Practical design criteria of interior permanent magnet synchronous motor for 42V integrated starter-generator

For the 42 V integrated starter-generator (ISG) application, the interior permanent magnet (IPM) synchronous machine has been a good candidate because of the field weakening capability, efficiency and high power density. However, the integration of starter and alternator requires contrary specifications, i.e. a high starting torque and a field-weakening capability in a wide speed range, and they make the design of the IPM machine a challenging job. In designing the IPM machine, there are many preceding design factors such as the pole number, the stator inductance value, the saliency ratio, and magnitude of the permanent-magnet flux and so on. These design factors provide the basic guideline for the detailed geometric design of the machine. However, because these design factors are dependent on each other, it is difficult to select a set of design factors which satisfies the required specifications. This paper presents the design criteria to select the set of design factors using the 42 V ISG specifications. The criteria are based on the three important specifications. From the specifications, the feasible ranges of stator inductance with current magnitude are illustrated with respect to the pole number and the saliency ratio. Using this analysis, the minimum saliency ratio and the range of stator inductance can be calculated with respect to the pole number. This paper also presents a practical design example of 8-pole IPM for crankshaft-mounted 42V ISG. In order to verify the validity of the design, a prototype 8-pole IPM is built and the parameters and performance of the machine are measured by the various experiments.