MEO CLASS II Marine ElectroTechnology 2024 QUESTION PAPER
MEO CLASS- II
Marine Electro Technology
JANUARY – 2024
Q1.A. Sketch a main engine shaft driven generator arrangement with an electronic system for frequency correction.
B. Describe the operation of the generator arrangement sketched in.
MAR2024, JAN 2024, DEC2019,
Q.2 With reference to testing High Voltage equipment:
(a) Explain why earthing down is considered essential.
(b) Briefly describe the procedures of earthing down
(c) describe how an insulation resistance test is carried out on High Voltage equipment, making reference to personnel safety;
(d) Describe, with the aid of a sketch, a method to detect earth leakage in EACH of the following systems:
(i) Earthed
(ii) Insulated
JAN 2024, SEP 2022
Q. 3 Explain What is meant by, and the significance of, four of the following terms.
I. Voltage Stabilization,
II. Filter choke;
III. Impedance,
IV. Rectification,
V. Grid bias voltage.
JAN 2024, OCT 2022, SEP2022, .
Q.5 With reference to electronic control systems
A. Draw a simple block diagram for temperature control
B. Describe each component shown in the diagram in (A
JAN 2024, MAR 2024, JAN2024, OCT 2023,
Answer – A. Simple Block Diagram for Temperature Control:
B. Description of Components:
Temperature Sensor: This component senses the temperature of the system or environment. It typically converts the temperature into an electrical signal that the controller can process. Common types include thermocouples, resistance temperature detectors (RTDs), and thermistors.
Controller: The controller receives input from the temperature sensor and compares it to a setpoint or desired temperature. Based on this comparison, it generates a control signal that dictates the operation of the actuator.
Actuator: The actuator is responsible for making changes to the system based on the control signal from the controller. In a temperature control system, the actuator might adjust the output of a heat source (e.g., a heater or cooler) to regulate the temperature.
Heat Source: This is the component that directly influences the temperature of the system. It could be a heater, cooler, or any other device that can alter the thermal conditions of the environment.
the temperature sensor provides feedback to the controller, which then determines the necessary action to maintain the desired temperature setpoint. The actuator executes this action, manipulating the heat source to achieve the desired temperature control.
Q.6 a) Derive the expression for current and voltage relations between line and phase values in the star and delta cases. Draw vector diagram.
JAN 2024,
Q.7 A. Discuss different methods of speed control of a d.c. series motor by adjusting field ampere turns. (6)
B. A 230 V, d.c. shunt motor runs at 1000 r.p.m and takes 5 amperes. The armature resistance of the motor is 0.025 and shunt field resistance is 230 Calculate the drop in speed when the motor is loaded and takes the line current of 41 amperes. Neglect armature reaction.
APR2024, JAN 2024, DEC 2023, JUL2023,
Q.8 A. What are the factors which determine the synchronous speed of a motor?
B. The star-connected rotor of an induction motor has a stand-still resistance of 4.5 ohms/phase and a resistance of 0.5/phase. The motor has an induced emf of 50 V between the slip-rings at stand- still on open circuit when connected to its normal supply voltage. Find the current in each phase and the power factor at start when the slip-ring is short-circuited
FEB 2024, JAN 2024, FEB 2023, SEP2022, AUG 2022,
Q.9 a) Explain the purpose of interpoles and state their magnetic polarity relative to the main poles of both generators and motors.
b) A 200V, long-shunt compound-wound generator has a full-load output of 20kW. The various resistances are as follows; armature (including brush contact) 0.15ohm, series field 0.025 ohm, interlope field 0.028 ohm, shunt field (including the field-regulator resistance. 115ohm. The iron losses at full load are 780W, and the friction and wind age losses 590W. Calculate the efficiency at full load.
JAN 2024, AUG 2023, SEP2022, AUG2022,
Q.10 A. (i) What is direct-connected alternator?
(ii) How is a direct-connected exciter arranged in an alternator?
B. A 440V shunt motor takes an armature current of 30A at 700 rev/min. The armature resistance is 0.7ohm. If the flux is suddenly reduced by 20 per cent, to what value will the armature current rise momentarily? Assuming unchanged resisting torque to motion, what will be the new steady values of speed and armature current? Sketch graphs showing armature current and speed as functions of time during the transition from initial to final, steady state conditions.
JAN 2024, MAR 2011, AUG 2022, SEP2022, FEB 2011,
FEBRUARY – 2024
Q.1 Differentiate with the aid of simple sketches between the following types of electronic circuits;
A. Rectifier circuit;
B. Amplifier circuit;
C. Oscillator circuit.
FEB 2024, MAR 2013, MAR 2022, FEB 2022, JAN 2022,
Q.2 With reference to U.M.S. operations:
(a) State with reasons the essential requirements for unattended machinery spaces. (8)
(b) As second Engineer, describe how you would respond to the irretrievable failure of the Machinery space fire alarm system whilst the ship is on voyage
FEB 2024, JAN 2023,
Q.3 Explain the matching of an induction electric motor to a pump required for main circulating duty, with the aid of pump characteristic and torque/ship diagrams.
