Position estimator and simplified current control strategy for brushless-DC motors, using DSP technology

This paper describes a different way to sense the phase currents and to estimate DC rotor position of a brushless DC (BLDC) motor. For current control, the current is sensed taking the absolute value of two of the three phases, transforming this information in a DC current I_MAX, which is finally compared with a reference value. With this method of control, all the transistors of the inverter are commutated with the same PWM signal. Based on the this characteristic, the paper proposes a method to estimate the instantaneous position of the rotor. The method is based on the determination of the current slopes of the PWM during the conduction periods. The main characteristic of this type of motor, fed with quasi-square-wave currents, is that it only needs a six-position sensor, and one current controller for its full torque control. The system was implemented using a fast digital signal processor (TMS320F241) which is programmed with a closed loop PI control for the phase currents. The processor also makes all the calculations required for position estimation. Because of a lack of BLDC in laboratory, the motor tested was a 12 kW (16 HP) permanent magnet synchronous motor (PMSM), with sinusoidal back EMF. This machine has been tested with quasi-square current waveforms using the method proposed, and fed with an IGBT inverter working at 15 kHz commutation frequency. Experimental results of the currents and the way the slopes of current locus are obtained are shown.