Recommendations for design and protection of induction motors for extruder applications

Extrusion processes are widely used in the petrochemical industry. Extruders have to be purged first of residue before the extrusion process can begin. This extruder purging and subsequent starting process is known as purge-start. Quite often the initial purge-start is unsuccessful and hence multiple starts are required on the main motor. This paper analyzes the detrimental effects of multiple purge-starts on an 8000 HP induction motor used for extruder application. Various cases of multiple purge-starts are analyzed with the motor assumed to be under various initial conditions such as purge-starts from ambient, from rated temperature, starts under reduced voltages and under various loads. In each of these cases, the temperature rise in the rotor bar has been predicted using finite element analysis based design tools. These results, along with cooling time constants, are used in developing the temperature rise and cooling profiles of the rotor bar during starts. The temperature profiles are used to predict the possible failure mechanisms on the rotor and to develop design guidelines for induction motors used for extruder applications. The analyses also help in setting up motor protection guidelines for extruders.