FEB 2024, NOV 2022, OCT 2022, JUL2019,
Q.4 With reference to a three-phase shipboard electrical distribution system:
(a) Enumerate the advantages of an insulated neutral system.
(b) Enumerate the disadvantages of an insulated neutral system.
(c) Describe how the Earthed neutral system is Earthed.
(d) Compare the use of an insulated neutral system as opposed to the use of an Earthed neutral system with regard to the risk of electric shock from either system.
MAR 2024, FEB 2024, OCT 2012, SEP 2017,
Answer –
(a) Advantages of an insulated neutral system in a three-phase shipboard electrical distribution:
Reduced Risk of Ground Faults: Insulating the neutral reduces the likelihood of ground faults occurring, as there's no direct connection between the neutral conductor and the ground.
Improved System Stability: Insulated neutral systems offer better stability by preventing unwanted current flow through the ship's hull or other unintended grounding paths.
Enhanced Protection for Equipment: With insulation, there's less chance of stray currents causing damage to sensitive equipment, as the neutral is isolated from the ship's structure.
Compliance with Safety Standards: Insulated neutral systems often comply with stringent safety standards, ensuring the safety of personnel and equipment aboard the ship.
(b) Disadvantages of an insulated neutral system in a three-phase shipboard electrical distribution:
Complexity of Installation: Installing and maintaining an insulated neutral system can be more complex and costly compared to other configurations due to the additional insulation requirements.
Potential for Insulation Failure: If the insulation on the neutral conductor degrades or fails over time, it could lead to unexpected grounding and associated hazards.
Limited Fault Detection: Insulated neutral systems may make it more challenging to detect faults, especially if insulation degradation goes unnoticed until a fault occurs.
Increased Risk During Maintenance: Maintenance activities on an insulated neutral system require careful attention to prevent inadvertent grounding, which could pose risks to personnel and equipment.
(c) Description of how the Earthed neutral system is Earthed in a three-phase shipboard electrical distribution:
In an Earthed neutral system on a ship, the neutral conductor is intentionally connected to the ship's hull or structure, which serves as the earth reference point. This connection is typically achieved through a grounding electrode or conductor that physically links the neutral to the ship's metallic structure. The earth connection ensures that any fault currents are safely directed to the ship's hull, which acts as a low-resistance path to dissipate the current harmlessly into the surrounding water.
(d) Comparison of using an insulated neutral system versus an Earthed neutral system regarding the risk of electric shock:
Insulated Neutral System: With an insulated neutral system, the risk of electric shock to personnel is reduced due to the isolation of the neutral conductor from the ship's structure. However, there's still a risk of shock if insulation fails or during maintenance activities.
Earthed Neutral System: In an Earthed neutral system, there's a risk of electric shock if a fault occurs and the ship's hull becomes energized. However, proper grounding ensures that fault currents are safely directed away from personnel and equipment, minimizing the risk compared to uninsulated systems.
Overall, both systems carry risks, but proper design, maintenance, and safety protocols can mitigate these risks to ensure the safety of personnel and equipment aboard the ship.
Q.5 (a) Sketch a magnetic overload device incorporating a dashpot and explain how the current and time settings of the device may be varied.
(b) With the aid of a sketch, outline the essential features of a three stage "preferential tripping" scheme for the main generators of a ship.
FEB 2024,
Q.6 a) Explain the applications of PN junction diode.
b) A full-wave, 1-phase rectifier employs a double diode valve, the internal resistance of each element of which may be assumed constant at 500 W. The transformer r.m.s. secondary voltage from the centre-tap to each anode is 300 V and the load has a resistance of 2000 W. Evaluate:
(i) mean load current.
(ii) r.m.s. value of load current
(iii) the d.c. output power
(iv) the input power to the anode circuit
(v) the refrigeration efficiency
FEB 2024, FEB 2023
Q.7 a) What is back emf? Derive the relation for the back emf and the supplied voltage in terms of armature resistance.
b) An 8kw, 230V, 1200 rpm d.c shunt motor has Ra 0.7W. The field current is adjusted until, on no-load with a supply of 250V, the motor runs at 1250 rpm and draws armature current of 1.6 amps. A load torque is then applied to the motor shaft which causes la to raise to 40 A and the speed falls to 1150 rpm. Determine the reduction in the flux per pole due to the armature reaction.
FEB 2024, FEB 2023,
Q.8 a) What are the factors which determine the synchronous speed of a motor? (6)
b) The star-connected rotor of an induction motor has a stand-still resistance of 4.5 ohms/phase and a resistance of 0.5/phase. The motor has an induced emf of 50 V between the slip-rings at stand- still on open circuit when connected to its normal supply voltage. Find the current in each phase and the power factor at start when the slip-ring is short-circuited.
