Class 4 Elect Oral Que With Ans

7/23/2019 Class 4 Elect Oral Que With Ans

1/15

What does different position of the synchroscope needle mean; what is the difference between 6 o

clock and 12 clock?

6 oclock means that out of synchronisation. We cannot parallel the incoming generator. 12oclock means the perfect synchronised condition (oltage! fre"uency and phase se"uence

match#.

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 waes

is approaching minimum (slip is &ero when the synchroscope is at 12 o$clock# and the oltagedifferential between the phases is minimal (it$s &ero when the synchroscope is at 12 o$clock#. 'ue

to time taken to close the synchronoscope switch! it is generally done at 11 oclock. 'ue to the

small delay to close! by doing at 11 oclock! we are achieing closing close to 12 oclock.

Why is a egger used for insulation test and not a multi meter?

With a megger! a oltage of not less than )** olts '+ is used for testing the insulation

resistance of windings. With a multi,meter! the oltage used is not more than -,olts '+.egger uses high impedance testing and is therefore more accurate than a multi,meter

What are the safeties on /?

+ircuit breakers! fuses and oer current relays are used. 0he 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.

When does eerse power flow?

ituation of a generator is feeding a system through switchgear haing seeral generators

connected in parallel with this generator. 0he flow of current! when the system is runningnormally! is from the generators to the switchgear. f one generator e3periences problems and its

terminal oltage falls below the system oltage! the generator will act as a motor! 4ust as a motor

can act as a generator! and current will flow from the switchgear to the generator. 0his is reersepower. 0he effects can range from minor to e3treme in the eent of a complete mechanicalfailure of the generator which fails.

What is the harm if reerse power flows?5ssume that the generator normally produces an amount of power e"ual to ! and that when

operating as a motor! it will absorb a similar amount! . 0herefore! the net effect on the grid will

be the same as if it e3perienced a step increase in load e"ual to 2. 'epending on the si&e of thegrid and the strength of the remaining generation! a step increase of 2 could result in a

significant fre"uency change on the grid.

0here could be prime moer damage in some instances (especially steam turbines#.

7ow is the protection against reerse power gien?

eerse power protection is used for anti,motoring. 0his function is used for protection of prime

moer not generator. t can cut,off the fuel supply and stop the prime moer.

7ow do you test reerse power trip?


2/15

When two generators are running in parallel and one generator can carry the load! reerse power

trip can be tested by load shifting using goernor control. When the load has shifted sufficiently

and the off loaded generator is carrying a small percentage of load! its breaker trips and fuelsupply to its prime moer cuts off. 0his means reerse power relay has operated. 0he relay can

be tested by simulation (using the test push button on the relay# to see if it initiates a trip signal.

What is the full form of 5+/?

5ir circuit breaker. t is normally used at 8**9 and higher current applications (generator

breakers#.

f you press the 5+/ +lose button on an idle generator what will happen?

:ormally! the breaker wont close until you synchronise! so een if you press the breaker close! it

will not close. 0he breaker has under,oltage protection which will not let you close it.

What is nder,oltage protection?

t preents closure of the breaker by mistake! or the generator that is coming on load duringparallel operation. t also proides protection against loss of oltage while machinery is

connected to the switchboard.

What is the meaning of preferential trip? Why is it proided?referential 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

oerload of the main supply. t lets the critical loads run (like steering gear# and trips the nonessential loads (like 5+ and galley# and is a safety feature.

What is the purpose of the earth fault indication on the switch board?

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

f you get an earth fault alarm what will you do?

0he fault can be inestigated by first identifying areas of ship that hae a high chance of earthfault (e3ample deck lighting or pantry e"uipment# 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 haing earth fault and we can go and repair it.

What is 'irect +urrent?

ni,directional flow of current.

What is alternating current?

/i,directional flow of current in a sinusoidal form with a positie and negatie peak.

What is meant by phase in 5+?0wo or more 5+ oltages or currents that are out of step with each other. 0heir peaks and &ero

points do not match up at the same points in time. 0his is meant by phase.

s there phase in '+?

:o

Why is '+ not much in use now?
http://www.brighthub.com/engineering/marine/articles/23934.aspxhttp://www.brighthub.com/engineering/marine/articles/23934.aspxhttp://www.brighthub.com/engineering/marine/articles/23934.aspx


3/15

5+ generators are a better technology and oltage transformation (by transformers# is easy is

possible in 5+ and not '+.

Where are you likely to see it used?

/attery powered '+ is generally used in naigation! control and safety systems mainly in

naigation! radar! safety systems! antenna and communication systems and emergency lighting.

