A ship consist of a power plant system which generates enough power for the propulsion plant machines, engine room and deck machines, and navigation equipment of the ship, along with daily life demands of the crew onboard which includes galley, lift and cabin power supply etc. For this, large quantities of fuel are burned within the prime mover which in turns rotates the rotator in the alternator and generates power. The burning of fuel increases the operational cost for owner, burdens the engine crew with maintenance of the generator and the most importantly, it causes air pollution.
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To eliminate the usage of independently driven generators when the ship is sailing in the mid sea, shaft generator concept is used. Shaft generator is a clean source of power, which means it does not burn any fuel to generate power, and for the same reason it is also called the green source of power.
Shat Generator: Principle of Working
In an A.C generator, in order to produce power, the stationary armature conductors are cut by the rotating magnetic field, produced by the rotation taken from the propeller shaft of the main propulsion plant or main engine.
The running machinery&#;s power is supplied through main switch board with constant voltage and frequency by diesel generator. In case of shaft generator, which is driven by main engine, the speed of the former may vary at different situations like ship sailing in traffic water and crossing canals, resulting in variation in voltage and frequency of shaft generator.
To overcome this deficiency, two systems are used Onboard ship
a) Power take off (PTO) system is incorporated with different kinds of frequency control system which makes sure of producing power with constant frequency.
b) Hybrid system consisting of an advanced power electric system for conditioning the power generated from shaft generator so that the supply to the switchboard always remains constant at any engine speed.
The modern application of the shaft generator includes its functioning as a motor by taking power from the electrical plant of the ship to drive the propeller at reduced speed.
This application is expensive to install and is used for vessels which moves very slowly or vessel which stays still most of the time.
Advantages of Shaft generator system:
1) The biggest advantage- it dose not cause air pollution unlike other traditional methods of power production in ship. Moreover, noise level is also low.
2) It is more cost effective as it dose not requires expensive fuel for power generation as main engine itself is a prime mover.
3) The wear and tear and hence the maintenance schedule and costs for the same reduces for independent driven generator.
4) Installation space is less as it is installed close or in line with the shaft of the main engine.
5) The investment cost depends on the type and system of the shaft generator but for a basic designed shaft generator it is low.
6) The installation cost for shaft generator is also low as it doesn&#;t require separate foundation, prime mover or exhaust system. Even time for installation is also less.
7) Low spare parts cost and man &#; hour cost as the schedule maintenance period for shaft generator has larger time gap as compared to diesel generator.
Disadvantages of Shaft generator:
1) For a basic shaft generator system, the efficiency of propeller and engine is reduced at low propulsion power. Since the frequency requirement is constant, for a main engine with a CPP, it has to run at constant speed even at low load.
2) No power generation in port as the prim mover is in stop condition.
3) Due to an additional attachment to shaft of the engine, the load in the engine also increases, resulting in increase in specific fuel and cylinder oil consumption when shaft generator is used.
4) Cannot cope up alone when the load demand is high as it may affect the main engine performance and maintenance.
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5) It requires gears, couplings and other complicated arrangement for installation in some system.
References : manbw
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The information contained in this website is for general information purposes only. While we endeavour to keep the information up to date and correct, we make no representations or warranties of any kind, express or implied, about the completeness, accuracy, reliability, suitability or availability with respect to the website or the information, products, services, or related graphics contained on the website for any purpose. Any reliance you place on such information is therefore strictly at your own risk.
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An ardent sailor and a techie, Anish Wankhede has voyaged on a number of ships as a marine engineer officer. He loves multitasking, networking, and troubleshooting. He is the one behind the unique creativity and aesthetics at Marine Insight.
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Brushless DC Motor &#; This type is the reverse of a brush-type DC motor: the permanent magnets are attached to the rotor and windings are attached to the housing. Since the windings don&#;t rotate, the need for brushes is eliminated. The advantages of a brushless DC motor are that it requires less maintenance than a DC motor with brushes, generates little or no EMI, and is more efficient. The disadvantages are that it is more expensive, generates a 3-phase AC output that requires a diode bridge to convert to DC, and the maximum allowable rpm are usually to rpm.
Alternator &#; A typical automotive or marine alternator is also a candidate for a prop shaft generator. It overcomes some of the issues of a DC motor. Since it&#;s meant to be coupled directly to an engine pulley, the maximum allowable speed is typically greater than 10,000 rpm; they are made by the millions, so the cost is relatively low; the output is easily regulated by varying the field current; they are very efficient; and they are self-cooling. There are a couple of disadvantages as well. Since they are meant to run at high rpm, unless the windings are rewound with finer wire, the output at low rpm is quite small. The biggest disadvantage, however, is that an alternator requires a typical field current of 3 to 5 amps. When an alternator is connected to an engine, it spins at thousands of rpm and the field current is negligible compared to the total output. When the boat is sailing, however, and the total output is only a few amps, the field current becomes significant. In fact, at lower boat speeds, the field current will be higher than the amperage produced by the alternator.
Note: To keep things simple, unless I&#;m referring specifically to one of the devices, I will use the generic term &#;generator&#; to refer to any of the three types of alternators/generators.
Prop Shaft Speed
When Nine of Cups is sailing, the prop shaft ranges from zero rpm at 3 knots to around 200 rpm on those rare occasions when she is doing 8 knots through the water. When we&#;re motoring, taking the speed reduction ratio of the transmission into account, the shaft rotates at between 400 and rpm. The generator speed will be some multiple of the shaft speed, depending on the ratio of the two pulleys. Ideally, I wanted the generator to produce at least a few amps at the low RPM of the shaft while sailing, yet be able to withstand the much higher RPM while motoring.
Pulley Sizes
The faster the generator spins, the more current it will generate. The ratio of the two pulley sizes determines how fast the generator will spin, which in turn determines how much current will be generated. To generate the maximum current, we want a very large pulley on the shaft and a very small pulley on the generator. On the other hand, when the engine is cranked on and we are motoring, the shaft will turn much faster than when we are sailing, potentially destroying the generator if it spins too fast. To determine the largest pulley ratio that can be used, divide the maximum allowable speed of the generator by the maximum prop shaft speed. For example, Nine of Cups has a maximum shaft speed of rpm. If I chose an alternator with a maximum speed of 10,000 rpm, the largest pulley I could safely use would have a diameter of 10,000/ = 7.1 times the diameter of the alternator pulley.
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