Q. What is the regular maintenance done on batteries?
Ans– 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.
Q. Purpose of AVR
Ans– During KVAr loading, voltages of alternators falls which can be gained by AVR, which increases the excitation until voltage is back to normal .
Q. How does this cause a rotation to change?
Ans– Due to the change in phase sequence, the rotating magnetic field changes direction and the rotor rotates in the opposite direction.
Q. How is protection provided for OL and SC?
Ans– Damage from the short circuits & overloads can be reduced or prevented by using 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 the short circuit due to very high currents but can higher (a few seconds) due to the currents say 1.1 to 1.5 times the normal current.
Q. What is a current transformer and voltage transformer?
Ans– Current transformers used for sensing load currents and as inputs to indicating instruments. Voltage transformers used for step-up / step-down voltages.
Q. What is a relay? How is it different from a magnetic contactor?
Ans– Contactor is a heavy duty switch whereas the relay is a light duty switch. Relays can be as simple as 12 V DC solenoids with 12 V, 0.5 A rated contacts. They are employed to detect faults on the power system & signal a local or remote switching device (breaker) to trip the circuit.
Q. What is colour code of resistors , Please explain ?
Ans– 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.
Q. What are the safeties on MSB?
Ans– 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.
Q. What is capacitance?
Ans– 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
Q. Even though the main engine runs at varying speeds, how does the alternator maintain constant frequency?
Ans– By use of thyristor controlled frequency (rpm) converters.
Q. How is this voltage achieved?
Ans– By connecting 12 cells in series. 12 x 2 V = 24 V is total battery bank voltage.
Q. How is the direction of rotation varied?
Ans– By changing the phase sequence (from say R-Y-B to R-B-Y)
Q. What is the harm if reverse power flows?
Ans– 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 the load equal to the 2P. Depending on the size of the grid & the strength of the remaining generation, a step increase of 2P could result in a significant frequency change on grid. There could be prime mover damage in some instances (especially steam turbines).
Q. If you press the ACB Close button on an idle generator what will happen?
Ans– Generally, the breaker won’t close till 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.
Q. Apart from renewing bearings is there any reason for opening up motors for the maintenance?
Ans– Insulation checks & renewal (example using insulation spray on stator winding insulation)
Q. What is the meaning of the overload?
Ans– An overload is a current over & 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 normally 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
Q. What is an induction motor?
Ans– An induction motor is an alternating current motor in which the primary winding on one member (generally the stator) is connected to the power source & a secondary winding or a squirrel-cage secondary winding on the other member (generally the rotor) carries the induced current.
Q. What is the meaning of excitation in an alternator?
Ans– An electric generator or electric motor consists of a rotor rotating in the magnetic field. The magnetic field may be produced by the permanent magnets or by field coils. In the case of a machine with the 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 producing a magnetic field by means of an electric current is called excitation.
Q. Permanent magnets are generally made of
Ans– Alnico alloys
Q. What is the full form of ACB?
Ans– Air circuit breaker. It is generally used at 400V and higher current applications (generator breakers).
Q. What is the meaning of ACB, MCCB, NFB?
Ans– Air circuit breaker for 400V higher current ratings.
Moulded case circuit breaker for 400V medium current ratings.
No Fuse Breaker – 75 A capacity
Q. What is a synchronous condenser?
Ans– 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.
Q. What is meaning of short circuit?
Ans– A short circuit occurs when conductors are connected (shorted) together by a fault. This is normally phase to phase fault due t failure of insulation by something cutting the wires, fires, etc.
Q. How is the direction varied?
Ans– A general purpose DC motor can be reversed by changing the polarity of either the armature or the field but not both.
Q. What is a magnetic contactor? Where is it used?
Ans– A contactor is an electrically controlled switch used for switching a power circuit, similar to a relay except with higher current ratings. Contactors are used to control electric motors, lighting, heating, capacitor banks, and other electrical loads.
Q. What does different position of the synchroscope needle mean; what is the difference between 6 o clock and 12 clock?
Ans– 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).
Q. What is the voltage used in a meggar?
Ans– 500V DC.
Q. What is the full battery voltage ?
Ans– 24 V DC.
Q. What is the voltage available from each cell in a lead acid battery?
Ans– 2 V DC.
Q. What is the specific gravity of electrolyte used in lead acid battery?
Ans– 1280 at full charge and 1180 at discharge
Q. Out at sea, if there is a black out during your watch, what action will you take?
Ans– Refer company manual for detailed procedure
Q. Why generally we use induction motor on board, not synchronus motor?
Ans– Easy to make. Serves our purpose on board. Easy to operate. Singly excited ac supply is more than sufficient to start and rotate the motor. Induction motors are Used where speed control is not a priority at all but continous running, easy start motors are needed.
Q. What is the usual value you see onboard?
Q. Can reverse power trip occur with one generator running? If yes, how? If no, why?
Q. Short circuit and Earth fault: Define
Q. Star-Delta starter: No diagram jus explain why used on-board
Q. What is KVA?
Q. What is meant by phase in AC?
Q. Why is DC not much in use now?
Q. Where are you likely to see DC used?
Q. What is a semi conductor?
Q. What is doping?
Q. What happens to insulation when temperature rises?
Q. What are Superconductors?
Q. What happens to resistance of conductors when temperature rises?
Q. What is “slip”?
Q. What is a synchronous motor?
Q. How do you use a multimeter?
Q. How do you check continuity?
Q. What do we use for welding on ships? AC or DC?
Q. What is the voltage?
Q. Does current flow through hull during welding?
Q. If so why don’t you get a shock?
Q. What is safe voltage to prevent shock?
Q. What is the meaning of earthing in a ship?
Q. What is the meaning of Star winding/ Delta winding?
Q. Is the ship’s alternator Star wound or Delta wound?
Q. Can you draw a DOL starter circuit?
Q. What is the normal setting of overload relays?
Q. How does a thermal overload relay work?
Q. What is single phasing?
Q. How can it happen?
Q. What is the harm caused by this?
Q. Why is it necessary that incoming alternator frequency is more than bus bar?
Q. What is dark lamp and bright lamp method for synchronizing? How is the connection made?
Q. Why is this reverse power used instead of reverse current in alternators?
Q. Is the field current in an alternator AC or DC ?
Q. Is the field rotating or stationary?
Q. How is it supplied?
Q. What is a brush less alternator?
Q. How is the excitation achieved in this?
Q. What is the meaning of residual magnetism?
Q. What is a zener diode?
Q. What is a thyristor? What is an SCR?
Q. How does an inverter work?
Q. How does a tube light work?
Q. What is the function of the choke?
Q. What is the difference between neutral and earth?
Q. If the AVR, is defective, when additional loads come on will the generator be able to take it?
Q. How do you test reverse power trip, high current trip, preferential trip?
Q. Why is a capacitor used in single phase motors, fluorescent tube lights and electronic circuits.*
Q. while welding onboard a ship the current passes through hull, then why people dont get electric shock?
Q. types of fuses used in ER ,how to test cartridge type fuse (ceramic )
Q. how to select a fuse ?
Q. Sketch the types of motor starters. Why timer required in auto transformer and start delta types?
Q. How do you trace earth faults?
Q. Trips on an alternator. Motor protection aids?
Q. There is no neutral on board then how do we get 220V supply?
Q. Safeties in Alternator