Low starting current cage induction motors for the offshore petrochemical industry

Offshore electrical power systems are generally compact with low fault levels, yet modem production needs high power electric motors. Examples are product-transfer or re-injection gas compressors, water injection pumps and oil transfer pumps. To start such motors on a small power system requires 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 and special cage design and the comparative costs of different solutions; the design of cage induction motors for low starting current performance <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 and pressurising and motor auxiliaries; experience in major offshore applications of the low starting current induction motors; and pitfalls in making these choices, at the specification stage and in operation.