Q.9 a) Explain how drooping characteristics cater for stable operation when running in parallel.
b) Two shunt generators X and Y work in parallel. Their external characteristics may be assumed to be a linear over their normal working range the terminal voltage of X falls 265V on no load 230V when delivering 350Ato the busbars, while the voltage of Y falls from 270 V on no load to 240V when delivering 400A to the bus bars. Calculate the current with each machine delivers when they share a common load of 500A, what is thw bus bar voltage under this condition and the power delivered by each machine.
FEB 2024, FEB 2023,
Q.10 a) Describe how protection against short circuit is provided in a 3-phase induction motor circuit.
b) An eight-pole alternator running at a speed of 720rev/min supplies current to synchronous and induction motors with forty- eight poles. Calculate the frequency and speed of rotation of the motors if thr induction motor runs with 2 percent slip.
FEB 2024, FEB 2023,
MARCH – 2024
Q1. A. Sketch a main engine shaft driven generator arrangement with an electronic system for frequency correction;
B. Describe the operation of the generator arrangement sketched in (A).
MAR 2024, JAN 2024, DEC 2019
Q2. With reference to electronic control systems:
A. Draw a simple block diagram for temperature control;
B. Describe each component shown in the diagram in.
JUN2024, MAR 2024, JAN 2024, OCT 2023, SEP 2022,
Q3. a) Sketch a circuit diagram for an automatic voltage regulator illustrating how the A.V.R. utilizes a Silicon-controlled rectifier to control the excitation system for an alternator.
(b) Describe how the A.V.R. monitors output and controls the excitation system.
MAR 2024, FEB 2023, DEC 2021
Q 4: -It is proposed to operate a Bow thruster unit from a 3.3 KV electrical supply.
Outline suitable options for the design of installation under each of the following heading.
i) Protection of main switch board
ii) Overload of a bow thruster motor
iii) Cable protection
MAR 2024, S EP 2017, S EP 2015,
5. With reference to a three-phase shipboard electrical distribution system:
-
Enumerate the advantages of an insulated neutral system.
-
Enumerate the disadvantages of an insulated neutral system.
-
Describe how the earthed neutral system is Earthed.
-
Compare the use of an insulated neutral system as opposed to the use of an Earthed neutral System
with regard to the risk of electric shock from either system.
Q6. A. A series circuit having resistance, Inductance and capacitance is to be operated on a constant voltage supply of available frequency. Indicate graphically how changes will take place in the current and voltage in resistance, inductance and capacitance, and also capacitive reactance and inductive reactance.
B. A resistance of 130 Ω and a capacitor of 30μF are connected in parallel across a 230 Volt, 50Hz supply. Find the current in each component, total current, phase angle and the power consumed.
Q7. A. Explain the working principal of a three-phase induction motor. What are the various types of rotors?
(B) An 18.65Kw, 6-pole, 50Hz, 3 phase slip ring induction motor runs at 960 rpm on full load with a rotor current per phase of 35A, allowing 1Kw for mechanical losses, find the resistance per phase of 3- phase rotor winding.
8. a) Explain why the iron losses in a transformer are substantially independent of the load current.
(B) The equivalent circuit for a 200/400-V step-up transformer has the following parameters referred to the low-voltage side. Equivalent resistance = 0.15Ω Equivalent reactance = 0.37 Ω Core-loss component resistance = 600ΩMagnetising reactance = 300 Ω When the transformer is supplying a load at 10 A at a power factor if 0.8 lag, Calculate,
i) the primary current
ii) secondary terminal voltage.
Q9): -. Sketch a graph of starting current, and torque against the speed of rotation for a single cage motor.
(B): -A 230V motor, which normally develops 10Kw at 1000 rev/min with an efficiency of 85%, is to be used as a generator. The armature resistance is 0.15Ohm and the shunt field resistance is 220Ohm. If it is driven at 1080 rev/min and the field current is adjusted to 1.1A by means of the shunt regulator what output in Kw could be expected as a generator, if the armature copper loss was kept down to that when running as a motor.
Q 10 (a): - Compare the effectiveness of a current limiting circuit breaker with that of a HRC fuse
Q10 (B): - A coil having a resistance of 10 Ohm, and an inductance of 0.15 H is connected in series with a capacitor across a 100V, 50Hz supply. If the current and the voltage are in phase what will be the value of the current in the circuit and the voltage drop across the coil.
Answer: - R = 10 ohm
L = 0.15 H V = 100 V
f = 50 Hz
To find –
1. Value of the current in the circuit
2. The voltage drop across the coil.
It is a series circuit , Voltage and current in phase. Therefore it satisfies the condition of resonance.
