Analysis of idle power and iron loss reduction in an interior PM automotive alternator.

Abstract

In recent years there has been considerable interest in high power automotive alternators. As part of earlier work, an interior permanent magnet machine had been built as a concept demonstrator for a 6 kW automotive alternator. Test results on this machine had shown good field-weakening performance but that the machine fell 16% short of the idle power requirement and had high iron losses. This thesis describes the development of a finite-element model for this machine and how it was validated using experimental test results and used to examine the cause of the low idle power and high iron losses. A limited optimisation of the machine design to reduce iron losses was examined and demonstrated experimentally. It was found that the cause of the low idle power was due to cross-saturation effects in the machine, particularly in the stator teeth. Both the finite-element analysis and experimental tests showed a substantial reduction in the q-axis inductance due to the presence of the magnet flux. It was predicted that the machine would have met the idle power requirement if saturation and cross-saturation had not been present. The high iron loss during field-weakening was identified as being due to large amplitude, higher harmonic flux components caused by the interaction of the rotor barriers and the d-axis flux distribution. This was seen in the spatial airgap flux density and the time-varying stator tooth flux waveforms. The analysis also showed that eddy-current loss was dominant in this machine at higher speeds, and that under short-circuit conditions the majority of the iron loss was due to large thirteenth and eleventh harmonic flux density components. This was confirmed experimentally using the stator tooth flux waveforms and the total iron loss measurement. A limited design optimisation was performed by examining three machine design changes to reduce the high stator iron loss under short-circuit conditions. Of these, increasing the stator slot opening using flat stator teeth offered the greatest calculated iron loss reduction (19%). This was tested experimentally by machining the stator teeth of the prototype machine. The measured iron loss reduction (21%) at high speed was comparable to the calculated effect, and the output power was shown to improve slightly. The results of the detailed iron loss investigation in this thesis forms a basis for development for more general machine design approaches for reducing stator iron loss under field-weakening conditions in interior permanent magnet machines.