ANALYSIS OF THE VOLTAGE REGULATION CHARACTERISTIC OF A PERMANENT-MAGNET ALTERNATOR WITH INVERSE SALIENCY

Abstract

ABSTRACT This paper concerns the peculiar voltage regulation characteristics of an alternator with buried permanent-magnet excitation. An introductory analysis shows that, with resistive load, the output voltage V rises above its open-circuit value E as current is increased from zero, provided inverse saliency r = Xq/Xd >2. V then reaches a maximum which is significantly greater than E if r has a typical value such as r = 4. Previous experimental work on a small machine demonstrated the phenomenon but suggested that a full analysis would require closer attention to magnetic saturation. This paper presents a method of performance prediction based on finite-element analysis. The approach adopted is to compute air-gap voltage and its phase angle for a range of pairs of values of stator current I and current angle β relative to the magnet axis. Voltage drops across stator leakage reactance and stator resistance are then added vectorially, yielding pairs of values of V and output phase angle φ. These results are used to interpolate values of V, and the corresponding I and β, which give φ= 0 as appropriate for resistive load. Experimental results are given for a 7.5kW, 4-pole machine with Nd-Fe-B magnets, again showing negative voltage regulation and exhibiting a maximum voltage at a particular value of output current. Results of the finite-element analysis are shown to follow the measured form of voltage regulation more closely than approximate analytical methods. The findings open up the possibility of design to achieve very low or zero voltage regulation on rated load