MARINE
ELECTRICAL KNOWLEDGE QUESTIONS & ANSWERS
1. What
is KVA?
Kilo volt-ampere (kVA) is the unit used for
the apparent
power in an AC electrical circuit. It is the product of root-mean-square
(RMS) voltage
and RMS current.
It is given by S
= P + jQ
Where S is the
apparent power in kVA
P is the real
power in kW and Q is the reactive power in kVAr
2. What
is special about Steering gear Overload safety?
Short Circuit
protection and were appropriate-single phase protection is fitted. In addition
instead of over current protection an Overload alarm is fitted set to operate
at not less than twice normal running current.
ALSO ASK YOUR
SHIP E/O FOR INFORMATION.
3. Explain
the condition for Paralleling of Alternators?
The following
conditions must be fulfilled for proper synchronizing of alternators.
The terminal
voltage of the incoming machine must be approximately equal to bus-bar voltage.
The frequency of
the incoming machine must be equal to that of the bus-bar.
In the case of
3-phase alternators an additional requirement is that phase sequence of the
incoming machine voltages must be the same as that of the bus-bars.
4. What
does different position of the synchroscope needle mean; what is the difference
between 6 o clock and 12 clock?
6 o’clock means that out of
synchronisation. We cannot parallel the incoming generator. 12 o’clock means
the perfect synchronised condition (voltage, frequency and phase sequence
match).
5. Why
do we close the switch at 11 o'clock and not at 12 o' clock?
When the synchroscope is approaching 12
o'clock the "slip" (differential) between the sine waves is
approaching minimum (slip is zero when the synch'scope is at 12 o'clock) and
the voltage differential between the phases is minimal (it's zero when the
synch'scope is at 12 o'clock). Due to time taken to close the synchronoscope
switch, it is generally done at 11 o’clock. Due to the small delay to close, by
doing at 11 o’clock, we are achieving closing close to 12 o’clock.
6. Why
is a Megger used for insulation test and not a multi meter?
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
7. What
are the safeties on MSB?
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.
8. When
does Reverse power flow?
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.
9. 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).
10. 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.
11. How
do you test reverse power trip?
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.
12. What
is the full form of ACB?
Air circuit breaker. It is normally used
at 400V and higher current applications (generator breakers).
13. If
you press the ACB Close button on an idle generator what will happen?
Ask E/O. 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.
14. 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.
15. 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.
16. What
is the purpose of the earth fault indication on the switch board?
It detects and indicates phase to earth
fault on a circuit.
17. 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.
18. What
is Direct Current?
Uni-directional flow of current.
19. What
is alternating current?
Bi-directional flow of current in a
sinusoidal form with a positive and negative peak.
20. 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.
21. Is
there phase in DC?
No
22. 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.
23. 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.
24. What
does three-phase current mean?
Three current sinusoidal waveforms equal
in magnitude but with a phase shift of 120 degrees.
25. What
is rms value?
Root mean square value is the peak value
divided by square root of 2.
26. What
is the meaning of power factor?
Power factor is the cosine of the angle
between voltage and current waveform.
27. What
is the usual value you see onboard?
Ask E/O, usually 0.8.
28. What
is the best value possible?
1 is the best value possible. That is
possible with a purely resistive load.
29. 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.
30. How
can it be improved?
Power factor improvement capacitor banks
are used if the loads are mainly resistive.
31. 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.
32. Why
are motor ratings given in KW and that of alternator and transformer given in
KVA?
kW is the output mechanical power of a
motor and is expressed in kW.
kVA is the net (apparent) power input to
the transformer. This input power is the output + losses.
kW = kVA x system power factor
33. 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.
34. What
is a semi conductor?
Substance as germanium or silicon whose
electrical conductivity is intermediate between that of a metal and an
insulator; its conductivity increases with temperature and in the presence of
impurities
35. 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.
36. 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
37. What
happens to resistance of conductors when temperature rises?
For conductors an increase in temperature will cause their resistance
to increase.
38. What is an induction motor?
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?
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.
41. Where
is it used?
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.
42. How
do you use a multimeter?
Ask E/O as you will have to see how to
use it on ship.
43. 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
44. 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.
45. 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.
46. What
is the voltage used in a meggar?
500V DC.
47. How
does current flow during welding?
