State of the art in electric propulsion — viewpoint on redundancy

Available solutions for redundancy in electric propulsion are outlined, emphasizing their benefits and drawbacks. Two different state-of-the art solutions of redundant electric propulsion are presented. The first example is a cruise ship with two azimuthal thrusters, both of which are equipped with 18 to 21 MW dual-stator synchronous motors and driven by two voltage source inverters (VSI) in a redundant configuration. The phase shift between the stator windings in each motor is 30 degrees, and the motors can also operate with only one stator system in case that another system is lost. The second example is LNG carriers, where one propeller is driven through a gearbox by two 12 to 15 MW dual-stator synchronous motors. The motors are supplied by VSI-type frequency converters with double inverter units and supply units, allowing each motor to operate in half power mode in case of a fault in any of these modules. The both configurations provide 75% redundancy in case of the most common single faults without notably increasing the cost and complexity of the system.