The value of current I = V / R
= 100 / 10
= 10 A
Reactance of inductor X = 2 x pi x f x L
= 2 x 3.14 x 50 x 0.15
= 47 ohm
Voltage drop across inductor = Current x Reactance
= I x X
= 10 x 47
= 470 V
Because of resonance condition,
Voltage drop across Inductor = voltage drop across Capacitor = 470V
Impedence of coil Z = √ (Resistance2 + Reactance2)
= √ (R^2 + X^2)
= √ (10^2 + 47^2)
= 48.05 ohm
-
Voltage drop across coil = Current x Impedence
= 10 x 48.05
= 480.5 V
APRIL-2024
1. Differentiate between squirrel cage and wound rotor motors, of the three phase a.c induction type, in respect of the following:
(a) rotor construction.
(b) torque characteristics
(c) speed variation.
APRI2024, DEC2023, JULY2023,Top of Form
2. With reference to an emergency source of electrical power in cargo ships:
a) Describe a typical power source..
b) Give a typical list of essential services, which must be supplied simultaneously.
c) Explain how the emergency installation can be periodically tested.
APRIL 2024, DEC 2023, OCT 2023, JUL2023,
3. The direct online start of squirrel cage motor is used for most electrical drives on a.c powered ships. Describe with sketches as necessary one method of overcoming each of the following problems:
(a) High starting current.
(b) Low starting torque.
, APRIL 2024, DEC 2023, OCT2023, JULY 2023
4. (a) (i) Discuss the various hazards and problems which are associated with electric cable insulation in the event of fire.
(ii) Suggest remedies for these problems.
(b) State how the spread of fire may be reduced by the method used for installing electric cables.
APRIL 2024, DEC 2023, JULY2023,
5. (a) What are the causes of overheating of an induction motor.
(b) What preventive measures are provided against damage to an induction motor in installed condition?
(c) What is the purpose of "fuse back up protection" provided to an induction motor?
d) How does an induction motor develop torque?
(e) What is the condition to be satisfied for achieving Maximum running torque in an induction motor?
APRI2024, DEC 2023, JULY 2023, MAR 2017,
SECTION-2
6. (a) What are the characteristics of PN junction diode? Point out its specifications. Also point out the significance of dynamic and static resistances.
(b) Draw the circuit of Half-wave rectifier and its output waveform. A diode whose internal resistance is 20 2 is to supply power to 1000 load from 110 V (RMS) source. Calculate
(i) peak load current, (ii) DC load current, (iii) AC load current.
APRL 2024, DEC 2023, JULY 2023, APRIL2022,
7(b) A. d.c. motor takes an armature current of 110 A at 480 V. The resistance of the armature circuit is 0.22. The machine has six poles and the armature is lap-connected with 864 conductors. The flux per pole is 0.05 Wb. Calculate;
(i) The speed
(ii) The gross torque developed by the armature.
APRIL 2024, DEC 2023, JULY 2023,
8. a) What is a commutator? Discuss its rectifying action in detail.
b) Calculate the e.m.f. generated by a 4-pole, wave wound armature having 40 slots with 18 conductors per slot when driven at 1000 r.p.m. The flux per pole is 0.015 wb.
APRIL 2024, DEC 2023, JULY 2023, JULY 2022,
9. (a)Discuss different methods of speed control of a d.c. series motor by adjusting field ampere turns.
(b)A 230 V, d.c. shunt motor runs at 1000 r.p.m. and takes 5 amperes. The armature resistance of the motor is 0.025 52 and shunt field resistance is 230 Ω. Calculate the drop in speed when the motor is loaded and takes the line current of 41 amperes. Neglect armature reaction.
APRIL 2024, JAN 2024, DEC 2023, JULY 2023,
JUNE-2024
1.With reference to electronic control systems
a) Draw a simple block diagram for temperature control
b) Describe each component shown in the diagram in
JUN 2024, MAR 2023, JAN 2024, OCT 2023,
2.Compare methods of obtaining speed regulation of three-phase induction motor generally used in tankers by means of:
A. Rotor resistance;
B. Cascade system;
C. Pole changing. Give examples where each system may be employed with advantage.
JAN 2024 APRIL 2023, OCT 2022, AUG 2022,
3. With reference to a 3 speed a.c. cage motor driven cargo winch:
a) Sketch a circuit diagram for a pole change motor
b) Describe how speed change and braking are achieved
3. Pole Change Motor for a 3-Speed A.C. Cage Motor Driven Cargo Winch
JUN 2024, SEP 2023, JAN 2023, APRI 2023,
4. What is a marine high voltage system? Sketch and describe a shipboard high voltage switch board and its protective devices.
JAN 2024, MAR 2023,
Q.5 Differentiate between half and full wave rectification. State where half wave rectification may be used and the purpose for which it is not well adapted. Sketch a bridge connection by which full wave rectification may be obtained.
JUN 2024, AUG 2023, MAR 2023,
SECTION-2
6.a) Explain what is meant by the terms wave form, frequency and average value.
b) A moving coil ammeter, a thermal ammeter and a rectifier are connected in series with a resistor across a 110 V sinusoidal a c. supply. The circuit has a resistance of 50Ω to current in one direction and, due to the rectifier, an infinite resistance to current in the reverse direction. Calculate:
(i) the readings on the ammeters.
