Enhancement of starting performance of an induction motor by improving rotor design

Abstract

The goal of the thesis is to develop a novel technique to ~nhance the starting torque and starting performance of a three phase squirrel cage induction motor by changing the material composition of the end connections of the rotor. Magnetic materials show significant skin effect properties at power frequency. This property has been exploited in this research work to change the resistance of the end connections of the rotor to enhance the starting perfonnanee of the squirrel cage induction motor. Commercially available iron strips having relative pem1eability of the order of 5000- 6000 have been introduced in the rotor end connections for the purpose. At start, the rotor induced current has a frequency of 50 Hz which shows skin effect large enough to increase the rotor resistance. As the starting torque of the motor is a function of rotor resistance, the starting torque increases appreciably together with the decrease in the starting current. As the motor acquires its rated speed, the frequency of the rotor current decreases, diminishing the skin effect and thereby reducing overall resistance of the rotor. Thus the overall performance of the motor remains virtually unaffected. At first, experimentally determined motor parameters were used in MATLAB and the performance of the motor was evaluated to ascertain the impact of skin effect on the modified rotor end connections. The simulation results show appreciable enhancement in the starting perfom1ance of the motor. Using the simulation software, the optimum size of the magnetic materials placed at the end connections has been determined to achieve high starting torque without affecting the overall running performance of the motor. The dimension thus obtained is used to modify the end connection of the rotor for experimental verification of the simulated results. The experimental results are in good agreement with the simulated ones. It is worth mentioning that, the starting current is also reduced significantly, as expected. For further validation of the experimental results analytical study has also been carried out with the help of software package SLIM.