| dc.description.abstract | Two types of prime movers are continuously applied for rotating machinery: mechanical and electric drives. This tutorial focuses on electric drives, which are actually composed of separate components: electric motor, frequency converter (optional), and electric network including transformers. The main aim of this tutorial is to explain the phenomena of electric drives related to the torsional vibrations. All the components of an electric drive may affect torsional dynamics of the system. Naturally the main concern is torsional excitations. Another aspect is the electromechanical interaction in the air-gap of the motor that produces electromagnetic torsional stiffness and damping. These and other similar phenomena, related to the electric drives, have been observed on test fields and site conditions. The scope of the paper includes the most common induction, synchronous and permanent magnet motors. First, the various rotor structures of electric motors are discussed. Then, the effects of electromagnetic stiffness and damping on the torsional dynamics are introduced. Next, the main electric excitations are presented and their effect on drivetrain dimensioning is discussed. Various modelling and analysis methods are also presented for drive-trains with an electric motor. The previous sections focus on the motor without any frequency converter. The rest of the paper deals with the variable speed drives. First the main voltage source inverter, current source inverter and load commutated inverter drive types, their operational principles and application areas are introduced. Then the harmonic and inter-harmonic torsional excitations are explained that depend on the configuration and control of the frequency converter. The main functionality of a variable speed drive system is the speed-control feedback loop. This loop is used to maintain the preset speed. Although this feedback loop is also a potential source of instability it can with proper tuning be used to damp low frequency torsional resonances. The analysis guidelines and design principles are distributed throughout the paper. The paper is concluded with summary and main recommendations. | en |