(ii)the form and peak factors of the current wave
Jun2024, Oct 2019 , July2019 , Nov 2019 , Sept 2024
Answer -
a) Definitions
Waveform A waveform is a graphical representation of the variation of a signal with respect to time. It shows how the value of the signal (such as voltage, current, or displacement) changes over time. Common waveforms include sinusoidal, square, triangular, and sawtooth shapes.
Frequency Frequency is the number of cycles of a waveform that occur in one second. It is measured in Hertz (Hz). For a sinusoidal waveform, one cycle is a complete oscillation from one peak to the next. Frequency is the reciprocal of the period (T), which is the time taken for one complete cycle. Mathematically,
Average Value The average value of a waveform is the arithmetic mean of all the instantaneous values over one complete cycle. For a symmetrical waveform like a sinusoidal wave, the average value over one complete cycle is zero, but if considering only the positive half-cycle, the average value can be calculated. For a full rectified sine wave, the average value can be calculated as:
7.a) Why is it important to maintain high efficiency of operation and low values of voltage regulation for power transformers?
b) A 100 kVA transformer has 400 turns on the primary and 80 turns on the secondary. The primary and secondary resistances are 0.3 2 and 0.010 respectively, and the corresponding leakage reactance’s are 1.1 Q and 0.035 Ω respectively. The supply voltage is 2200 V. Calculate:
(i) The equivalent impedence referred to the primary circuit;
(II) The voltage regulation and secondary terminal voltage for full load having a power factor of (i) 0.8 lagging and (ii) 0.8 leading
JUN2024, JAN 2023, MAR 2023, OCT 2022,
8.a) List the factors that determine the starting torque of the three-phase induction motor, how does this torque generally compare with the value of the rated torque?
b) The low-voltage release of an a.c. motor-starter consists of a solenoid into which an iron plunger is drawn against a spring. The resistance of the solenoid is 35 ohm.When connected to a 220 V, 50 Hz, a.c. supply the current taken is at first 2 A, and when the plunger is drawn into the "full-in" position the current falls to 0.7 A. Calculate the Inductance of the solenoid
JUN2024, OCT 2023, JUN2023, JAN 2023,
9 a) With the aid of delta and star connection diagrams, state the basic equation from which the delta-star and star- delta conversion equation can be derived.
b) Three batteries A, B, and C have their negative terminals connected together, between the positive terminals of A and B there is a resistor of 0.5 ohm and between B and C there is a resistor of 0.3 ohm
Battery A 105 V, Internal resistance 0.25 ohm.
Battery B 100 V, Internal resistance 0.2 ohm
Battery C 95 V, Intemal resistance 0.25 ohm
Determine the current values in the two resistors and the power dissipated by them.
JUN 2024, OCT 2023, JUN2023, JAN 2023,
10) Compare the effectiveness of a current limiting circuit breaker with that of a HRC fuse
b)A coil having a resistance of 10 ohm and an inductance of 0.15 H is connected in series with a capacitor across a 100 V, 50 Hz supply. If the current and the voltage are in phase what will be the value of the current in the circuit and the voltage drop across the coll?
JAN 2024, MAR 2024, SEP 2023, MAR 2022,
JULY – 2024
1.a) Give a brief outline of the care and maintenance that should be given to the stator and rotor of an A.C. generator.
b) Explain what is likely to occur if the driving power of one A.C. generator suddenly fails when two generators are running in parallel. What safety devices are usually provided for such events?
ANSWER- a)
Care and Maintenance of the Stator and Rotor of an AC Generator Stator Maintenance:
Insulation Inspection: Regularly inspect the insulation of the stator windings for any signs of wear, damage, or contamination. Use insulation resistance testers to check the insulation resistance values and ensure they meet the manufacturer's specifications.
Cleaning: Keep the stator windings clean and free of dust, dirt, and moisture. Use compressed air or a vacuum cleaner to remove debris from the stator slots and core. Avoid using abrasive materials that might damage the insulation.
Cooling System Maintenance: Ensure that the cooling system, whether air or liquid-cooled, is functioning properly. Check coolant levels, cleanliness, and temperature controls. Clean or replace cooling fans and filters as needed.
Ventilation: Ensure proper ventilation around the stator to prevent overheating. Check that ventilation openings are not blocked and that cooling ducts are clear.
Alignment and Balancing: Verify that the stator is correctly aligned and that any attached components are securely fastened. Regularly check for signs of imbalance that could affect the generator's performance.
Rotor Maintenance:
Inspection of Rotor Windings: Check the rotor windings for signs of damage or overheating. Inspect slip rings, brushes, and commutators for wear and proper alignment.
Cleaning: Clean the rotor and associated components to prevent buildup of dust, carbon, or other contaminants. Ensure that slip rings and commutators are clean and free from debris.
