Open Loop Control of Synchronous Motor using Inverters

Abstract

A distinctive feature on speed controls of synchronous motor is that the steady state speed error can always be reduced to zero as long as the stator frequency can precisely be identified. However, the problem of stepping-out in both steady and dynamic states is inevitable and, in usual, the so called closed control system has been constituted. Thus, if any open loop control (OLC) system without rotor position sensors can be constituted in pursuing to overcome the problem, it is quite desirable for extendind the application areas of OLC.
First, voltage source drive (VSD) and current source drive (CSD) are compared for OLC and it is made clear that CSD is more profitable for the constitution of OLC than VSD is. However, CSD has the following features. (i) Large torque is generated in the low speed range. (ii) The voltage of synchronous motor becomes excessively high in the high speed range. (iii) The dynamic characteristic is not good. (vi) The current of synchronous motor is large in the light load.
Second, we show a new control method which the torque of synchronous motor is proportional to the angle between rotor position and stator current vector and use lead-lag compensation. The torque transfer function becomes constant using this control method. As a result, the good characteristics are obtained.