INVESTIGATION OF IRON LOSSES IN A HIGH FLUX INTERIOR PM AUTOMOTIVE ALTERNATOR V. Zivotic-Kukolj, W.L. Soong and N. Ertugrul University of Adelaide Abstract This paper examines the iron loss characteristics of a high-flux interior PM machine. This was designed as a concept demonstrator for a 6kW automotive alternator and has a wide field- weakening range. Initial experimental tests showed a high iron loss during high-speed field- weakening conditions. This paper examines the causes of this high iron loss using finite-element simulations and experimental tests. It was found that the high iron loss was due to high harmonic airgap flux density components during field-weakening. Means to reduce these losses are discussed. 1 INTRODUCTION Proposed new features in cars such as active suspension systems, electric air-conditioning and sophisticated electronic controls have meant that the existing automotive alternator cannot satisfy the growing electrical power demands. A proposed specification for a high power alternator is a linearly increasing output power from 4kW to 6kW as the engine speed increase from idle (600rpm) to maximum operating speed (6krpm) [1]. Figure 1 shows the output power specification versus alternator speed assuming a 3:1 belt ratio between the alternator and engine. This challenging requirement requires a 10:1 constant power speed range. Interior permanent magnet (PM) machines are one of the few machine types capable of delivering such a wide constant power speed range. 0 1 2 3 4 5 6 7 0 3 6 9 12 15 18 Alternator Speed (krpm) Output Power (kW) requirement calculated curve test points Figure 1. Output power characteristics of the interior PM alternator showing the measured performance (triangles) versus the calculated performance (lines) at rated voltage. The high power alternator specification is also shown [1]. An interior PM concept demonstrator for a 6kW automotive alternator was built in [1]. Figure 1 shows the measured and calculated output power characteristics of the interior PM machine showing the potential to meet the high power alternator specification. The experimental results were limited to 6krpm by dynamometer limitations. The concept demonstrator used a high flux interior PM machine based on rare-earth magnets (NdFeB). The machine was based a commercial 415V, 2.2kW, 4 pole induction motor stator and used a custom designed rotor which uses three flux-barriers per pole (see Fig. 2). The motor stack length was 95mm, the stator outer diameter was 153mm, the rotor diameter was 92mm, and the airgap was 0.39mm. The rotor uses a number of rounded bridges to mechanically retain the magnets at high speed. Figure 2. Rotor lamination cross-section [1]. Iron loss during field-weakening operation of interior PM machines has been examined in [2] using time- stepping finite-element analysis. They found that in these machines the iron loss is significant, especially at high speeds. Their finite-element analysis results showed that a significant amount of iron loss is due to high frequency harmonic flux components near the airgap. This is likely to also be the case with the interior PM machine considered in this paper. The paper is organised as follows : firstly, the experimental obtained machine loss measurements are discussed; secondly, finite-element analysis is used to examine the spatial distribution of the airgap flux density during field-weakening is examined; thirdly, additional measurements on the stator tooth flux waveforms are examined; and finally, motor design methods for reducing the iron loss are discussed. 2 MACHINE LOSS MEASUREMENTS In this section, experimental loss measurements were taken on the machine to examine the variation of iron losses with speed and operating conditions. Figure 3 shows the measured efficiency at rated voltage of the interior PM machine corresponding to the output curves shown in Fig. 1. Below 2.5krpm, the