Electrical Propulsion

Electrical propulsion systems offer numerous advantages for ships that are subject to specific requirements. They are rated as particularly economical, environmentally friendly and reliable, offer considerable comfort in terms of operation and control, have optimal manoeuvring and positioning properties, low vibration and noise levels and additionally suitable the best possible utilisation of space owing to their reduced volume.

The electrical side of the system will be based on a direct current or an alternating current motor, coupled to the ship's propeller shaft, with speed and direction of propeller rotation being governed by electric control of the motor itself or by the alternation of the power apply.

An electrical propulsion arrangement for a ship is often described as a diesel-electric or turbo-electric system. It is characterized only by the type of prime mover, with no reference to lie type of electrical propulsion motor, the generator or the electrical power system.

The most commonly used diesel electric propulsion systems are not a new concept. In the past these systems were usually diesel engine driven d.c. generators that supplied d.c. motors. Their applications were generally limited to vessels that required a degree of low-speed manoeuvring.

Vessels such as ferries, harbour tugs and various other applications used diesel electric systems for features that were not available in mechanical systems at that time. Till date, electrical propulsion systems have been used mainly for specialized vessels rather than for cargo ships in general. These include dredgers, tugs, trawlers, lighthouse tenders, cable ships, icebreakers, research ships, floating cranes and vessels for the offshore industry.

Electric-drive systems have made substantial progress in recent years. The two systems dominating the market today are frequency controlled a.c. motors and SCR controlled d.c. motors. Frequency controlled a.c. motor drive systems were generally more cost effective below 500 H.P. and SCR controlled d.c. motor systems more cost effective at the higher end. The offshore drilling industry favours SCR controlled DC drives. Modern SCR and frequency controlled systems have efficiencies approaching 97% in power conversion. The selection of one over the other is an application issue.

The deep draft cruise ship industry, due to the very high hotel power requirements, is adopting high-power diesel-electric (a.c.) propulsion systems in most of its new builds. Both technologies have a proven record of efficiency and reliability. For a direct current propulsion motor, the electrical power may be from one or more d.c. generators or may be from an alternator and then delivered through a rectifier as a d.c. supply. The power for direct current motors is limited to about 8 MW and so a.c. machines are used for higher outputs unless an effort is made to install d.c. motors in tandem. The rectification scheme can incorporate speed control and the means of reversing. Power for an a.c. propulsion motor is supplied obviously by an alternator. The prime mover may be a diesel engine, a gas turbine or a boiler and steam turbine installation.

The choice of a diesel electric system as the power source for a propulsion system of a vessel has nothing to do with hydro-dynamic efficiency. A propulsion system of a vessel provides thrust to move the vessel and is still chosen by the designer based on its merits for the vessel's application. Conventional propellers, controllable pitch propellers, azimuthing Z drives, transverse tunnel thrusters and low speed water jet systems can all be driven with equal effectiveness by a diesel-electric system.

Diesel-electric systems become viable when the installed KW for propulsion approaches or is exceeded by the installed KW for other purposes. The convenience of electric power distribution makes it possible to locate the primary power source i.e. diesel generators exclusive of consideration as to where the power must be applied, whether it be propulsion, thrusters or cargo handling purposes. A large variation in propulsion power requirements i.e. long periods of low speed or a necessity to shift power from main propulsion to thrusters for dynamic positioning purposes can also justify diesel electric systems.

Modern turbo-charged diesel engines are efficient over a relatively narrow operating load and RPM range. They are not suitable for long periods of low speed, low load, or low RPM, high torque requirements for reversing large propellers. Modern generator systems with load sharing, auto-start, and load shedding features make it possible to efficiently utilize the installed horsepower of a diesel electric system.

Diesel electric propulsion can overcome the following design problems:

1. When propulsive or station-keeping power requirements are a small or relatively small percentage of the total power requirements. Drill rigs, offshore vessels with special positioning requirements, research vessels with special manoeuvring requirements, and gaming vessels where speed is inconsequential such as gaming vessels operating on a river.

2. When space and/or propulsion machinery limitations either exclude the use of direct diesels or adversely affect the construction cost resulting from using direct diesels.

a. Vessels with hulls and/or struts too small to accommodate diesel engines, access, ventilation, etc.

b. Vessels with potential trim problems, such as stern wheelers, where machinery needs to be located forward to avoid trim problems.

c. Vessels that require, due to space limitations, more than one machinery space are subject to increased construction cost due to duplication of or increased in systems such as:

i. Engine cooling

ii. Space ventilation

iii. Control facilities

iv. Exhausts, etc.

3. Vessels that require maximum torque at minimum propeller speeds.

4. Vessels that have a large variation in power consumption.

The fact that the propulsion power may be supplied by an electric motor instead of a direct driven diesel engine does not make equipment aboard the vessel any less familiar to the operator. The utilization of the diesel engines is transferred from direct propulsion power to generator power. This provides greater flexibility of the use of the installed KW and in some instances reduces the number of diesel engines installed. The ability to generate only the power required to meet the needs of the duty cycle of the vessel utilizing multiple generator sets reduces fuel consumption and maintenance costs. It also provides redundancy in power capacity.

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