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(a) Write short note on “Electric shock” . ( 6 Marks)
Ans: Electric shock is a jarring , shaking sensation. Usually it feels like receiving a sudden blow. If the voltage and current are sufficiently high unconsciousness ocours. Electric shock may severely burn the skin. Muscular spasms may cause the hands to clasp the apparatus or the wire making it impossible to let go.
(b) How do you treat a person who has Suffered an Electric Shock onboard from a AC supply? Jan 20
(i) Try to switch off the power.
(ii) Remove the victim from the electrical contact with the help of dry stick,rope, belt, coat, blanket, shirt or any other non conductor of electricity to drag or push the victim to safety.
(iii) Determine the cardiopulmonary status of the casualty. If required start CPR.
(iv) Check the other physical injuries
(v) Lay the victim face up in a prone position.
(vi) The feet should be 12″ (Inches) higher then the head. In case of head or chest injuries then the head to be slightly elevated.
(vii) Keep the victim warm. The injured person body heat must be conserved. Cover the victim with one or more blankets.
(viii) Avoid artificial means of warming such as hot water bottles.
(ix) Do not give drugs, food and liquids Never give alcohol and other depressant substances
(x) Do not leave the victim unattended till the medical team arrive/ doctor.
Q – With respect to Alternators onboard
(a) Briefly explain True power, Apparent Power & Power factor. (8 Mark). Jan20
Ans- True Power-The actual amount of power being dissipated or performs the useful work in the circuit is called as active or true or real power. It is measured in watts, practically measured in KW(kilowatts) & MW(megawatts) in power systems. It is denoted by the letter P (capital) and it is equal to the average value of P = VI cos ϕ. It is the required outcome of an electrical system which drives the circuit or the load.
P = VI cos ϕ
Reactive Power – The average value of the second term in the above derived expression is zero, so the power given by this term is zero. The component, which is proportional to the VI sin ϕ is called as reactive power, symbolized by the letter Q. Even though it is a power, but not measured in watts as it is a non active power & hence, it is measured in Volt-Amperes- Reactive(VAR). The value of this reactive power can be negative or positive depends on the load power factor. This is because inductive load consumes the reactive power while capacitive load generates the reactive power.
Q = VI sin ϕ
Apparent Power – The complex combination of true or active power & reactive power is called apparent power. Without reference to any phase angle, the product of voltage & current gives the apparent power. The apparent power is very useful for rating the power equipment. It can also be expressed as the square of the current multiplied by the circuit’s impedance. It is denoted by the letter S & measured in Volt-Amperes (VA), practical units include KVA (Kilo volt-amperes) & MVA (mega volt-amperes).
Apparent power = RMS voltage × RMS current
Apparent power, S = V × I
In complex form, S = V I
S = V ∠0° I ∠ ϕ (for lagging load current)
S = V I ∠ ϕ
S = V I cos ϕ + jV I sin ϕ
S = P + jQ
Or S = I²Z
The power factor is the cosine angle between the voltage & current. The power factor can be expressed in terms of the above discussed power forms. Power factor defines the efficiency of the circuit.
Power factor (PF) = (Active power in watts) / (Apparent power in volt amps)
PF = VI cos ϕ / VI
PF = cos ϕ
Power Triangle and Power Factor
Electrical power consumed in an AC circuit can be represented by the three sides of a right angled triangle, known as a power triangle.
Q – State the applications of the synchronous motors. Discuss how a synchronous motors can function as a synchronous capacitor. Compare synchronous motors with induction motors. (16 Marks) Dec 20, Aug 2019
The Applications of the synchronous motor are as follows:-
1. Power factor correction
2. Voltage regulation
3. Constant speed, constant load drives
Synchronous motor can function as a synchronous capacitors:-
Here, in a three-phase system, we connect one 3-phase synchronous motor & run it at no load.
Suppose due to a reactive load of the power system the system draws a current IL from the source at a lagging angle θL in respect of the voltage. Now the motor draws a IM from the same source at the leading angle θM. Now the total current drawn from the source is the vector sum of the load current IL & motor current IM. The resultant current I drawn from the source has an angle θ in respect of the voltage. The angle θ is less than angle θL. Hence power factor of the system cosθ is now more than the power factor cosθL of the system before we join the synchronous condenser to the system.
