Thyristor frequency changer using discrete frequency modulation technique

Ali Kadhem Jaber Al-Huseini
Dr. Mohammed T. Lazim

Abstract : In recent years modulation techniques techniques have been introduced into power semiconductor switching devices have proved to be suitable elements for achieving the required modulation processes. Due to the development of these devices, modulation find widespread application in the power frequency conversion and power control. In the present work an attempt is made to use a frequency modulation of discrete form as a technique to development a frequency changer which may compete with the conventional power frequency changer. Mixed modulation systems such as amplitude / phase (AM/PM) and frequency/ phase (FM/PM) are also introduced to solve many disadvantages and undesirable features associated with pure (FM) modulation. The mixed modulation technique lead to the development of a generalized equation of modulation that is applicable for all types of modulation concerned with the present work. A prototype power frequency changer employing various type of discrete modulation is designed and built in the laboratory. The system is tested with three-phase resistive, inductive and motor loads. The design of the circuit is simple, inexpensive, and can be assembled easily using commercially available components . It is found that there is a very good agreement between the theoretical and experimental results. The problem of phase unbalance associated with a discrete modulation when it is used with a three-phase system is also studied . Phase shifting technique with the aid of the computer is employed as an attempt to solve the problem of unbalanced phase of the generated harmonic components.