Question 1: WHAT IS ACTIVE POWER AND REACTIVE POWER? Answers: Consider an ideal alternating current (ac) circuit consisting of a source and a generalized load, where both the current and voltage are sinusoidal.
If the load is purely resistive, the two quantities reverse their polarity at the same time, the direction of energy flow does not reverse, and only real power flows.
If the load is purely reactive, then the voltage and current are 90 degrees out of phase and there is no net power flow.
This energy flowing backwards and forwards is known as reactive power Since there is no any power loss when current flows through the pure inductor or capacitor, but the energy is stored in the form of magnetic field(in inductor) or static field (in capacitor).
Question 2: INSULATED NEUTRAL SYSTEM AND EARTH NEUTRAL SYSTEM. Answers: Insulated Neutral System: When the neutral of the generator is completely insulated from earth or ship`s hull it is called insulated neutral system.
It is very advantageous because it does not pose any danger to human life even if there is a single earth fault because the whole ship will be at same potential leading to no damage to machinery and life. N alarm system will detect this earth fault.
Earthed Neutral System: When the generators neutral is connected to the earth or ship`s hull it is called earth neutral system. A single fault can cause very large current to flow causing damage to the machinery.
But still it is used in HV systems because if there is only one earth fault the machinery will trip and protect the equipment.
Question 3: BRUSHLESS ALTERNATOR Answers: A brushless alternator is composed of two sections:
The 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).
Question 4: SELF-EXCITED GENERATOR. Answers: A kind of excitation of generators where the magnetic field of the main poles is excited by a current supplied to the windings of the main poles from the armature (rotor) winding.
A self-excited generator is thus distinguished from a separately excited generator, in which the windings of the main poles receive current from an external source.
Self-excitation is made use of most often in DC generators. When a selfexcited generator is started, the initial current in the field winding is produced by the electromotive force (emf) induced in the armature winding by the residual magnetic field of the main poles.
To sustain selfexcitation, the initial current must reinforce this field. The additional magnetic flux increases the emf in the armature and, consequently, the current in the windings of the main poles.
Because, however, of magnetic saturation in the magnetic circuit, the increments in magnetic flux corresponding to equal increments in current become smaller as the current builds up.
The process of self-excitation continues as long as the emf in the armature exceeds the voltage drop in the field winding. At a certain magnitude of the magnetic flux, electric equilibrium is reached, and there is no further increase in magnetic flux, armature emf, and excitation current.
Self-excitation can be achieved when the value of the resistance of the field winding does not exceed a certain limit, which depends on the electric parameters of the generator.
Question 5: PARALLING OF A/E MANUALLY. Answers:
All meters and indicators must have in good working order.
Start the incoming generator with the correct staring procedure.
Check the working condition by readings pressure gauges, thermometer and audible and visual.
Watch for a minutes until prime mover come to stable.
Check the voltage of existing and incoming generator on the switchboard,
Move the selector switch to incoming generator. At that time, synchroscope pointer will rotate clockwise or counter clockwise direction. Check the frequency and voltage of existing and incoming.
Carry out the adjustment by means of speed adjuster of prime mover to obtain the condition such that synchroscope pointer rotate in the clockwise direction at a speed of about 4sec/rev.
Close the circuit breaker of incoming generator when the pointer reaches just before 12’ O Clock position.
Made off the selector switch.
Made load shearing the two generators by the speed adjuster of generator.
Question 6: MAINTENANCE OF ALTERNATOR. Answers:
Ventilation passages &air filters to clean.
Insulation resistance to check for Stator, rotor winding. If low, then varnish to be done
Air gap to check using plastic feeler gauge. Bearing oil to be renewed.
Use vacuum cleaner to remove dirt/ dust.
Terminal box cover gasket integrity to check
Terminal connections to check for tightness.
AVR components, diodes to keep free from oil, moisture or any dirt.
Heaters to be checked.
Question 7: ALTERNATOR SAFETIES AND HOW WILL YOU TRY OUT REVERSE POWER TRIP AND WHY REVERSE POWER TRIP? Answers: The three main type alternator protection are:
Over current protection.
Reverse power trip
Under voltage 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. Reverse power trip is provided to protect generator.
Situation of a generator is feeding a system through switchgear having several generators connected in parallel with this generator. The flow of current, when the system is running normally, is from the generators to the switchgear.
If one generator experiences problems and its terminal voltage falls below the system voltage, the generator will act as a motor, just as a motor can act as a generator, and current will flow from the switchgear to the generator.
