Direct torque control of brushless dc motor with non

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DIRECT TORQUE CONTROL OF BRUSHLESS DC MOTOR WITH NON-SINUSOIDAL BACK-EMF USING TWO-PHASE CONDUCTION MODE 2.1. Introduction Permanent magnet synchronous motor (PMSM) with sinusoidal shape back-EMF and brushless dc (BLDC) motor with trapezoidal shape back-EMF drives have been extensively used in many applications. They are used in applications ranging from servo to traction drives due to several distinct advantages such as high power density, high efficiency, large torque to inertia ratio, and better controllability [41]. Brushless dc motor (BLDC) fed by two-phase conduction scheme has higher power/weight, torque/current ratios. It is less expensive due to the concentrated windings which shorten the end windings compared to three-phase feeding permanent magnet synchronous motor (PMSM) [42]. The most popular way to control BLDC motors is by PWM current control in which a two-phase feeding scheme is considered with variety of PWM modes such as soft switching, hard-switching, and etc. If the back-EMF waveform is ideal trapezoidal with 120 electrical degrees flat top, three hall-effect sensors are usually used as position sensors to detect the current commutation points that occur at every 60 electrical degrees. Therefore, a relatively low cost drive is achieved when compared to a PMSM drive with expensive high-resolution position sensor, such as optical encoder.
30 Several current and torque control methods have been employed for BLDC motor drives to minimize the torque pulsations mainly caused by commutation and non- ideal shape of back-EMF. The optimum current excitation method, considering the unbalanced three-phase stator windings as well as non-identical and half-wave asymmetric back-EMF waveforms, is reported in [43]. Each phase back-EMF versus rotor position data is stored in a look-up table. Then, they are transformed to the dq –axes synchronous reference frame. The d –axis current is assumed to be zero and the q –axis current is obtained from the desired reference torque, motor speed, and the q –axis back- EMF. Consequently, inverse park transformation is applied to the dq –axes currents to obtain the abc frame optimum reference current waveforms. Minimum torque ripple and maximum efficiency are achieved at low speeds for a BLDC motor. However, three hysteresis current controllers with PWM generation which increases the complexity of the drive are used to drive the BLDC motor. Several transformations are required in order to get the abc frame optimum reference current waveforms. These transformations complicate the control algorithm and the scheme could not directly control the torque, therefore fast torque response cannot be achieved. In [44], estimating the electromagnetic torque from the rate of change of coenergy with respect to position is described. However, the stator flux linkage, the coenergy, and the torque versus the estimated position look-up tables are needed to generate the optimized current references for the desired torque, therefore more complicated control algorithm is inevitable. Moreover, open-loop position estimation

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