Operation and maintenance of large Exd machines

The increasing demand for oil and gas has led not only to the discovery of new sources both onshore and offshore but also for electrical machines to power the necessary pumps and compressors associated with petrochemical processes. Such equipment is usually situated in Zone 1 or Zone 2 areas where there is a risk that potentially explosive gases may be present at various times during normal operation. There is an increasing tendency for machines in these areas to be subjected to exceptionally low ambient temperatures which can present problems for the motor designer. There is a need to achieve the highest practical level of safety in a hazardous areas. Even Zone 2 areas may have machines which require purging and pressurisation. This plant must be correctly specified and designed for the application. It is also essential to ensure that a proper maintenance regime is in place which will not prejudice the original design intent. The effects of nonsinusoidal supplies or variable frequency drives also need to be considered. Flameproof (Exd) cage induction motors can be used in many such applications. They give a very high degree of assurance against the risk of an explosion and may be a practical and cost effective alternative to other types of drive motor. The author discusses the design considerations, motor certification and testing, and the technical problems presented by low ambient temperature conditions. inverter supplies and maintenance and servicing are also discussed