This is reverse power. The effects can range from minor to extreme in the event of a complete mechanical failure of the generator which fails.
Question 8: SYNCHRONIZING METHODS. Answers: Synchro scope is the main method. Back-up methods are a. Lamp dark method b. Lamp bright method c. Rotating lamp method or Sequence methods (preferable).
Question 9: PARTS OF ALTERNATOR AND ITS CONSTRUCTION. Answers: The main parts of alternator are its stator and rotor. Stator core is assembled from laminated steel with the windings housed in slots around the inner periphery of the cylindrical core. And stator coils are interconnected to form three separate phase winding with six ends.
Two constructional forms of rotor are a. Salient pole type b. Cylindrical type The salient pole type has projecting poles bolted or keyed onto the shaft hub. Field excitation windings are fitted around each pole.
In cylindrical type rotors the excitation windings are wedged into axial slots around the steel rotor. Unwound sections of the rotor form the pole faces between the winding slots.
Question 10: WRITE DOWN THE PROCEDURE FOR IR TEST. Answers: Test procedure
Test your tester.
Connect both probe, press test button, it shows zero.
Connect for proper earthing, to connect the negative terminal to ground and positive also to the ground at a different place test the reading, it must show zero.
Carry out insulation test of the conductor without disturbing the earth (-) terminal of the megger. Connect the (+) probe in to the conductor.
Press the test button note down the reading. It must be more than min. (1.5 m ohm) it should be less, do not operate high voltage machineries. It must be min. (KV + 1) m ohm.
For Example:
If 440 V then IR = 0.44 + 1 = 1.44M ohm. For 11 KW, IR = 11 + 1 = 12 M ohm.
Question 11: WHAT IS AN ALTERNATOR? Answers: An alternator is an electro-mechanical device comprising of stator, rotor winding and an external exciter for supplying excitation voltage. Alternator generates electricity when coupled with a prime mover.
Alternator on a ship is exposed to harsh weather and sea conditions, due to which, its capacity and efficiency tends to reduce.
It is very important to have proper maintenance on the alternator part of the generator as per planned maintenance or as and when required.
Question 12: ALTERNATOR MAINTENANCE Answers: Before starting any maintenance work on the alternator, all safety precaution should be taken and the alternator should be shut and locked down. Also, post notice and ply cards on relevant places and alternator heater to be isolated. Clean the alternator ventilation passage and air filter.
Check the Insulation resistance of stator and rotor winding.
Air gap between stator and rotor to be checked and maintained between 1.5 to 2 mm.
Slip rings to be checked for even wear down to be renewed if required.
Carbon brushes to be clean and checked for free movement.
The brush contacting pressure to be checked by spring balance.
Automatic Voltage Regulator to be checked and cleaned off oil and dust.
The lube oil level of pedestal bearing to be maintained and renewed as per planned maintenance.
A vacuum cleaner can be used to remove dust accumulated in the inner parts of alternator.
The terminal box cover gasket to be checked for proper oil and water tightness.
All the connection in the terminal box to be tightened properly.
Cable gland to be checked for integrity.
Forced Ventilation around alternator must be maintained all the time.
Check heater for proper operation.
The foundation bolts of the alternator to be checked for tightness.
After maintenance is performed, a no load test should be carried out and general condition such as noise, temperature, voltage generated etc. of the alternator should be observed and noted.
Question 13: BRUSHLESS ALTERNATOR WITH DIAGRAM Answers:
A brushless alternator is composed of two alternators built end-to-end on one shaft. Smaller brushless alternators may look like one unit but the two parts are readily identifiable on the large versions.
The larger of the two sections is the main alternator and the smaller one is the 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 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).
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.
AVR
AVR stands for Automatic Voltage Regulator. The AVR maintains a constant or stable output voltage during operation, usually by varying the field voltage.
The voltage sensing unit transforms down, rectifies and smoothest the generator output voltage .this produces a low voltage dc signal that is proportional to the ac generator voltage.
This actual dc signal is compared with a set dc value produced by a reference circuit of zener diodes and resistors.an error signal output from comparator is then amplified and made suitable for driving the field circuit regulating thyristors.
Question 14: HOW DOES REVERSE POWER HAPPEN? Answers:
Reverse power generally describes a condition where the prime mover of a generator is not supplying sufficient torque to keep the generator rotor spinning at the same frequency as the grid to which the generator is connected.
