Voltage harmonics analysis and efficiency of three-phase induction motor with change in coil pitch of the stator winding.

Variable speed drives employing induction motors have been widely used in industry for decades. Today there is a continually increasing demand for more precise and flexible speed control usually with close attention to energy efficiency. The inverter is used because of its reliability, flexibility and relatively low cost. However its output a.c. voltage is not sinusoidal so the core losses in the induction motors consequently increase. This research is centred on the design and testing of the stator winding configuration of three phase induction motors with various coil pitches and measure the dynamic performance under sinusoidal and PWM supplies. Measurements were carried out to determine the behavior of harmonic losses and the efficiency of four identical three-phase 746 W induction motors with stator coil pitches of 180 , 160 , 140 and 120 . The motors were fed from either a three-phase inverter or a three-phase sinusoidal voltage supply. The switching frequency was varied from 4 kHz to 16 kHz and the modulation frequency was varied between 30 Hz to 60 Hz. Simulations were carried out using OPERA 2D software under sinusoidal voltage supply. The phenomenon of chording by l/n* of pole pitch to suppress the n* harmonic was particularly followed by motors with 120 and 160 coil pitches under sinusoidal voltage supply. This phenomenon was also followed by 120 , 140 and 160 coil pitch motors under PWM voltage supply at all the switching frequencies and modulation * frequencies. The motor with 120 coil pitch showed a drastical increase in the lower order voltage harmonic components with simulation under sinusoidal supply when compared to full pitch motor. The total voltage harmonic distortion due to the third, fifth and ninth harmonics was less for the motor with 120 coil pitch under PWM voltage at higher switching frequencies and under over modulation condition. The efficiency of the same motor was higher at full load and over loads under all the switching frequencies and modulation frequencies. The measurement results and discussion enable motor manufacturers to consider 120 coil pitch motor under PWM voltage supply and 160 coil pitch motor under sinusoidal voltage supply for the 746 W induction motors as the increase in the efficiencies were 12% and 5% respectively when compared to full pitch motor