Bearings and Shaft: Regularly inspect and lubricate the rotor bearings. Check for signs of wear, vibration, or unusual noise. Ensure proper alignment of the rotor shaft.
Inspection of Cooling: Verify that the rotor cooling system (if applicable) is working properly. Clean or replace any cooling components as necessary.
Balance and Vibration: Check the rotor for balance and vibration. Imbalances can lead to excessive wear on bearings and other components.
General Maintenance Tips:
Monitoring and Testing: Regularly monitor the performance of the generator, including voltage, current, and temperature. Perform routine tests to identify potential issues before they become serious problems.
Documentation: Keep detailed records of maintenance activities, inspections, and repairs to track the condition of the generator and plan for future maintenance.
b) Failure of One AC Generator in Parallel Operation
Effects of One Generator Failing:
Load Redistribution: When one generator fails, the remaining generator(s) must handle the entire load. This sudden increase in load can cause a temporary drop in voltage or frequency if the remaining generators are not capable of handling the additional load.
System Stability: The sudden failure of a generator can affect the stability of the power system, potentially leading to voltage fluctuations, frequency deviations, or even blackouts if the system is not properly managed.
Overloading: The remaining generators may become overloaded, leading to potential damage or failure if they are not designed to handle such sudden changes in load.
Safety Devices and Measures:
Automatic Voltage Regulators (AVR): AVRs help maintain the voltage levels by adjusting the excitation of the remaining generators. They ensure that voltage remains stable despite the loss of one generator.
Load Shedding: Load shedding devices automatically reduce the load on the remaining generators by disconnecting non-essential loads. This helps prevent overloading and ensures system stability.
Generator Protection Relays: Protection relays monitor generator performance and detect faults. They can isolate a faulty generator from the system to prevent further damage and maintain system stability.
Synchronization Equipment: Synchronization equipment ensures that the remaining generators are properly synchronized with the power grid, helping to prevent issues with voltage and frequency.
Emergency Backup Systems: Some power systems are equipped with emergency backup generators or uninterruptible power supplies (UPS) that can take over the load in the event of a generator failure.
Alarm and Notification Systems: Alarm systems alert operators to the failure of a generator and any associated issues. Proper notification allows for a quick response to mitigate potential problems.
Q.2.Compare methods of obtaining speed regulation of three-phase induction motors generally used in tankers by means of
a) Rotor resistance
b) Cascade system
c)Pole-changing
Give examples where each system may be employed with advantage.
3.a) Explain how the efficiency and regulation of a transformer can be assessed by open circuit and short circuit tests?
b) What is meant by equivalent resistance?
c)What is meant by all day transformer efficiency?
4. With reference to a 3 speed A.C. cage motor driven cargo winch:
a) Sketch a circuit diagram for a pole change motor
b) Describe how speed changes and braking are achieved.
5. With reference to the condition monitoring of electrical machinery:
a) State the important parameters that may be recorded
b) Explain how the parameters are measured and what defects may be revealed.
SECTION-II
6.a) Explain the significance of the root-mean- square value of an alternating current or voltage waveform. Define the form factor of such a wave form.
b) A total load of 8000 kW at 0.8 power factor is supplied by two alternators in parallel. One alternator supplies 6000kW at 0.9 power factor. Find the kVA rating of the other alternator and the power factor.
7.a) Electric motors contain a stationary member as well as a rotating member. For each of the following machines, identify in which part of the motor three field winding and the armature winding are located: three phase induction motor, three phase synchronous motor, d.c. motor.
b) A 220 V, d.c. shunt motor has an armature resistance of 0.5 ohm and an armature current of 40 A on full load. Determine the reduction in flux necessary for a 50 per cent reduction in speed. The torque for both conditions can be assumed to remain constant.
8.a) Show how the power that is transferred across the air gap of the three-phase induction motor is represented. Explain the terms. What portion of this is useful power?
b) A 440 V load of 400 kW at 0.8 (lagging) power factor is jointly supplied by two alternators A and B. The kW load on A is 150 kW and the kVAr load on B is 150 kVAr (lagging).
Determine the kW load on B, the kWAr load on A, the power factor of operation on each machine and the current loading of each machine.
9. Explain the preference for a 60 Hz system. Describe the dangers of running a 50 Hz system from a 60 Hz supply.
b) A ring-main, 900m long, is supplied at a point A at a p. d. of 220V. At a point B, 240m from A, a load of 45A is drawn from the main, and at a point C, 580m from A, measured in some direction, a load of 78A is taken from the main. If the resistance of the main (lead and return) is 0.25 ohm per kilometer, calculate the current which will flow in each direction round the main from the supply point A and the potential difference across the main, at the load where it is lowest.
10 a) (i) What is direct-connected alternator?
(ii) How is a direct-connected exciter arranged in an alternator?
b) Find the synchronous impedance and reactance of an alternator in which a given field current produces an armature current of 200 A on short circuit and a generated e. m. f. of 50V on open- circuit. The armature resistance is 0.1 ohm. To what induced voltage must the alternator be excited if it is to deliver a load of 100 A at a p.f. of 0.8 lagging, with a terminal voltage of 200 V.
