON BATTERIES?

The room needs to be ventilated to prevent explosions due to gases generate by the battery. Protective clothing, gloves and eye protection should be used to prevent acid splash over. No flames or ignition sources should be brought near batteries. Never short both positive and negative terminals as this will cause short circuit and high energy flashover. When disconnecting batteries, always disconnect negative terminals first and vice versa when reconnecting.

91. WHY IS SPECIAL LIGHTING USED IN BATTERY ROOMS?

Spark proof lighting is used as explosive gases may be generated during battery charging and so there should be no sparks in the room.

92. WHAT IS THE VOLTAGE AVAILABLE FROM EACH CELL IN A LEAD ACID BATTERY?

2 V DC.

93. WHAT IS THE FULL BATTERY VOLTAGE? Ans: 24 V DC. Q189. HOW IS THIS VOLTAGE ACHIEVED?

By connecting 12 cells in series. 12 x 2 V = 24 V is total battery bank voltage.

94. WHAT IS THE MEANING OF A NORMALLY OPEN CONTACT?

The contact is open in the normal (de-energised condition).

95. HOW DOES A SALINOMETER FUNCTION?

It detects the salinity in parts per million (ppm) and provides a alarm signal and also initiates a command to the bypass valve of a fresh water generator. Ask E/O and look in your ship Fresh water generator manual for more details.

96. HOW DOES THE AIR COMPRESSOR START AND STOP AUTOMATICALLY?

The air reservoir has pressure switch which is set for cut-off and cut-in depending on the pressure desired. When the pressure drops below the cut-in setting, the switch energises the contactor coil and the compressor starts. When the pressure reaches the cut-off setting, the switch opens its closed contact in series with the power circuit to contactor and cuts supply to the contactor.

97. HOW IS THE COLD ROOM TEMPERATURE MAINTAINED?

The cold room has temperature switch (thermostat) which has a cut-in and cut-off temperature setting. Depending on the setting, the compressor cuts when the temperature rises and cuts-off when the temperature cools down to the set value.

98.WHAT IS A SELF-MONITORING ALARM CIRCUIT?

It self-monitors the health of the alarm circuit. That is, it senses whether the power supply to the alarm circuit is healthy and all the relays and contacts are functioning normally. Check with E/O for more details. Out at sea, if there is a black out during your watch, what action will you take? After a black out the emergency generator comes on; On restoring the main supply we are closing the circuit breaker of the main generator without bothering about synchronizing. How is this possible? There is a power link connection breaker between the main generator and the emergency generator. During black out, this breaker trips. The emergency generator starts and its breaker closes and feeds only the emergency loads. During restoration, the main generator is started again and its breaker is closed. As the link breaker is still open, there is no paralleling between main and emergency generator.

99. HOW DOES THE EMERGENCY GENERATOR START AUTOMATICALLY?

It is activated by a under voltage relay. When there is blackout, the under voltage relay senses loss of voltage and starts up the emergency generator. Similarly when the power is restored, the relay stops the emergency generator.

100. IF ALL YOUR AIR BOTTLES ARE AT LOW PRESSURE AND THERE IS A BLACKOUT HOW DO YOU START THE GENERATORS?

If main air bottles are empty, the emergency air bottle can be used to start the main generator. If even the emergency air bottle is empty, then it can be filled by running emergency air compressor (powered by emergency generator). Once the emergency air bottle is filled, this air can be used to start the main generator.

101. WHAT IS A SHAFT GENERATOR?

A shaft generator is coupled to the main engine. It uses the main engine as its prime mover and has a frequency converter (thyristor controlled) that converts the variable engine speed to near constant speed and produces electrical power. It can only be employed at sea speed (full speed) and not at manoeuvring speed.

102.EVEN THOUGH THE MAIN ENGINE RUNS AT VARYING SPEEDS, HOW DOES THE ALTERNATOR MAINTAIN CONSTANT FREQUENCY?

