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- Felicetti, Roberto, 1972-
(författare)
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Run-Up Transient Analysis for Salient Pole Wound Field Synchronous Motors : Field winding and power electronics implications for a smoother and cooler asynchronous start
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Salient Pole Wound Field Synchronous Machines have better efficiency at full load than asynchronous machines of the same capability. Moreover, where asynchronous machines need inherently to draw a magnetizing current from the grid, Salient Pole Wound Field Synchronous Machines can work efficiently at unity power factor. When needed, their power factor can even turn leading in order to provide reactive power to the grid. It is well known, that the asynchronous machines performances deteriorate very quickly in presence of voltage drops, while the Wound Field Synchronous Machines prove to be capable of riding through voltage sags without stalling of falling out of step. Besides, for short-time overload, machines with a field winding can provide a pull-out torque even three times larger than the nominal one. Finally, when working as generators, Wound Field Synchronous Machines can perform a black starting where asynchronous machines cannot. Why are then asynchronous machines so successful in comparison to synchronous ones when used as motors? Robustness, low-maintenance, simplicity are the key-features that translate immediately in low fixed costs for asynchronous motors. But, even though lower variable costs for a synchronous motor should envisage a break-even starting form a given power rate on, there are still some technical issues to be overcome. A synchronous motor must be first sped-up to the synchronism for it to deliver mechanical power. And when it is directly on-line started like an asynchronous motor, it draws large reactive currents due to its large airgap. Therefore, additional means are needed for avoiding the motor overheating and voltage disturbances on the grid, such as autotransformers, transformers and reactors. Besides, in Salient Pole Wound Field Synchronous Machines. due to the inherent magnetic and electric rotor asymmetry, the starting asynchronous torque presents a nasty large pseudo-periodic ripple. In this work, a novel starting technique is presented, which mitigates all problems related to the run-up by means of a special rotor winding and excitation-system arrangement. The suggested start strategy is validated by an experimental test carried out on a build prototype and through a field-to-circuit coupled transient simulation. Since AC currents are essentially injected into the rotor winding during the machine start, in its first part the thesis explores what does happen in a winding traditionally designed for the direct current supply when an alternated current is injected into it by a power electronics device. To that end, changes in the main field winding parameters are firstly investigated, which take place under AC-supply. The obtained results make it possible to build a wide-band frequency electromagnetic model for the field winding. The model is useful for studying the potentially harmful voltage surges and voltage gradients triggered by fast rising/falling voltage-edges. Finally, within the specific application of the field current control and with the aim to prevent large voltage surges, the research provides some hints for the design of the field winding and for the coordination of the power electronics and the field winding properties.
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