Starting Performance of Large Grid-Fed Solid-Rotor Salient-Pole Synchronous Motors for the Oil&Gas Industry: Simulation Challenges and Factory Test Experiences

In the Oil&Gas Industry, the use of large medium-voltage salient-pole synchronous motors (SMs) directly fed from the mains with power ratings in the multi-MW range is still of great importance. These motors have to drive gas compressor, either directly or through a gearbox, at a fixed speed so that power electronics converters with relevant cost, reliability and control issues can be removed. In absence of a variable-frequency supply, the start-up process for these machines is critical and usually requires a solid-steel rotor. The starting torque is produced by the currents induced in the solid rotor as well as in the short-circuited excitation winding. The prediction of the starting performance for this kind of machines is a challenge that power engineers have been facing for decades. State-of-the-art simulation tools, based on 2D and 3D Finite Element Analysis (FEA), have brought advances but the problem of a reliable, computationally affordable prediction of the start-up-process is still an open and timely issue. This paper attempts to provide a contribution reporting on computation and testing experiences on a multi-MW SM. The impact of the various possible uncertainties in material properties and the need to model various details of the machine design are investigated in depth through sensitivity analyses in comparison with measured data. It is shown that acceptably accurate results in the starting performance prediction can be obtained through a set of 2D FEA simulations suitably combined with some analytical formulas and simple considerations.