In other words, the generator has actually become a motor and is drawing current from the grid to which is connected and is supplying torque to the prime mover which is supposed to be supplying torque to the generator
Question 15: TWO GEN RUNNING IF ONE GEN LOAD DROPS WHAT IS THE FAULT. Answers: Reasons- Engine is not developing power,some problem with governor. Speed drop set too tight
Question 16: HOW TO SYNCHRONIZE GENERATORS ON A SHIP? Answers: Synchronizing of an incoming generator or alternator is very important before paralleling it with another generator. The synchronizing of the generator is done with the help of synchroscope or with three bulb method in case of emergency.
It is of utmost importance that before paralleling the generators the frequency and voltage of the generators need to be matched.
Synchro scope method
The synchro scope consists of a small motor with coils on the two poles connected across two phases. Let’s say it is connected in red and yellow phases of the incoming machine and armature windings supplied from red and yellow phases from the switchboard bus bars.
The bus bar circuit consists of an inductance and resistance connected in parallel.
The inductor circuit has the delaying current effect by 90 degrees relative to current in resistance.
These dual currents are fed into the synchro scope with the help of slip rings to the armature windings which produces a rotating magnetic field.
The polarity of the poles will change alternatively in north/south direction with changes in red and yellow phases of the incoming machine.
The rotating field will react with the poles by turning the rotor either in clockwise or anticlockwise direction.
If the rotor is moving in clockwise direction this means that the incoming machine is running faster than the bus bar and slower when running in anticlockwise direction.
Generally, it is preferred to adjust the alternator speed slightly higher, which will move the pointer on synchro scope is in clockwise direction.
The breaker is closed just before the pointer reaches 12 o clock position, at which the incoming machine is in phase with the bus bar.
When the synchro scope is approaching 12 o’clock the “slip” (differential) between the sine waves is approaching minimum (slip is zero when the synchro scope is at 12 o’clock) and the voltage differential between the phases is minimal (it’s zero when the synchro scope is at 12 o’clock).
Due to time taken to close the synchro scope 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.
Question 17: EXPLAIN BRUSHLESS ALTERNATORS? Answers: 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).
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.
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.
Basic Theory
When an electric current is passed through a coil of wire, a magnetic field is produced (an electromagnet). Conversely, when a magnetic field is moved through a coil of wire, a voltage is induced in the wire. The induced voltage becomes a current when the electrons have some place to go such as into a battery or other
load. Both of these actions take place in alternators, motors and generators or dynamos. Voltage is generated when a coil of wire is moved through a magnetic field. It doesn’t matter whether the coil is moving or the magnetic field is moving.
Either configuration works equally well and both are used separately or in combination depending on mechanical, electrical and other objectives. The old DC generators (dynamos) used a stationary field and rotating armature.
Automotive alternators use the opposite configuration with a rotating field and stationary armature. In a brushless alternator, both configurations are used in one machine.
Terminology
The stationary part of a motor or alternator is called the stator and the rotating part is called the rotor.
The coils of wire that are used to produce a magnetic field are called the field and the coils that produce the power are called the armature.
Question 18: WHAT IS REVERSE POWER TRIP? Answers:
There is not much difference between an alternator and electric motors from the engineer’s perspective. They are both based on similar principles.
So just imagine what would happen if an alternator suddenly would act as a motor. This is only possible in systems where two or more generators are runningin parallel.
Hence this type of protection system is used only if there is more than one alternator on board a ship. The system is designed in such a way that it will release the breaker and prevent motoring of alternator if a reversal of power occurs.
This protection device is also used to prevent damage to the prime mover, which might be stopped due to some fault. Though 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.
Situation of a generator is feeding a system through switchgear having several generators connected in parallel with this generator. The flow of current, when the system is running normally, is from the generators to the switchgear. If one generator experiences problems and its terminal voltage falls below the system voltage, the generator will act as a motor, just as a motor can act as a generator, and current will flow from the switchgear to the generator. This is reverse power. The effects can range from minor to extreme in the event of a complete mechanical failure of the generator which fails.
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).
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. 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).
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.
Question 19: HOW ELECTRICITY PRODUCED ONBOARD SHIP? Answers: The electrical power demand aboard ship will vary according to ship type and day by day operational needs. The meet of power demand, two or more generator and is backed up by an emergency generator.
Power generation On board
Shipboard power is generated using a prime mover and an alternator working together. For this an alternating current generator is used on board. The generator works on the principle that when a magnetic field around a conductor varies, a current is induced in the conductor.
