Electrical oral question & answers

Electrical oral question & answers

Servicing a motor effected/washed by seawater:

1. Cut out power supply by circuit breaker & taking out fuse. Mark & disconnect supply wire. Took Megger reading & recorded.
2. Take out the motor, open up & dismantle. (Make sure marking on both cover & body)
3. Clean and wash with warm fresh water.( About 180′ F)
4. Cover by canvas, dry with positive ventilation & 500 watt lamp
5. Clean with Electro cleaner.
6. Baking by 500 Watt lamp for few hours.
7. Take Megger reading. (test stable or constant reading) Apply insulation varnish to the winding while warm.
8. Baking & taking the Megger.
9. Reassemble & put back into service.
10. When test run check sound, ampere & temperature.

Safety device on alternator:

1. Heater
2. Cooling fan.
3. Reverse power trip.
4. Preferential trip.
5. Over current trip.
6. Under voltage trip.

Windlass safety device:

1. Slipping clutch.( Fitted between hydraulic motor and gear)
2. Over load trip.

Winch safety device.

1. Over load trip.
2. Centrifugal brake.
3. Magnetic brake.
4. Limit switch for runner, topping & slowing.
5. Hydraulic oil high temperature alarm & cut out.
6. Hydraulic oil over head tank low level alarm.

Megger:

A Megger is an ohmmeter to measure insulation resistance in million of ohms. (Pole to pole, pole to earth). Good insulation has high resistance; poor insulation, relatively low resistance. The actual resistance values can be higher or lower, depending upon such factors as the temperature or moisture content of the insulation (resistance decreases in temperature or moisture).

Purpose of Megger Test:

1. To verify insulation resistance.
2. To detect any insulation fault.

Correct Procedure to Check Megger:

Switch off main switchboard by means of circuit breaker & taking out fuse, put label of MEN AT WORK, disconnect the connection from stator, teat with Megger Meter. Measure during hot condition resistance for accurate reading.

The Megger insulation tester is essentially a high-range resistance meter (ohmmeter) with a built-in direct-current generator. This meter is of special construction with both current and voltage coils, enabling true ohms to be read directly, independent of the actual voltage applied.

This method is non-destructive; that is, it does not cause deterioration of the insulation.

(Image Credit: electrical-engineering-portal.com)

Fig: Megger test instrument hook-up to measure insulation resistance.

The generator can be hand-cranked or line-operated to develop a high DC voltage which causes a small current through and over surfaces of the insulation being tested (Fig). This current (usually at an applied voltage of 500 volts or more) is measured by the ohmmeter, which has an indicating scale.

Safety device on switch board:

1. Circuit breakers
2. Over current relay (OCR) for protection from high current
3. Reverse power trip
4. Preferential trip
5. Under voltage trip
6. Fuse
7. Earth lamp
8. Synchroscope, synchronising lamp.
9. Meter (ampere, frequency, volt, watt)
10. Dead front panel safety device provided on the Main switch board individual panels wherein you cannot open the panel until the power of that panel is switched off.

Shore supply connections

• Where arrangements are made for the supply of electricity from a source on shore or other location a suitable connection box has to be installed in a position in the ship suitable for the convenient reception of flexible cables, it should contain a circuit breaker or isolating switch, fuses, and terminals of adequate size to receive the cable ends.
• For three phase shore supplies with earthed neutral terminals are to be provided for connecting hull to shore earth
• An indicator for shore side connection energised is to be provided.
• A means for checking polarity or phase rotation is to be provided
• At the connection box a notice indicating ships requirements with respect to supply as well as connection procedure.
• Alternative arrangements may be submitted for consideration.

How will you know the shore power supply is correct or not?

• Phase sequence indicator.(inside shore connection box, turn clock wise direction)
•  Operate E/R vent fan & check airflow direction.

When excitation loss:

•  Tapping with hammer to field coil core of excitation motor.
• Energise with battery.

Preferential trip:

 If a generator overload condition develops, its preference overload trip will operate to energise the timing relay. The timing relay then operates to disconnect non-essential services in a definite order and at definite time intervals.