What does three,phase current mean?

0hree current sinusoidal waeforms e"ual in magnitude but with a phase shift of 12* degrees.

What is rms alue?oot mean s"uare alue is the peak alue diided by s"uare root of 2.

What is the meaning of power factor?ower factor is the cosine of the angle between oltage and current waeform.

What is the usual alue you see onboard?

sually *. losses.

kW k95 3 system power factor

Where does reactie power go?

eactie power is not $lost$. t is deliered to the motor where it sustains the electric field that

enables the motor to conert the real power (electrical# into mechanical tor"ue.

What is a semi conductor?

ubstance as germanium or silicon whose electrical conductiity is intermediate between that of

a metal and an insulator; its conductiity increases with temperature and in the presence ofimpurities

What is doping?


4/15

'oping intentionally introduces impurities into an e3tremely pure (also referred to as intrinsic#

semiconductor for the purpose of modulating its electrical properties. 0he impurities are

dependent upon the type of semiconductor. @ightly and moderately doped semiconductors arereferred to as e3trinsic. 5 semiconductor doped to such high leels that it acts more like a

conductorthan a semiconductor is referred to as degenerate.

What happens to insulation when temperature rises?

Aor insulators! an increase in temperature will cause their resistance to decrease ,which is why

e3cessie temperature is often the main reason why insulation fails

What happens to resistance of conductors when temperature rises?

Aor conductors an increase in temperature will cause their resistance to increase.

What is an induction motor?5n 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 s"uirrel,cage

secondary winding on the other member (usually the rotor# carries the induced current.

What is %slip%?

5n 5+ (5mplitude +urrent# induction motor consists of two assemblies , a stator and a rotor.0he interaction of currents flowing in the rotor bars and the stators$ rotating magnetic field

generate a tor"ue. n 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 tor"ue. 0his speed

difference is called the slip.

What is a synchronous motor?

5 constant,speed motor! the speed being dependent on the fre"uency of the ac supply and thenumber of poles for which it is designed.

Where is it used?

ynchronous motors find applications in all industrial applications where constant speed isnecessary. mproing the power factor as ynchronous condensers. Blectrical power plants

almost always use synchronous generators because it is important to keep the fre"uency constant

at which the generator is connected. @ow power applications include positioning machines!where high precision is re"uired! and robot actuators.

7ow do you use a multimeter?

What is an 59C meter?

0he 5ometer was a /ritish brand of multimeter! latterly owned by egger. t is often called

simply an 59C and deries its name from the first letter of the words amperes! olts!ohms

7ow do you check continuity?

et the multimeter in resistance mode and then check with probes the resistance between thepoints or circuit under check. f the resistance is low or &ero! it means continuity. f it is high or

infinite! it means open circuit.

What limits the electrical load?
http://en.wikipedia.org/wiki/Extrinsic_semiconductorhttp://en.wikipedia.org/wiki/Conductor_(material)http://en.wikipedia.org/wiki/Degenerate_semiconductorhttp://www.engineeringtoolbox.com/electrical-motors-torques-d_651.htmlhttp://www.engineeringtoolbox.com/electrical-motor-slip-d_652.htmlhttp://en.wikipedia.org/wiki/Multimeterhttp://en.wikipedia.org/wiki/Meggerhttp://en.wikipedia.org/wiki/Amperehttp://en.wikipedia.org/wiki/Volthttp://en.wikipedia.org/wiki/Ohmhttp://en.wikipedia.org/wiki/Ohmhttp://en.wikipedia.org/wiki/Extrinsic_semiconductorhttp://en.wikipedia.org/wiki/Conductor_(material)http://en.wikipedia.org/wiki/Degenerate_semiconductorhttp://www.engineeringtoolbox.com/electrical-motors-torques-d_651.htmlhttp://www.engineeringtoolbox.com/electrical-motor-slip-d_652.htmlhttp://en.wikipedia.org/wiki/Multimeterhttp://en.wikipedia.org/wiki/Meggerhttp://en.wikipedia.org/wiki/Amperehttp://en.wikipedia.org/wiki/Volthttp://en.wikipedia.org/wiki/Ohm


5/15

0he aailable power is fi3ed and the oltage is fi3ed! so the setting of the breaker or fuse is the

normal current of the load. f the load e3ceeds the rated current! then the breaker or fuse will

operate and limit the load.

What is the oltage used in a meggar?

)**9 '+.

7ow does current flow during welding?

0he current flows through the electrode and to the clamp. 0hat$s how it flows in D! stick! flu3

cored. Cn 0ig! the electricity flows through the electrode also e3cept that now that$s the onlyfunction of it! the rod is held in your hand and electrode in the other.

