Marine alternator oral questions and answers |ETO| Marine Electrical

What will happen if we overexcite the alternator?
ORIf we overexcite what will happen to the power factor?

If the generator overexcited so and it will supply a lagging current to the system. Because the lagging current will produce the flux that will act against the rotor flux to reduce the action.

In Case of overexcitation, we increase the voltage and correspondingly current will increase and rest as same as above.

What is the effect in PF of the bus bar when overexcitation?

Power factor will be lagging because the current increases when excitation increases and loss increases when the current increases. Therefore power factor will be lagging on the bus bar.

Two generators are running parallel with different KW (200,800kw) with unequal kW sharing what is your action?.

The generators provide power in accordance with its rating. The 200kW generator and the 800kW generator (200+800=1000kW and 1000kW / 2 = 500kW) in parallel serving a 500kW load, the serve load is one half of the total source rating. The 200kW generator will be giving 100kW and the 800kW generator will give 400kW.

If kW different we have to adjust the governor droop settings.

What causes the generator under a voltage problem?

Cause 1: Prime speed is low as compared to the rated speed.

Action: Have to adjust the speed of the generator according to the specified rated speed.

Cause 2: Open diode

Action: Change the whole bank of rotating diodes

Cause 3: Ground fault or short-circuiting error especially at the field winding or stator winding.

Action: perform the megger test

Cause 4: Loose connections on the control circuit

Action: Check the connections as per the manufacturer guide.

How to check the air gap and purpose of the air gap?

To prevent the mechanical contact between rotor and stator; required mechanical balance.

To determine the bearing condition.

Measured with the help of plastic feeler gauge and follow the manufacturer procedures.

Generally, the air gap should be 1.5 to 2 mm or follow the manufacturer guidelines.

What is the result of an uneven air gap between rotor and stator of the generator?

Due to uneven air gap- Flux linkage will get distorted.

The generator output voltage will not be pure sine wave due to stator linkage flux will not be a pure sine wave.

Unbalance magnetically will be there.

Due to uneven air gap magnetic pull experienced by the rotor.

Why there are two tie breakers in the circuit diagram of emergency gen auto start?

• One tie-breaker is in the MSB Where we can manually isolate the ESB if we want to do any maintenance or repair work.
• And the other one is interlocked with the main breaker of EM. Gen
• The tie-breaker is an ESB it will not allow the power from ESB to MSB
• It will give the power from only one direction from MSB to ESB
• It will not allow the power from ESB to MSB
• If the main and emergency generator is running this breaker will not switch on
• If it is in MSB it divides MSB into two parts for maintenance purpose or any fire hazard purpose.

What is the role of a Voltage sensing relay in a generator panel?

Voltage sensing relays identify overvoltage, under-voltage, or both. They can only detect an abnormal condition on the line side of where the relay is connected. This allows the device to provide pre-start protection.

Why alternator is connected in star?

#Lesser Stress on Insulation and Copper Saving:

Voltage per phase is less for a given line voltage (V_ph= V_L / root 3) which reduces insulation requirement and this also reduces the number of turns hence copper is also saved.

#Easy Protection/ Neutral:

Neutral grounding is necessary to allow zero-sequence currents to flow to the ground in case of a fault.

#Elimination of harmonics:

Star connection facilitates a neutral connection which is instrumental in eliminating triple harmonics.

#No Circulating Currents:

In star, connection don’t have circulating currents like in delta which leads to heating losses.

What will happen if the generator voltage is not the same while paralleling?

If the voltage is not the same then large circulating current will develop between machines will produce large magnetic force to pull the generator's voltage into synchronism. So, it will cause rapid acceleration of one machine and retardation of another. Large force may damage the prime mover large circulating current will trip the overcurrent relay of the generator. Resulting in a blackout.

What will happen if air gap reduces?

• Air gap reduced means pedestal bearing is damaged.
• For alternator, the rotor will touch the stator and damage it.
• Alternator characteristics and efficiency will be reduced drastically.
• Motor same thing bearing is damage or shaft is bend. The consequence will be the same.

Why is Reverse Power Relay required?

• Prevent the motor effect
• Prevent the mechanical failure of the prime mover.
• Prevent total power failure.

When does reverse power flow?

• Faulty Governor of the prime mover.
• Loss of excitation in the alternator.
• Faulty synchroscope.

What is sequential starting?

Automatic starting of essential necessary equipment when power restored after a blackout.

What are the reason for the loss of residual magnetism in the generator?

If the alternator is not started from so long, improper maintenance and Moisture in air.

What happens if we increase the excitation of one alternator, when two alternators are in parallel condition?