•  None essential (without effecting the ship operation.)           5 sec:
• Essential     (running the ship properly)                                     5 sec:
• Top polarity     (propulsion, navigation)                                   15 sec:

Static electricity:

• Electricity produced on dissimilar materials through physical contact & separation
• Out going material negative, remaining material positive.

Spontaneous combustion:

The ignition of material brought about by a heat producing exothermic chemical actin within the material itself, without exposure to an external source of ignition. (Wet oily rags, wet saw dust)

Armature reaction.

 –  When a D.C generator or a motor is operating with load, sparking between the carbon brushes and the commutator results due to Armature Reaction. Armature reaction takes place when the armature current flows in the armature.

–  When current flows in armature, the armature core is magnetising by this current. Magnetising effect of the armature current can be divided into two effects: –

• Cross magnetising or distorting effect.
• Demagnetising or weakening effect.

To compensate the Armature reaction, modern Electrical Machines are provided with

• Interpoles
• Carbon brush rocker.
• Neutralising Winding.

A.C motor starters: 

A motor starter is an apparatus used for controlling the starting of an electric motor.

1. Direct on line starter.(without current limiting element)
2. Resistor or rheostatic starters.(with current limiting device)
3. Star delta starter.
4. Auto Transformer starter.(reduced voltage starting 55%, 60%, 70%, full voltage running
5. Rotor resistance starter.(starting slip ring induction motor)

Dash pot 

Mechanical device to produce a time delay action. Operation of switch gears, arc lamps, motor starters, Electro magnetic bake, etc. Time delay is adjusted by oil viscosity used.

Earth detecting lamps

• The earth fault detector consists of three incandescent lamps which are connected in star. These three lamps are supplied from the secondaries of three single phase step down transformers. The primaries of three transformers are connected in star. The star point of the primaries is connected the frame of the ship. The primaries of these transformers are fed from the red phase, yellow phase and blue phase of the ship supply.
• Giving visual signals and buzzer will sound when there is an Earth fault in the system on board.
• Three incandescent lamps which are connected in star. These three lamps are supplied from the secondaries of three single-phase step down transformers.
• The primaries of these three transformers are connected in star. The star point of the primaries is connected the Frame of the ship. The primaries of these transformers are fed from the Red phase, Yellow Phase and Blue Phase of the ship supply main.
• No Earth Fault in the system, Lamps will glow with equal brightness.
• When earth fault occurs one of the phases, the lamp on that phase will become dark and other two lamps will burn with extra brightness.
• Earth fault can be traced by switching off the branch circuit breaker one by one. When the branch circuit with the fault is switched off, the earth lamp will return to its normal glow.

Emergency power supply:

1. Emergency lights.
2. Navigation lights.
3. International communication equipment.
4. Day light signalling lamp.
5. Ship’s whistle.
6. Fire detecting and alarming installation.
7. Manual fire alarm.
8. Other internal emergency signal.
9. Emergency fire pump.
10. Steering gear.
11. Navigation aid and other equipment.

Single phasing:

• Single phasing is the term used to denote the condition arising in a three-phase circuit when one phase becomes open circuited.
• The open circuit in a phase, often from a blown fuse, faulty contact or broken wire, will prevent a motor from starting but running motor may continue to operate with a fault.

1. Excessive current in the remaining supply cable.
2. Unequal distribution of current in motor winding.

• Can be detected by overload device in the supply line or through the overheating.
• Overheating in a stalled or running motor will cause burn out of the overloaded coil.
• In a lightly loaded motor to remain undetected by electromagnetic trips on the supply line which monitor only current. Improve protection is given by thermisters placed in the winding to measure thermal effects.

Insulated neutral system

Advantages

• This system avoids the risk of loss of essential services e.g. steering gear
• If the neutral was earthed and a short circuit on one phase causes the fuse in that phase to blow the system would now be singled phasing and may burn out motors
• In an insulated neutral, one earth fault does not interrupt the supply but an earth leakage detection system will give warning.
• Low earth fault currents in insulated systems gives a much less fire risk.