What do we use for welding on ships? 5+ or '+?

What is the oltage?

What is the oltage used for ignition in boilersEincinerators etc.?

:ormally 1*!***9 (1*k9#.Why is it different from that for welding?

t is higher as the ignition re"uires break down of air gap for park to occur. 0he air breaks down

at this high oltage of 1*k9. 0herefore! it is higher than normal welding oltage.

'oes current flow through hull during welding?

Fes think.

f so why don$t you get a shock?. /ecause the whole hull is at the same potential! so there is no potential difference to get a

shock.

What is safe oltage to preent shock?:ormally )*9. t depends on the body resistance and how conducting the path is (e3ample use of

insulating boots or mat may allow slightly higher oltage to be safe#

What is the meaning of earthing in a ship?

What is the meaning of tar windingE 'elta winding?

efer 'ennis 0 7all

s the ship$s alternator tar wound or 'elta wound?

efer 'ennis 0 7all

What is the meaning of 5+/! ++/! :A/?5ir circuit breaker for 8**9 higher current ratings.

oulded case circuit breaker for 8**9 medium current ratings.

:o Ause /reaker G H) 5 capacity

What is a magnetic contactor? Where is it used?


6/15

5 contactor is an electrically controlled switch used for switching a power circuit! similar to a

relaye3cept with higher current ratings.

+ontactors are used to control electric motors! lighting! heating! capacitor banks! and otherelectrical loads.

What is a relay? 7ow is it different from a magnetic contactor?+ontactor is a heay duty switch whereas the relay is light duty switch. elays can be as simple

as 12 9 '+ solenoids with 12 9! *.) 5 rated contacts. 0hey are used to detect faults on the power

system and signal a local or remote switching deice (breaker# to trip the circuit.

+an you draw a 'C@ starter circuit?

efer to 'C@ diagram in 'ennis 0 7all

What is the meaning of oerload?

5n oerload is a current oer and aboe the normal load current (oer the load#.

n other words greater than the original design current. 0his can be a momentary oerload suchas the starting current for a motor which is usually ignored or it can be a sustained oerload such

as plugging in too many appliances at once or a motor that has faulty bearings or gets 4ammed

What is meaning of short circuit?

5 short circuit occurs when conductors are connected (shorted# together by a fault. 0his is

normally phase to phase fault due to failure of insulation by something cutting the wires! fires!etc.

7ow is protection proided for C@ and +?'amage from short circuits and oerloads can be reduced or preented by employing fuses!

circuit breakers! or otheroerload protection! which disconnect the power in reaction to

e3cessie current. 0he tripping time is much less (in milli seconds# for short circuit due to ery

high currents but can higher (a few seconds# due to currents say 1.1 to 1.) times the normalcurrent.

What is the function of a fuse?Ause places a limit on the amount of current that can be drawn by an electric circuit by opening

(blowing or melting# when the current e3ceeds a preset limit. 0his protects the circuit and the

surroundings from fire or damage in the case of an oerload or short circuit.

What is the normal setting of oerload relays?

:ormally I*,1**J of rated normal current. ometimes can be set to 1*)J of rated current.

7ow does a thermal oerload relay work?

0he bi,metallic strip in the relay heats up by the oer load current and the moement in the bi,metallic strip causes the breaker to trip. 0he breaker has to be reset (turn off# before turning onagain. ometimes it takes a time delay for the bi,metallic strip to cool down after which the

breaker can be reset and switched on.

What is single phasing?ingle phasing is a condition in three phase motors and transformers wherein the supply to one

of the phases is cut off.
http://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Electric_motorhttp://en.wikipedia.org/wiki/Electric_lightinghttp://en.wikipedia.org/wiki/Electric_heatinghttp://en.wikipedia.org/wiki/Capacitorhttp://en.wikipedia.org/wiki/Fuse_(electrical)http://en.wikipedia.org/wiki/Circuit_breakerhttp://en.wikipedia.org/wiki/Circuit_breakerhttp://en.wikipedia.org/wiki/Overload_protectionhttp://en.wikipedia.org/wiki/Overload_protectionhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Electric_motorhttp://en.wikipedia.org/wiki/Electric_lightinghttp://en.wikipedia.org/wiki/Electric_heatinghttp://en.wikipedia.org/wiki/Capacitorhttp://en.wikipedia.org/wiki/Fuse_(electrical)http://en.wikipedia.org/wiki/Circuit_breakerhttp://en.wikipedia.org/wiki/Overload_protection


7/15

7ow can it happen?

ingle phasing is caused by the use of single,phase protection deices such as fuses. 0hree phaseloads should be protected by deices which cause the interruption of power to all three phases

simultaneously when a fault occurs. 'efectie contacts or loose connections in three phase

breakers can also cause single phasing.

