It has been explained that one cycle of alternating current covers a period in which the current value increases from zero to maximum in one direction, returns to zero, increases to maximum in the opposite direction, and then returns to zero.

The number of cycles occurring per second is the frequency of the current and is measured in a unit called the Hertz, named for Heinrich Rudolph Hertz, a German physicist of the late nineteenth century who made a number of important discoveries and valuable contributions to electrical science.

The repetition rate of a periodic signal used to represent or process a communication signal. Frequency is expressed in units of hertz (Hz). 1 Hz represents one cycle per second, 1 MHz represents one million cycles per second, and 1 GHz represents one billion cycles per second.

City lighting and power systems in the United States generally operate at a frequency of 60 Hz.  In the U.S., this is the standard frequency for ac. In some places it is 50 Hz. (Some remote places even use dc, but they are definitely the exception, not the rule.)

In radio practice, higher frequencies are common, and you’ll hear about kilohertz (kHz), megahertz (MHz) and gigahertz (GHz). Alternating currents in airplane circuits usually have a frequency of 400 Hz.

This frequency is commonly used for modern jet aircraft as well as for a number of other applications. The word alternation is frequently  used in discussing alternating current, and it means one half cycle. It is apparent, therefore, that there i: are 120 alternations in a 60-Hz current.

Usually, but not always, the waveshapes are of the type shown in Fig. 2-8. This waveform is known as a sine wave or a sinusoidal waveform.

The frequency of an alternating current has considerable effect on the operation of a circuit, for many units of electrical equipment operate only on current of a certain frequency. Wherever such equipment is used, it is important to make sure it is designed for the frequency of the current in the circuit in which it is to be used.

Units such as synchronous motors operate at speeds proportional to the frequency of the current even though the voltage is somewhat lower or higher than the rated voltage of the machine.

It is also important to remember that a circuit designed for a given frequency may be easily overloaded by using a current of a different frequency, even though the voltage 65 may remain the same. This is because of effects of inductive and capacitive reactance.

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