A number of different electric propulsion systems, as for example diesel-electric systems on board ships or locomotives, are well known. Common to most of these systems is that they meet requirements as regards the control of the speed or the power, or both, of the means of propulsion such as ship propellers or the traction wheel sets of locomotives. These parameters are controlled in a number of different ways, including the use of rotating or static converters, of which electronic frequency converters have become increasingly common as advances in power electronics have been made, and have become more available and increasingly cost-efficient. It is a standard feature of these frequency converters that they cover the whole of the relevant power range.

However, in many cases such converter-based systems are expensive and complex, in particular for the propulsion of ships, when they are intended to cover a wide range of power and speed. To illustrate current operational requirements, it may be mentioned that when sailing normally between two harbours, a ship may require propeller speeds of 60-200 revolutions per minute or more, whilst the power demand may be very high, whereas the speed requirement during navigation within harbours may be as low as zero revolutions per minute with a small power demand which may be a matter of percentages of the maximum rated power of the propulsion system.

A number of documents describe power generation, power transmission, power control and complete systems for electric propulsion systems, of which : DE-2908270-A1 describes an electric two-phase propulsion system having electric transmission via electronic inverters; DE-3445535-A1 relates to a method and a system for controlled power supply to an alternating current network having a diesel engine driven generator ; WO-88/08924 describes a method for controlling the rotational speed of a heat-operated motor ;

WO-98/08287 discloses a power supply system having an alternating current generator with a variable speed of rotation for a number of separate networks which can be switched using switch devices. A frequency converter is arranged between the alternating current generator and the network, where all transmission of electric power between generator and network passes via the converter. The separate networks are divided into two-a mains power network and a secondary network for power supply of other electric power consumers; WO-98/11643 relates to a system where a main motor is connected to an electric asynchronous motor which together with a combined rectifier/inverter can function as either a generator or a propulsion motor; RU-2093378 describes autonomous electric propulsion machinery having power transmission via a frequency converter formed by three pairs of single phase thyristor bridges connected in anti-parallel relation; RU-2087340-C1 describes an arrangement for adjusting the electric power supplied to a diesel-electric locomotive; SU-1699862-A1 relates to power supply for ships with a generator connected to the shaft of the main engine ; SU-167777-A1 describes a diesel-electric propulsion system having an inverter provided with a commutation circuit having contact groups that provide inverter operation from batteries in a start-up phase; SU-821244 describes a reversed control system for effective control of frequency and rheostat braking in an electric propulsion system having a heat-operated motor; and WO 94/14649 describes a propeller drive system for a vessel or the like for operating two contra-rotating propellers, comprising two electric propulsion motors, two controllable frequency converters, a device for controlling the frequency converters, and a power source such as a combination of a diesel engine and a generator. The electric motors are fed by power supply mains having switch devices, wherein the power, as mentioned, can be provided diesel-electrically via the frequency converters. If required,

the propeller drive system can be operated with only one propeller, i. e., either of the propellers.

Thus, there is a need for an electric propulsion system where the generator can be driven by a combustion engine or turbine, and which can cover a wide speed range in a wide power range with high overall efficiency and a preferably simpler and more cost- efficient structure.

The objects of the present invention are therefore to provide a system that meets the aforementioned needs.

The aforementioned objects are achieved in that through the present invention there is provided a generator set driven electric propulsion system having the features that are disclosed in the attached patent claims.

In the following the invention will be described in more detail with reference to the attached drawings, wherein: Fig. 1 is a schematic illustration of an example of a known propulsion system having electromotor operation with power supply from a generator driven by a diesel engine; Fig. 2 is a schematic illustration of an example of a propulsion system according to the invention; and Fig. 3 is a schematic illustration of another example of a combined system for electricity supply and propulsion according to the invention.

Referring first to Fig. 1, this figure shows a known arrangement of a propulsion system having an electromotor (2) connected to a diesel engine driven generator set (1) via a circuit breaker device (5). Such systems, it is true, show good efficiency in the higher power ranges and when operated close to or at a synchronous speed. The dynamics of the speed range are often increased by inserting between the generator set (1) and the electromotor (2) a controllable frequency converter (not shown) capable of controlling across the entire power range. However, the frequency converter introduces a loss somewhere in the order of a few percent which, admittedly, is of minor importance at low powers, but which, however, is of major significance when the propulsion

machinery is to run at almost full power. Moreover, converters for high powers are very expensive.

Fig. 2 shows an example of an arrangement for a diesel-electric ship propulsion system according to the invention, where the system comprises a diesel engine driven generator set (1), a propulsion electromotor (2), a controllable frequency converter (3), a control unit (not shown), a first controllable electric contactor (5), and a second controllable electric contactor (4). In an arrangement of the kind according to the invention, the operation of the propulsion electromotor (2) in a typical low speed range, such as 0-50% of the maximum propeller speed, is obtained by closing the contactor (4) whilst keeping the first contactor (5) open and supplying the propulsion electromotor (2) with electric power from the frequency converter (3) which, in turn, is supplied with power from the generator set (1). When a greater speed and power demand is required for the motor (2), the frequency converter is switched off by opening the contactor (4), and the propulsion electromotor (2) is connected directly to the generator set by closing the first contactor (5). Contactor control and any synchronisation of the switching is effected by the control unit (not shown). One of the purposes in switching from power transmission via frequency converter (3) to direct power transmission between generator set (1) and motor (2), as disclosed above, is to cut costs by installing only a small (i. e., low-power) frequency converter (3). At the same time, better efficiency is achieved at higher loads through direct power transmission.

However, it should be understood that the control unit to which reference is made in the above example can be replaced by an operator who manually, or perhaps by using remote control, effects the control of the opening and closing of the various electric contactors (4,5), and the operational control of the generator set (1) and the frequency converter (3). However, it will in any case be necessary to introduce some protective devices in order to prevent critical operations such as switching under such operating conditions as operations of this type can cause damage or danger, as for instance damage to the machinery or injury to the crew.

Fig. 3 shows another example of an arrangement for a diesel-electric ship propulsion system according to the invention, where besides the elements shown in the example in Fig. 2, the system also includes a further converter (6) which, for example, can convert from variable frequency and/or voltage to fixed frequency and/or voltage, and optionally an additional isolating contactor (7), which allows the vessel's other electrical equipment to be supplied with electric power from the propulsion generator

set. In the example in Fig. 3, an additional circuit breaker device has also been introduced by expanding the contactor (4) into a contactor group comprising two contactors, which makes it possible to disconnect the converter (3) completely when, for example, it is to be inoperative.

Again, with reference to Figs. 3 and 4 it is possible that the contactor (4), alternatively the contactor group (4) or the contactor (7), or both, can be omitted in that either the converter (3) or the converter (6), or both, are of a type that can be controlled into the off-state or an inoperative state without any contactors or other circuit breaker devices being required to isolate them from the power input or power output circuits.

Thus, by means of the present invention a major advantage is obtained in that the invention provides a generator set driven electric propulsion system having a wide speed and power range that can be produced at a lower cost than conventional systems including converters, since only for low powers is there a need for a converter, preferably a frequency converter. Furthermore, another major advantage is obtained in that the invention provides a generator set driven electric propulsion system having a wide speed and power range that exhibits a minimum of power loss, particularly when the system is working at high powers.