Analysis of Losses in Inverter-fed Synchronous Machines with Salient-pole Solid Rotors

Abstract: The work that is presented here considers loss analysis of inverter-fed field-excited synchronous machines. The project was initiated, as a co-operation between indus­trial partners and KTH, with the aim of providing knowledge about the losses that take place in inverter-fed large scale synchronous machines. The use of inverter con­trolled machines is becoming prominent in heavy industry, as VSD (Variable Speed Drives) allow for energy savings compared to the more primitive methods of process control (such as throttling). The machines being considered have a solid rotor and rotor pole-plates and are driven by a DTC (Direct Torque Control) inverter. DTC has several advantages over other, more conventional control techniques. However its main disadvantage, from an analysis point of view, is its stochastic nature. This means that DTC lacks a fixed switching frequency, and hence a predictable pattern. One of the challenges that are faced in dealing with DTC controlled machines is modelling their operation in FEM (Finite Element Method) software. The use of FEM has become an inseparable part of machine design and analysis, as it provides for a detailed insight into the machine and its various components.One of the initial aspects as part of this project was the development of a method in which DTC could be modelled using commercially available FEM software. Ini­tially the method developed relied on analytical simulations to produce DTC switch­ing signals, which were then inserted in FEM. This was later developed further to allow switching signals obtained from measurements to be used. Apart from eval­uating simulation methods and results, the research has also involved intensive measurements. The focus of the research is on analysing the losses that take place in the machine due to the inverter drive (time harmonic losses). As the rotor and its pole-plates are solid, it would be expected that a significant portion of the time harmonic losses would take place on the rotor. Furthermore it is expected that, since the time harmonics are of a high frequency, the pole-plate will act to shield the rest of the rotor. Hence most of the rotor losses will be in the pole-plates. The fact that the pole-plates are solid also means that the eddy currents will compose the majority of the losses in the rotor.The thesis gives a detailed overview of how the time, as well as the space, harmonic losses are located on the pole-plates. Moreover, the research also considers the effect of varying material properties on the different losses that take place in the machine as a whole and in the pole-plates in particular. Qualitative explanations are given to how these different losses vary with material property. The research covers the effect of different variables - such as switching frequency and material properties - on the losses in the machine, with specific focus on the time harmonics. The study is conducted using both measurements as well as simulations. The knowledge acquired from the evaluations carried out, reveal that significant power savings are possible by modifications to the machine structure, material as well as to the control technique. Various methods are investigated and recommendations provided into how these benefits can be attained.