aux 2 new pp1 Flashcards
Describe, with the aid of a sketch, a variable frequency drive for speed control of a three phase motor. (10)
A variable frequency drive (VFD) is a device used to control the speed of a three-phase AC motor. It works by adjusting the frequency of the voltage supplied to the motor, which in turn adjusts its speed. A typical VFD system consists of several components, including an AC/DC rectifier, a DC link, an inverter, and a microprocessor control unit.
AC/DC rectifier: The AC power supply is fed into the rectifier, which converts it into a DC voltage.
DC link: The DC voltage from the rectifier is stored in a DC link, which acts as an energy reservoir.
Inverter: The inverter takes the DC voltage from the DC link and converts it back into AC voltage, but with a variable frequency. The frequency of the voltage is adjusted based on the desired speed of the motor.
Microprocessor control unit: The microprocessor control unit manages the overall operation of the VFD, including controlling the frequency of the voltage supplied to the motor. The control unit receives input from various sensors and adjusts the frequency of the voltage accordingly.
In summary, a VFD works by converting the AC power supply into DC voltage, storing the DC voltage in a DC link, converting it back into AC voltage with a variable frequency, and controlling the frequency based on the desired speed of the motor.
| | |
With reference to a crane operated by a constant pressure hydraulic system incorporating unidirectional, fixed displacement pumps that run continuously, explain EACH of the following:
(a) the purpose of the accumulator; (2)
(b) how the hydraulic pressure is regulated; (2)
(c) how the speed and direction of the hoist motor is varied; (3)
(d) how the torque available from the hoist motor can be varied. (3)
(a) The purpose of the accumulator:
The purpose of the accumulator in a constant pressure hydraulic system is to store energy and provide additional fluid volume to the system when necessary. When the fluid volume in the system decreases, the accumulator acts as a reservoir, supplying additional fluid to maintain a constant pressure. This helps to ensure that the hydraulic system can operate smoothly, even when the demand for fluid flow changes suddenly.
(b) How the hydraulic pressure is regulated:
In a constant pressure hydraulic system, the hydraulic pressure is regulated by a pressure control valve. The valve monitors the pressure in the system and adjusts the flow of fluid to the actuators to maintain a constant pressure. If the pressure in the system increases, the valve restricts the flow of fluid to the actuators, reducing the pressure. Conversely, if the pressure in the system decreases, the valve increases the flow of fluid to the actuators, increasing the pressure.
(c) How the speed and direction of the hoist motor is varied:
The speed and direction of the hoist motor in a crane operated by a constant pressure hydraulic system is controlled by a directional control valve. The valve directs the flow of fluid to the hoist motor in either direction, depending on the desired direction of travel. The speed of the hoist motor is controlled by adjusting the flow rate of fluid to the motor. By controlling the flow rate, the valve regulates the speed of the motor and, in turn, the speed of the hoist.
(d) How the torque available from the hoist motor can be varied:
The torque available from the hoist motor in a constant pressure hydraulic system can be varied by adjusting the pressure of the fluid supplied to the motor. By increasing the pressure, the torque available from the motor increases, and vice versa. This allows for precise control of the hoist, enabling it to lift heavy loads with ease or to perform delicate operations with precision. Additionally, the torque can also be adjusted by controlling the displacement of the fixed displacement pumps, which determine the flow rate of fluid to the hoist motor.
With reference to variable speed control of a 3 phase ac induction motors:
(a) explain why EACH of the following is not preferred:
(i) variable voltage, constant frequency; (3)
(ii) variable frequency, constant voltage. (3)
(b) explain why voltage and frequency should both be varied. (4)
(a)
(i) Variable voltage, constant frequency:
Variable voltage, constant frequency control of a 3-phase AC induction motor is not preferred because it can cause mechanical stress and result in reduced efficiency of the motor. When the voltage supplied to the motor is increased, the magnetic field intensity in the motor also increases. This increase in magnetic field intensity can cause increased mechanical stress on the motor, leading to damage or failure. Additionally, the efficiency of the motor decreases as the voltage is increased, reducing its overall performance.
(ii) Variable frequency, constant voltage:
Variable frequency, constant voltage control of a 3-phase AC induction motor is not preferred because it can result in reduced starting torque and power factor. The starting torque of an AC induction motor is directly proportional to the frequency of the voltage supplied to it. If the frequency is reduced while the voltage remains constant, the starting torque of the motor is reduced, making it more difficult to start the motor or to move heavy loads. Additionally, the power factor of the motor decreases as the frequency is reduced, reducing its overall efficiency.