By use of thyristor controlled frequency (rpm) converters.

103.WHY IS 440 V USED FOR MOTORS AND 110/220 V USED FOR LIGHTING?

Motor load currents are large. Motors are 3-phase loads. A higher 440V voltage means lesser current for the same power and hence losses and size of cable wires is lower. Lighting is single phase loads and its load currents are small, hence lower voltage means less insulation in the cable wires.

104. FOR A GIVEN LINE VOLTAGE, FOUR HEATING COILS WILL PRODUCE MAXIMUM HEAT WHEN CONNECTED IN-

Parallel

105. IN PRACTICE, EARTH IS CHOSEN AS A PLACE OF ZERO ELECTRIC POTENTIAL BECAUSE IT-

Has almost constant potential

106. CURRENT CARRIED BY EACH OF THE TWO LONG PARALLEL CONDUCTORS IS DOUBLED. IF THEIR SEPARATION IS ALSO DOUBLED, FORCE BETWEEN THEM IS

Increase two fold

107. THE DIRECTION OF INDUCED EMF CAN BE FOUND WITH THE HELP OF – Ans: Lenz’s Law Q206. PERMANENT MAGNETS ARE GENERALLY MADE OF-

Alnico alloys

108.The rate of rise of current through an inductive coil is maximum-

At the start of the current flow

109.What is power factor? What are the ways to improve it?

Ration of true power and apparent power. Using capacitors and synchronous motors.

110.What is star / delta?

In star VL = 3 Vph In delta VL = Vph

111.WHY IS A CAPACITOR USED IN SINGLE PHASE MOTORS, FLUORESCENT TUBE LIGHTS AND ELECTRONIC CIRCUITS.

To provide a 900 phase shift to the incoming supply necessary for starting the motor. To provide boost voltage for starting, power factor correction and to minimize radio interference. For stabilizing voltage, filtration and coupling.

112.WHAT IS A SOLENOID?

Single layered coil.

113.WHAT IS A PHOTO CELL?

A cell whose resistance varies with intensity of light.

114. WHAT IS A MAGNETIC PICK-UP?

An electromagnet used as a speed or position sensor.

115.WHAT IS AN ELECTRIC TACHOMETER?

A generator producing voltage proportional to speed.

116.WHAT IS AN OVERLOAD RELAY?

A safety trip for an electrical equipment which carries over-rated current.

117. WHAT IS COLOUR CODE OF RESISTORS, PLEASE EXPLAIN?

Colours to denote value. In four band colour coding, the first band indicates the first digit, the second band: the second digit and the third band: the number of zeroes. The values are as given below: Black : 0, Brown : 1, Red : 2, Orange : 3, Yellow : 4, Blue : 5, Green : 6, Violet : 7, Grey : 8, White : 9.

118. WHAT IS RESIDUAL MAGNETISM?

Magnetism remaining in a Ferro magnetic material after the removal of magnetizing force.

119. EXPLAIN WHAT HAPPENS IF A DC MOTOR IS FED WITH AC SUPPLY?

Motor will run at low speed, sparking at brushes, heat due to eddy current will finally burn the motor.

120. WHAT ARE THE BASIC PARTS OF A DC MOTOR?

Rotor and stator, field poles, commutator, brushes, fan impeller, bearing & housing, end covers and name plate.

121. WHAT ARE THE EXCITATION METHODS USED IN AN ALTERNATOR?

Rotary : Using rotating diode rectifiers, primary exciter and main exciter. Static : Excitation given by brushes and slip rings.

122. WHAT ARE THE PARTS OF A BRUSHLESS AC GENERATOR?

Yoke, armature, stator, rotor, primary exciter, main exciter, rotary diode rectifier assembly, impeller, bearing & housing, end cover, terminal block, AVR.

123. HOW CAN THE DIRECTION OF ROTATION OF AN AC MOTOR BE REVERSED?

By interchanging any two supply lines.

