Abstract: A power system includes an a-c power supply which constitutes a voltage source for a user of d-c current. The a-c power supply is designed to generate an output voltage having a trapezoidal wave form. This is achieved by generating a sinusoidal voltage and then suppressing the waves at a predetermined amplitude to obtain flat-topped voltage waves. The edges of the trapezoidal waves representing the output voltage of the a-c power supply have slopes which equal or exceed the corresponding slopes of a sinusoidal curve having the same maximum amplitude and period as the trapezoidal output voltage. The a-c power supply is further designed to maintain the mean or average value, as well as the maximum value, of its output voltage constant.