(b) Why voltage and frequency should both be varied:
Voltage and frequency should both be varied in a 3-phase AC induction motor for efficient and reliable operation. By varying both the voltage and the frequency, the magnetic field intensity and the speed of the motor can be adjusted, allowing for precise control of the motor. This precise control results in reduced mechanical stress on the motor, increased efficiency, and improved starting torque and power factor. By controlling both the voltage and the frequency, the motor can be operated at its optimal performance levels, ensuring reliable and efficient operation.
Describe, with the aid of a sketch, a Direct Expansion Regrigeration System for an air cooler in an air conditioning installation. (10)
A Direct Expansion (DX) refrigeration system for an air cooler in an air conditioning installation consists of several key components, including a compressor, an evaporator, an expansion valve, and a condenser. The system works by removing heat from the indoor air and rejecting it to the outdoor environment.
DX System Diagram
Compressor: The compressor is the heart of the DX refrigeration system. It compress the refrigerant gas, raising its temperature and pressure, and circulates it throughout the system.
Evaporator: The refrigerant gas is then passed through the evaporator, which is installed inside the air cooler. The refrigerant absorbs heat from the indoor air as it evaporates, lowering the temperature of the air.
Expansion Valve: The high-pressure refrigerant gas from the compressor then flows through the expansion valve, which reduces its pressure, causing it to cool and become a low-pressure refrigerant.
Condenser: The low-pressure refrigerant gas then flows to the condenser, which is installed outside the air cooler. The condenser releases the absorbed heat from the indoor air to the outdoor environment, and the refrigerant condenses back into a liquid form.
Refrigerant Cycle: The liquid refrigerant then returns to the compressor, and the cycle repeats, continuously removing heat from the indoor air and rejecting it to the outdoor environment.
This system is used to cool the air inside the air cooler, which is then distributed throughout the air conditioning installation to provide comfortable indoor air temperatures. The DX refrigeration system is reliable, efficient, and widely used in air conditioning installations.
Describe, with the aid of a sketch, how the relative humidity may be controlled in an Air Conditioning System.
Relative humidity in an air conditioning system can be controlled by using a humidifier or dehumidifier. The goal is to maintain the relative humidity within a comfortable range, typically between 30% and 60%.
A humidifier adds moisture to the air, increasing the relative humidity, while a dehumidifier removes moisture from the air, decreasing the relative humidity.
Humidity Control in Air Conditioning System
Air Handling Unit (AHU): The air handling unit (AHU) is responsible for circulating the air throughout the air conditioning system. The air passes through filters to remove any particles and is then cooled or heated as required.
Humidifier: The air from the AHU is then passed through a humidifier, where it is moistened by a stream of water. The amount of water added can be controlled by adjusting the flow rate of the water or the amount of time the air is in contact with the water.
Dehumidifier: If the relative humidity is too high, the air is passed through a dehumidifier, where moisture is removed from the air. This can be done using a desiccant material that adsorbs moisture from the air or by using a refrigeration-based system that condenses the moisture from the air.
Temperature and Humidity Sensors: The relative humidity is monitored by temperature and humidity sensors, which provide feedback to the control system. The control system adjusts the humidifier and dehumidifier as required to maintain the relative humidity within a comfortable range.
By controlling the relative humidity in this way, the air conditioning system can provide a comfortable indoor environment while also reducing the growth of mold, bacteria, and other contaminants.
Describe the effects of EACH of the following contaminants when found in air required for breathing and diving use:
(a) oil vapour; (4)
(b) water vapour; (4)
(c) overheated oil. (2)
(a) Oil Vapor:
Oil vapor in air that is required for breathing and diving can have several negative effects on human health and safety. These include:
Respiratory Irritation: Oil vapor can irritate the eyes, nose, and throat, causing symptoms such as coughing, sneezing, and shortness of breath.
Toxicity: Depending on the type of oil, exposure to high levels of oil vapor can be toxic and can cause symptoms such as headaches, dizziness, and nausea.
Asphyxiation: In high concentrations, oil vapor can displace the oxygen in the air, causing asphyxiation.
Fire Hazard: Oil vapor can be flammable and can increase the risk of fire in the air supply system.
(b) Water Vapor:
Water vapor in air that is required for breathing and diving can also have negative effects:
Reduced Oxygen Concentration: Water vapor can reduce the concentration of oxygen in the air, which can be dangerous for diving operations.
Corrosion: Water vapor can cause corrosion in the air supply system and other equipment.
Condensation: Water vapor can condense into droplets, which can clog air filters and affect the performance of air compressors and other equipment.
Reduced Visibility: Water vapor can also reduce visibility in diving operations.
