Electric motors and drives for modern ship thruster propulsion: Design and system integration issues

In modern ships of large size, the main propulsion system, consisting of either an internal-combustion engine or an electric drive, is assisted by auxiliary propulsion equipment composed of thruster propellers, used for low-speed, transversal, docking and station-keeping maneuvers. Thruster motors are typically induction machines, connected to the on-board grid either directly or through autotransformer or variable-frequency converters. This paper describes the main issues connected with the choice and design of thruster motors. In particular, the problems of integrating thruster motors into the on-board electric system are addressed. Moreover, the typical requirements and constraints imposed on thruster motor performance are highlighted along with the strategies adopted to fulfill them in the design and development stage.