Frequency Changer

(1) 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.

Transistors and thyristors are commonly used as rectifiers in rectifier-type changers, where they have superseded thyratrons and mercury-arc tubes. Static frequency changers of the transistor type are used mainly in radio-engineering equipment; their power rating can be as high as 2 to 3 kilovolt-amperes (kVA). Thyristor-type static frequency converters can be of three kinds: with direct coupling, with an intermediate DC stage, and with an intermediate AC stage at an increased frequency.

Static frequency changers with direct coupling (current rectifiers can be considered as being in this class) are made use of in high-power industrial AC electric drives, in AC electric drives of self-contained power systems with generators that operate at an increased frequency, and in the equipment of centralized power supply systems for passenger trains. The efficiency of such frequency changers reaches 95–98 percent.

A frequency changer with an intermediate DC link is a two-stage frequency changer; a rectifier is built into the input stage, and an inverter unit is provided in the output stage. Such static frequency changers are used in industrial and traction AC electric drives where a smooth regulation of frequency and voltage is required; their power rating can reach 3 to 5 megavolt-amperes. Their efficiency is somewhat lower than that of frequency changers with direct coupling.

Static frequency changers with an intermediate AC stage operated at a frequency higher than that of the power supply are used less widely. They have an inverter unit built into the input stage. The output stage includes a static frequency changer with direct coupling. Loads operating at the increased frequency can be connected to the intermediate buses. The efficiency of this type is lower than that of frequency changers with an intermediate DC stage.

Rotating frequency changers can be built either as two machines or as one machine. The two-machine type usually combines an electric driving motor and a generator of alternating or direct current. Two-machine frequency changers with a synchronous generator supplying current at frequencies of 50 to 400 hertz are used in self-contained power systems. Their efficiency can reach 85 percent, and their power ratings range from 30 to 800 kVA. Rotating frequency changers can also be built as a single electric machine with a common armature.

(2) In radio engineering, the circuit in a superheterodyne receiver that changes, or converts, the frequency of the incoming oscillations to an intermediate frequency that is usually lower than the incoming frequency. Often called a frequency converter, it consists of a frequency mixer and a local oscillator (heterodyne) with transistor components or of a single converter tube. In its broad sense, the term “frequency changer” often designates other radio-engineering devices associated with frequency conversion, such as frequency synthesizers, frequency dividers, and frequency multipliers.