Question

Diesel electric propulsion, is now being chosen as the power plant for an increasingly wide variety of vessels.

Sketch a simple layout of such an installation.

Explain the advantages of selecting such a plant.

Simple Ward-Leonard system for diesel-eieciric propulsion

The simple Ward-Leonard arrangement for diesel electric propulsion (Figure.) is an all-d.c. scheme with a diesel engine as the prime mover driving the single d.c. generator at constant speed.

An exciter mounted on an extension of the generator shaft provides field current both for the generator and for the direct current propulsion motor.

The exciter is itself a d.c. shunt generator.

At start-up, the armature windings of the exciter have current generated in them when they pass through the field emanating from the residual magnetism of the exciter poles.

The small current generated initially, circulates through the windings of the exciter poles, strengthening their magnetic fields until full output is reached.

The current generated in the d.c. exciter is delivered unchanged to its own field poles and to the field poles of the d.c. propulsion motor.

It is available to the field poles of the generator, but only through the regulating resistances of the manoeuvring control.

If the control contacts are at the raid positions of the resistances, then no current flows to the main generator poles and there is no output from it to the propulsion motor.

Rotation of the manoeuvring handwheel and gears turns the threaded bars to move the contacts along the resistances, in opposite directions.

As the contacts travel toward the extremities and resistance lessens, current from the exciter flows to the generator field poles.

The direction of current flow and the level are used to control the output of the generator and, in turn, the propulsion motor. Propeller speed is proportional to the actual voltage produced in the generator and fed to the propulsion motor.

The advantages claimed for electrical transmission for main propulsion are :—

Reliability in so far as a number of small generating sets can be used, should one or more of the sets require attention or should break down take place, the ship could still be run at slightly reduced speed until repairs were effected.

Economy.

The whole power can be transmitted through one propeller running at its most economical revolutions reducing the losses due to cavitation also losses due to friction of extra bossing and brackets where multiple screws are fitted.

The electrical transmission is as efficient at half-load as at full load, as the main generator prime movers are running at a constant speed and do not need to be reversed when ship has to go astern.

A uniform torque is maintained at all loads, thus eliminating the variable twisting stresses obtained in reciprocating direct coupled prime movers.

Intermediate shafting can be dispensed with as the motor on propeller shaft can be placed well aft and the current led to it, thus allowing more room for cargo.

The generating sets can be placed in any convenient position in the ship and not especially in line with the propeller shafting. The same power is available for going astern as for going ahead.

No reverse turbine required.

Ease of Control.

Manoeuvring is very much simplified as shopping, reversing or slowing down can be done by one switch.

Low noise.

An electric motor is able to provide a drive with very low vibration characteristic and this is of importance in ocengraphic survey vessel and cruise ships where for different reasons a low noise signature is required.

Auxiliary Power.

Since the main generating sets can be made small, all or any may be used in port for lighting, working winches, or cooling chambers.