During operation of compressor trains by a variable speed drive system (VSDS), integer and non-integer harmonic currents are generated in the inverter. Via the electrical system of the inverter and the motor, an excitation torque is transferred across the motor air gap into the main mass of the motor rotor. The frequency of this excitation may cause torsional resonances. Due to the rapid increase in excitation frequency of integer harmonics, intersections with relevant torsional natural frequencies (TNFs) can in general be avoided within the operating speed range. In contrast, the intersections of the noninteger harmonic excitation frequencies, also called interharmonics, with TNFs within the operating speed range may have an essential impact on the vibration behaviour of the rotating equipment. This aspect has to differentiate between two train configurations. The first are direct driven trains and the second, trains including an intermediate gear. For direct driven trains, only fatigue problems have to be considered. In trains with an intermediate gear, on top of that, interaction of torsional and lateral movement may have a negative effect on the lateral vibration behaviour of the gear rotors.
The main focus of this publication is on a simple but effective method for turbo compressor applications that allows avoiding main resonances within the operating speed range caused by intersections of interharmonic excitations with relevant TNFs. This method is based on detailed knowledge of the inverter behaviour and possible design options of the motor itself. This in-depth understanding was developed by correlating numerical and experimental results based on dynamic torque measurements of real turbo compressor trains. During this investigation the mechanically relevant torsional excitations were identified. Therefore, the different types of inverters and their corresponding characteristics had to be analyzed and understood in detail. This knowledge, in combination with possible motor designs, with regard to the number of pole pairs and the most common train configurations (direct driven and/or trains including intermediate gears), is incorporated in this report.