Practical requirements for starting very large machines

Large drives, usually exceeding a few MW in rating, are likely to require reduced-current starting. The drive motors usually adopted are solid salient-pole synchronous machines, in order to take advantage of better acceleration, torque per ampere and efficiency as compared with cage induction machines. The solid-pole synchronous motor has a drooping torque-speed characteristic whereas a turbo-compressor presents a square-law rising torque-speed load characteristic. The motor must accelerate smoothly past the critical speed of coupled train, which occurs at around 85 to 90% of synchronous speed, and synchronise under reduced-current starting. This discussion addresses the practicalities of applying large solid salient-pole synchronous motors to drive turbo-compressors. It is intended to be of interest to applications engineers concerned with larger compressor drives and high efficiency