Induction motors fed by PWM MV7000 converters enhance electric propulsion performance

Recent progress in power electronics has opened a new area in power conversion and has enabled to design high-performance medium voltage PWM converters. Such converters have allowed the use of induction motors in electric propulsion systems bringing many advantages in terms of robustness and efficiency. In the recent years, there is a huge demand by ship owners and ship yard operators for electric propulsion based solutions in order to ease their maintenance efforts and to enhance their efficiency and durability levels. The expected propulsion solution needs to be compact, robust, competitive and easily installable. Two vital factors that decide the performance of an electric propulsion system are choice of proper power electronic converter and type of motor for a given application size. This paper presents a new electric propulsion strategy with a high performance IGBT converter and a high torque density induction motor. To achieve higher power density, higher switching frequencies and improved fault current limitation capabilities, high performance press-pack IGBT devices were used in the PWM converter realization. Use of NPC based 3-L converter also improves the motor control in terms of increased speed range, constant magnetizing current for constant flux control and excellent control on motor torque via motor active current. On the other hand, a novel high torque density induction motor is designed and developed to suit particularly for navy and merchant applications. The optimized HTD motors were able to satisfy all marine requirements such as larger airgaps (typically 5 to 8 mm), high efficiency about 97.5%, optimized power density through pin ventilation laminations and safe operation in any condition including a single-shaft line operation. In a large cruise liner and LNG carrier applications, how reduced fuel consumption with savings in weight and space were achieved is also presented.