Field-weakening performance of brushless synchronous AC motor drives

The paper examines the theoretical and practical limitations to the field-weakening performance of surface permanent magnet, synchronous reluctance and interior permanent magnet motors when driven from an inverter with a limited volt-ampere rating. It is shown that the `optimal´ field-weakening performance consists of an infinite constant-power speed range but is limited to an inverter utilisation of about 0.7. The new concept of the interior permanent magnet parameter plane is introduced. This graphically illustrates the effect of varying the drive parameters on the shape of the field-weakening characteristic. The interior permanent magnet parameter plane is used to show that there are three types of optimal field-weakening designs. When practical factors and considerations are taken into account, the optimal high-saliency interior permanent magnet motor design is the most promising for applications requiring a wide field-weakening range. A 7.5 kW design was built and a constant-power speed range exceeding 7.5:1 was demonstrated