ABSTRACT

In this Thesis, finite element analysis technique is used in the analysis of a 7.5 kW q cage induction motor for improved performance. Different loading conditions (no > load, blocked rotor, and intermittent loading) were analyzed by finite element simu ations, their influence on the performance of the machine was investigated.The effect of damping coefficient on the behaviors of the motor was also presented. A study of the basic characteristics ofthe motor at two operating temperatures of 75°C and 30°C was investigated. It was observed that the motor recorded increase in the phase current (against speed and output power), output power (against speed) and torque (against speed) when the operating temperature was 30°C. A study of the losses occurring in the motor performed at the two operating conditions (temperature and frequency) showed that stator copper loss and rotor copper loss were higher at 75°C and 50 Hz. The motor efficiency based on material design was also studied at 75°C and 30°C. The materials considered in this thesis were iron, nickel and cobalt. It was observed that iron at 30°C provided the best efficiency than other materials. It should be noted that the motor should not be operated on intermittent basis in 0.2s time intervals when given loads that are above 32Nm since the operation led the motor into generating mode. The flux levels, magnetic vector potential and magnetic flux density at these loading conditions were also monitored. Co-simulation of the maxwell model with the PWM inverter was also studied which showed that the performances of the motor fed by PWM inverter at supply frequencies of 50Hz and 100Hz. At 100Hz, a high speed with low electromagnetic torque and low stator current were observed while at 50Hz, a lower speed with high electromagnetic torque and high stator current were observed.Co-simulation of the maxwell model with MATLAB model was also presented. The simulated results from FEA and MATLAB softwares were compared. The overshoots from the two softwares at startup were presented and compared for the no-load and rated load conditions.FEA software recorded overshoots of 1777.906rpm, 37.43999A and 143.0724Nm while MATLAB software has overshoots of 1810.138475rpm, 117.4587A and 111.7447Nm on no-load while FEA software recorded overshoots of 1888.231rpm, 41.61333A and 167.2048Nm and MATLAB software has overshoots of 1807.744rpm, 117.6073A and 111.8348Nm at rated load. The results obtained for the transient behavior of the motor from the two models differed because of the assumptions made in development of the conventional model. However, there was agreement between the FEA and conventional methods at steady state operation. It can be inferred that FEA software (Ansys Maxwell2D) is highly suited for machine analysis under steady and dynamic conditions.