Electrical systems in pod propulsion
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Typ
Examensarbete för masterexamen
Master Thesis
Master Thesis
Program
Electric power engineering (MPEPO), MSc
Publicerad
2007
Författare
Berg, Lena
Helldén, Ulrika
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
Since middle of 1990´s pod propulsion in marine application have become common. There are mainly two constructors on the pod market. ABB is the biggest producer and have two types of pods, Azipod (5-30MW) and Compact (0.4-5MW). Rolls-Royce (RR) is the second biggest company on the market, producing a pod called Mermaid (5-25MW). The electrical system in pod propulsion normally consists of a transformer, a frequency converter and an electric motor. The transformer is used to divide the system into several parts in order to obtain different voltage levels but also for phase shifting. The purpose of the frequency converter is to control the speed and torque of the motor by changing constant frequency into variable frequency. The electric motor is used for conversion from electrical to mechanical power. In all applications with electric motors, current that travel from the shaft trough the bearings is a phenomena. There are two categories of causes of bearing current, a classical problem where low frequency current and a newer high frequency current problem that occur when using high switching control. To mitigate currents in bearings grounding of the shaft and insulation of the bearings is used. Azipod has a synchronous motor controlled by a cyclo converter and dry-type transformer. The non-drive end bearing is insulated and the shaft is grounded to earth in drive end and to the bearing house in non-drive end. Mermaid has a dry type transformer and a synchronous motor controlled of a load commutated inverter (LCI). Both bearings are insulated and the shaft is grounded in drive-end. The biggest change in pod propulsion within a close future is the switch to voltage source inverter with pulse-width-modulation (VSI-PWM). Permanent magnets synchronous motors (PMSM) are an alternative in smaller pods and the development of magnets can make PMSM to an alternative also for bigger pods. Also induction motor is an alternative motor in the future.
Beskrivning
Ämne/nyckelord
Elkraftteknik , Electric power engineering