What is the harm caused by this?

ingle phasing causes higher than rated currents in the healthy phases of loads such as motors

causing oer heating of the motor and conse"uently motor failure.ingle phasing can sometimes cause e3cessie noise and ibration in motors.

What protection is proided against this?

ingle phasing can be identified by special protectie relays which can identify and isolate theconnected loads. maller motors rely on oer current and negatie phase se"uence relays. otor

protection relays for larger motors come readily fitted with protection against single phasing.

Why is it necessary that incoming alternator fre"uency is more than bus bar?

rior to paralleling alternators together! the fre"uency of the incoming alternator is made slightlyhigher. 0his is to ensure that the incoming generator will take some of the bus load. f the

incoming generator is slow it will become motori&ed and will add load onto the bus. f thishappens it is possible to cause other generators to trip off line.

What is dark lamp and bright lamp method for synchroni&ing? 7ow is the connection made?Aormerly! three light bulbs were connected between the generator terminals and the system

terminals. 5s the generator speed changes! the lights will rise and fall in intensity at a rate

proportional to the difference between generator fre"uency and system fre"uency. When theoltage at the generator is opposite to the system oltage (either ahead or behind in phase#! the

lamps will be bright. When the oltage at the generator matches the system oltage! the lights

will be dark. 5t that instant! the circuit breakerconnecting the generator to the system may beclosed and the generator will then stay in synchronism with the system.

5part from renewing bearings is there any reason for opening up motors for maintenance?

nsulation checks and renewal (e3ample using insulation spray on stator winding insulation#.

Why is this reerse power used instead of reerse current in alternators?

t is e3tremely difficult to detect reerse current with an alternating current system! reerse

power can be detected and protection can be proided by reerse power relay.

What is the meaning of e3citation in an alternator?

5n electric generator or electric motor consists of a rotor spinning in a magnetic field. 0he

magnetic field may be produced bypermanent magnetsor by field coils. n the case of a machinewith field coils! a current must flow in the coils to generate the field! otherwise no power is

transferred to or from the rotor. 0he process of generating a magnetic field by means of anelectric current is called e3citation.

s the field current in an alternator 5+ or '+ ?

'+. 0o confirm check 'ennis 0 7all.

s the field rotating or stationary?
http://en.wikipedia.org/wiki/Light_bulbhttp://en.wikipedia.org/wiki/Phase_(waves)http://en.wikipedia.org/wiki/Circuit_breakerhttp://en.wikipedia.org/wiki/Electric_generatorhttp://en.wikipedia.org/wiki/Electric_motorhttp://en.wikipedia.org/wiki/Rotor_(electric)http://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Permanent_magnethttp://en.wikipedia.org/wiki/Field_coilhttp://en.wikipedia.org/wiki/Light_bulbhttp://en.wikipedia.org/wiki/Phase_(waves)http://en.wikipedia.org/wiki/Circuit_breakerhttp://en.wikipedia.org/wiki/Electric_generatorhttp://en.wikipedia.org/wiki/Electric_motorhttp://en.wikipedia.org/wiki/Rotor_(electric)http://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Permanent_magnethttp://en.wikipedia.org/wiki/Field_coil


8/15

otating. 0o confirm check 'ennis 0 7all.

7ow is it supplied?0he rotor$s magnetic field is supplied by a rotor winding energi&ed with direct current through

slip ringsand brushes.

What is a brush less alternator?

5 brushless alternator is composed of two sectionsK main alternator and the smaller e3citer. 0he

e3citer has stationary field coilsand a rotating armature (power coils#. 0he main alternator uses

the opposite configuration with a rotating field and stationary armature. 5bridge rectifier! calledthe rotating rectifier assembly! is mounted on a plate attached to the rotor. :either brushes nor

slip rings are used! which reduces the number of wearing parts. 0he main alternator has a

rotating field as described aboe and a stationary armature (power generation windings#.

7ow is the e3citation achieed in this?

9arying the amount of current through the stationary e3citer field coils aries the -,phase outputfrom the e3citer. 0his output is rectified by a rotating rectifier assembly! mounted on the rotor!

and the resultant '+ supplies the rotating field of the main alternator and hence alternator output.0he result of all this is that a small '+ e3citer current indirectly controls the output of the main

alternator.

What is the meaning of residual magnetism?

esidual magnetism is a property in which certain amount of e3citation remains back in theconductor een after the remoal of the magnets.

