Sensorless technique and drive setup for high pole number brushless DC motors operated with single stage inverters

This paper describes the electronics and sensorless control algorithm implemented in an experimental setup to operate a 3 phase, high pole number, brushless DC motor through a single stage inverter. The use of a high PWM switching frequency imposed on the motor phases for torque and speed control generates noise on the sensing and drive circuitry. Thus the first part of this paper focuses on designing noise immune feedback and control circuitry containing the required galvanic isolation. The second part of this paper focuses on the development of a sensorless algorithm based on back-emf voltage sensing to commutate between phases. The outcome of this algorithm is useful when the motor is operated from medium to high speed. This sensorless algorithm uses synchronous sampling of direct terminal voltages measurement proposed in [1]. Then through an innovative digital estimation technique, the zero crossing of the back-emf of the concurrently unenergized phase is calculated. This technique avoids the use of passive filters on terminal voltage sensing thus eliminating undesired phase shifts that cause erratic phase commutation. Validity of this technique during steady state and transient operation is shown through simulation and practical results obtained with the experimental setup. Moreover the limitations encountered on employing the designed algorithm on the experimental setup are also discussed.