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.
48. What
do we use for welding on ships? AC or DC?
Ask E/O on your ship.
49. What
is the voltage?
Ask E/O on your ship.
50. What
is the voltage used for ignition in boilers/incinerators etc.?
Normally 10,000V (10kV). But ask E/O to
confirm.
51. 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.
52. Does
current flow through hull during welding?
Ask E/O. Yes I think.
53. If so
why don't you get a shock?
Ask E/O. Because the whole hull is at
the same potential, so there is no potential difference to get a shock.
54. 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)
55. What
is the meaning of earthing in a ship?
Ask E/O
56. What
is the meaning of Star winding/ Delta winding?
Refer Dennis T Hall
57. Is
the ship's alternator Star wound or Delta wound?
Refer Dennis T Hall
58. 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
59. 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.
60. 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.
61. Can
you draw a DOL starter circuit?
Refer to DOL diagram in Dennis T Hall
62. 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
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
63. 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.
64. 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.
65. 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 preset limit. This protects the circuit
and the surroundings from fire or damage in the case of an overload or short
circuit.
66. What
is the normal setting of overload relays?
Ask E/O. Normally 90-100% of rated
normal current. Sometimes can be set to 105% of rated current.
67. 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.
68. 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.
69. 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.
70. 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.
71. 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 overcurrent and negative phase sequence relays. Motor
protection relays for larger motors come readily fitted with protection against
single phasing.
72. 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.
73. What
is dark lamp and bright lamp method for synchronizing? How is the connection
made?
Formerly, three light bulbs
were connected between the generator terminals and the system terminals. As the generator speed changes, the lights
will rise and fall in intensity at a rate proportional to the difference
between generator frequency and system frequency. When the voltage at the
generator is opposite to the system voltage (either ahead or behind in phase),
the lamps will be bright. When the voltage at the generator matches the system
voltage, the lights will be dark. At that instant, the circuit
breaker connecting the generator to the system may be closed and the
generator will then stay in synchronism with the system.
74. Apart
from renewing bearings is there any reason for opening up motors for
maintenance?
Ask E/O. Insulation checks and renewal
(example using insulation spray on stator winding insulation).
75. 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.
76. 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.
77. Is
the field current in an alternator AC or DC ?
DC. To confirm check Dennis T Hall.
78. Is
the field rotating or stationary?
Rotating. To confirm check Dennis T Hall.
79. How
is it supplied?
The rotor's magnetic field is supplied
by a rotor winding energized with direct current through slip rings
and brushes.
80. 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).
81. 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.
82. 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.
83. Why
is an air compressor 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.
84. 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.
85. How
is the direction of rotation varied?
By changing the phase sequence (from say
R-Y-B to R-B-Y)
86. How
does this cause a rotation to change?
Due to the change in phase sequence, the
rotating magnetic field changes direction and the rotor rotates in the opposite
direction.
87. How
is the speed of a DC motor varied?
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.
88. 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.
89. 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
90. 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."
91. 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.
92. How
does an inverter work?
An inverter converts DC supply into AC
by use of electronic devices such as diodes and thyristors.
93. How
does a tube light work?
It is well explained in Dennis T Hall
When voltage
is applied to the fluorescent lamp, here's what happens:
· The
starter (which is simply a timed switch) allows current to flow through the
filaments at the ends of the tube.
· The
current causes the starter's contacts to heat up and open, thus interrupting
the flow of current. The tube lights.
· Since
the lighted fluorescent tube has a low resistance, the ballast now serves as a
current limiter.
When you turn on a fluorescent tube, the
starter is a closed switch. The
filaments at the ends of the tube are heated by electricity, and they create a
cloud of electrons inside the tube. The fluorescent starter is a time-delay switch that opens after a
second or two. When it opens, the voltage across the tube allows a stream of
electrons to flow across the tube and ionize the mercury vapor.
Without the starter, a steady stream of
electrons is never created between the two filaments, and the lamp flickers.
Without the ballast, the arc is a short circuit between the filaments, and this
short circuit contains a lot of current. The current either vaporizes the
filaments or causes the bulb to explode.
94. 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.
95. 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.
96. What
is capacitance?
Capacitance is the ability of a body to
store an electrical charge. Any body 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
97. 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.
