Design Considerations For A 66,000 HP Motor Driving An Air Compressor.

The design of a 66,000 hp electric motor to drive an air compressor presented significant design challenges. Using conventional assumptions in assessing the bearing support and foundation stiffness yielded a scenario where it would be impractical to achieve a 20 percent separation margin between operating speed and the lateral critical speeds of the motor. Structural stiffnesses well above conventional values were required in order to achieve the specified separation margin. Finite element studies were performed on the bearing pedestals and motor base. As a result of these studies, it was deemed necessary to perform a modal finite element study on the complete motor assembly. Eventually, it was decided that a modal analysis would be performed on the entire drive train (motor, gear, compressor, and foundation). These studies confirmed that the structural stiffness needed was achievable. Factory and field tests substantiated the analysis. Another challenge in this design was the ability of the motor to accelerate a drive train with a polar inertia in excess of 355,000 lbm-ft. The rotor construction is such that there is not a discrete amortisseur winding. On startup, currents are induced in the rotor pole face, which lead to significant heating. Surface temperatures in excess of 400°C (750°F) were calculated. Plastic deformation of the rotor pole face will occur at this temperature due to large thermal strains. Consistency of the shaft forging mechanical properties is paramount if shaft bending is to be avoided. A stringent forging specification was written, which limited the number of forging suppliers who could meet the requirements. Finite element studies were performed to calculate the stresses and plastic strains. High temperature mechanical testing was performed on the forging material to confirm that the stress-strain hysterisis loop would close after repeated cycling. Furthermore, care needed to be taken to ensure that the rotor winding and insulation system was protected from damaging temperatures. Once again, factory and field tests confirmed the analytical work. INTRODUCTION Large quantities of waste petroleum coke remain after the refining process at the Motiva Enterprises Delaware City Refinery. The refinery has recently installed integrated gasification combined cycle (IGCC) power technology featuring the proprietary Texaco Gasification Power System (TGPS), to produce electrical power and steam from the petroleum coke. The TGPS oxygen blown entrained gasification process will be used to produce synthetic gas from coke production. The synthetic gas will be cleaned and used as fuel in two new 90 MW gas turbines and modified power plant facilities designed to produce steam and electrical power for the refinery. The pure oxygen for the gasifier is produced by means of an air separation unit (ASU). The ASU is made up of a 66,000 hp air compressor, a 43,000 hp nitrogen compressor, a 12,000 hp oxygen compressor, and a 6000 hp nitrogen recycle compressor. Four-pole synchronous motors were selected as the compressor drivers. Synchronous motors can offer significant benefits when compared to mechanical turbine drivers. These benefits include low capital cost, low maintenance, good reliability, and high efficiency. Users and purchasers are taking advantage of these benefits by utilizing larger and larger ratings, so much so that machines approaching 100,000 hp are now being considered. The 66,000 hp motor was significantly larger than any former four-pole synchronous motor produced by the selected manufacturer. This paper describes two of the many design considerations that required careful attention in order to achieve smooth and reliable operation at site.