The generator consists of a stationary set of conductors wound in coils on an iron core. This is known as the stator. A rotating magnet called the rotor turns inside this stator producing magnetic field. This field cuts across the conductor, generating an induced EMF or electro-magnetic force as the mechanical input causes the rotor to turn.
The magnetic field is generated by induction (in a brushless alternator) and by a rotor winding energized by DC current through slip rings and brushes. Few points to be noted about power on board are:
AC, 3 phase power is preferred over DC as it gives more power for the same size.
3 phases is preferred over single phase as it draws more power and in the event of failure of one phase, other 2 can still work
Question 20: WHAT IS THE PROCEDURE FOR TESTING EMERGENCY GENERATOR ONBOARD? Answers: Emergency generator on ship provides power in case the main generators of the ship fails and creates a “dead or blackout condition”. According to general requirement, at least two modes of starting an emergency generator should be available. The two modes should be – battery start and hydraulic or pneumatic start.
Testing of Emergency Generator
The testing of ship’s emergency generator is done every week (as part of weekly checks) by running it unloaded to check ifit starts on battery mode.
The hydraulic start is done every month to ensure that it is working fine. Also every month automatic start of generator is also done to check its automatic operation and to see whether it comes on load
Procedure for Battery Start
Go to the emergency generator room and find the panel for emergency generator.
Put the switch on the test mode from automatic mode. The generator will start automatically but will not come on load.
Check voltage and frequency in the meter.
Keep the generator running for 10-15 min and check the exhaust temp and other parameters. 5 Check the sump level.
6 For stopping the generator, put the switch in manual and then stop the generator
Procedure for Hydraulic Start
Out the switch in manual mode as stated above and check the pressure gauge for sufficient oil pressure.
Open the valve from accumulator to generator.
Push the spring loaded valve and the generator should start.
4 Check voltage and frequency.
Keep the generator running for 10-15 min and check the exhaust temp and other parameters.
Check the sump level
For stopping, use the manual stop button from the panel.
After stopping the generator, pressurize the hydraulic accumulator to desired pressure. 9 Close the valve from accumulator to generator.
Procedure for Automatic Start
For automatic start, we know that there is a breaker which connects Emergency Switch Board (ESB) and Main Switch Board (MSB); and there is also an interlock provided due to which the emergency generator and Main power of the ship cannot be supplied together.
Therefore, we simulate by opening the breaker from the tie line, which can be done from the MSB or the ESB panel.
After opening the breaker, the emergency generator starts automatically with the help of batteries and will supply essential power to machinery and pumps connected to ESB.
For stopping the generator, the breaker is closed again and due to the interlock the generator becomes off load.
Now again put the switch to manual mode to stop the generator.
6 Press stop and the generator will stop
Question 21: HOW TO START DIESEL GENERATOR? Answers: Procedure for preparing a diesel generator for starting
Set the engine to local control.
Set up the fuel oil service system.
Set up the auxiliary fresh water cooling system.
Check the level of oil in – the sump, the governor and the alternator free end bearing and top up if required.
Switch the generator engine pre lubricating oil pump to ‘auto’ operation and check that the lubricating oil pressure builds up. The engine should be pre lubricated at least 2 minutes prior to start. The pre lubricating pump starter is:
mounted on a panel separate from the generator local
control panel near the generator.
Check the pressure before and after the lube oil filters.
Check the air pressure in the starting air receiver.
Turn the engine at least one complete revolution using the turning gear with the cylinder indicator cocks open. Remove the turning gear.
Close the cylinder indicator cocks.
Procedure to start a diesel generator engine locally.
Prior to starting the generator engine following maintenance once all the steps above have been completed bar the engine over manually as follows:
Select the control switch to BLOCKED.
Remove the cover over the flywheel near the starter motor.
Lift the release button on the top of the starter motor of engage the starter motor pinion wheel. The pinion wheel will move forward under spring pressure.
With the indicator cocks open, engage the ratchet spanner fitter with the dedicated engine barring square socket on the starter motor shaft and turn the engine over at least two full turns.
Remove the tool, lift the release push button and push the pinion wheel back until it latches in position and replace the flywheel cover.
Blow through the engine manually from local.
Close the indicator cocks
Select the control switch to the LOCAL position and manually start the engine by pressing the START push button on the local
control panel main starting valve. Allow the engine to run up to normal speed.
