Electric current power - the speed of the work performed by the circuit. Simple definition, confusion with understanding. Power is divided into active, reactive. And it starts. ..
Work of electric current, power
As the charge moves along the conductor, the field performs work on it. The value is characterized by stress, as opposed to tension in free space. The charges move in the direction of decreasing potentials, to maintain the process a source of energy is required. The voltage is numerically equal to the work of the field when moving in the area of a single charge( 1 C).In the course of interactions, electrical energy is transferred to other types. Therefore, it is necessary to introduce a universal unit, a physical, freely convertible currency. In the body, ATP is the measure, electricity is the work of the field.
In the diagram, the moment of energy conversion is displayed as EMF sources. If the generators are directed in one direction, the consumer - necessarily in the other. A clear fact reflects the process of power consumption, selection from energy sources. EMF carries a reverse sign, often called counter-emf. Avoid confusing the concept with the phenomenon that occurs in the inductors when the power is turned off. Counter-emf means the transition of electrical energy into chemical, mechanical, light.
The consumer wants to do the work for a certain unit of time. Obviously, the lawnmower does not intend to wait for the winter, hoping to cope for lunch. The power source must provide a given speed of execution. Work carries an electric current, therefore, the concept also applies. Power is active, reactive, useful and power loss. The areas denoted by physical circuits by resistances are harmful in practice, they are costs. On the resistors of the conductors of heat, the Joule-Lenz effect leads to unnecessary power consumption. An exception is heating devices, where the phenomenon is desirable.
Useful work on physical circuits is indicated by back-EMF( a normal source with a direction opposite to the generator).For power, there are several analytical expressions. Sometimes it is convenient to use one thing, in other cases it is different( see fig.):
- Power is the speed of work performance.
- Power equal to the product of voltage per current.
- The power spent on the thermal effect is equal to the product of the resistance per square of current.
- The power expended on the thermal effect is equal to the ratio of the square of the voltage to the resistance.
With a current clamp it is easier to use the second formula. Regardless of the nature of the load we calculate the power. Only active. Power is determined by many factors, including temperature. Under the nominal value for the device we understand, developed in the steady state. For heaters, use the third, fourth formula. Power depends entirely on the parameters of the power supply. Designed to work with 110 volts AC in European conditions will quickly burn.
Beginners three-phase circuits seem difficult, in fact, it is a more elegant technical solution. Even the house is supplied with electricity by three lines. Inside the entrance is divided into apartments. More embarrassing is the fact that some devices are three-phase devoid of grounding, neutral wire. Schemes with isolated neutral. The zero wire is not needed, the current is returned to the source via phase lines. Of course, the load on each vein is increased. The requirements of the OLC separately specify the type of network. For three-phase circuits, the following concepts are introduced that need to be understood in order to correctly calculate power:
- neutral Phase voltage, current is called, respectively, the potential difference and the rate of charge movement between phase and neutral. Understandably, in the above case, with complete isolation, the formulas will be invalid. Since there is no neutral.
- Linear voltage, current is called, respectively, the potential difference or the speed of movement of the charge between any two phases. The numbers are clear from the context. When talking about networks of 400 volts, imply three wires, the potential difference with the neutral is 230 volts. Line voltage above phase.
Between voltage and current there is a phase shift. What is silent school physics. Phases coincide if the load is 100% active( simple resistors).Otherwise, a shift appears. In the inductance, the current lags the voltage by 90 degrees, in the capacitor it is ahead. The simple truth is easily remembered as follows( smoothly approaching reactive power).The imaginary part of the inductance resistance is jωL, where ω is a circular frequency equal to the usual( in Hz) multiplied by 2 pi numbers;j is the operator denoting the direction of the vector. Now we write Ohm’s law: U = I R = I jωL.
Equality shows: the voltage needs to be put up 90 degrees when plotting, the current will remain on the x-axis( horizontal axis X).Rotation according to the rules of radio engineering occurs counterclockwise. Now the fact is obvious: the current is 90 degrees behind. By analogy, we make a comparison for a capacitor. Resistance to alternating current in the imaginary form looks like this: -j / ωL, the sign indicates: it will be necessary to put off the voltage down, perpendicular to the x-axis. Consequently, the current is ahead in phase by 90 degrees.
In reality, in parallel with the imaginary part, the real is present - called active resistance. The coil wire is represented by a resistor, being twisted, acquires inductive properties. Therefore, the real phase angle will not be 90 degrees, a little less.