Why is an air compressor started unloaded?

0he air pressure inside the compressor cylinder offers resistance to the moement of the piston.7ence started unloaded. 5sk 8EB or 2EB in ship.

7ow is the speed of a -,phase induction motor aried?0he speed of a normal -,phase induction motor is a function of the fre"uency of the supply

oltage. +hanging the speed of such a motor hence re"uires building a -,phase power fre"uency

conertor. 0his can be realised by using power CAB0s (or D/0s# capable of handling higholtages and fast switching speeds.

7ow is the direction of rotation aried?

/y changing the phase se"uence (from say ,F,/ to ,/,F#

7ow does this cause a rotation to change?

'ue to the change in phase se"uence! the rotating magnetic field changes direction and the rotor

rotates in the opposite direction.

7ow is the speed of a '+ motor aried?

0o ary the speed of a '+ motor we need a constant oltage power supply with a series C0resistor of higher watt rating. 5 ulse Width odulator (W# can be used.

7ow is the direction aried?
http://en.wikipedia.org/wiki/Slip_ringshttp://en.wikipedia.org/wiki/Field_coilhttp://en.wikipedia.org/wiki/Bridge_rectifierhttp://en.wikipedia.org/wiki/Slip_ringshttp://en.wikipedia.org/wiki/Field_coilhttp://en.wikipedia.org/wiki/Bridge_rectifier


9/15

5 general purpose '+ motor can be reersed by changing the polarity of either the armature or

the field but not both.

What is a diode?

n electronics! a diode is a two,terminal electronic component with asymmetric transfer

characteristic! with low (ideally &ero# resistance to current flow in one direction! and high(ideally infinite# resistance in the other. 5 semiconductor diode! the most common type today! is

a crystallinepiece of semiconductormaterial with a p,n 4unctionconnected to two electrical

terminals

What is a &ener diode?

5 &ener diode is a special kind of diodewhich 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 reerse direction whenthe oltage is aboe a certain alue known as thebreakdown oltage! %&ener knee oltage% or

%&ener oltage.%

What is a thyristor? What is an +?5 thyristor is a solid,state semiconductor deicewith four layers of alternating : and ,type

material. 0hey act asbistableswitches! conducting when their gate receies a current trigger! andcontinue to conduct while they are forward biased (that is! while the oltage across the deice is

not reersed#.5 silicon,controlled rectifier (or semiconductor,controlled rectifier# is a four,layer solid state

deice that controls current. t is a type of thyristor.

7ow does an inerter work?

5n inerter conerts '+ supply into 5+ by use of electronic deices such as diodes and

thyristors.

7ow does a tube light work?

t is well e3plained in 'ennis 0 7allWhen oltageis applied to the fluorescent lamp! here$s what happensK

0he starter (which is simply a timed switch# allows current to flow through the filaments at the ends

of the tube.

0he current causes the starter$s contacts to heat up and open! thus interrupting the flow of current.0he tube lights.

ince the lighted fluorescent tube has a low resistance! the ballast now seres as a current limiter.

When you turn on a fluorescent tube! the starter is a closed switch. 0he filaments at the ends ofthe tube are heated by electricity! and they create a cloud of electrons inside the tube. 0he

fluorescent starter is a time,delay switch that opens after a second or two. When it opens! the

oltage across the tube allows a stream of electrons to flow across the tube and ioni&e the

mercury apor.

Without the starter! a steady stream of electrons is neer created between the two filaments! and

the lamp flickers. Without the ballast! the arc is a short circuit between the filaments! and thisshort circuit contains a lot of current. 0he current either apori&es the filaments or causes the

bulb to e3plode.

