Asynchronous Solid Rotors as High-Speed Drives in the Megawatt Range

In the field of large electrical machines, highspeed direct gearless drives have become the focus of increasing user interest in recent years. Their growing popularity can be attributed to the enhanced efficiency and improved plant availability that are characteristic of gearless drives. These motors are especially popular for use as compressor drives in the power range from 5 MW to 100 MW at speeds of between 3,600 rpm and 15,000 rpm. While the synchronous machine has proven its drive credentials in the speed range up to about 6,000 rpm, asynchronous machines with solid rotor and fed by a voltage-source DC-link converter have recently become available for the upper speed range with outputs of between approximately 5 MW and 15 MW. This article describes the electrical and mechanical design used for this type of asynchronous high-speed motor range. The motors can be constructed with either magnetic bearings or conventional friction bearings. As compared to variants with laminated rotor, this solid rotor design significantly improves the mechanical operational performance of the motor, a property which is especially reflected in its low-level, stable vibration characteristics. In order to obtain good electrical operating characteristics in spite of the solid rotor design, a copper cage is diffusion welded to the rotor. The machine model required for advance calculation of the motor\´s electrical operating characteristics will be explained. The possible power ranges based on the currently available models of power converter will also be examined. Advance calculations of machine data will be compared to measurement results. Special attention will be paid to the subject of optimizing the "machine -converter" system, an essential process for this type of drive so as to avoid the risk of unacceptably high stray losses caused by the solid rotor and high machine frequency.