(c) Overheated Oil:
Overheated oil in air that is required for breathing and diving can have the following effects:
Toxicity: Overheated oil can release toxic fumes that can be harmful to human health.
Fire Hazard: Overheated oil can also be a fire hazard and can increase the risk of fire in the air supply system.
It is important to monitor air quality in breathing and diving operations and to take steps to remove or mitigate the effects of contaminants such as oil vapor, water vapor, and overheated oil. This can include using filters, drying systems, and monitoring equipment to ensure that the air supply remains safe and clean.
With reference to the Code of Safe Working Practices for Merchant Seamen and maintenance of lifting equipment:
(a) outline FIVE maintenance procedures to be carried out on lifting equipment; (5)
(b) state who should carry out the examination of the lifting equipment, the interval
between examinations and the defects that may be found. (5)
(a) Five Maintenance Procedures to be Carried Out on Lifting Equipment:
Inspections: Regular visual inspections should be carried out to identify any visible damage, wear, or corrosion on the lifting equipment.
Testing: Lifting equipment should be tested regularly to ensure that it is safe to use and that it meets the required safety standards. This can include load tests, proof load tests, and functional tests.
Lubrication: Regular lubrication of the lifting equipment should be carried out to reduce wear and extend its lifespan.
Repairs: Any defects identified during inspections or testing should be repaired promptly to ensure that the lifting equipment remains safe to use.
Record Keeping: Detailed records of maintenance and inspection activities should be kept to ensure that the lifting equipment is maintained to the required standards and to demonstrate compliance with safety regulations.
(b) Examination of Lifting Equipment:
Who: The examination of lifting equipment should be carried out by a competent person, such as a qualified engineer or inspector.
Interval: The frequency of the examination depends on the type of equipment and the conditions in which it is used, but it should be carried out at least annually, or more often if required by regulations.
Defects: The examination should identify any defects in the lifting equipment, including cracks, corrosion, wear, or damage to components such as chains, ropes, and hooks. The examination should also assess the overall condition of the equipment and determine whether it is safe to use.
Describe the procedure that should be followed if an outboard motor has been submerged in
sea water. (10)
The following steps should be followed if an outboard motor has been submerged in sea water:
Disconnect the battery: Disconnect the battery to prevent any electrical damage to the motor or the battery.
Rinse with fresh water: Rinse the entire outboard motor with fresh water to remove any salt or debris that may have accumulated on the surface. This should be done as soon as possible after removal from the water to minimize corrosion.
Drain the motor: Remove the spark plugs and drain any water from the motor’s internal components. Also, drain the fuel system and carburetor to remove any water or contaminants.
Check for damage: Carefully inspect the motor for any signs of damage, such as corrosion or bent components. If any significant damage is found, it should be repaired by a qualified mechanic.
Lubricate: Lubricate all moving parts, such as the shifter and throttle cables, to prevent corrosion and ensure smooth operation.
Clean the motor: Clean the motor thoroughly, including the exterior and all internal components, to remove any remaining salt or debris.
Replace the spark plugs: Replace the spark plugs with new ones to ensure that the engine starts easily.
Reinstall the battery: Reinstall the battery and check the electrical system for any damage or corrosion.
Fill the fuel tank: Fill the fuel tank with fresh fuel to minimize the risk of contamination.
Test run: Finally, run the motor on a test stand or in the water to ensure that it is functioning properly and that there are no leaks or other problems. If any issues are found, they should be addressed before operating the motor.
It is important to follow these steps carefully to ensure that the outboard motor is restored to good working order and that it remains safe to use.
If your engine cannot be serviced within three or four hours after recovery, it should be submerged in fresh water until a complete recovery procedure can be performed. Once the engine is exposed to air, the cor- rosive action of the water in and on the engine will begin, so you need to minimize this exposure.
The first step is to disable the ignition by discon- necting the plug that connects your ignition power pack to the charge coil under the flywheel. Next, remove the spark plugs and lay the engine in a position so that the spark-plug holes are facing downward. Next, spin the engine over by turning the flywheel by hand. You will see water pumping out of the spark-plug holes.
Once all the water is drained from the cylinders, mount the engine in a vertical position and disconnect the fuel lines. If you can reach it, there may be a drain screw in the float bowl of your engine’s carburetor. Remove the screws and drain the water from all the carburetors.
Next, spray some outboard engine two-stroke oil into the spark-plug holes to coat the cylinder walls.
This will take care of the emergency first aid part of the recovery. The next step will require the assistance of your dealer.