What is the function of the choke?
http://en.wikipedia.org/wiki/Electronicshttp://en.wikipedia.org/wiki/Electronicshttp://en.wikipedia.org/wiki/Terminal_(electronics)http://en.wikipedia.org/wiki/Electronic_componenthttp://en.wikipedia.org/wiki/Transfer_characteristichttp://en.wikipedia.org/wiki/Transfer_characteristichttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductancehttp://en.wikipedia.org/wiki/Infinityhttp://en.wikipedia.org/wiki/Crystallinehttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/P-n_junctionhttp://en.wikipedia.org/wiki/Diodehttp://en.wikipedia.org/wiki/Breakdown_voltagehttp://en.wikipedia.org/wiki/Solid_state_(electronics)http://en.wikipedia.org/wiki/Solid_state_(electronics)http://en.wikipedia.org/wiki/Semiconductor_devicehttp://en.wikipedia.org/wiki/N-type_semiconductorhttp://en.wikipedia.org/wiki/P-type_semiconductorhttp://en.wikipedia.org/wiki/Bistablehttp://en.wikipedia.org/wiki/Solid_state_(electronics)http://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Thyristorhttp://science.howstuffworks.com/environmental/energy/question501.htmhttp://en.wikipedia.org/wiki/Electronicshttp://en.wikipedia.org/wiki/Terminal_(electronics)http://en.wikipedia.org/wiki/Electronic_componenthttp://en.wikipedia.org/wiki/Transfer_characteristichttp://en.wikipedia.org/wiki/Transfer_characteristichttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductancehttp://en.wikipedia.org/wiki/Infinityhttp://en.wikipedia.org/wiki/Crystallinehttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/P-n_junctionhttp://en.wikipedia.org/wiki/Diodehttp://en.wikipedia.org/wiki/Breakdown_voltagehttp://en.wikipedia.org/wiki/Solid_state_(electronics)http://en.wikipedia.org/wiki/Semiconductor_devicehttp://en.wikipedia.org/wiki/N-type_semiconductorhttp://en.wikipedia.org/wiki/P-type_semiconductorhttp://en.wikipedia.org/wiki/Bistablehttp://en.wikipedia.org/wiki/Solid_state_(electronics)http://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Thyristorhttp://science.howstuffworks.com/environmental/energy/question501.htm


10/15

0he choke has two main functions. n con4unction 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.

Where is a slip ring used and where is a commutator used?

lip rings are commonly found in slip ring motors! electrical generatorsfor alternating currentsystems and alternators. 0hey can be used on any rotating ob4ect to transfer power.

5 commutator is a common feature of direct currentrotating machines. /y reersing the current

direction in the moing coil of a motor$s armature! a steady rotating force (tor"ue# is produced.

What is capacitance?

+apacitance is the ability of a body to store an electrical charge. 5ny body or structure that is

capable of being charged! either with static electricity or by an electric current e3hibitscapacitance. + LE9 where L is the charge and 9 is the oltage

'oes current flow though a capacitor?:o. +urrent (or more specifically! charge# flows into one plate! and an opposing current (charge#

flows out of the other plate! but the current (e3cept for leakage current# does not flow across thedielectric. 0he result is that there is a charge differential between the plates.

What is the unit for capacitance ?

Aarad

What is a neutral?

n multiphase circuits! the conductor used to carry unbalanced current. n single,phase systems!

the conductor used for a return current path.

What is the difference between neutral and earth?

0he ground or %earth% wire is a circuit$s safetyprotectie wire that normally carries no current.

What is a synchronous condenser?

5 synchronous condenser is a deice identical to a synchronous motor! whose shaft is notconnected to anything but spins freely. ts purpose is not to conert electric powerto mechanical

power or ice ersa! but to ad4ust conditions on the electric power transmission grid. ts field is

controlled by a oltage regulator to either generate or absorb reactie poweras needed to ad4ustthe grid$s oltage!or to improepower factor.

Where and why is it used?

sed for power factor correction as a synchronous capacitor.f a motor is wound star and you want to change to delta what should you do?

Cpen the terminal connection bo3. emoe the shorting link connecting the common connection(star# point of 52! /2 and +2. With the - phase windings marked 5152! /1/2! +1+2! connect52 to /1! /2 to +1 and +2 to 51.

f the 59! is defectie! when additional loads come on will the generator be able to take it?

:o. and refer to 'ennis 0 7all.

7ow do you test reerse power trip! high current trip! preferential trip?
http://en.wikipedia.org/wiki/Slip_ring_motorhttp://en.wikipedia.org/wiki/Electrical_generatorhttp://en.wikipedia.org/wiki/Alternating_currenthttp://en.wikipedia.org/wiki/Alternatorhttp://en.wikipedia.org/wiki/Direct_currenthttp://en.wikipedia.org/wiki/Torquehttp://en.wikipedia.org/wiki/Electric_chargehttp://wiki.answers.com/Q/What_is_the_difference_between_earth_and_neutralhttp://en.wikipedia.org/wiki/Electric_powerhttp://en.wikipedia.org/wiki/Electric_power_transmission_gridhttp://en.wikipedia.org/wiki/AC_power#Real.2C_reactive.2C_and_apparent_powershttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Power_factorhttp://en.wikipedia.org/wiki/Power_factorhttp://en.wikipedia.org/wiki/Slip_ring_motorhttp://en.wikipedia.org/wiki/Electrical_generatorhttp://en.wikipedia.org/wiki/Alternating_currenthttp://en.wikipedia.org/wiki/Alternatorhttp://en.wikipedia.org/wiki/Direct_currenthttp://en.wikipedia.org/wiki/Torquehttp://en.wikipedia.org/wiki/Electric_chargehttp://wiki.answers.com/Q/What_is_the_difference_between_earth_and_neutralhttp://en.wikipedia.org/wiki/Electric_powerhttp://en.wikipedia.org/wiki/Electric_power_transmission_gridhttp://en.wikipedia.org/wiki/AC_power#Real.2C_reactive.2C_and_apparent_powershttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Power_factor


11/15

eerse power trip G refer answer for "uestion 11..

