Why should the motor choose 50HZ AC?

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

What if the frequency is lower?

Cute Dickson also lost to Tesla in the end, and AC beat DC with the advantage of easily changing the voltage level. In the case of the same transmission power, increasing the voltage will reduce the transmission current, and the energy consumed on the line will also decrease. Another problem of DC transmission is that it is difficult to break, and this problem is still a problem until now. The problem of DC transmission is the same as the spark that occurs when the electrical plug is pulled out at ordinary times. When the current reaches a certain level, the spark cannot be extinguished. We call it “arc”.

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

The transformer relies on the change of the magnetic field on the primary side to sense the step-up or step-down of the secondary side. The slower the frequency of the magnetic field changes, the weaker the induction. The extreme case is DC, and there is no induction at all, so the frequency is too low.

For example, the speed of the car engine is its frequency, such as 500 rpm when idling, 3000 rpm when accelerating and shifting, and the converted frequencies are 8.3Hz and 50Hz respectively. This shows that the higher the speed, the greater the power of the engine.

In the same way, at the same frequency, the larger the engine, the greater the output power, which is why diesel engines are larger than gasoline, and the big and powerful diesel engines can drive heavy vehicles such as bus trucks.

In the same way, the motor (or all rotating machinery) requires both a small size and a large output power. There is only one way – to increase the speed, which is why the frequency of the alternating current cannot be too low, because we need a small size but high power. electric motor.

The same is true for inverter air conditioners, which control the output power of the air conditioner compressor by changing the frequency of the alternating current. In summary, power and frequency are positively correlated within a certain range.

Let’s talk about loss first. Transmission lines, substation equipment, and electrical equipment all have reactance. The reactance is proportional to the frequency. less.

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

Corresponding to reactance, there is also capacitive reactance, which is inversely proportional to frequency. The higher the frequency, the smaller the capacitive reactance and the greater the leakage current of the line. If the frequency is high, the leakage current of the line will also increase.

Another problem is the speed of the generator. The current generator set is basically a single-stage machine, that is, a pair of magnetic poles. In order to generate 50Hz electricity, the rotor rotates at 3000 rpm. When the engine speed reaches 3,000 rpm, you can clearly feel the engine vibrating. When it turns to 6,000 or 7,000 rpm, you will feel that the engine is about to jump out of the hood.

Because the scenery changes rapidly, the rotors weighing dozens of tons are very slow to reduce or increase the output due to the huge inertia (the concept of ramp rate), which cannot keep up with the changes of wind power and photovoltaic power generation, so sometimes it has to be abandoned. Wind and abandoned light.

It can be seen from this

The reason why the frequency cannot be too low: the transformer can be highly efficient, and the motor can be small in size and large in power.

The reason why the frequency should not be too high: the loss of lines and equipment can be small, and the generator speed does not need to be too high.

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