124. WHAT CAN BE POSSIBLE REASONS FOR A MOTOR FAILING TO START?

No power , Fuse blown , Overload trip, contactor coil burnt , Contactors contacts bad, control circuit relay faulty, stop switch open circuit and start switch does not operate, hold on contact does not make and motor burnt.

125. WHAT IS MEANT BY ‘SINGLE PHASING’?

When one supply is open circuit in a three phase motor.

126.WHERE IS A ROTOR WOUND AC MOTOR USED?

For starting heavy torques, mostly used on portable machines and small winches.

127.WHAT ARE THE DIFFERENCES BETWEEN SYNCHRONOUS AND INDUCTION MOTOR?

Synchronous motor: Constant speed on all loads, can be operated on a wide range of power factors, not self-starting, requires dc excitation. Induction Motor: Speed varies with load, operates on lagging power factor, self-starting, no dc excitation required.

128. WHAT IS THE SPECIFIC GRAVITY OF ELECTROLYTE USED IN LEAD ACID BATTERY?

1280 at full charge and 1180 at discharge.

129. COMPARE BETWEEN LEAD ACID AND NICKEL-CADMIUM BATTERIES?

Lead acid : Dilute H2SO4, spongy and PbSO2, low efficiency, 2 volt per cell, less strong, requires more maintenance, less efficient wrt temperature, discharges fast, low cost, problem of sulfation. NiCd : KOH, Ni & Cd, high efficiency, 1.2V per cell, robust, less maintenance, more efficient wrt temp difference, retains charge for longer periods, expensive, no sulfation.

130. WHAT IS A CURRENT TRANSFORMER AND VOLTAGE TRANSFORMER?

Current transformers used for sensing load currents and as inputs to indicating instruments. Voltage transformers used for step-up / step-down voltages.

131. WHAT ARE THE SAFETY PRECAUTIONS WHEN WORKING ON ELECTRICAL EQUIPMENTS?

Switch off power, wear proper protective clothing, safety shoes, Notice board : Do not switch on – Men at work, stand on rubber mat, remove metal rings, watch bracelets, use proper insulated tools, check power supply with voltmeter and double check the voltmeter with known power supply, keep one person stand-by with proper safety gears, inform authority and get proper work permit. In case of electrical shock, give proper first aid, check heart beat and pulse.

132. EXPLAIN HOW THE EMERGENCY GENERATOR STARTS UP IN THE EVENT OF TOTAL POWER FAILURE

The start up of the emergency generator is initiated by an electrical relay which monitors the normal mains power supply. Falling mains frequency or voltage causes the ‘start up’ relay to operate the generator starting equipment. The prime mover may be electrically cranked for its own 24 v battery and starter motor or air started from its own air reservoir fitted local to the generator engine. A manual start up may also be initiate by push buttons in the main control room and in the emergency generator room. Also when power loss occurs, the breaker feeding the emergency switch board from the main switch board opens. This breaker is interlocked with the emergency generator breaker which is normally open under normal circumstances but in the event of total power failure this breaker will close when the breaker feeding the emergency generator will feed the emergency switch board

133. NAME THE TRIPS FOUND ON A GENERATOR CIRCUIT BREAKER

Over current, under voltage, reverse power

134.WHEN WORKING ON BATTERIES, WHAT PRECAUTIONS SHOULD BE TAKEN?

Ensure the space is well ventilated, do not smoke or use naked lights and also wear protection clothing such as apron, gloves and face shield.

135.WHAT IS MEANT BY THE TERM TYPE EX’P?’

ex’p’ – pressurization This is where equipment is pressurized to prevent any gases entering equipment

136.WOULD YOU ENTER AN ENCLOSED SPACE IF THE O2 LEVELS WERE RECORDED AS 19% OR BELOW?

