ABSTRACT

This thesis presents the stability analysis of three horse-power squirrel cage induction motor for enhanced dynamic performance. It includes the machine Simulink modelling under dynamic condition, its steady state analysis and stability study. Simulink software is used for step by step dynamic modelling of the machine. Matlab program is also developed for the study of the steady state behaviour of the machine, this includes comparative analysis of the torque-speed characteristics of the machine under transient and steady-state, the torque-speed characteristics of the machine at various values of rotor resistance and also the torque-slip characteristics of the machine at various slip ranges. For the stability studies, the ordinary differential equations that described the motor electrical and mechanical models is linearized for the computation of the eigenvalues, transfer functions, poles and zeros. From the transfer function developed the step response of change in electromagnetic torque divided by change in load torque at different operating slip is analysed. The transfer function is also used to analyse the change in electromagnetic torque divided by change in stator voltage at different operating slip. The zero and pole plots are also presented on the plane. The results from the dynamic modelling of the machine reveal the oscillations or ripples experienced by the machine at start-up. The torque-speed graph at various values of rotor resistance shows that the maximum torque is independent of rotor resistance, but the starting torque, the speed at which the maximum torque is developed and the duration at which it is sustained depend on the rotor resistance. The study of the torque-slip characteristics of the machine at various slip ranges shows that at the slip range of zero to one, the machine is in the motoring mode. Considering the slip range of -2 to 2, the machine experiences both the motoring mode (slip range of 0 to 2) and the generating mode (slip range of -2 to 0). The step response graphs, the eigenvalues and the zero-pole plots are used for the stability analysis of the machine for enhanced dynamic performance. The results show that the machine is stable at the two different cases presented when the steady state slip values are 0, 0.03, 0.1, 0.4 and 0.6, but is unstable at steady state slip values of 0.8 and 1.0. This study gives an idea of better operating regions of the induction machine for enhanced dynamic performance. The stability analysis is carried out using eigenvalue method which takes care of absolute stability of the machine. An improvement can be achieved by the use of other stability control techniques such as Nyquist plot and Root locus which can account for the motor’s relative stability.