An Approach to Solving Ramp Start Issues in Sensorless Field Oriented Control with Sliding Mode Observer for Permanent Magnet Synchronous Motors

There are several issues in motor drive systems implementing sensorless field oriented control with a sliding mode observer (SMO) for permanent magnet synchronous motors (PMSM). One of those issues is stability during initial motor start-up when using a ramp. This is due to the fact that at low or zero speed there isn’t a significant back-EMF signal to properly drive the observer into accurate angle tracking. Usually step commands are used to help with the transition from zero to the commanded speed as it allows for a very fast transition out of low speed operation. This paper discusses an approach to overcome issues when a ramp start is required by using an open loop control strategy for some portion of the start-up procedure.