Disadvantages

• On the insulated system the voltage to earth is 1.73 Vph e.g. 440v vs 250v
• Tracing an earth fault is more difficult because although selective tripping may trace the earthed circuit, the actual position on the circuits may still be difficult to locate. Resonant or intermittent faults in say a contactor solenoid or a transformer with an insulated neutral can cause voltages to be magnified to say 4 times the normal voltage to earth (250v x 4 = 1000v)

Note: electrical shock is not reduced by using a non-earthed neutral as large voltages are involved. Both systems are equally dangerous

Earthed neutral system

When an earthed neutral system of generation is used earthing is to be through a resistor. The resistor is to be such that it limits the earth fault current to a value not greater than the full load current of the largest generator on the switchboard section and not less than three times the minimum current required to operate any device against

Back E.M.F

•  When a current flows through the conductors of an armature of a motor, a force will be exerted on the conductors. This force produces a torque, which will cause the armature of the motor to rotate.
• When the armature conductors are moving across the magnetic field, these conductors will cut the magnetic lines of force and therefore generate an E.M.F. The direction of this generated E.M.F is opposite to the applied voltage of the motor. Since the generated E.M.F in the motor is opposite to the applied voltage to the motor, this E.M.F is called Back E.M.F.

Applied Voltage = Voltage drop to armature resistance + Back E.M.F

V          =                 Ir                                +           Eb

Automatic voltage regulator (AVR):    

Sudden load current surges (sudden great increase) (e.g. due to motor starting) on a generator cause a corresponding change in its output voltage. This is due to an internal voltage drop in the generator ac windings and the effect is usually called ‘voltage dip’.Similarly, load shedding (to get rid of it) will produce an over voltage at the bus-bars. An unregulated or non-compounded generator excitation system would not be realistic on board ship due to the varying voltage caused by the fluctuating load demand. Automatic voltage regulation (AVR) equipment is necessary to rapidly correct such voltage change.

1. Carbon pile regulator.
2. Vibration contact regulator.

The alternator output is transformed, rectified and output effect through magnetic coil against spring which are voltage reference and apply to dc exciter shunt field and rotor of alternator.

3. Static automatic voltage regulator.

The direct current derived from the alternator output through transformer & rectifier, is applied to a bridge which has fixed resistances on two arms and variable resistances (zener diode voltage references) on the other two. When change in applied voltage, by Whetstone bridge produce an error signal. The error signal can be amplified and used to control alternator excitation in different ways. (Thyristor, Transistors in series)

Diode

•   An electronic device that allows electrons to flow in one direction but in highly resistant to current flow in the opposite direction. (Similarly check in the piping system.)
• The purpose of the diode is to function as a rectifier of higher frequency and low frequency oscillation. It is used as detector in a receiver circuit.

1. Thermionic diode consists of a heater, a cathode and an anode, in an indirectly heated valve.
2. Semi conductor diode is constructed with Mono crystalline germanium or silicon wafers with two layers differently doped; ‘pn’ junction acting as rectifier junction.

Transistor

• Three layer device, either NPN or PNP
• It has emitter, collector & base
• Used small signal power from a side circuit controlling the larger power in other circuit. (Power gain or amplification)

Thyristor

• Power control element. Multi -layer device of alternate P and N junction.
• E.g.         Silicon controlled rectifiers (SCRs) ~ Four layer device.

Triacs ~ Greater number.

• Thyristors are solid state switches, which are turned on by application of a low-level signal voltage through a trigger connection known as a gate electrode
• No moving parts to wear, or contacts which can be damaged by arcing.
• Ideal for remote operation. Small size ~ convenient component of control circuit. Can control current greater than 1000 amps and voltage in excess of 1000 volts. Replace large conventional switches.
• Operate faster rate (25,000 times per second).

Exciter:

• It is a small D.C generator. It supplies the exciting current to the rotor of the A.C generator.
• Exciting current is current required to create any magnetic field.

Equaliser

An equaliser is a connection between the generators of different capacities running in parallel so that the running load is divided proportionately between the two.

