Observability criteria for winding currents in three-phase brushless DC drives

A rigorous method for applying observability criteria to the estimation of winding currents in brushless DC drives is presented. Observability is related to the drive´s operating condition, both motoring and regenerating, and to the pulse-width modulated (PWM) switching strategy being employed. The precise conditions under which motor winding currents can be estimated from measurements of DC link current are clearly defined. A modified single current sensor arrangement, in which switching device currents are measured, is shown to improve the observability of currents with complementary switching, but to have an adverse effect on observability with the basic switching strategy. An arrangement involving two current sensors allows discontinuous estimation of the winding currents for all operating conditions and both switching strategies. The primary advantage of this technique is that measurements by each sensor are made equally on all three phases of the motor