Since the 70's modulation techniques have been introduced into power engineering application. The semiconductor devices in the form of thyristors, triacs and power transistors prove to be suitable switches for achieving the required modulation, therefore modulation methods find widespread applications in power frequency conversion and power control. In the present work a power frequency changer based on the discrete phase modulation technique is introduced. This type of modulation, although it is complicated in theory and circuitry, it has many advantages compared with amplitude modulation. The supply component can be completely eliminated, and the power associated with this component transferred to the other desirable frequency components in the generated frequency spectrum, therefore a process of frequency changing with high power is achieved. A prototype power frequency changer employing discrete phase modulation is designed and built with the aid of SDK-85 system to verify the predicted results. The system is tested with three-phase resistive, inductive and induction motor loads. It is found that there is a well agreement between the theoretical and the experimental results. The problem of phase unbalance associated with phase modulation when it is used with a three phase system is also studied. Phase shifting technique with the aid of the microprocessor - is employed as an attempt to solve the problem of unbalanced phases of the generated harmonic components.