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This paper proposes an alternative position estimation method of surface permanent magnet (SPM) machines at low speed using high frequency (HF) voltage injection. SPM machines traditionally do not have enough spatial saliency signals because of their symmetric rotors. The proposed solution is to track the HF eddy-current-reflected asymmetric resistance for position estimation. It is shown that the spatial variation of resistance can be enhanced when an HF voltage is superimposed. Finite element analysis is performed to investigate the asymmetric resistance in different SPM machines. By superimposing a d-axis pulsating vector HF voltage, SPM machines can achieve the position estimation performance with reduced degradation at high load conditions. Implementation methods are developed to track the resistance-based saliency signal for speed and position closed-loop control. An SPM machine prototype with a ring magnet rotor is used for the experimental evaluation.
Date of Publication: May 2012