Some countries in the world, such as the United States and the United States, use 60Hz alternating current because they use the dodecal system, which 12 constellations, 12 hours, 12 shillings equal to 1 pound and so on. Later countries adopted the decimal system, so the frequency was 50Hz.

So why do we choose 50Hz alternating current instead of 5Hz or 400Hz?

What happens if the frequency is low?

The lowest frequency is 0, which is DC. In order to prove that Tesla's alternating current was dangerous, Edison killed a lot of small animals with alternating current. If elephants were also small animals Objectively speaking, under the same current, the human body's time to withstand direct current is longer than that to withstand alternating current, which has something to do with ventricular tremor, that is, alternating current is more dangerous.

Lovely Edison also lost to Tesla in the end, alternating current defeated direct current with the advantage of convenient changing voltage level. In the case of the same transmission power, when the voltage is increased, the transmission current will be reduced, and the energy consumed on the line will also be reduced.

The other problem of DC power transmission is that it is difficult to disconnect. Until now, this problem is still a problem. The problem of DC power transmission is the same as that of electric spark when pulling out the plug. When the current reaches a certain level, the electric spark cannot be extinguished. We call it "arc".

For alternating current, the current will change direction, so there is a time when the current passes zero. Using this small current time point, we can cut off the line current through the arc extinguishing device. But the direction of DC current will not change. Without this zero crossing point, it is very difficult for us to extinguish the arc.

What's the problem with low frequency alternating current?

一、The problem of transformer efficiency
Transformer is based on the original side of the magnetic field changes, induced by the secondary side of the step-up or step-down. The slower the frequency of magnetic field changes, the weaker the induction is. In the extreme case, it is DC. There is no induction at all, so the frequency is too low.

二、Power problem of electric equipment
For example, the engine speed of an automobile is its frequency, such as 500 rpm at idle speed and 3000 rpm at acceleration and shifting, which are converted into 8.3Hz and 50Hz respectively. This shows that the higher the speed, the greater the power of the engine.

In the same way, under the same frequency, the larger the engine, the greater the output power, which is also why the diesel engine is larger than gasoline. Only a powerful diesel engine can drive heavy-duty vehicles such as buses and trucks.

In the same way, the motor (or all rotating machinery) requires small size and high output power. There is only one way to improve the speed, which is why the AC frequency cannot be too low, because we need a small but high-power motor.

The same is true for variable frequency air conditioning. The output power of air conditioning compressor is controlled by changing the frequency of alternating current. In short, power and frequency are positively correlated in a certain range.

What about the frequency? How about 400 Hz？

There are two problems, one is the increase of loss of lines and equipment, the other is that the speed of generator is too fast。

First of all, the loss, transmission lines, substation equipment, electrical equipment, are reactance, reactance is proportional to the frequency, the higher the frequency, the greater the reactance, the greater the reactive power consumed, the less the active power can be transmitted.

At present, the reactance of 50Hz transmission line is about 0.4 ohm, about 10 times of the resistance. If it is increased to 400Hz, the reactance will be 3.2 ohm, about 80 times of the resistance. For high voltage transmission lines, reducing reactance is the key to improve transmission power.

The higher the frequency is, the smaller the capacitive reactance is and the larger the leakage current of the line is. If the frequency is high, the leakage current of the line will also increase.

Another problem is the speed of the generator. Today's generator set is basically a single-stage machine, that is, a pair of magnetic poles. In order to generate 50Hz power, the rotor speed should reach 3000 rpm. When the engine speed reaches 3000 rpm, you can obviously feel the engine vibrating. When you turn to 67000 rpm, you will feel the engine jumping out of the hood.

The automobile engine is still like this, not to mention a solid iron lumpy rotor and steam turbine weighing up to 100 tons, which is also the reason for the noise of the power plant. It is not easy for a 100 ton steel rotor to turn 3000 rpm. If the frequency is three or four times higher, it is estimated that the generator can fly out of the plant.

Such a heavy rotor has a considerable inertia, which is also the premise of the power system known as the inertial system to maintain safe and stable operation. It is also the reason why wind power and solar energy intermittent power supply challenge the traditional power supply.

Because of the rapid change of the wind and the wind, the speed of reducing or increasing the output of the dozens of tons of rotor is very slow (the concept of climbing rate) due to the huge inertia, which can not keep up with the changes of wind power and photovoltaic power generation, so sometimes it has to abandon the wind and light.

综上所述

The reason why the frequency can't be too low: the transformer has high energy efficiency, and the motor can be small and powerful.

The reason why the frequency can't be too high: the line and equipment can lose less, and the speed of the generator doesn't have to be too high.

Therefore, according to experience and habits, our electric energy is set at 50 or 60 Hz.