At one time, Edison and Tesla were opponents in the use of electric current in energy. Tesla believed that it was necessary to use alternating current, and Edison - that it was necessary to apply direct current. The second scientist had more opportunities, since he was engaged in business, but Tesla eventually managed to win, since he was simply right.
Introduction
Alternating current is much more efficient to use for energy transfer. We will discuss how AC power is calculated, because AC is the power that is transmitted from a distance.
Power calculation
Suppose we have an alternator that is connected to a load. At the output of the generator, between two points on the terminals, the voltage changes according to a harmonic law, and the load is taken arbitrarily: coils, active resistance, capacitors, electric motor.
A current flows in the load circuit, which changes according to a harmonic law. Our task is to establish what the power consumption of the generator is equal to. We have a generator at our disposal. The input direction, which will change according to a harmonious rule, is presented as input data:
(U (t) = U (m) cos wt)
Load is the most arbitrary concept.
The current strength in the load and, accordingly, in the wires that bring power to the load will change. The frequency of current oscillations will come out the same as the frequency of voltage fluctuations, but there is also the concept of phase shift in the intervals of current and voltage fluctuations:
(I (t) = I (m) cos wt)
Further calculations
Power indicators will be equal to the product:
P (t) = I (t) U (t)
This law remains valid both for alternating current with the power that needed to be calculated, and for direct.
(I (t) = I (m) cos (wt + J)
AC power for AC is calculated using three formulas. The above calculations relate to the basic formula, which follows from the determination of current strength and voltage.
If the chain section is homogeneous and Ohm's law can be used for this section of the chain, such calculations cannot be used here, since we do not know the nature of the load.
We determine the result
We substitute the current and voltage indicators into this formula, and here knowledge of trigonometric formulas will come to our aid:
cosa cosb = cos (a + b) + cos (a - b) / 2
We use this formula and get the calculations:
P (t) = I (m) U (m) cos (wt + J) cos wt
After simplifying the results, we get:
P (t) = I (m) U (m) / 2 cos (wt + J) + I (m) U (m) cosJ
Let's look at this formula. Here, the first term depends on time, changing according to a harmonic law, and the second is a constant value. AC power with alternating current is composed of a constant and a variable component.
If the power is positive, then the load consumes energy from the generator. With negative power, on the contrary, the load spins the generator.
Find the average power over a period of time. For this, the work done by electric current is divided by the value of this period.
The power of a three-phase AC circuit is the sum of the variable and DC components.
Active and reactive power
Many physical processes can be represented by analogies of each other. On this basis, we will try to reveal the essence of the concepts of active power of an alternating current circuit and reactive power of an alternating current circuit.
A glass is a power station, water is electricity, a tube is a cable or wire. The higher the glass rises, the greater the stress or pressure.
The power parameters in an active or reactive type alternating current network depend on those elements that consume such energy. Active - the energy of inductance and capacitance.
We show it on the capacitor, capacity and glass. Active are those elements that are able to convert energy into another form. For example, in heat (iron), light (light bulb), movement (motor).
Reactive energy
When reactive energy is simulated, the voltage increases and the capacitance fills. When the voltage decreases, the stored energy is returned through the wire back to the power plant. This is repeated cyclically.
The very meaning of reactive elements is the accumulation of energy, which is then returned back or used for other functions. But it’s not going anywhere. The main disadvantage of this derivative is that the virtual pipeline, through which energy is somehow flowing, has resistance, and a percentage of savings are spent on it.
A full AC power circuit requires a certain percentage of effort. For this reason, large enterprises are struggling with the reactive component of full power.
Active power is that energy that is consumed or converted into other types - light, heat, movement, that is, in some kind of work.
Experience
For experience, take a glass that serves as the active component of power. It represents part of the energy that needs to be consumed or converted into another form.
Part of the energy of water can be drunk. The full AC power power factor is an indicator that consists of the reactive and active components: the energy flowing through the water supply and the one that is converted.
