| Abstract |
This paper discusses design optimization strategies for electric drive systems for the thrusters of unmanned underwater vehicles (UUVs). These are intended to replace current hydraulic systems, with considerable advantages in terms of reliability, efficiency, ease of maintenance, quiet operation and compactness. The paper emphasises the need for the design of the motor and propeller as a coupled unit if good performance is to be achieved. Two types of drive system are considered. In the first, a direct-coupled in-line electric motor is used. The main difficulty with this system is that conventional electric motors are very bulky and their diameter can be a considerable proportion of the propeller diameter, thus obstructing the flow of water. They are considerably larger than hydraulic motors, which is the main reason for using hydraulic systems at present. However, by optimizing the design of propeller pitch and diameter, so as to reduce torque without unduly increasing the power requirement of the thruster, the paper demonstrates that the proportions of commercially available PM electric motors can just about be made acceptable. Using recently developed designs ofhigh-output motor, and redesigning the propeller, motor proportions comparable with hydraulic motors are shown to be possible. In the second form of drive system, which is preferred, the motor and possibly the power and control electronics are integrated within the volume of the thruster duct. This reduces the bulk of the thruster, increases its ruggedness, improves efficiency, reduces cost, and improves the cooling of the motor and electronics. However, fluid friction loss in this arrangement can be very high, because of the relatively large diameter and high peripheral speed of the rotor. But, again by giving attention to propeller hydrodynamics at the design stage, the paper establishes the feasibility, and indeed attractiveness, of this concept. The paper presents details of the design of an integrated thruster which fits well within the structure of a modified standard duct and produces 1800 N of thrust using a 300 mm diameter propeller, with a motor efficiency of 88%. Its overall diameter and length are 370 mm and 150 mm, respectively. |
