Our common alternating current is a sine wave, and a sine wave is described using: amplitude, frequency and phase, called the three elements.
The so-called sine wave, that is, its voltage and current waveform (or instantaneous value) is changed with time in accordance with the sine wave law on the horizontal coordinate, half of the time is in the positive half cycle and half of the time is in the negative half cycle, and the change period is 0.02 seconds. It changes 50 times a second, and this is called frequency.
Hz= Hertz, the number of times the alternating current changes per minute. The prescribed alternating current frequency in our country is 50Hz. Some countries have 40Hz. Generally, it should be used together, such as the motor standard 50Hz, if the current is lower than 50Hz, the number of revolutions will be reduced.