No

137. WHAT IS THE PURPOSE OF THE OVER CURRENT PROTECTION TRIP?

The purpose of the over current protection trip is to trip the generator in over load situations. The trip is usually set at 150% operating capacity and has a time delay of usually 20 sec. this allows for short periods of overload currents. If also protects against a short circuit in the generator

138.WHAT IS DONE TO ENSURE EMERGENCY GENERATOR IS ALWAYS AVAILABLE AND WILL START?

The system should be checked regularly and operated weekly to ensure its 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 charged fully or air receiver full

139.DESCRIBE HOW A SELF EXCITED A.C.GENERATOR WOULD WORK.

The self-excited a.c. generator basically consists of a synchronies motor where 3 phase supply is generated an a.c. exciter and a rotary rectifier and a static excitation device. The rotating diodes change a.c. into D.C. for the d.c. excitation current, which is supplied through bushes. The exciter in conjunction with the AVR is used to monitor the correct voltage under load changes.

140.HOW DO YOU ENSURE THAT BATTERIES ARE ALWAYS IN GOOD WORKING ORDER?

Where lead acid batteries are used they require a constant trickle charge, but for both types of batteries a regular charge of up is necessary. Check state of battery reaching a hydrometer; lead acid – specific gravity 1270. Alkaline is 1190 but SB for alkaline batteries does not change much during charge or discharge Ensure cells are covered with electrolyte if not top up with distilled water. Keep batteries dry and clean Terminals should be smeared with petroleum jelly

141.HOW WOULD YOU KNOW ABOUT 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 breaker were being opened and closed as they may be supply essential at the time. It would be good practice to start with places such as the galley and laundry where fault are common

142.WHAT CONDITIONS ARE NECESSARY TO PARALLEL TWO GENERATORS?

The speed of both machines must be same i.e., the frequency and the voltages must be same in phase

143.WHAT ARE THE CAUSES OF SINGLE PHASING?

Single phasing can be caused when one of the three back up fuses blows or if one of the conductor conducts is in open circuit.

144. WHAT ARE THE ADVANTAGE 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 effects 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.

145.WHAT IS SEQUENTIAL STARTING?

Sequential starting is the automatic starting of essential equipment when power is returned after a power failure, i.e. equipment such a steering gear

146.DESCRIBE HOW SYNCHRONIZING LAMPS ARE SUED TO PARALLEL A GENERATOR

This is normally used as a backup alternative way to the synchroscope; the lamps are connected between the incoming generator and the bus bars The sequence method is the most preferred one as it displays a rotation of lamp brightness, which indicates whether the incoming machine is running fast (clockwise) or slow (anticlockwise). As with the synchroscope the lamp sequence must appear to rotate slowly clockwise. Correct synchronization occurs when the top or keys lamp is dark and the two bottom lamps are equally bright

147.WHAT GAS IS GIVEN OFF BY BATTERIES WHILE CHARGING?

Hydrogen

148.WHAT ARE THE TWO MAIN TYPES OF BATTERIES COMMONLY USED ON BOARD SHIPS?

Lead acid and alkaline

149. WHAT IS MEANT BY PREFERENCE TRIPPING?

Preference tripping is the tripping of non-essential load 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 load may trip at set intervals depending on how essential the .load first trip 5 sec, second trip 10 sec

150. WHAT ART THE ESSENTIALS SUPPLIED BY THE EMERGENCY SWITCH BOARD?

Essentials are emergency lighting, alarm, communication, water tight doors and other services necessary to maintain safety and to permit safe evacuation of the ship by life boats. Also you will find a steering gear motor, emergency air start compressor, and emergency fire pump motor are supplied from the emergency switch board

151. Describe How An A.C. Motor Works?

Most ac motors on ships and the induction or squirrel cage motor types, the principle of operation for which is as follows. The starter is made up of 3 separately phased windings, to which a 3 phase supply is connected. The rotor has a series of copper conductors along its axis, which are joined by rings at the ends to form a cage. When the motor is started, the rotating magnetic field induces an EMF in the cage and thus a current flow. The current carrying conductor in a magnetic field produces the motor effect, which turns the rotor. The motor speed builds up to a value just less than the speed of rotation of the magnetic field. The motor speed depends upon the EMF induced in the rotor and this depends upon the difference in speed between the conductors and the magnetic field. The number of different fixed speeds is possible by changing the number of poles (conductors).

