Large Cage Induction Motors for Offshore Machinery Drive Applications

Offshore electrical power systems are of necessity very compact with low fault levels. Despite this limitation, modern production increasingly requires high power electric motors as a clean and cost effective alternative to other forms of prime mover. In order to start such motors on a small power system necessitates equipment that is reliable, compact, cost-effective and designed with the system in mind. This paper sets out the main design criteria that need to be satisfied to meet these difficult requirements, as an aid to the specifiers and operators of such equipment. In particular the following topics are covered: offshore specification requirements, including NEMA, EEMUA, API and IEC; power system considerations, including configuration and modelling; practical solutions, including transformer assistance, star-delta starting, soft-starting, special cage design & the comparative costs of different solutions; the design of cage induction motors for low starting current applications <2.8 times full load current, including the prediction of performance, comparison with test and experience in service; the hazardous environment implications of low starting current cage induction motor applications, including cage temperatures, purging & pressurising and motor auxiliaries; experience in major offshore applications of the low starting current induction motors enables potential pitfalls to be identified and avoided