Simplified Sensorless Control for BLDC Motor, Using DSP Technology

This paper describes a simple way to control, in a sensorless way, a Brushless DC (BLDC) motor for electric vehicle applications. To control this machine it is generally required to count with a position sensor because the inverter phases, acting at any time, must be commuted depending on the rotor position. Encoders and resolvers have been used for sensing rotor position with respect to stator. These sensors, however, make the motor system more complicated and mechanically unreliable. In this paper, a simple solution is presented to determine the commutation sequence of a BLDC motor with a sinusoidal flux distribution. The method is based on a two phase current sensing and the determination of the back emf. For trapezoidal flux distributions the solution may be implemented with some minor changes. The main characteristic of this type of motor, fed with quasi-square-wave currents, is that it only needs a six position sensor, and only one current controller for its full torque control. In contrast, the sinusoidal current type, the angular position needs to be known at any moment in order to control each of the three phase currents. The solution proposed makes use of information contained in the back emf, calculating the six commutation points required. This method is only applicable while currents can be sensed, so it needs to be complemented with a starting method. The system was implemented using a fast digital signal processor (TMS320F241) which is programmed with a closed loop PI current control in order for the motor to produce a constant torque. Additionally, a fiber optic link is used between the controller and the inverter. This minimizes noise production and possibilities of error on commutations. As current transducers LEM sensors were used. The motor tested is a 12kW (16HP) Brushless Permanent magnet Motor, with an IGBT inverter with a commutation frequency of 15 kHz. Experimental results of the currents are shown. Copyright 2002 EVS19