KVAR load varies but KW load remains the same.

In balanced generator Circulating current flows in the increased excitation one. So that generator will share only KVAR load.

Power factor will reduce, heat will increase and the current drawing will be more.

Can we use a 50Hz supply for a 60Hz system and if we use it then what are the effects on Voltage, torque and speed and current on the machinery?

Vice versa 60Hz on 50Hz rated machine

If the frequency is reducing then the corresponding voltage should also reduce to make sure voltage by frequency ratio should be same. Speed depends upon frequency and torque depends upon voltage. And for smooth operation speed and torque characteristics should follow the linear curve. For taking shore supply always make sure voltage by frequency ration should be same.

For 50 Hz voltage should be 380.

So if we have 50Hz, 380Volts on a 440v, 60Hz supply.

What are the effects on the Motor?

V/f ratio should be the same. The motor will run without any problems if it is designed for that frequency but efficiency of the equipment will be less

Starting torque will increase with a low V/f ratio

But maximum torque is constant.

What is the importance of power factor in paralleling two generators?

If the power factor is not same then losses of both the alternator are not the same then there is a possibility that actual power or current will also not be the same. So for perfect paralleling power factor should be same. Then only both the alternator will share equal kW and will have equal current.

IF we increase the excitation of one generator, Will the impedance of both generators be equal or different?

Their impedances will be different as they have different power factor.

Is there any time delay for under-voltage, over-voltage, under frequency and over frequency?

Usually Under Voltage and over-voltage 2 sec.

In which all condition reverse power can happen?

• Mechanical failure
• Improper synchronising 
• ACB not opening after the load shifted.

What is magnetic resistance?

Reluctance which opposes the flow of the magnetic flux in a magnetic circuit Reluctance = MMF/FLUX.

Can we parallel emergency generator and main generator?

Emergency generator normally cannot be parallel with main generator until and unless it is designed to be run in parallel with main generator.

There are 2 modes one is propulsion mode and another one is a harbour mode.

In propulsion mode, it works as a normal emergency generator, whereas in harbour mode it can be used as a normal generator.

Practically emergency generator is required to run parallel with main generators in the ships with shaft generator and CPP propulsion and high load even in case of propulsion is not used.  That is in container ships with high reefer capacity, reefer ships etc.  As the size of the main generators is small they may not be able to supply power to entire load in case of one generator failure, especially in ports where we are not using shaft generator.  So arrangements are made such that an emergency generator can be used in parallel with the main generator.

Consequences of difference in power factor during the parallel running of generators.

Reduces the efficiency of a generator with less power factor, I square R losses increases in the generator with less power factor.

What are the function of reactor coils and capacitors in static excitation?

Reactor coil maintains the rated voltage and. Capacitor release store energy to induce the primary side in case voltage drops. Voltage drop that time inductance allows to increase voltage.

What is the form factor?

The ratio of RMS value by average value is called the form factor.

What is the reason to fail auto Start of Emergency Generator?

• Switch in manual mode
• Fuel valve is closed
• The battery is fully discharged
• Fuel is empty in the tank
• Auto circuit problem.

There is blackout for 10 hours; how you will prepare the ship (for cleaning) for maintenance of MSB?

• Carry out a risk assessment and follow the company procedures
• Inform to dry dock ship repair manager (SRM)
• Inform chief cook to prepare the food.
• General-purpose battery should be fully charged.
• Will make sure that emergency lights are working.
• Make an arrangement to switch off the unwanted load in dry dock.
• Portable torches should be fully charged and ready for use.
• Make sure shore connection is disconnected and lockout
• Make sure MSB is death condition and Earthing has to be done prior to work.

Why we don't parallel with the unknown generating station?

Because the characteristics of the prime mover of unknown source generating station is not known (drooping characteristic).

If two generators are running in parallel and kW is showing same but current is lower than the other what may be the problem and how to rectify?

AVR setting is disturbed and KVAR will be more for gen showing more current and power factor will be reduced for the same generator.

Rectify by changing AVR setting

It can be rectified by adjusting their field excitation. Increase excitation which supplies lower current.

Why generator rpm slightly drops instantaneously when a heavy load cuts in?

Due to the armature reaction.

The armature current will increase so stator flux will increase and it will increase the electromagnetic-torque. This torque will resist the prime mover torque.

When sudden load increases in alternator the rotating magnetic field opposes rotor to rotate this phenomenon is electromagnetic torque.

Rpm is directly proportional to fuel injection. After monitoring speed governor will increase fuel so speed gains up.

What will happen if the load is not reduced after the preferential trip?

Stand by generator will start automatically when it’s in standby mode.

