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/sub 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.

I. Introduction

In electric traction, like in other applications, a wide range in speed and torque control for the electric motor is desired. The DC machine fulfils these requirements, but this machine needs periodic maintenance. The AC machines, like induction motors, and brush less permanent magnet motors do not have brushes, and their rotors are robust because commutator and/or rings do not exist. That means very low maintenance. This also increases the power-to-weight ratio and the efficiency. For induction motors, flux control has been developed, which offers a high dynamic performance for electric traction applications [1], [2]. However, this control type is complex and sophisticated. The development of brushless permanent magnet machines [3] – [5] has permitted an important simplification in the hardware for electric traction control. Today, two kinds of brushless permanent magnet machines for traction applications are the most popular: i) the Permanent Magnet Synchronous Motor (PMSM), which is fed with sinusoidal currents, and ii) the Brushless DC Motor (BDCM), which is fed with quasi-square-wave currents. These two designs eliminate the rotor copper losses, giving very high peak efficiency compared with a traditional induction motor (around 95 % and more in Nd-Fe-E machines in the range 20 to 100 kW). Besides, the power-to-weight ratio of PMSM and BDCM is higher than equivalent squirrel cage induction machines. The aforementioned characteristics and a high reliability control make this type of machine a powerful traction system for electric vehicle: applications [6]. However, sensing the phase currents and the position of the rotor are two of the drawbacks that this type of machine have. In this paper, a control system for brushless DC motors, based on a DSP from Texas Instruments is proposed. The DSP used is the TMS320F241, which has been programmed to produce a simple way to control the machine currents, and to evaluate the instantaneous position of the rotor. The method is based on 1) the measurement of the currents, based on a common dc current , which is obtained taking the absolute values of two of the three real phase currents, and 2) the calculation of the commutation instants based on the slope variations of this current .