Trouble-free operation of the device depends on the compliance of the technical characteristics of the device with the norms of the supply network. Knowing the voltage, resistance and amperage in a circuit, an electrician will understand how to find the power. The formula for calculating an important parameter depends on the properties of the network to which the consumer is connected.
Content
- The labor of electricity
- DC performance
-
Variable network power
- Active ingredient
- Reverse losses
- In full force
- Utility criterion
The labor of electricity
Mechanical devices and electrical appliances are designed to get the job done. According to Newton's second law, kinetic energy that acts on a material point for a certain period of time performs a useful action. In electrodynamics, a field created by a potential difference transfers charges along a section of an electrical circuit.
The amount of work produced by the current depends on the intensity of the electricity. In the middle of the 19th century, D. NS. Joule and E. NS. Lenz solved the same problem. In the experiments carried out, a piece of high-resistance wire was heated when a current was passed through it. Scientists were interested in the question of how to calculate the cardinality of a chain.
To understand the process taking place in the conductor, the following definitions should be introduced:-
P - power. - A is the work done by the charge in the electrical circuit.
- U is the voltage drop in the conductor.
- I is the current strength.
- Q is the amount of electric charges transferred per unit of time.
Power is the work done by current in a conductor over a period of time. The statement is described by the formula: P = A ∕ ∆t.
On the circuit section, the potential difference at points a and b performs work to move electric charges, which is determined by the equation: A = U ∙ Q. The current is the total charge passed in the conductor per unit of time, which is mathematically expressed by the ratio: U ∙ I = Q ∕ ∆t. After the transformations, the formula for the power of the electric current is obtained: P = A ∕ ∆t = U ∙ Q ∕ ∆t = U ∙ I. It can be argued that work is carried out in the circuit, which depends on the power determined by the current and voltage at the contacts of the connected electrical device.
DC performance
Ohm's law is observed in a linear circuit without capacitors and inductors. A German scientist discovered the relationship between current and voltage and the resistance of a circuit. The opening is expressed by the equation: I = U ∕ R. With a known value of the load resistance, the power is calculated in two ways: P = I ² ∙ R or P = U ² ∕ R.
If the current in the circuit flows from plus to minus, then the network energy is absorbed by the consumer. This process takes place when the battery is being charged. If the current moves in the opposite direction, then the power is given off to the electrical circuit. This happens in the case of mains supply from a running generator.
Variable network power
The calculation of variable circuits is different from the calculation of the performance parameter in the DC line. This is due to the fact that voltage and current change in time and direction.
In a circuit with a phase shift of current and voltage, the following types of power are considered:
- Active.
- Reactive.
- Complete.
Active ingredient
The active part of the useful power takes into account the rate of irreversible conversion of electricity into thermal or magnetic energy. In a streamline with one phase, the active component is calculated by the formula: P = U ∙ I ∙ cos ϕ.
In the international system of units, SI, the value of productivity is measured in watts. The angle ϕ defines the offset of the voltage with respect to the current. In a three-phase circuit, the active part consists of the sum of the powers of each individual phase.
Reverse losses
For the operation of capacitors, inductors, electric motor windings, the power of the network is expended. Due to the physical properties of such devices, energy, which is determined by reactive power, is returned to the circuit. The recoil value is calculated using the equation: V = U ∙ I ∙ sin ϕ.
The unit of measurement is watt. It is possible to use an off-system measure of counting var, the name of which is made up of the English words volt, amper, reaction. Translation into Russian, respectively, means "volt", "ampere", "reverse action".
If the voltage is ahead of the current, then the phase displacement is considered greater than zero. Otherwise, the phase shift is negative. Depending on the value of sin ϕ, the reactive component is positive or negative. The presence of an inductive load in the circuit allows us to speak of the reversible part greater than zero, and the connected device consumes energy. The use of capacitors makes the reactive performance negative and the device adds energy to the grid.
To avoid overloads and changes in the set power factor, compensators are installed in the circuit. Such measures reduce power losses, reduce current waveform distortion, and allow the use of smaller conductors.
In full force
The total electrical power determines the load that the consumer places on the network. The active and reversible components are combined with the total power by the equation: S = √ (P ² + V ²).
With an inductive load, V 0, and using capacitors makes V ˂ 0. The absence of capacitors and inductors makes the reactive part equal to zero, which returns the formula to its usual form: S = √ (P ² + V ²) = √ (P ² + 0) = √ P ² = P = U ∙ I. Apparent power is measured in a non-SI unit "volt-ampere". Abbreviated version - B ∙ A.
Utility criterion
The power factor characterizes the consumer load in terms of the presence of the reactive part of the work. In the physical sense, the parameter determines the current shift from the applied voltage and is equal to cos ϕ. In practice, this means the amount of heat generated on the connecting leads. The heating level can reach significant values.
In power engineering, the power factor is denoted by the Greek letter λ. The range of change is from zero to one or from 0 to 100%. When λ = 1, the energy supplied to the consumer is consumed for work, the reactive component is absent. Values λ ≤ 0.5 are considered unsatisfactory.
Fail-safe operation of devices in the electrical line is due to the correct calculation of the technical parameters. A set of formulas derived from the Joule-Lenz and Ohm laws helps to find the power of the current in the circuit. The schematic diagram, competently drawn up taking into account the characteristics of the devices used, increases the performance of the power grid.