Make a thorough check of the engine to ensure that there are no leaks and the engine is running smoothly and firing on all cylinders.
Check that the LO pressures and temperatures are normal.
Check that the pressure drop across the filters is normal.
Check that the FO pressure and temperature are normal.
Change over the control switch to REMOTE
Question 22: WHAT IS THE HARM IF REVERSE POWER FLOWS? Answers:
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).
Question 23: HOW IS THE PROTECTION AGAINST REVERSE POWER GIVEN? Answers: 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.
Question 24: HOW DO YOU TEST REVERSE POWER TRIP? Answers: 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.
Question 25: WHY ARE MOTOR RATINGS GIVEN IN KW AND THAT OF ALTERNATOR AND TRANSFORMER GIVEN IN KVA? Answers:
A volt is not the same thing as watt.
A volt ampere is the unit of measurement of apparent power which is the product of supply voltage and load current.
The Watt on the other hand is the unit of measurement for true power which is the product of supply voltage, load current and the power factor of the load.
The reason that a motor is rated in Watts and not in VA is twofold. Firstly, it is in-phase component of the load (and supply the losses), while the reactive component provides the magnetic field. The in-phase component of current, multiplied by the supply voltage is true power in watts. Secondly the mechanical load of the motor is measured is Watts so it makes an absolute sense to express the output power of a motor in watts too, so that the two can be matched.
A transformer on the other hand is rated accordingly to the product of its rated voltage and rated current, in the other words in terms of its apparent power in volt amperes. This is because the transformer designer has no way to know what type of load (resistive, inductive, capacitive) is going to be connected to his transformer , so has to allow for the ‘worse case’ scenario, so that the windings do not become overheated. The only way to do this is to specify the max. rated current which, when applied by the rated
voltage, results in volt amperes, not watts.
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
Question 26: WHERE DOES REACTIVE POWER GO? Answers: 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.
Question 27: WHAT HAPPENS TO INSULATION WHEN TEMPERATURE RISES? Answers: 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
Question 28: WHAT HAPPENS TO RESISTANCE OF CONDUCTORS WHEN TEMPERATURE RISES? Answers: For conductors an increase in temperature will cause their resistance to increase.
Question 29: WHAT IS AN INDUCTION MOTOR? Answers: 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.
Question 30: WHY IS IT NECESSARY THAT INCOMING ALTERNATOR FREQUENCY IS MORE THAN BUS BAR? Answers: 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.
Question 31: WHY IS THIS REVERSE POWER USED INSTEAD OF REVERSE CURRENT IN ALTERNATORS? Answers: 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.
Question 32: WHAT IS THE MEANING OF EXCITATION IN AN ALTERNATOR? Answers: An electric generator or electric motor consists of a rotor spinning in a magneticfield. 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.
Question 33: HOW IS IT SUPPLIED? Answers: The rotor’s magnetic field is supplied by a rotor winding energized with direct current through sliprings and brushes.
Question 34: WHAT IS A BRUSH LESS ALTERNATOR? Answers: 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).
Question 35: HOW IS THE EXCITATION ACHIEVED IN THIS? Answers: 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.
Question 36: WHAT IS THE MEANING OF RESIDUAL MAGNETISM? Answers: Residual magnetism is a property in which certain amount of excitation remains back in the conductor even after the removal of the magnets.
Question 37: WHAT IS RESIDUAL MAGNETISM? WHERE IS IT IMPORTANT? Answers: 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
Question 38: HOW DOES THE EMERGENCYGENERATOR START AUTOMATICALLY? Answers: 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.
Question 39: IF ALL YOUR AIR BOTTLES ARE AT LOW PRESSURE AND THERE IS A BLACKOUT HOW DO YOU START THE GENERATORS? Answers: 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.
Question 40: WHAT IS A SHAFT GENERATOR? Answers: 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.
Question 41: EVEN THOUGH THE MAIN ENGINE RUNS AT VARYING SPEEDS, HOW DOES THE ALTERNATOR MAINTAIN CONSTANT FREQUENCY? Answers: By use of thyristor controlled frequency (rpm) converters.
Question 42: WHY IS 440 V USED FOR MOTORS AND 110/220 V USED FOR LIGHTING? Answers: 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.
Question 43: WHAT IS RESIDUAL MAGNETISM? Answers: Magnetism remaining in a Ferro magnetic material after the removal of magnetizing force.