(B) Solution :
Given Data :
Open circuit voltage – 50V
, V = 200 V
= 0.8 (lagging)
Find : E
= 
= 0.25 Ω (synchronous impedance)
= 
(synchronous reactance)
As , cos ф = 0.8 
0.6
Now ,
E = 
E = 
E = 222.082 V
Induced voltage must the alternator be excited to deliver 100 A load at p.f. of 0.8 lagging is 222.083 V.
AUG – 2024
Q.1 a) Describe the working principle of a Programmable Logic Controller (PLC) used in ship automation systems. How does a PLC enhance the safety and efficiency of shipboard operations?
b) Explain the process of programming a PLC for an emergency shutdown sequence on a marine engine. What considerations should be taken into account to ensure reliable operation in critical situations?
Q.2 a) Explain the construction and working principle of a three-phase induction motor used on ships. How is the direction of rotation of the motor reversed?
b) Discuss the maintenance procedures for an electric propulsion motor on a ship What common faults can occur in the motor, and how would you diagnose and repair them?
Q.3 With reference to electronic control systems
(a) Draw a simple block diagram for temperature control.
(B) describe each component shown in the diagram in (a).
Q.4 a) Explain the concept of power factor in electrical systems. Why is it important to maintain a high-power factor on board a ship?
b) Discuss the methods used to correct power factor on ships. How does power factor correction improve the efficiency of the electrical system?
Explain the matching of an induction electric motor to a pump required for main circulating duty, with the aid of pump characteristic and torque/ship diagrams.
Q.6A. Explain what is meant by the terms wave form, frequency and average value.
B. A moving coil ammeter, a thermal ammeter and a rectifier are connected in series with a resistor across a 110 V sinusoidal a.c. supply. The circuit has a resistance of 50 to current in one direction and, due to the rectifier, an infinite resistance to current in the reverse direction. Calculate:
(i) The readings on the ammeters; (ii) The form and peak factors of the current wave.
Q.7A. Why is it important to maintain high efficiency of operation? And low values of voltages regulation for power transformers?
B. A 100 KVA transformer has 400 turns on the primary and 80 turns on the secondary. The primary and secondary resistances are 0.3 and 0.01 respectively, and the corresponding leakage reactance's are 1.1 and 0.035 respectively. The supply voltage is 2200 V. Calculate:
(i) The equivalent impedance referred to the primary circuit;
(ii) The voltage regulation and secondary terminal voltage for full load having a power factor of (i) 0.8 lagging and (ii) 0.8 leading.
Q.8 A. List the factors that determine the starting torque of the three- phase induction motor. How does this torque generally compare with the value of the rated torque?
B. The low-voltage release of an a.c. motor-starter consists of a solenoid into which an iron plunger is drawn against a spring. The resistance of the solenoid is 35 ohm. When connected to a 220 V, 50 Hz, a.c. supply the current taken is at first 2A, and when the plunger is drawn into the "full-in" position the current falls to 0.7 A.
Calculate the inductance of the solenoid for both positions of the plunger, and the maximum value of flux-linkages in weber-turns for the "full-in" position of the plunger.
Q.9A. With the aid of delta and star connection diagrams, state the basic equation from which delta - star - delta conversion equation can be derived.
B. Three batteries A, B and C have their negative terminals connected together, between the positive terminals of A and B there is a resistor of 0.5 ohm and between B and C three is a resistor of 0.3 ohm,
i. Battery A 105 V, Internal resistance 0.25 ohm'
ii. Battery B 100 V, Internal resistance 0.2 ohm
iii. Battery C 95 V, Internal resistance 0.25 ohm
Determine the current values in the two resistors and the power dissipated by them
Q.10a) Compare the effectiveness of a current limiting circuit breaker with that of a HRC fuse.
(b) A coil having a resistance of 10 Ohm, and an inductance of 0.15 H is connected in series with a capacitor across a 100V, 50Hz supply. If the current and the voltage are in phase what will be the value of the current in the circuit and the voltage drop across the coil?
SEPT – 2024
Q1. a) Explain the construction and the periodic maintenance required of a Vacuum Circuit Breaker (VCB).
b) Compare the construction, operation, and usage of Vacuum Circuit Breakers (VCB) with Air Circuit Breakers (ACB).
Q2. a) Explain the construction and working principle of Star-Delta Starter with the help of circuit diagram. What are the advantages and limitations of using a Star-Delta Starter for starting an induction motor.
b) Describe the maintenance procedures for a motor starter. What are the common faults, and how would you troubleshoot them.
Q3. Explain the methods used to control the speed of a 3 Phase induction motors. Draw and Explain a Variable Frequency Drive used for optimization of energy efficiency of auxiliary machineries on board vessel.
Q4. What is the meant by "excitation" in an alternator? With the help of a neat diagram of brushless alternator labelling all the important parts. explain how the excitation is achieved in a brushless alternator.