What does the full power look like in our analogy? We drink part of the water, and the rest will continue to run along the tube. Since we have a reactive capacitive element - a capacitor or a capacitor, we lower the water and begin to simulate an increase and decrease in voltage. At the same time, one can see how water flows in two directions. Therefore, in this process both the active and reactive components are used. Together, this is full power.
Power conversion
Active power is converted into another form of energy, for example, into mechanical motion or heating. The reactive power that accumulates in the reactive element later returns back.
Gross power is the geometric sum of active and reactive power.
To perform the calculations, we use trigonometric functions. The physical meaning of the calculations is as follows. Take a right-angled triangle in which one of the sides is 90 degrees. One of the parties is his hypotenuse. There is an adjacent and opposite relative to the right angle of the leg.
The cosine is represented by a ratio that determines the length of the adjacent leg relative to the length of the hypotenuse.
The sine of the angle is the kind of relationship that makes up the length of the opposite leg relative to the hypotenuse. Knowing the angle and length of either side, all other angles and length can be calculated.
In this triangle, you can take the length of the hypotenuse and the adjacent leg and calculate this angle using the trigonometric function of cosines. The power of direct and alternating current is calculated using such knowledge.
To calculate the angle, you can use the inverse function of the cosine. We get the necessary calculation result. To calculate the length of the opposite side, you can calculate the sine and get the ratio of the opposite side to the hypotenuse.
The calculation of the power of the AC circuit according to the formula proposed in this description.
In DC circuits, power is equal to the product of voltage by current. This rule also works in AC circuits, but its interpretation will not be entirely correct.
Inductance
In addition to the active elements, reactive elements act inductance and capacitance. In DC circuits, where the amplitude value of the current voltage does not change in time, the work of this resistance will occur only in time. Inductance and capacitance can negatively affect the network.
The active power that a three-phase AC circuit has can do useful work, and reactive does not do any useful work, but only is spent on overcoming the reactance of the inductance and capacitance.
Let's try to complete the experience. Take an AC voltage source of 220 W with a frequency of 50 Hz, a voltage and current sensor of the source, a load that is active 1 Ohm and inductive 1 Ohm resistance.
There is also a switch that will be connected at a certain moment, an active-capacitive load. Run such a system. For convenience, we introduce the voltage correction factors.
We start the device
When you start the device, it is clear that the voltage and current of the network do not coincide in phase. There is a transition through 0 at which there is an angle - the power factor of the network. The smaller this angle, the higher the power factor, which is indicated on all AC devices, for example, electric machines or welding transformers.
The angle depends on the magnitude of the inductive load resistance. When the shift decreases, the network current increases. Imagine that the coil resistance cannot be reduced, but the cosine of the network must be improved. For this, capacitors are needed, which, in contrast to inductance, are ahead of voltage and can mutually compensate for reactive power.
At the moment of connecting the capacitor bank, a sharp decrease in the cosine occurs for 0.05 s, practically to 0. There is also a sharp decrease in current, which without the capacitor bank had an amplitude value much lower than when the capacitor bank was turned on.
In fact, by connecting a capacitor bank, it was possible to reduce the power of the current consumed from the network. This is a positive point and allows you to reduce the network current and save on cable cross-sections, transformers, power equipment.
If the inductive load is disconnected and the resistance remains, the process will happen when the cosine of the network after connecting the capacitor bank leads to a phase shift and a large jump in the current that goes into the network, and is not consumed from it, which happens in the reactive power generator mode.
Summary
Active power again remains constant and equal to zero, since there is no inductive resistance. The process of generating reactive power into the network has begun.
Therefore, to compensate for reactive power at large enterprises consumed by its colossal volumes from power systems is a priority task, as it allows saving not only on electrical equipment, but also on the cost of paying for the reactive power itself.
This concept is regulated, and the company pays for both consumed and generated capacity. Automatic compensators are installed here, providing support for the power balance at a given level.
When a powerful load is disconnected, if the compensating device is not disconnected from the network, reactive power will be generated in the network, which will create problems in the power system.
In everyday life, reactive power compensation does not make sense, since the power consumption here is much lower.
Active and reactive power are the concepts of a school course in physics.