Difference between Synchronous motors and induction motors :-
|Synchronous motor||Induction motor|
|Not self starting||Self Starting|
|Zero slip||Slip is not zero|
|AC & DC Power is required||AC power is required|
|It can be used for improvement of power factor||It is not used for improvement of power factor|
|Prime mover is required||Prime mover is not required|
|Moves at constant speed||Variable speed|
|Slip ring is required||No slip ring required|
|More maintenance||Less maintenance|
|It runs at lagging, leading & unity power factor||Lagging power factor|
Q. With reference to an emergency source of electrical power in cargo ship:- Dec 2019,
a) Describe a typical power source. ( 6 Marks )
b) Give a typical list of essential services, which must be supplied simultaneously. ( 6 Marks )
c) Explain how the emergency installation can be periodically tested. (5 Marks )
Ans: In the event of power failure in ship, Emergency power source is required to meet the essential services by the emergency equipments.
SOLAS chapter II-1 Part D, regulation 43 tells about emergency source of power and regulation 44 states about starting arrangement of emergency source.
a) One of the emergency power source is Emergency Generator
The Emergency source of electrical power and associated transition source of emergency power, emergency switchboard and emergency light switchboard shall be located at above the uppermost continuous deck and readily accessible from open deck. They shall not be located at the forward of the collision bulkhead, except where permitted by the administration on certain circumstances.
Fire or other casualty on main source of electrical power or machinery space of category A should not interfere the source, control and distribution of emergency source of power, switchboard or any transforming aid should not be contiguous to the boundaries of machinery space of category A.
Emergency Generator sets shall be readily started in their cold condition at a temperature of 0 degree C , if not possible than suitable heating arrangement should be present.
In auto mode on failure of the main source , emergency generator should start and connect to load within 45 seconds of power failure.
There must be two independent way of starting emergency generator.
Batteries: Act as a transition source of power to start emergency generator and capable of giving three consecutive start before discharging, connected with an independent battery charger. Battery charger fail and battery low voltage audio and visual alarm arrangement should be present.
Air starter Motor: With provision of supplying air from main and emergency air reservoir.
Hydraulic starter: With independent tank and manual pump for building sufficient pressure.
Spring start: Provision to charge the spring by manual handle and then releasing the potential energy stored in the spring to crank the engine
Fuel Oil and its supply:
Emergency Generator must have provision of cooling either by water or air.
Water cooled: Level of cooling water in the radiator should be periodically checked and audio and visual alarm for low water level should be present. Associated pipes for filling cooling water should not be chocked.
Air Cooled: Proper arrangement of circulation of air should be ensured. It must be provided with forced ventilation. Hot air after cooling should have provision of escaping outside the compartment.
(b) The essential services supplied by the emergency sources are the following
i) Emergency Fire Pump
ii) One of the steering gear motor.
iii) Emergency Air compressor
iv) Emergency Lightings
v) Battery Charger for Emergency battery and GMDSS battery
vi) Electrical Test Panel
vii) Fire Detection System
viii) Telephone Exchange system
ix) Cargo Control Console
x) Engine Control Console
xi) Emergency Bilge pump (440 V)
xii) Navigation Equipment (220 V), RADAR, Gyro, etc
xiii) Radio Communication (220 V),
xiv) General Alarm and PA system.
(c) Periodic Testing and attention required.
Emergency generator must be used for short period to check for services such as routine testing, operation and functions of all related audio and visual alarms.
Daily: Daily visual checks should be done for all indication and parameters on emergency switchboard, battery charger panel and generator starting box.
Weekly: Once in a week normally Saturday following should be checked.
Quarterly: Addition to daily and weekly procedure, emergency generator should be tested on load by simulation, and start all emergency and safety equipments including emergency light and check the performance of the generator.
Yearly: Intentional Black out test should be carried out for checking of operation and time taken by generator to come on load.
Q. What are semiconductor devices? What are its advantages over thermionic devices? With respect to semiconductor devices describe working principle and application of the Following:- a) Zener Diode, b) Transistor, c)Photo cell, d) Thyristor ( 16Marks ) Jan 20, Nov 19
Ans:- A semiconductor device is an electronic component that utilizes the electronic properties of semiconductor material, principally silicon, germanium, & gallium, arsenide, as well as organic semiconductor devices have replaced vacuum tubes in most of the applications. They use electrical conduction in the solid state rather than the gaseous state or thermionic emission in the vacuum. Semiconductor materials are useful because their behavior can be easily changed by the deliberate addition of impurities, called as doping. Semiconductor conductivity can be regulated by the introduction of an electric or magnetic field, by exposure to light or heat, or by the mechanical deformation of the doped mono crystalline silicon grid; thus, semiconductors can make excellent sensors. Current conduction in a semiconductor occurs due to mobile or “free” electron & electron holes, collectively called as the charge carriers.