152.WHAT WOULD YOU DO IF YOU FOUND AN ENGINEER IN ELECTRIC SHOCK?

First raise alarm then isolate or switch off the current. If this is not possible, pull or push the victim away from the source, taking care not to make electrical contact with the victim or the electrical source, i.e., use something non conductive Once the victim has been removed from the electrical source if the breathing is feasible or ceased artificial respiration should be started at once. Delay on starting artificial respiration can prove fatal Note: if artificial respiration is started once 70% of the victims recover. If there is 3 minutes delay only 20% of the victims recover.

153.WHAT IS THE AVR?

The function of the AVR is to regulate the exciter fields 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.

154. WHAT LOADS ARE CONSIDERED AS NON ESSENTIALS?

Loads considered as non-essentials are air conditioning and ventilation, laundry, galley, refrigeration and deck equipment

155. WHAT IS SINGLE PHASING OF MOTOR?

Single phasing is where are the 3 phases supplying the motor becomes disconnected. The motor will continue to run it 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.

156. NAME THE TRIPS FOUND IN A GENERATOR CIRCUIT BREAKER.

 Over current  Under voltage  Reverse power

157. EXPLAIN REGULATION REGARDING POWER SUPPLY FOR STEERING GEAR.

The electrical supply must be from two widely separated supplies i.e., one from the main switch board and the other from emergency board Electrical leafs and fuses are allow 100% over load, giving only short circuit protection If failure does occur on one system, the standby system will cut in within 45 sec The steering motor will have sequential starting.

158.WHAT WOULD YOU CONSIDER A MINIMUM INSULATION READING RESISTANCE?

Insulation resistance must be kept above at least 1 MW, the higher the insulation resistance the better, companies have their own regulation but most companies only allow reading above 5 M ohms

159. WHAT IS MEANT BY THE TERM TYPE EX’E?’

Ex’e’ – increased safety equipment Increased safety equipment is based primarily on the elimination of ‘open sparking’ i.e., equipment does not have contacts

160. WHAT IS THE PURPOSE OF THE OVER CURRENT PROTECTION TRIP?

It is to trip the generator in overload situations. The trip ix usually set at 150% operating capacity and has a time delay of usually 20 sec. this allows for short periods of overload currents. It also protects against a short circuit in the generator.

161. WHAT IS MINIMUM CURRENT CAN BE REGARDED AS FATAL?

A shock current as low as 15 mA ac or dc can be fatal

162. HOW CAN YOU REVERSE THE DIRECTION OF MOTOR?

By swapping any two supply line connections over.

163. WHAT IS LOAD SHARING?

It is equal balance of loads between generators irrespective of load changes.

164. WHEN WORKING ON BATTERIES WHAT PRECAUTIONS SHOULD BE TAKEN?

Ensure the space is well ventilated, do not smoke or use naked lights and also wear protective clothing such as apron, gloves and face shields.

165. WHAT IS THE NORMAL READING ON AND INSULATI0N TEST METER?

Infinity is normal reading, but reading should be kept above 5 ohms

166. WHAT IS MEANT BY PROTECTION DISCRIMINATION?

This is the ability of a protection system to disconnect only faulty circuits and to maintain the electrical supplies to healthy one Discrimination is advised by coordinating the current ratings and the time settings of the fuses and over current relays used between the generator and the load The devices nearest the load having the lowest current rating and shortest operating time; these nearest the generator having the highest current rating and the longest operating time i.e., a motor fault on a centrifugal pump would not cause the generator breaker to trip.

