Calculation of power in electrical circuits
Any electrical appliance is characterized by several basic parameters, including rated voltage, current and power. Sometimes in the technical passport only power and voltage are indicated, the current in this case is easy to find using the famous Ohm formulas (of course, with a number of reservations - for example, cos should be known). The converse is also true: knowing the current and voltage, you can perform a power calculation. In the global network there are many materials on this topic, but most of them are intended for specialists.
Let's look at what is meant by the term "electrical power", what kind of variants exist and how to calculate the power. The physical meaning of power indicates how quickly the conversion (power) of electricity to a particular type of useful work takes place in the installation (device). That's so simple! For non-electrical devices, it is entirely permissible to use the term "performance".
In electrical engineering, a separation is accepted, according to which active and reactive power exists. The first one is directly converted into a useful work, therefore it is considered the main one. The unit of measurement is Watt and the derivatives - Kilowatt, Megawatt, etc. On household electrical appliances it is indicated exactly. Although this does not mean that there is no reactive component. In turn, the second - undesirable, because in the performance of the work does not participate, but is wasted on various types of losses. Measured in "var" (volt-ampere reactive) and derivatives - kilovolt-ampere reactive, etc. The sum of the active and reactive components forms the total power (volt-ampere, VA).
A bright example of a consumer with a clean active load is an electric heater. When electric current passes through it, heat is generated, and in direct dependence. Similarly, the consumer of reactive energy - a classical transformer. When it operates, a magnetic field is created in the turns of the winding, which in itself is not necessary (the property of electromagnetic induction is used). The magnetic circuit is magnetized, losses occur. In other words:
Q = U * I * sin Fi,
where sin Fi is the sine of the angle between the current and voltage vectors. Its sign depends on the nature of the load (capacitive or inductive).
The calculation of power begins with the definition of the type of current: constant or variable, since the formulas are not universal.
In the first case we use a consequence of the classical Ohm's law. The power P is the product of the current I by the voltage U:
P = I * U (W = A * B).
In the circuit with the power supply, the direction of the EMF is taken into account: this is necessary for calculating the resistance of the source itself. So, the generator or battery in which the current flows from "-" to "+", giving out energy to the load of the circuit, gives power. If the current flow is opposite to the applied potential (charging the battery), then there is power absorption by the EMF source.
The formula for calculating the power for an alternating current (single-phase circuit) takes into account the coefficient - "cosine phi". It is the ratio of the active component of power to the total. Obviously, in the case of TEN, the cosine will equal 1 (ideal), since there is no reactive component. Otherwise, various compensators or other technical solutions are used to reduce losses on the generator side.
P = U * I * cos Fi.
The calculation of the power in three-phase circuits is performed for each phase, and the resulting values are then summed. For AC, the total power is calculated as the square root of the sum of the squares of the active and reactive components. For generating devices (substations) it is more important to know exactly the full power, since on this basis all other elements of subsequent circuits are selected. Obviously, in most cases it is impossible to know in advance the nature of the load.