Principle of alternating current

At present, most of the electric energy in the power supply network is sine wave alternating current.

So there are three questions. First, why is the electricity in the grid alternating current, not direct current? Second, why sine waves and not other waveforms? Third, why use the adjective "vast majority"?

In fact, these three questions respectively correspond to electrical knowledge points: first, long-distance transmission of electric energy; Second, the principle of generator generation; Third, UHV and power electronics. The detailed principles of the first, and the third will be updated later, and in this article, the space can only briefly explain the principles and conclusions. The second issue will be explored in detail in this article.

To answer the first question, the establishment of the power grid from the era of Edison-Tesla began, the power station issued electricity, need to go through a long line to send the power to the load end, including the electric machinery of the factory, household appliances, etc., and with the expansion of the power grid, the current of the grid will be more and more large, and at this time must consider the loss of the line and voltage drop. If the use of direct current transmission, according to the technology at that time to boost the voltage is difficult, if the use of low-voltage transmission, then the line loss is great, the voltage drop may even be reduced to the point of not being able to use, more importantly, the transmission cable will be too large current can not carry high-power applications. Ac due to the existence of transformers, convenient voltage boost, the same power not only the line loss and voltage drop greatly improved, more importantly, the current in the line changes, more save the cable consumption, the economy is much higher than direct current, so although Edison through AC can easily electrocution a large animal experiment to prove the superiority of direct current, But it still lost to alternating current, as represented by Tesla.

To answer the third question, let's first take a piece of news report that the "UHV 800kV DC transmission project" won the National Science and Technology Progress Award. Uhv 800kV DC transmission technology is the world's highest voltage level, the largest transmission capacity, the longest transmission distance, the most advanced technical level of transmission technology, is to solve the problem of energy and power load reverse distribution, the implementation of the national "west to east power transmission" strategy of the core technology. Does that negate the answer to the first question? The answer is no. The reason why UHV direct current transmission can be successful in recent years, the most basic progress comes from the great progress of semiconductor technology, especially the rapid development of power semiconductor technology, so that UHV direct current transmission can be successful.

The above are science popularization, now enter the main topic of this article:

Answer the second question:

Principle of generator generation

Since middle school, we know that the conductor cutting the magnetic inductance line will generate electromotive force at both ends of the conductor, that is, magnetic electricity. As for why can magnetic electricity be generated? The conductor cuts the magnetic inductance line, which is equivalent to the motion of the magnetic inductance line relative to the conductor, so an electric field perpendicular to the direction of the magnetic field will be generated. This led to Maxwell's theory of electromagnetic fields. But specifically, how to understand Maxwell's electromagnetic field theory from the micro level? I don't know, or didn't learn. There's a deeper nature of charge and magnetic moment that I don't think humans have yet discovered. After that, I felt I was entering the world of philosophy.

All right, this is going too far. Come back. Human scientists have found that the electromotive force generated by a conductor cutting a magnetic inductance line is proportional to the rate of change in the magnetic flux.

Suppose that the strength of the uniform magnetic field is B, the length of the side of the coil cutting the magnetic inductor is L, the area of the coil is S, the speed of cutting the magnetic inductor is V, and the coil rotates uniformly around the axis of symmetry perpendicular to the magnetic field at the angular velocity ω.

From the previous we can see that the electromotive force ε is equal to the derivative of the magnetic flux with respect to time, and the magnetic flux Φ=B*S=B*L*L,

The derivative of the magnetic flux Φ with respect to time is B*L*V is ε=B*L*V (this is when the wire completely vertically cuts the magnetic induction line)

When the coil moves to a general position, as shown in the figure below, the speed is V, but the actual cutting speed of the magnetic induction line is V1, then in general ε=B*L*V1, vector operations, V1=Vsin (ωt)

Therefore, the electromotive force of a single wire ε=B*L*Vsin (ωt), and V= ωL /2, and the total electromotive force of the coil e=2B*L*Vsin (ωt) =BL^2 sin (ωt) when there are two wires cutting the magnetic induction wire. The actual generator has many turns of coil n, then the total electromotive force of a generator e=nBL^2 sin (ωt), once a generator is completed, then nBL^2 is a fixed value, no longer change, so the electromotive force is a sine function of time.

When the coil passes through the neutral plane, the current will change direction, the coil rotates once, twice through the neutral plane, the current direction will change twice. The logarithm of the positive and negative magnetic fields in the actual motor can have 1 pair, 2 pairs, 3 pairs, and 5 pairs. Therefore, the frequency of the AC power emitted F=1/T=nP/60, where n represents the number of poles, P represents the speed of the coil, the unit is rap/min, F represents the frequency, the unit is Hz, 60 is per minute (second).

China stipulates that the standard power supply frequency is f=50 cycles/second, that is, the power frequency is 50Hz, so the speed of the rotating magnetic field is only related to the number of magnetic poles. The number of magnetic poles is large, and the rotational speed of the rotating magnetic field is low. When the pole number P=1, the rotational speed n=3000; When the number of poles P=2, the rotational speed n=1500; When the pole number P=3, the rotational speed n=1000; When the pole number P=4, the rotational speed n=750; When the pole number P=5, the rotating magnetic field speed n=600 (in fact, due to the existence of slip rate, motor. The actual speed is slightly lower than that of the rotating magnetic field)

In the frequency conversion speed regulation system (the basic principle of the engine and the motor is the same), according to the formula n=60f/p, we can see:

Change the frequency f to change the speed

Reduce the frequency ↓f, the speed becomes smaller: that is, 60 f↓ / p = n↓

Increasing the frequency ↑f increases the speed: 60 f↑ / p = n↑.

In summary, alternating current is a sine wave, and the frequency of electricity is determined by the number of poles and the speed of the generator.