7igh current (oer load trip# G et your oerload relay high current trip setting to say H)J of the

rated current. un the motor. 0he relay will trip the breaker. 5nother way of testing it is pushingthe trip button on the relay which trips the breaker.

referential trip G With one generator running! try to switch on additional loads to oerload the

generator. referential trip will trip all the non,critical loads..

What is residual magnetism? Where is it important?

esidual magnetism is a property in which certain amount of e3citation remains back in theconductor een after the remoal of the magnets.

t is re"uired as we need some magnetism to start off the generation before the field winding has

power to produce the full field.

ometimes! when you change the running direction of a BE blower (from supply to e3haust# it trips.

Why?

f the flaps of the blower are not set or opened properly or the filters are clogged! the air supplyis affected and the blower may be oerloaded causing it to trip. 5sk BEC for more information.

What is the function of the friction clutch mechanism used in purifiers?

t is a mechanical "uestion.

What is the regular maintenance done on batteries?

Bery week the cell oltage is checked. 0he specific graity is monitored using the batteryhydrometer. 0he terminals are lubricated to preent corrosion. ometimes load tests are done on

the batteries.

What safety precautions need to be taken during maintenance on batteries?0he room needs to be entilated to preent e3plosions due to gases generate by the battery.

rotectie clothing! gloes and eye protection should be used to preent acid splash oer. :o

flames or ignition sources should be brought near batteries. :eer short both positie andnegatie terminals as this will cause short circuit and high energy flashoer. When disconnectingbatteries! always disconnect negatie terminals first and ice ersa when reconnecting.

Why is special lighting used in battery rooms?park proof lighting is used as e3plosie gases may be generated during battery charging and so

there should be no sparks in the room.

What is the oltage aailable from each cell in a lead acid battery?2 9 '+.

What is the full battery oltage ?

28 9 '+.

7ow is this oltage achieed?

/y connecting 12 cells in series. 12 3 2 9 28 9 is total battery bank oltage.

What is the meaning of a normally open contact?

0he contact is open in the normal (de,energised condition#.

7ow does a alinometer function?


12/15

t detects the salinity in parts per million (ppm# and proides a alarm signal and also initiates a

command to the bypass ale of a fresh water generator. 5sk BEC and look in your ship Aresh

water generator manual for more details.

7ow does the air compressor start and stop automatically?

0he air reseroir has pressure switch which is set for cut,off and cut,in depending on thepressure 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 thecontactor.

7ow is the cold room temperature maintained?

0he cold room has temperature switch (thermostat# which has a cut,in and cut,off temperaturesetting. 'epending on the setting! the compressor cuts when the temperature rises and cuts,off

when the temperature cools down to the set alue.

What is a self,monitoring alarm circuit?

t self monitors the health of the alarm circuit. 0hat is! it senses whether the power supply to thealarm circuit is healthy and all the relays and contacts are functioning normally. +heck with BEC

for more details.

Cut at sea! if there is a black out during your watch! what action will you take?

5fter a black out the emergency generator comes on; Cn restoring the main supply we are closingthe circuit breaker of the main generator without bothering about synchroni&ing. 7ow is this

possible?

0here is a power link connection breaker between the main generator and the emergencygenerator. 'uring black out! this breaker trips. 0he emergency generator starts and its breaker

closes and feeds only the emergency loads. 'uring restoration! the main generator is started

again and its breaker is closed. 5s the link breaker is still open! there is no paralleling betweenmain and emergency generator.

7ow does the emergency generator start automatically?

t is actiated by a under oltage relay. When there is blackout! the under oltage relay senses

loss of oltage and starts up the emergency generator. imilarly when the power is restored! therelay stops the emergency generator.

f all your air bottles are at low pressure and there is a blackout how do you start the generators?

f main air bottles are empty! the emergency air bottle can be used to start the main generator. feen the emergency air bottle is empty! then it can be filled by running emergency air

compressor (powered by emergency generator#. Cnce the emergency air bottle is filled! this air

can be used to start the main generator.

What is a shaft generator?

