Method for directly changing mains-frequency power into low-frequency rational number frequency power

The invention provides a method for directly changing mains-frequency power into low-frequency rational number frequency power. The method includes the steps: firstly, numbering six silicon controlled rectifiers; secondly, dividing a variable frequency target value by a power grid mains-frequency value to obtain a decimal, and truncating the decimal according to needed precision to form a rational number; thirdly, amplifying the rational number into an integer, dividing the obtained integer by an amplification factor, reducing a fraction, denoting a numerator as an N and denoting a denominator as an M; fourthly, gaining a reciprocal of the mains-frequency value to obtain a mains-frequency cycle; fifthly, multiplying the M by the mains-frequency cycle to obtain a control cycle, and multiplying the N by the mains-frequency cycle to obtain a conduction cycle; finally, forming an open pulse signal of each silicon controlled rectifier according to the phase, the conduction direction, the mains-frequency cycle, the control cycle and the conduction cycle of the silicon controlled rectifier, controlling on and off of each silicon controlled rectifier through the open pulse signal of each silicon controlled rectifier, and outputting power with a target frequency. A scheme for continuously adjusting the speed below an engineering mains-frequency is provided for a motor.