Question 44: WHAT ARE THE EXCITATION METHODS USED IN AN ALTERNATOR? Answers: Rotary :
Using rotating diode rectifiers, primary exciter and main exciter.
Static :
Excitation given by brushes and slip rings.
Question 45: WHAT ARE THE PARTS OF A BRUSHLESS AC GENERATOR? Answers: Yoke, armature, stator, rotor, primary exciter, main exciter, rotary diode rectifier assembly, impeller, bearing & housing, end cover, terminal block, AVR.
Question 46: EXPLAIN HOW THE EMERGENCY GENERATOR STARTS UP IN THE EVENT OF TOTAL POWER FAILURE Answers:
The start up of the emergency generator is initiated by an electrical relay which monitors the normal mains power supply. Falling mains frequency or voltage causes the ‘start up’ relay to operate the generator starting equipment.
The prime mover may be electrically cranked for its own 24 v battery and starter motor or air started from its own air reservoir fitted local to the generator engine. A manual start up may also be initiate by push buttons in the main control room and in the emergency generator room.
Also when power loss occurs, the breaker feeding the emergency switch board from the main switch board opens.
This breaker is interlocked with the emergency generator breaker which is normally open under normal circumstances but in the event of total power failure this breaker will close when the breaker feeding the emergency generator will feed the emergency switch board
Question 47: NAME THE TRIPS FOUND ON A GENERATOR CIRCUIT BREAKER Answers: Over current, under voltage, reverse power
Question 48: WHAT IS DONE TO ENSURE EMERGENCY GENERATOR IS ALWAYS AVAILABLE AND WILL START? Answers: The system should be checked regularly and operated weekly to ensure its availability if required. Fuel tanks should be kept full, ample cooling water in radiator cooling system and starting equipment should be functional i.e, batteries should be charged fully or air receiver full
Question 49: DESCRIBE HOW A SELF EXCITED A.C.GENERATOR WOULD WORK. Answers: The self-excited a.c. generator basically consists of a synchronies motor where 3 phase supply is generated an a.c. exciter and a rotary rectifier and a static excitation device. The rotating diodes change
a.c. into D.C. for the d.c. excitation current, which is supplied through bushes. The exciter in conjunction with the AVR is used to monitor the correct voltage under load changes.
Question 50: WHAT CONDITIONS ARE NECESSARY TO PARALLEL TWO GENERATORS? Answers: The speed of both machines must be same i.e., the frequency and the voltages must be same in phase
Question 51: NAME THE TRIPS FOUND IN A GENERATOR CIRCUIT BREAKER. Answers:
Over current
Under voltage
Reverse power
Question 52: WHAT WOULD YOU CONSIDER A MINIMUM INSULATION READING RESISTANCE? Answers: Insulation resistance must be kept above at least 1 MW, the higher the insulation resistance the better, companies have their own regulation but most companies only allow reading above 5 M ohms
Question 53: WHAT IS REVERSE POWER PROTECTION FOR? Answers: Generators intended to operate in parallel must have a reverse power protection trip. A reverse power relay monitors the direction of power flowing between the generator and the switch board.
If a prime mover failure occurred the generator would act as a motor. The reverse power relay detects the fault and acts to trip the generator circuit breaker.
Question 54: WHAT IS THE FUNCTION OF FUSE? Answers: The function of the fuse is to give short circuit protection and also over load protection and operates in milliseconds.
Question 55: EXPLAIN WHAT WOULD HAPPEN IF YOU WERE TO LOSE A GENERATOR Answers: First the standby generator set would start up and automatically put itself on the board. If this did not happen, the ship would ‘black out’ and after a time delay usually 30 seconds the eme.
Question 56: EXPLAIN HOW THE EMERGENCY GENERATOR STARTS UP IN THE EVENT OF TOTAL POWER FAILURE?Answers:
The startup of the emergency generator is initiated by an electrical relay, which monitors the normal mains power supply. Failing mains frequency or voltage causes the ‘start up’ relay to operate the generator starting equipment.
The prime movers may be electrically cranked from its own 24v battery and starter motor or air started from its own air reservoir filled local to the generator engine.
A manual start up may also be initiated by push buttons in the main control room and in the emergency generator room
Also when power loss occurs, the breaker feeding the emergency switch board from the main switch board opens.
This breaker is interlocked with the emergency generator breaker, which is normally open under normal circumstances, but in the event of total power failure, this breaker will close when the breaker feeding the emergency board from the main board opens. Thus the emergency generator will feed the emergency switch board.