5 shaft generator is coupled to the main engine. t uses the main engine as its prime moer and

has a fre"uency conerter (thyristor controlled# that conerts the ariable engine speed to nearconstant speed and produces electrical power. t can only be employed at sea speed (full speed#

and not at manoeuring speed.

Ben though the main engine runs at arying speeds! how does the alternator maintain constantfre"uency?


13/15

/y use of thyristor controlled fre"uency (rpm# conerters.

s the alternator connected star or delta@ook up in 'ennis 0 7all. :ormally star.

Why is 88* 9 used for motors and 11*E22* 9 used for lighting?otor load currents are large. otors are -,phase loads. 5 higher 88*9 oltage means lesser

current for the same power and hence losses and si&e of cable wires is lower. @ighting is single

phase loads and its load currents are small! hence lower oltage means less insulation in the

cable wires.Aor a gien line oltage! four heating coils will produce ma3imum heat when connected in,

arallel

0he unit of absolute permittiity of a medium is,Farad/ metre

130. 0he unit of electric intensity is,

Volt/ metre

131. n practice ! earth is chosen as a place of &ero electric potential because it,

Has almost constant potential

132. +urrent carried by each of the two long parallel conductors is doubled. f their separation

is also doubled ! force between them is

Increase two fold

133. 0he direction of induced BA can be found with the help of ,@en&s @aw

134. ermanent magnets are generally made of,

5lnico alloys

135. 0he rate of rise of current through an inductie coil is ma3imum,

5t the start of the current flow

136. What is power factor? What are the ways to improe it?M

ation of true power and apparent power. sing capacitors and synchronous motors.

137. What is star E delta ?M

n star 9@ - 9

n delta 9@ 9

138. Why is a capacitor used in single phase motors! fluorescent tube lights and electronic

circuits.M0o proide a I**phase shift to the incoming supply necessary for starting the motor.


14/15

o pro!ide "oost !olta#e for startin#$ power factor correction and to minimi%e

radio interference.For sta"ili%in# !olta#e$ filtration and co&plin#.

13'. What is a solenoid?Mingle layered coil.

140. What is a photo cell?M

5 cell whose resistance aries with intensity of light.

141. What is a magnetic pick,up?M

5n electromagnet used as a speed or position sensor.

142. What is an electric tachometer?M

5 generator producing oltage proportional to speed.

143. What is an oerload relay?M

5 safety trip for an electrical e"uipment which carries oer,rated current.

144. What is colour code of resistors ! lease e3plain ?+olours to denote alue. n four band colour coding! the first band indicates the first digit! the

second band K the second digit and the third band K the number of &eroes.

0he alues are as gien belowK/lack K *! /rown K 1! ed K 2! Crange K -! Fellow K 8! /lue K )! Dreen K 6! 9iolet K H! Drey K

145. What is residual magnetism?M

agnetism remaining in a Aerro magnetic material after the remoal of magneti&ing force.

146. B3plain what happens if a '+ motor is fed with 5+ supply?otor will run at low speed! sparking at brushes! heat due to eddy current will finally burn the

motor.

147. What are the basic parts of a '+ motor?Motor and stator! field poles! commutator! brushes! fan impeller! bearing N housing! end coers

and name plate.

148. What are the e3citation methods used in an alternator?M

otary K sing rotating diode rectifiers! primary e3citer and main e3citer.tatic K B3citation gien by brushes and slip rings.

14'. What are the parts of a brushless 5+ generator?

Foke! armature! stator! rotor! primary e3citer! main e3citer! rotary diode rectifier assembly!

impeller! bearing N housing! end coer! terminal block! 59.

150. 7ow can the direction of rotation of an 5+ motor be reersed?M/y interchanging any two supply lines.


15/15

151. What can be possible reasons for a motor failing to start?

:o power ! Ause blown ! Cerload trip! contactor coil burnt ! +ontactors contacts bad! control

circuit relay faulty! stop switch open circuit and start switch does not operate! hold on contactdoes not make and motor burnt.

152. What is meant by Oingle hasing?MWhen one supply is open circuit in a three phase motor.

153. Where is a rotor wound 5+ motor used?Aor starting heay tor"ues! mostly used on portable machines and small winches.

154. What are the differences between synchronous and induction motor?M

ynchronous motor K +onstant speed on all loads! can be operated on a wide range of power

factors! not self starting! re"uires dc e3citation.nduction otor K peed aries with load! operates on lagging power factor! self starting! no dc

e3citation re"uired.

155. What is the specific graity of electrolyte used in lead acid battery?

12

Documents

Class 4 Elect Oral Que With Ans