D.C loads on A.C ship service system:

1. General alarm system.
2. Emergency lighting.
3. Emergency radio power.
4. Rectifying for battery charging.

Maintenance of motors:

1. Keep air passage clean.
2. Remove deposits of dust, oil and grease.
3. Take insulation resistance reading.
4. Prevent sparking at brushes and commutator.
5. Mechanical check at bearings, holding bolts couplings.
6. Regular check on motor starters and controllers, maintenance of contacts, resistance and connection.

Galvanometer:

It is an instrument for measuring the small electric current with its magnitude and direction.

Resistor:

It is a device, which conducts electricity but converts part of the electrical energy into heat.

Resistivity:

It is a resistance of a material and expressed in ohms/unit length.

Rheostat

It is a resistor, which is provided with means for readily adjusting its resistance.

Fire fighting at switchboard

1. First of all the circuit breaker to be break out. Then extinguished with the fire with CO2 portable extinguisher.
2. After fire is stop, the switch board is cool down
3. All wire system must be traced and continuity test.
4. After tracing and renewing all fittings, the switchboard can be put back into service.

Fire Prevention:

1. Use correct fuse.
2. All contacts of circuit breakers and switches are well contact.
3. All wire connections are well lapped.

Causes of Fire:

1. Loose contact.
2. Incorrect fuse.
3. Insulation break down

Residual magnetism:

When starting up a generator, the necessary magnetism is provided by the iron cores of the field poles. This magnetism is known as residual magnetism.

Battery installation and safety measures:

1. Require good ventilation for H2 involving. Require suitable paint to outlet vent ducts. Ventilation inlet should be below battery level.
2. Naked light and smoking are prohibited in battery room.

Lap wound.          Multi parallel circuit in armature.

Wave wound        Two parallel circuit in armature.

Failure to excite:

1. Loss of residual magnetism.
2. Too high resistance in the field circuit.

Universal motor:

1. Series wound.
2. Single phase.
3. Either A.C or D.C can be used.
4. Fractional(very small) power. (Less than 1 HP.)

Used on sewing machine, portable drill, etc.

(When the motor is attached to D.C line, the current is in the same direction all the time. In an A.C  line the current is reversing itself every cycle.)

What are the likely consequences of attempting to close the incomer’s Air Circuit breaker (ACB) when the generators are not in synchronism?

At the instant of closing the breaker, the voltage phase difference causes a large circulating current between the machines which produces a large magnetic force to ‘pull’ the generators into synchronism. This means rapid acceleration of one rotor and deceleration of the other. The large forces may physically damage the generators and their prime movers and the large circulating current may trip each generator breaker. Result? Blackout, danger and embarrassment!

Synchronising:

1. Synchroscope
2. Synchronising lamp:

• ‘Dark’ lamp method. (2 lamps)
• ‘Bright’ lamp method. (2 lamps)
• ‘Sequence’ method.(3 lamps)

How could you monitor the correct instant for synchronising without the aid of a Synchroscope or synchronising lamps?

Connect pair of 500 V voltmeter probes across one phase of the incoming machine circuit breaker. Adjust the generator speed until the voltmeter slowly fluctuates from zero to maximum. Close the breaker when the voltmeter passes through zero.

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).

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 synchroscope is at 12 o'clock) and the voltage differential between the phases is minimal (it's zero when the synchroscope 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.

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

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.

When does Reverse power flow?

Situation of a generator is feeding a system through switch gear 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 switch gear. 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 switch gear 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.

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).

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.

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.

What is the full form of ACB?

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

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.

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.

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.

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

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

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.

What is Direct Current?

Uni-directional flow of current.

What is alternating current?

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

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.

Is there phase in DC?

No

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.

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.

What does three-phase current mean?

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

What is rms value?

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

What is the meaning of power factor?

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

What is the usual value you see onboard?

Usually 0.8.

What is the best value possible?

1 is the best value possible. That is possible with a purely resistive load.

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.

How can it be improved?

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

What is the meaning of saying I and V are in phase?

