Automatic Frequency Control

automatic frequency control: see automatic tuning control.

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automatic frequency control [¦ȯd·ə¦mad·ik ′frē·kwən·sē kən‚trōl]

(electronics)

Abbreviated AFC.

A circuit used to maintain the frequency of an oscillator within specified limits, as in a transmitter.

A circuit used to keep a superheterodyne receiver tuned accurately to a given frequency by controlling its local oscillator, as in an FM receiver.

A circuit used in radar superheterodyne receivers to vary the local oscillator frequency so as to compensate for changes in the frequency of the received echo signal.

A circuit used in television receivers to make the frequency of a sweep oscillator correspond to the frequency of the synchronizing pulses in the received signal.

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Automatic Frequency Control 

a radio circuit for automatically maintaining a given frequency of electrical oscillations of an oscillator. Automatic frequency control is utilized in a transmitter for maintaining a given frequency of the master oscillator, in super-heterodyne radio receivers for accurate tuning of stations being received, in a frequency synthesizer for frequency multiplication or division, and for other purposes. In a widely used automatic frequency control circuit, deviation from a given frequency (frequency separation) is converted by the discriminator into a constant voltage of corresponding sign (polarity), proportional in amplitude to the separation (in the absence of separation the voltage at the discriminator output is equal to zero). This output voltage is then passed to the control element (reactance tube, reactance transistor, variable capacitor, and others), regulating the oscillator frequency.

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Automatic Frequency Control 

a process of maintaining the frequency of alternating current in electrical energy systems within the limits permissible by technical requirements and conditions of economic exploitation of its operation.

The normal functioning of a significant portion of users of electrical energy is dependent upon the frequency of the supply current. In certain manufacturing establishments—for example, in paper production and textile machinery—significant frequency deviations lead to spoilage in production and sometimes to accidents. As a result of frequency decrease, the productivity of the feed pumps of thermoelectric stations is substantially decreased, and this can cause damage to the operation of the energy system. In the USSR the nominal frequency of alternating current in electrical systems is equal to 50 hertz. Long-term deviations not exceeding 0.1 Hz (that is, 0.2 percent) are permissible.

The frequency of alternating current of an electrical energy system is determined by the rotary speed of synchronous generators operating in parallel. Changes in power requirements lead to an increase or decrease in their speed, and the system’s frequency changes. In order to guarantee the required frequency level, turbine power must be altered. This is achieved by speed governors, which regulate the energy supply being admitted (water, steam, gas) to the turbine. Automatic frequency control, attained by means of speed governors is designated as primary control. With the exception of special cases, primary regulation is not sufficient for maintaining a normal frequency level in the electrical system. Therefore, supplementary, secondary frequency regulation is used. For this purpose special devices are introduced, which act upon the speed governor and thus contribute to supplementary alteration of the input of the energy carrier to the turbine.

When secondary control of frequency is utilized on several motors or stations of a system, the need to secure a stable load distribution between them arises. This problem is related to the regulation of active power and optimum load distribution between units.

REFERENCES

Solov’ev, I. I. Avtomatizatsiia energeticheskikh sistem, 2nd edition. Moscow-Leningrad, 1956.
Moskalev, A. G. Avtomaticheskoe regulirovanie rezhima elektricheskoi sistemy po chastote i aktivnoi moshchnosti. Moscow-Leningrad, 1960.

V. P. VASIN and V. A. STROEV