Doping a semiconductor with a small proportion of an atomic impurity, such as phosphorus or boron, increases the number of free electrons or holes within the semiconductor. When a doped semiconductor consists of excess holes, it is called a p-type semiconductor(p for positive electric charge); when it consists of excess free electrons, it is called an n-type semiconductor (n for negative electric charge). A majority of mobile charge carriers have negative charge. The manufacture of the semiconductors controls precisely the location & concentration of p- & n-type dopants. The connection of n-type & p-type semiconductors form p-n junctions.
1. Low Power Consumption
2. Many possible applications through diodes & transistors. E.g. in solar cells, photovoltaic cells, zener diodes for voltage regulators, variety of transistors used as switches
3. Used in many integrated circuits due to their minute size
4. Longer life
5. Conductivity lies between that of conductor & insulator, thereby, they find application in almost any & every electronic device you can think of today like TV, Computer Motherboards, Microcontrollers, Microprocessors, Microwave Ovens and so on.
6. As semiconductor devices have no filaments, hence no power is required to heat them to cause the emission of electrons. During operation, semiconductor devices do not produce any humming noise. Semiconductor devices require low voltage operation as compared to vacuum tubes.
7. Semiconductor devices are manufactured both as single discrete devices & as integrated circuits (ICs), which consist of the number of two to billions of devices manufactured & interconnected on a single semiconductor substrate, or wafer.
8. Power semiconductor devices are discrete devices or integrated circuits intended for “high current or high voltage” applications. Power integrated circuits combine IC technology with the power semiconductor technology.
(a) Zener Diode- Zener diodes are normal PN junction diodes operating in the reverse biased condition. Working of the Zener diode is similar to a PN junction diode in forward biased condition, but the uniqueness lies in the fact that it can also conduct when it is connected in reverse bias above its threshold / breakdown voltage. These are among the basic types of diodes used frequently, apart from the normal diodes.
Application: Zener diodes are extensively used as voltage references & as shunt regulators to regulate the voltage across small circuits. When connected in parallel with a variable voltage source so that it is reverse biased, a Zener diode conducts when the voltage reaches the diode’s reverse breakdown voltage.
( b)Transistor – Transistors consists of 3 parts ‘ a base, a collector, & an emitter. The base is the gate controller device for the larger electrical supply. The collector is the larger electrical supply, & the emitter is the outlet for that supply. By sending varying levels of current from the base, the amount of current flowing through the gate from the collector may be regulated. In this way, a very small amount of the current may be used to control a large amount of current, as in an amplifier. The same process is used to form a binary code for the digital processors but in this case a voltage threshold of 5 volts is required to open the collector gate. In this way, the transistor is being used as a switch with a binary function: 5 volts ‘ ON, less than 5 volts ‘ OFF.
(C) Photocell: Photocells are sensors that enable you to detect light. A Photocell is generally a resistor that changes its resistive value(in ohms) depending on how much light is shining on it. They are very low cost, easy to get in many sizes & specifications, but are very inaccurate. Each photocell sensor will act a little differently than the other, even if they are from the same batch. The variations can be really large, 50% or higher. For this reason, they should not be used to try to find precise light levels in lux or millicandela. Instead, you can expect to only be able to find basic light changes.
a) Thyristor or SCR( Silicon Controlled Rectifier):
Silicon Controlled Rectifier(SCR) is a unidirectional semiconductor device made up of silicon. This device in the solid state equivalent of thyratron & hence it is also referred to as thyristor or thyroid transistor. In fact, SCR (Silicon Controlled Rectifier) is a trade name given to the thyristor by the General Electric Company. Generally, SCR is a three-terminal, four-layer semiconductor device consisting of the alternate layers of p-type & n-type material. Hence it has 3 pn junctions J1, J2 & J3. The figure below shows an SCR with the layers p-n-p-n. The device has terminals Anode(A), Cathode(K) & the Gate(G). The Gate terminal(G) is attached to the p-layer nearer to the Cathode(K) terminal.
It is seen that a single SCR is the combination of one pnp transistor (Q1) and one npn transistor (Q2). Here, the emitter of Q1 acts as the anode terminal of the SCR while the emitter of Q2 is its cathode. Further, the base of Q1 is connected to the collector of Q2 and the collector of Q1 is connected to the base of Q2. The gate terminal of the SCR is connected to the base of Q2, too.
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