**9. Acknowledgment**

The authors would like to acknowledge the financial support of THRIP and TESP.

### **10. References**


**1. Introduction**

are issues that should receive significant attention.

Distinctive features of synchronous machines like constant operation-speed, producing substantial savings by supplying reactive power to counteract lagging power factor caused by inductive loads, low inrush currents, and capabilities of designing the torque characteristics to meet the requirements of the driven load, have made them optimal options for a multitude of industries. Economical utilization of these machines and also increasing their efficiencies

**On the Application of Optimal PWM of**

**High Power Ratings** 

*1USA 2Iran* 

**16**

Arash Sayyah1 and Alireza Rezazadeh2 *1ECE Department, Boston University, Boston, MA 2ECE Department, Shahid Beheshti University, Tehran* 

**Induction Motor in Synchronous Machines at** 

At high power rating operations, where high switching efficiency in the drive circuits is of utmost importance, optimal pulsewidth modulation (PWM) is the logical feeding scheme (Holtz, 1992). Application of optimal PWM decreases overheating in machine and therefore results in diminution of torque pulsation. Overheating, resulted from internal losses, is a major factor in rating a machine. Moreover, setting up an appropriate cooling method is a particularly serious issue, increasing in intricacy with machine size. Among various approaches for achieving optimal PWM, *harmonic elimination method* is predominant ((Mohan et al., 1995), (Enjeti et al., 1990), (Sun et al., 1996), (Chiasson et al., 2004), (Czarkowski et al., 2002), (Sayyah et al., 2006c)). Since copper losses are fundamentally determined by current harmonics, defining a performance index related to undesirable effects of the harmonics is of the essence in lieu of focusing on specific harmonics (Bose, 2002). Herein, the total harmonic current distortion (THCD) is the objective function for minimization of machine losses.

Possessing asymmetrical structure in direct (*d*) and quadrature (*q*) axes makes a great difference in modeling of synchronous machines relative to induction ones. Particularly, it will be shown that the THCD in high-power synchronous machines is dependent upon some internal parameters of the machine; particularly *lq* and *ld*, the inductances of *q* and *d* axes, respectively. Based on gathered input and output data at a specific operating point, these parameters are determined using online identification methods (Ljung & Söderström, 1983). In light of the identified parameters, the problem is redrafted as an optimization task, and the optimal pulse patterns are sought through genetic algorithm (GA). Indeed, the complexity and nonlinearity of the proposed objective function increases the probability of trapping the