167. WHAT IS THE TERM EX’I?’

Ex’I’ – intrinsically safe Equipment with circuits that do not generate enough heat to ignite a gas. This generally means limiting circuits to less than 30v. 50mA

168.WHY DO YOU REQUIRE EARTH LAMPS ON 220V DISTRIBUTION BOARD WHEN YOU HAVE THEM ON THE MAIN SWITCH BOARD?

You require earth lamps on the 220v distribution board as well the main switch board, due to the air gap in step down transformer i.e., 440v earth lamps cannot detect earth on 220v systems due to the gap

169. WHEN WOULD A STAR/DELTA STARTER USED ON A MOTOR?

Where slow moving high criteria load obis involves the starting time must be considered because of the heating effect of the starting current and in this case a star/delta starter would be used The star/delta starter first connects the starter windings in star and when running charges over to delta. The star connection results in about 58% line voltage being applied to each phase with therefore a reduction in starting current. The starting torque reduced to about 1/3 of its direct on line value

170.WHAT SORT OF STARTING CURRENT DO YOU GET WITH A DIRECT ON LINE STARTER?

6 – 8 times full load current

171. WHAT IS MEANT BY THE TERMS TYPE EX’P’?

ex’p’; – pressurization This is where equipment is pressurized to prevent any gases entering equipment.

172. WHAT IS MEANT BY THE TERM 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 should be classed as Ex’d’.

173.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 switch board If a prime mover failure occurred the generator would act as a motor. The reverse power relay detects the fault and acts to trip the generator circuit breaker.

174. WHAT IS THE PURPOSE OF UNDER VOLTAGE TRIP?

It is fitted to all generator breakers. Its main function is to trip the breaker when severe voltage dip (around 50%0. the under voltage trip on a generator circuit breaker also prevents it trip on a generator circuit breaker also prevents it being closed when the generator voltage is very low or absent

175. WHAT IS THE FUNCTION OF FUSE?

The function of the fuse is to give short circuit protection and also over load protection and operates in milliseconds.

176. 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 eme.

177. 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 normal safe working current and the other against overload, which may also have a time delay.

178. WHAT IS BATTERY USED FOR A BOARD SHIPS?

They are used for supplying essential power to radio equipment, telephone exchange, fire detection, general alarm circuit etc. these system are after supplied from two sets of batteries worked on a regular charge/discharge cycle They are also used as emergency supplies i.e., emergency generator startup and emergency lighting being used in this case in a ‘stand by’ role to give power when main supplies fail

179. WHERE ON THE SHIP IS THE EMERGENCY GENERATOR LOCATED?

The emergency generator is located remotely from the engine room, usually on the Accommodation in deck or at weather deck level or above.

180. WHAT ARE DISADVANTAGES AND ADVANTAGES OF A LEAD ACID BATTERY?

The advantages of a lead acid battery are they require fewer cells to achieve particular voltage an d are reasonably priced The disadvantages are they require regular attention to keep fully charged, always require a trickle charge and they discharge on open circuit

181. EXPLAIN HOW THE EMERGENCY GENERATOR STARTS UP IN THE EVENT OF TOTAL POWER FAILURE?

The startup of the emergency generator is initiated by an electrical relay, which monitors the normal mains power supply. Failing mains frequency or voltage causes the ‘start up’ relay to operate the generator starting equipment. The prime movers may be electrically cranked from its own 24v battery and starter motor or air started from its own air reservoir filled local to the generator engine. A manual start up may also be initiated by push buttons in the main control room and in the emergency generator room Also when power loss occurs, the breaker feeding the emergency switch board from the main switch board opens. This breaker is interlocked with the emergency generator breaker, which is normally open under normal circumstances, but in the event of total power failure, this breaker will close when the breaker feeding the emergency board from the main board opens. Thus the emergency generator will feed the emergency switch board.