On-line dead time compensation technique for open-loop PWM-VSI drives

A new online dead-time compensation technique for low cost open-loop PWM VSI-fed drives is presented. Because of the growing numbers of open-loop drives operating in the low speed region, the synthesis of accurate output voltages has become an important issue where low cost implementation plays an important role. The so-called average dead time compensation techniques rely on two basic parameters to compensate for this effect: the magnitude of the volt-seconds lost each PWM cycle; and the direction of the current. In a low cost implementation, it is impractical to attempt an online measurement of the volt-seconds error introduced each cycle; instead an offline measurement is favored. On the other hand, the current-direction detection must be done online. This becomes increasingly difficult at lower frequencies and around the zero crossings leading to erroneous compensation and voltage distortion. This paper presents a simple and cost effective solution to this problem by using an instantaneous back calculation of the phase angle of the current. Given the closed loop characteristic of the back calculation, the zero crossing of the current is accurately obtained, thus allowing for a better dead time compensation. Experimental results validating the proposed method are presented