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Marine power systems are also known as marine engines or marine propulsion systems. They are used in workboats, ferries and military craft and have been around for centuries. A marine engine can be either a separate unit or built into the drive system of the boat itself. Marine power is measured by its ability to produce torque, which is the turning force that moves your boat through the water. While it may not seem obvious at first, the more powerful an engine is, the faster your boat will go. The electrical plant on a ship typically includes generators, a distribution system, and many devices that use electricity for their motor drives. Electricity is also used for various auxiliaries, deck machinery and equipment such as ventilation and air conditioning units. Electrical distribution on ships consists of a central switchboard called a DC power system and an AC distribution system. The DC power system provides direct current (DC) electrical power or 3-phase, 4-wire AC at 400 Hz or 50 Hz to all locations in the ship. In contrast, the AC distribution system provides single-phase, 2-wire and 3-phase, 3-wire voltages in addition to the 4-wire voltages.

A ship's machinery and equipment require a constant source of electricity for safe operation, so standby or emergency power generators are necessary. Emergency power generators may consist of automatic-starting alternators or batteries. The electrical equipment used in a power plant will include generators, switches for control and distribution of electricity, motors for turbines and their associated starting equipment, and backup systems in case of emergency.

AC or DC?

Alternating current has largely replaced direct current as the standard source of power for marine installations. AC has several advantages: for example, reduced first cost, less weight and space required, and lower maintenance requirements. Direct current, however, is useful in controlling motors to provide different speeds using a system known as the Ward-Leonard control method.

Machine Capacity and Ratings

DC and AC motors and generators are both classified as Continuous-service machines, which means they can handle momentary and even moderate overloads for an extended period.

Temperature affects the performance and useful life of electrical equipment. The total temperature of an operating machine is a result of the ambient air temperature, the heating effect of current in the windings, and power dissipation within the machine. Temperature rise is calculated based on the total temperature. Proper ventilation is crucial for electrical equipment. Different insulation classes have established requirements set by Classification Societies. Classes E, B and F are commonly used in marine installations and each class has a specific insulation material requirement and allows for increasing temperature rises in that order.

Enclosures

Motor and generator enclosures vary depending on their location. Drip-proof enclosures are the most common and protect against liquid drips or air drawn into the enclosure by ventilating air.  A watertight enclosure offers protection for submergence under a low water level for up to one hour. The term 'weatherproof' is used to describe the ability of a machine to function in a wide variety of weather conditions. The term 'hose proof' refers to a machinery space that will not allow moisture to enter via rubber hoses. The term 'deck watertight' refers to the ability of machinery spaces on boats to prevent water from entering through any opening in the ship's deck. A flameproof enclosure can withstand the explosion of flammable gas within it. The enclosure must also contain any fire or explosion that occurs within it.

High-efficiency electric motors for the marine industry

Electric motors are not 100% efficient, but their energy efficiency can be as high as 75% and often hovers around 90%. Thus, it makes sense to choose electric motors with high energy efficiency for a ship because this will save energy in the long term. With the advent of energy-saving standards for electric motors and increased efforts to make them more energy efficient, the choice of electric motors for ships must take into account energy efficiency early in the shipbuilding process. For example, when an electric motor needs to be replaced on an existing ship, procurement of an energy-efficient electric motor should be considered.

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