Some countries in the world, such as the UK and the US, use 60Hz alternating current because they use a binary system, which is used for things like the 12 constellations, 12 hours, and 12 shillings equaling 1 pound sterling. Later countries adopted the decimal system, so the frequency is 50Hz.
So why do we choose 50Hz AC power instead of 5Hz or 400Hz?
What happens if the frequency is low?
The lowest frequency is 0, which is direct current. Back then, to prove that Tesla's alternating current was dangerous, Edison electrocuted a number of small animals with alternating current. If elephants are considered small animals... Objectively speaking, under the same current magnitude, the human body can tolerate direct current for a longer time than alternating current, which is related to things like ventricular fibrillation. In other words, alternating current is more dangerous.
Edison ultimately lost to Tesla; alternating current (AC) triumphed over direct current (DC) due to its advantage of easily changing voltage levels. When transmitting the same power, increasing the voltage reduces the current supplied, thus decreasing the energy consumed in the circuit.
Another problem with DC power transmission is its difficulty in switching off, a problem that remains a challenge to this day. Similar to the electric sparks that occur when unplugging an appliance, when the current is high enough, these sparks cannot be extinguished; this is called an "electric arc."
For alternating current (AC), the current changes direction, resulting in a zero-crossing point. We can use this small current point to interrupt the current in the circuit using an arc-extinguishing device. However, direct current (DC) does not change direction; without this zero-crossing point, arc extinguishing is very difficult.
What are the problems with low-frequency alternating current?
I. The problem of transformer efficiency
Transformers work by inducing voltage increases or decreases on the secondary side by changing the magnetic field on the primary side. The slower the frequency of the magnetic field change, the weaker the induction. In extreme cases, there is no induction at all in direct current. Therefore, a frequency that is too low is not suitable.
II. Power Consumption Issues of Electrical Equipment
For example, the engine speed of a car is its frequency. For instance, 500 RPM at idle and 3000 RPM when accelerating and shifting gears translates to frequencies of 8.3 Hz and 50 Hz respectively. This shows that the higher the engine speed, the greater its power output.
Similarly, at the same frequency, the larger the engine, the greater the output power. This is why diesel engines are larger than gasoline engines. Only large and powerful diesel engines can power heavy vehicles such as buses and trucks.
Similarly, electric motors (or any rotating machinery) need to be small in size and have high output power. There is only one way to achieve this: increase the rotational speed. This is why the frequency of alternating current cannot be too low, because we need electric motors that are small in size but have high power.
Inverter air conditioners work on the same principle, controlling the output power of the air conditioner compressor by changing the frequency of the alternating current. In short, power and frequency are positively correlated within a certain range.
Let's talk about what happens if the frequency is too high? For example, what about setting it to 400Hz?
There are two problems: first, the losses in the lines and equipment are increasing; second, the generator is running too fast.
Let's talk about losses first. Transmission lines, transformers, and electrical equipment all have reactance. Reactance is directly proportional to frequency. The higher the frequency, the greater the reactance, the greater the reactive power consumed, and the less active power can be transmitted.
Currently, the reactance of a 50Hz transmission line is about 0.4 ohms, approximately 10 times its resistance. If the frequency is increased to 400Hz, the reactance will be 3.2 ohms, about 80 times its resistance. For high-voltage transmission lines, reducing reactance is key to increasing transmission power.
Corresponding to reactance is capacitive reactance, which is inversely proportional to frequency. The higher the frequency, the lower 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 issue is the generator's rotational speed. Modern generator sets are basically single-pole machines, meaning they have only one pair of magnetic poles. To generate 50Hz electricity, the rotor needs to rotate at 3000 revolutions per minute. When a car engine reaches 3000 revolutions per minute, you can clearly feel the engine vibrating and making noise; at 6000 or 7000 revolutions per minute, you'll feel like the engine is about to jump out of the hood.
If a car engine is this noisy, imagine the noise from a solid iron rotor and turbine weighing hundreds of tons. This is one of the reasons why power plants are so noisy. A steel rotor weighing hundreds of tons rotating 3000 revolutions per minute is no easy feat. If the frequency were three or four times higher, the generator would probably fly off the plant.
Such a heavy rotor possesses considerable inertia, which is why power systems are called inertial systems—a prerequisite for their safe and stable operation. This is also why intermittent power sources like wind and solar power pose a challenge to traditional power sources.
Because wind and solar power change rapidly, the rotor, weighing tens of tons, has a very slow speed to reduce or increase its output due to its huge inertia (the concept of ramp rate), and cannot keep up with the changes in wind and solar power generation. Therefore, sometimes it is necessary to curtail wind and solar power.
In summary
The reason why the frequency cannot be too low is that transformers have high efficiency, and motors can be small in size but have high power.
The reason why the frequency cannot be too high is that the line and equipment can have low losses, and the generator speed does not need to be too high.
Therefore, based on experience and habit, our electrical energy is set at 50 or 60 Hz.
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