Analysis of harmonic content and power losses of a frequency-converter fed high-speed induction motor system
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Suurnopeuskäyttöisen oikosulkumoottorin syöttöjärjestelmän harmonisen sisällön ja tehohäviöiden analysointi
High-speed high-power applications used in vacuum system applications have some unique features compared to other frequency-converter fed motor applications. Operating frequencies and rotational speeds are usually high and significant mechanical stresses occur in rotor. Therefore, solid rotor motors are used due to their mechanical strength. Solid rotor motors are sensitive to supply voltage distortion in terms of eddy current losses and their power factor is relatively low. Therefore, output sinusoidal filters are used at supply converter output. In addition, switching frequencies are relatively low compared to operating frequency. Harmonic contents, sinusoidal filter losses and other relevant vacuum system’s supply system characteristics are investigated in this thesis. Due to low switching frequency and high operating frequency, switching frequency harmonics and base band harmonics might occur at same frequency ranges causing high harmonic amplitudes. Harmonic voltage patterns at different operating frequencies might also be very different. Output voltage harmonics in relevant operating frequencies of Runtech Systems turbo blower application are investigated and analyzed. Output sinusoidal filter power losses at different operating points are investigated based on measured filter currents. Simplified analytical method to calculate filter power losses is derived based on filter parameters and structure. Due to limited compensation capacity of fixed output filter, filter current distortion is different at different operating frequencies. Therefore, frequency dependency of filter power losses is considered. Different filter structures are used in different vacuum applications. Therefore, filter power loss information is relevant in terms of cooling system design. Various supply cable configurations and cable lengths are used in different vacuum system field applications. Some power cables are not suitable for certain variable-frequency applications due to their electromagnetic asymmetry. Different power cables are analyzed, and certain cable configurations are suggested for vacuum applications. Purpose of the thesis is to produce valuable information for both product development in future and for vacuum system projects.