All of the electrical equipment on your engine will have to be removed, thoroughly rinsed in fresh water, dried, and sprayed with WD-40 or a similar product to displace any remaining water, and then dried again and reinstalled. Note: Depending on the design of the electrical coils under your flywheel, your dealer may recommend replacement instead of simply cleaning and lubricating them. Unless the coils are completely epoxy-sealed (and many aren’t) water will have migrated into the metal laminates of the coil winding core, and rusting will have started already. As the rust progresses, it may actually expand the core and break the wire winding around it, causing premature failure of the charge, sensor, or lighting coil. Ask your dealer for his experience with engines of your type, and take his recommendation. This procedure will include the complete disassembly of electric starter motors, and disconnecting all electrical connector plugs for a thor- ough cleaning and drying. In addition, you or your dealer will have to remove and disassemble all carbu- retors on the engine and flush them out. Note that you can use the cleaning procedure described in Chapter 7 to take care of this.
It’s important to note that in many cases, simply re- moving the water from the cylinders, draining the car- buretors, and drying off the ignition system will get the engine to a point where it can probably be started and run. But neglecting the starter motor (if it has one) and the charge coils and sensor coils located under the fly- wheel will cause these components to fail prematurely, so don’t leave them out of the cleaning procedure just because the engine is running.
Once the engine is ready to run, give it a double oil mix (the same mixture that you’d use for new en- gine break-in) for the first few hours of operation to completely coat internal parts and force out any re- maining water.
Once the mechanical and electrical recovery steps have been completed, the exterior of the engine should be washed and waxed with a good grade of car wax to complete the procedure. If proper care has been taken in this revival process, you can rest assured your en- gine will live a normal life. But carelessness here will reduce your engine’s life expectancy by many years. Don’t cut corners with these procedures.
With reference to a vessel’s motion control:
(a) outline the SIX degrees of freedom; (3)
(b) explain the term damping; (4)
(c) state THREE considerations to be made, before the installation of a motion reduction
system. (3)
The six degrees of freedom for a vessel’s motion control are:
Surge: This is the forward and backward motion of the vessel along its longitudinal axis.
Sway: This is the side-to-side motion of the vessel along its transverse axis.
Heave: This is the vertical motion of the vessel, up and down, along its vertical axis.
Roll: This is the rotational motion of the vessel around its longitudinal axis.
Pitch: This is the rotational motion of the vessel around its transverse axis.
Yaw: This is the rotational motion of the vessel around its vertical axis.
(b) The term damping refers to the reduction of a system’s oscillation or vibration over time. In the context of vessel’s motion control, damping is the process of reducing the magnitude of the vessel’s movements in response to waves or other external forces.
(c) Three considerations to be made before the installation of a motion reduction system include:
Vessel type and specifications: The type and specifications of the vessel must be taken into account to determine the most appropriate motion reduction system for its size, weight, and operating conditions.
Operational conditions: The operational conditions of the vessel, such as sea state, operating area, and cargo, must be taken into account when selecting a motion reduction system.
Maintenance requirements: The maintenance requirements of the motion reduction system must be considered to ensure that it is easy to maintain and that it will not cause significant downtime for the vessel.
- With regards to Recycling of Ships:
reference to the International Convention for the Safe and Environmentally Sound
The operational requirements for compliance with the International Convention for the Safe and Environmentally Sound Recycling of Ships (known as the Hong Kong Convention) include:
Safe and environmentally sound ship recycling facilities: All ship recycling facilities must be designed and operated to ensure the safety of workers and the protection of the environment.
Inventory of hazardous materials: Each ship must maintain an inventory of hazardous materials, which must be made available to the ship recycling facility.
Management of hazardous waste: The proper management of hazardous waste, including its storage, treatment, and disposal, must be ensured.
Compliance with regulations: All ships and ship recycling facilities must comply with relevant international and national regulations and standards.
(b) Four prohibited hazardous materials listed in the Hong Kong Convention include:
Asbestos: Asbestos is a fibrous mineral that is known to cause lung cancer and mesothelioma.
Ozone-depleting substances: These are chemicals that are harmful to the ozone layer and contribute to climate change.
Polychlorinated biphenyls (PCBs): PCBs are toxic chemicals that are harmful to human health and the environment.
Persistent organic pollutants (POPs): POPs are toxic chemicals that are persistent in the environment and can cause harm to human health and wildlife.
(c) Two hazardous materials whose use should be restricted according to the Hong Kong Convention include:
Mercury: Mercury is a toxic heavy metal that is harmful to human health and the environment.
Lead: Lead is a toxic heavy metal that is harmful to human health and the environment.
