This thesis investigates and evaluates the operation of an AC variable speed drive consisting of a squirrel cage induction motor (SCIM), controlled by a pulse width modulation (PWM) based controller. PWM is used to provide the variable voltage/variable frequency supply necessary for the effective speed control of this type of motors. The combination of the SCIM and the PWM controller offers one of the best solutions in the present industrial drive applications and is promising to have an even wider range of applications in the future. However, some of the problems associated with this type of drives have to be surmounted in order to achieve this aim. The primary objective of this research work has dealt with one these problems, namely the poor performance of these drives at low speeds caused by the stator resistance effects. Before tackling the above problem, the drive system as a whole, the SCIM and its speed controllers, is introduced. particular attention is given to the PWM controller and a classification of the different types of PWM is proposed. As an example, a typical PWM controller, used in this research, is then presented in detail. Tests and measurements performed on that latter, in order to establish its limitations and hence deduce solutions and design guide-lines for its improvement in general and to the above mentioned problem in particular are described and their results presented. This study has led to the development of a boost modulator . To compensate for the increased effects of the stator resistance at low speeds, usually, a certain value of voltage boost is selected and comes into operation below a given speed. This solution is found here to be not effective since it optimizes the drive operation for one operating point only or at its best for a very small operating range but not for the whole of it. For example, for a given speed, the amount of boost applied remains the same for a no-load or full-load operation and consequently, extra losses and overheating in the motor with a malfunctioning of In this the sis the proposed boost modulator which overcomes this problem is presented and its implementation is described. It applies the required value of voltage for each operating point which allows for not only the low speed resistance effects to be compensated but for the drive operation ta be optimized over its whole range of speed and torque. Its principles are based on the knowledge of the actual speed and torque. However, one of its important features is that it requires no transducers from the motor, which is necessary for keeping the simplicity and ruggedness of the SCIM since these are an essential requirements and two of the main criteria for selecting the SCIM in many applications. The present boost modulator uses only parameters measured within the drive for its operation. The most significant use is that of the DC link current in the optimization of torque.