If not in standby mode it will trip depends up LTD-long-time delay set up

(early days where 3 trips are given in preferential trip, 3rd one i.e. 15-second one will cause the main breaker to trip and it lead to a blackout. But in new ships, we don’t have the third trip).

Why we use a short time delay in ACB?

The short-time delay function determines the amount of current the breaker will carry for a short period of time, allowing downstream protective devices to clear short-circuit without tripping the upstream device.

Why synchroscope pointer moves clockwise when incoming generator speed higher and vice versa?

The incoming generator frequency is higher than the bus bar frequency and the pointer is moving in clockwise (more frequency)

When low frequency and pointer will rotate anticlockwise direction. (Low frequency)

Speed is directly proportional to frequency.

Generator output voltage is ok at 50 % load but when you increase beyond 50 % then the voltage drops, what was the reason?

The diode is open.

Two generators running one generator lube oil attached pump stuck what will happen? Which alarm come first? Directly trip or before any alarm came and what happened in other generator

• Lub oil low-pressure alarm will come first
• L.O low-pressure trip activate
• So generator breaker trip
• When the load increases up to 80-85% of total capacity than a standby generator will come to load in case it fails then at 90-95% of total load capacity preferential trip will activate.

Why are generator and motor bearings insulated?

To prevent circulating rotor currents which can damage bearings.

How did the surge resistor protect the diodes in the brushless generator?

It’s actually a variable resistor. When the voltage increases its resistance decreases. So in case of voltage surge, it will make a low resistance path.

What will happen if any of the diodes get open in running condition?

There will be momentarily voltage clip but that dip will be compensated by AVR. But the excitation system will start getting heated up. If that generator will start after stopping the rated voltage will not come.

What will happen if Diode gets short in running condition?

A short circuit diode is more serious as it leads to short circuit the exciter. Loss of Excitation can be happened due to rapid heated up of excitation system. And voltage will drop.

After stopping the generator if you will run, you will not get any voltage.

Generator started but no voltage producing

• Voltmeter may be faulty-Replace it
• Residual magnetism may be lost-flash the alternator
• Diode may be a shorted-replace whole bank
• Surge suppressor resistor may be shorted-replace the whole bank
• AVR malfunctioning-change it
• Circuit may open-make the connection according to drawings.
• Very low insulation resistance to earth (ground), on exciter stator or main stator.
• Winding fault. Open circuit or short circuit on any winding in the machine.

How to restore residual magnetism?

• Do the flashing
• Please follow the maker procedure
• Stop the generator.
• Then bring the 12v DC battery.
• Remove terminal F1 & F2 (J and K) from AVR (if you not disconnect may damage the AVR)
• Check the resistance between F1 and F2 it should show some value (Please refer the maker manual). If it is zero means field winding shorted. And have to check the resistance with respect to the ground also.
• Touch the terminals of batteries with F1 & F2 terminals. With same
• Polarity (positive to positive and negative to negative), for a very short time (5-6 seconds).

What will happen if the generator is immersed in seawater?

Its residual magnetism will be Lost because seawater is a good conductor of electricity. In freshwater or distilled water the residual magnetism will not be lost.

After starting generator, it is producing less voltage than rated Voltage

• Any of the diodes may be open.
• AVR may be malfunctioning.
• Excitation trimmer is disturbed.
• Somebody touched accidentally.
• With vibration, it disturbed.

After starting generator, it produces more voltage than rated Voltage

• AVR may be malfunctioning.
• Excitation trimmer is disturbed.

The generator started but the breaker is not closing (or)Individual generator is not coming on load.

• The generator may not be developing voltage.
• UV coil may be malfunctioning.
• Control supply may not be available.
• Charging motor may be malfunctioning.
• Charging spring defective.
• There may be mechanical damage in ACB.
• Racking handle is still inside/inserted.
• During dry clock, shore supply is available.
• Any of the interlocks are active.
• You were trying to close the wrong breaker.

Generator started but not paralleling

• Paralleling condition is not met.
• Synchronous relay malfunctioning.

During parallel the operation, one generator is showing less voltage, what the reason is and what action Will you take?

• AVR trimmer may be disturbed
• Rectifier diode may be open circuit
• AVR may be a malfunction
• Faulty voltmeter.

Difference between shunt coil and trip coil?

SHUNT COIL: - To trip breaker from a remote location.

TRIP COIL:-Built in the coil by the manufacturer for safety to keep the ACB for any abnormality.

Governor motor control for auxiliary Engine

It is a DC series motor.

Interlock in raise and lower- so that you can do one operation at a time.

When in auto, the signal comes to upper Land R and governor motor gets DC voltage accordingly.