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

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

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.

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

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.

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

What happens to resistance of conductors when temperature rises?

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

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.

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.

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.

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.

How do you use a multi-meter?

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

How do you check continuity?

Set the multi-meter 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.

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.

What is the voltage used in a meggar?

500V DC.

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.

What do we use for welding on ships? AC or DC?

What is the voltage?

What is the voltage used for ignition in boilers/incinerators etc.?

Normally 10,000V (10kV).

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.

Does current flow through hull during welding?

Yes I think.

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.

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)

What is the meaning of earthing in a ship?

What is the meaning of Star winding/ Delta winding?

Refer Dennis T Hall

Is the ship's alternator Star wound or Delta wound?

Refer Dennis T Hall

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

What is a magnetic contractor? Where is it used?

A contractor is an electrically controlled switch used for switching a power circuit, similar to a relay except with higher current ratings.

Contractors are used to control electric motors, lighting, heating, capacitor banks, and other electrical loads.

What is a relay? How is it different from a magnetic contractor?

Contractor 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.

Can you draw a DOL starter circuit?

Refer to DOL diagram in Dennis T Hall

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

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.

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.

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.

What is the normal setting of overload relays?

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

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

Is the field current in an alternator AC or DC ?

DC. To confirm check Dennis T Hall.

Is the field rotating or stationary?

Rotating. To confirm check Dennis T Hall.

How is it supplied?

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

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).

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.

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.

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.

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.

How is the direction of rotation varied?

By changing the phase sequence (from say R-Y-B to R-B-Y)

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.

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.

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.

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 acrystalline piece of semiconductor material with a p-n junction connected to two electrical terminals

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."

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.

How does an inverter work?

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

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.

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.

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.

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

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.

What is the unit for capacitance ?

Farad

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.

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.

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.

Where and why is it used?

Used for power factor correction as a synchronous capacitor.

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.

If the AVR, is defective, when additional loads come on will the generator be able to take it?

No. and refer to Dennis T Hall.

How do you test reverse power trip, high current trip, preferential trip?

Reverse power trip – refer answer for question 11..

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..

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.

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

It is a mechanical question.

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.

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.

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.

What is the voltage available from each cell in a lead acid battery?

2 V DC.

What is the full battery voltage ?

24 V DC.

How is this voltage achieved?

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

What is the meaning of a normally open contact?

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

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.

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.

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.

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. 

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.

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.

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.

Even though the main engine runs at varying speeds, how does the alternator maintain constant frequency?

By use of thyristor controlled frequency (rpm) converters.

Is the alternator connected star or delta

Look up in Dennis T Hall. Normally star.

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.

For a given line voltage, four heating coils will produce maximum heat when  connected in-

Parallel

The unit of  absolute permittivity of a medium is-

Farad/ metre

130.          The unit of electric intensity is-

Volt/ metre

131.          In practice , earth is chosen as a place of zero electric potential because it-

Has almost constant potential

132.          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

133.          The direction of induced EMF can be found with the help of -

Lenz’s Law

134.          Permanent magnets are generally made of-

Alnico alloys

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

At the start of the current flow

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

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

137.          What is star / delta ?*

In star V_L = Ö3 V_F

In delta  V_L = V_F

138.          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.

139.          What is a solenoid?*

Single layered coil.

140.          What is a photo cell?*

A cell whose resistance varies with intensity of light.

141.          What is a magnetic pick-up?*

An electromagnet used as a speed or position sensor.

142.          What is an electric tachometer?*

A generator producing voltage proportional to speed.

143.          What is an overload relay?*

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

144.          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.

145.          What is residual magnetism?*

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

146.          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.

147.          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.

148.          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.

149.          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.

150.          How can the direction of rotation of  an AC  motor be reversed?*

By interchanging any two supply lines.

151.          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.

152.          What is meant by ‘Single Phasing’?*

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

153.          Where is a  rotor wound AC motor used?

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

154.          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.

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

1280 at full charge and 1180 at discharge.

156.          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.

157.          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.

158.          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.