Q5. a) Explain the construction, working principle, and characteristics of a Zener Diode. Discuss its applications in electronic circuits.
b) What is a Zener Barrier? With the help of a diagram, explain how a Zener Barrier works in an intrinsic Safe Circuit and discuss its importance in hazardous environments.
SECTION-II
Q6. A. What are the characteristics of PN junction diode? Point out its specifications. Also point out the significance of dynamic and static resistances.
B. A 72 KVA transformer supplies a heating and lighting load of 12 KW at unity power factor and a motor load of 70 kVA at 0.766 (lagging) power factor: Calculate the minimum rating of the power-factor improvement capacitors which must be connected in the circuit the ensure that the transformer does not become overloaded.
Q7. A. Which of the following three motors has the poorest speed regulation: shunt motor, series Motor or cumulative compound motor? Explain.
B. A 440V shunt motor takes an armature current of 30A at 700 rev/min. The armature resistance is 0.7ohm. If the flux is suddenly reduced by 20 per cent, to what value will the armature current rise momentarily? Assuming unchanged resisting torque to motion, what will be the new steady values of speed and armature current? Sketch graphs showing armature current and speed as functions of time during the transition from initial to final, steady-state conditions.
Q8. A. What is back emf? Derive the relation for the back emf and the supplied voltage in terms of armature resistance.
B. A three phase induction motor is wound for four poles and is supplied from a 50 Hz system. Calculate.
i. The synchronous speed:
ii. The speed of the rotor when the slip is 4 per cent:
iii . The rotor frequency when the speed of the rotor is 600 r/min.
Answer – Back electromotive force (back EMF) is the voltage generated by a motor when it is running and its armature (rotor) is turning in a magnetic field. It opposes the supply voltage and is a direct consequence of Faraday's Law of Electromagnetic Induction. In a DC motor, as the armature rotates, it cuts through the magnetic field lines, inducing a voltage that opposes the applied voltage.
Working Principle: When a DC motor is powered, the armature starts to rotate due to the torque generated by the interaction between the current in the armature windings and the magnetic field. As the armature rotates, it cuts the magnetic flux, which induces a voltage (back EMF) in the opposite direction to the supply voltage.
Back EMF limits the speed of the motor. The faster the motor runs, the higher the back EMF, which reduces the net voltage driving the current through the motor, and thus limits the speed.
Protective mechanism: At high speeds, back EMF reduces the armature current, protecting the motor from drawing excessive current and overheating.
Derivation of Back EMF:
-
V be the applied supply voltage.
-
Eb be the back EMF generated in the armature.
-
Ia be the armature current.
-
Ra be the armature resistance.
From the voltage equation of a DC motor, the net voltage across the armature is given by the difference between the supply voltage V and the back EMF Eb, which causes current Ia to flow through the armature resistance Ra.
Using Ohm’s Law:
V−Eb = IaRa
This can be rearranged to express the back EMF in terms of the applied voltage, armature current, and armature resistance:
Eb = V − IaRa
Relation Between Back EMF and Armature Speed:
The back EMF Eb is also proportional to the speed of the motor. It can be expressed as:
Eb = ke Φ N
Where:
-
ke is a constant based on the construction of the motor.
-
Φ is the flux per pole of the magnetic field.
-
N is the speed of the motor (in RPM).
Thus, as the motor speed N increases, the back EMF Eb increases, reducing the effective voltage across the armature, and limiting the armature current Ia. This keeps the motor from accelerating indefinitely under constant supply voltage.
In summary, the back EMF opposes the applied voltage, and its magnitude depends on the motor’s speed and the magnetic field. The equation Eb=V−IaRa relates back EMF to the supply voltage and armature resistance.
(B) solution :
p = H, f = 50 H2
i) synchronous speed. Ns = 
= 
NS = 1500rpm
ii) speed of rotor, when slip is 4 %
S% = 
×
100
0.04 = 
NR = 1440 rpm
iii) speed of rotor is 600 r/min.
= 0.6
Rotor frequency = 50× 0.6
Rotor frequency = 30 H2
Q9. A. What are the factors which determine the synchronous speed of a motor?
B. Three conductors fitted side by side in the stator of a salient-pole alternator. Each generates maximum voltage of 200V (sinusoidal). The angle subtended at the center of the stator between adjacent conductors is 20 electrical degrees. If the three conductors are connected in series, find (i) the r.m.s. value of the effective voltage and
(ii) the 'breadth factor. Using the theory that is the basis of this problem, give one reason why three-phase current has been introduced.
Q10. A. Compare the effectiveness of a current limiting circuit breaker with that of a HRC fuse.
B. A coil having a resistance of 10 Ohm, and an inductance of 0.15 H is connected in series with a capacitor across a 100 V, 50 Hz supply. If the current and the voltage are in phase what will be the value of the current in the circuit and the voltage drop across the coil?