In electrical engineering, a device for changing the frequency of a voltage or current. It is used in power supply systems for adjustable electric drives and magnetic amplifiers. Its other uses include the matching of two or more AC systems with differing frequencies. Frequency changers can be classified into static, rotating and combined types. Static frequency changers can be further divided into electromagnetic and rectifier types. The operation of electromagnetic frequency changers is based on changing the shape of a sinusoidal alternating voltage by means of magnetic nonlinear elements, such as chokes and transformers with saturable cores, and then extracting the voltage component with the required frequency. Electromagnetic frequency changers are used as frequency dividers and frequency multipliers; their efficiency is between 70 and 80 percent.