And now you can go to the formulas of the power of the current of three-phase circuits. Here the line forms a phase shift. Between voltage and current, and relative to another line. Agree, without carefully knowing the knowledge of the authors, the fact cannot be realized. Between the lines of the industrial three-phase network, a shift of 120 degrees( full rotation - 360 degrees).It will provide uniform field rotation in engines; for ordinary consumers it is indifferent. It is more convenient for HPS generators - the load is balanced. The shift goes between the lines, in each the current leads the voltage or lags behind:
- If the line is symmetrical, the shift between any phases of the current is 120 degrees, the formula is extremely simple. But! If the load is symmetrical. Let's look at the image: the phase φ is not 120 degrees, it characterizes the shift between the voltage and current of each line. It is assumed that the engine was turned on with three equivalent windings, this result is obtained. If the load is not symmetrical, take the trouble to do the calculations for each line separately, then add the results together to get the total power of the current.
- The second group of formulas is given for three-phase circuits with isolated neutral. It is assumed that the current of one line flows through the other. Neutral is missing as unnecessary. Therefore, the voltages are taken not phase( not from what to count), as the previous formula, and linear. Accordingly, the numbers indicate which parameter should be taken. Wait a while to be intimidated by the Greek letters - the phase between the two parameters to be multiplied. The numbers are swapped( 1,2 or 2,1) to correctly take into account the sign.
- In the asymmetric circuit, phase voltages and currents reappear. Here the calculation is carried out separately for each line. There are no options.
In practice, measure the current power
Hinted, you can use current clamps. The device will determine the cruising parameters of the drill. Acceleration can be detected only with repeated experiments, the process is extremely fast, the frequency of changing the display is not higher than 3 times per second. Current tongs exhibit an error. Practice shows: to reach the error specified in the passport is difficult.
Most often for the evaluation of power use counters( for payments to suppliers), wattmeters( for personal and work purposes).The switch device contains a pair of fixed coils, through which the current of the circuit flows, the moving frame, in order to establish the voltage by parallel connection of the load. The design is designed to immediately implement the formula for total power( see fig.).The current is multiplied by the voltage and a certain coefficient taking into account the graduation of the scale, also by the cosine of the phase shift between the parameters. As mentioned above, the shift fits within 90 - minus 90 degrees, therefore, the cosine is positive, the torque of the arrow is directed in one direction.
There is no possibility to say whether the load is inductive or capacitive. But if the inclusion in the circuit is incorrect, the readings will be negative( blockage to the side).A similar event will occur if the consumer suddenly begins to give power back to the load( it happens).In modern devices, something similar happens, calculations are carried out by an electronic module that integrates the power consumption or reads the power readings. Instead of an arrow there is an electronic indicator and many other useful options.
Special problems are caused by measurements in asymmetric circuits with insulated neutral, where the power of each line cannot be directly added. Wattmeters are divided by the principle of operation:
- Electrodynamic. Described by section. Consist of one moving, two fixed coils.
- Ferrodynamic. Resembles a split-pole motor( shaded-pole motor).
- With a square. The amplitude-frequency characteristic of a nonlinear element( for example, a diode), resembling a parabola, is used to square the electrical quantity( used in calculations).
- With Hall sensor. If induction is made using a coil proportional to the magnetic field voltage in the sensor, applying current, the emf will be the result of multiplying two quantities. The desired value.
- Comparators. Gradually increases the reference signal until equality is achieved. Digital devices achieve high accuracy.
In circuits with a strong phase shift, a sinus wattmeter is used to estimate losses. The design is similar to that considered, the spatial position is such that reactive power is calculated( see fig.).In this case, the product of current and voltage is multiplied by the sine of the phase angle. Reactive power is measured by a normal( active) wattmeter. There are several techniques. For example, in a three-phase symmetric circuit, you need to connect a series winding in one line, parallel - in the other two. Then calculations are made: the instrument readings are multiplied by the root of three( taking into account that the indicator shows the product of current, voltage and sine of the angle between them).
For a three-phase circuit with simple asymmetry, the task is more complicated. The figure shows the technique of two wattmeters( ferrodynamic or electrodynamic).The beginning of the windings are indicated by asterisks. The current passes through the series, the voltage from two phases is applied to the parallel( one through a resistor).The algebraic sum of the readings of both wattmeters is added, multiplied by the root of three to obtain the value of reactive power.