98. What
is the unit for capacitance ?
Farad
99. 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.
100.
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.
101.
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.
102.
Where and why is it used?
Used for power factor correction as a
synchronous capacitor.
103.
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 A1A2, B1B2, C1C2, connect A2 to B1, B2 to
C1 and C2 to A1.
104.
If the AVR, is defective, when
additional loads come on will the generator be able to take it?
No. Ask E/O and refer to Dennis T Hall.
105.
How do you test reverse power trip, high
current trip, preferential trip?
Reverse power trip – refer answer for
question 11. Also ask E/O.
High current (over load trip) – Set your
overload relay high current trip setting to say 75% of the rated current. Run
the motor. The relay will trip the breaker. Another way of testing it is
pushing the trip button on the relay which trips the breaker.
Preferential trip – With one generator
running, try to switch on additional loads to overload the generator.
Preferential trip will trip all the non-critical loads. Also ask E/O.
106.
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.
107.
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.
108.
What is the function of the friction
clutch mechanism used in purifiers?
Ask 4/E or 2/E. It is a mechanical
question.
109.
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. Ask E/O also for more information.
110.
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.
111.
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. Ask E/O for more details.
112.
What is the voltage available from each
cell in a lead acid battery?
2 V DC.
113.
What is the full battery voltage ?
24 V DC.
114.
How is this voltage achieved?
By connecting 12 cells in series. 12 x 2
V = 24 V is total battery bank voltage.
115.
What is the meaning of a normally open
contact?
The contact is open in the normal
(de-energised condition).
116.
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.
117.
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.
118.
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.
119.
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.
120.
Out at sea, if there is a black out
during your watch, what action will you take?
Ask your C/E, 2/E or 4/E as they will
give you the best answer based on Univan company policy.
121.
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. Ask E/O
for more details.
122.
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. Ask E/) for more details.
123.
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.
124.
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. Ask E/O and 2/E for more details.
125.
Even though the main engine runs at
varying speeds, how does the alternator maintain constant frequency?
By use of thyristor controlled frequency
(rpm) converters.
126.
Is the alternator connected star or
delta
Look up in Dennis T Hall. Also in your
ship ask E/O. Normally star.
127.
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. Ask E/O
for more information
128.
For a given line voltage, four heating
coils will produce maximum heat when
connected in-
Parallel
129.
The unit of absolute permittivity of a medium is-
Farad/ metre
- The
unit of electric intensity is-
Volt/ metre
- In
practice , earth is chosen as a place of zero electric potential because
it-
Has almost constant potential
- 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
- The direction of induced EMF can be
found with the help of -
Lenz’s Law
- Permanent magnets are generally
made of-
Alnico alloys
- The rate of rise of current through
an inductive coil is maximum-
At the start of the current flow
- What is power factor? What are the ways to improve it?*
Ration of true power and apparent
power. Using capacitors and synchronous
motors.
- What is star / delta ?*
In star VL = Ö3
VF
In delta
VL = VF
- 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.
- What is a solenoid?*
Single layered coil.
- What is a photo cell?*
A cell whose resistance varies with intensity
of light.
- What is a magnetic pick-up?*
An electromagnet used as a speed or
position sensor.
- What is an electric tachometer?*
A generator producing voltage
proportional to speed.
- What is an overload relay?*
A safety trip for an electrical
equipment which carries over-rated current.
- 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.
Black : 0, Brown : 1, Red : 2, Orange : 3, Yellow : 4, Blue : 5, Green : 6, Violet : 7, Grey : 8, White : 9.
- What is residual magnetism?*
Magnetism
remaining in a Ferro magnetic material after the removal of magnetizing force.
- 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.
- 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.
- 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.
- 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.
- How can the direction of rotation
of an AC motor be reversed?*
By interchanging any two supply lines.
- 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.
- What is meant by ‘Single Phasing’?*
When one supply is open circuit in a
three phase motor.
- Where is a rotor wound AC motor used?
For starting heavy torques, mostly used
on portable machines and small winches.
- 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.
- What is the specific gravity of
electrolyte used in lead acid battery?
1280 at full charge and 1180 at
discharge.
- 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.
- 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.
- 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.
good .... helpful for marine engineers. http://thamizhmozhiyan.blogspot.in/
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