Frequency is defined as the number of occurrences of a repeating event per unit of time. In electrical engineering, it is also known as line frequency or mains frequency of an alternating signal.

In the late 19^{th} and 20^{th} centuries, the development of electrical equipment of a commercial power system uses different frequencies. It makes a slow process of development due to a large investment in different equipment. Hence, it needed standardization of frequency in all over the world.

But in present also, it is not done. Some countries use 50Hz frequency and others use 60Hz frequency. And Japan always stays advance in technology and Japan uses both.

The 50Hz frequency uses a 220-240V voltage level. And similarly, 60Hz frequency uses a 100-127V voltage level. There is no technical reason to use 50Hz or 60Hz. And there is not a perfect technical answer to which frequency is better.

This frequency also known as nominal frequency. The frequency of the grid varies around the nominal frequency.

In AC signals, several factors are affecting the frequency. It also affects most of the equipment of power system networks like Transmission line, Motors, Transformer, Generators, Lighting, Household equipment, and industrial machinery.

All the equipment designed to operate on the nominal frequencies. In the late 19^{th} century, the designer chooses the high frequencies in the transformer to reduce the cost. And also, it reduces the visible flicker of the lamps.

But some designers, choose the lower frequency for transmission lines, motors, and rotary converters.

## Frequency in Engineering

Frequency defined as the number of cycles per unit time for rotating, oscillating, or wave signals. Generally, it is denoted as f.

The frequency is inversely proportional to the time period. The SI unit of frequency is Hertz (Hz). It is the name of German physicist Heinrich Hertz.

One Hertz defines as that event repeat one time in one second. SI unit of time period (T) is second. So, frequency is 0.5Hz, then time period T=2s.

## Angular Frequency

Angular frequency defined as a rate of change of angular displacement (θ) of the sinusoidal waveform. It is nothing but a rate of change of phase of oscillations and waves.

Angular frequency denoted as omega (ω). The mathematical form of angular frequency is as below equation.

The unit of angular frequency is radian per second (rad/s). Angular frequency is larger than regular frequency by factor of 2π.

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