Reactive power

Reactive power is part of the electrical energy returned by the source load. The occurrence of a situation is considered harmful.

Occurrence reactive power

Suppose the circuit contains a DC power supply and an ideal inductance. The inclusion of the chain generates a transient. The voltage tends to reach the nominal value, the intrinsic flux linkage of the inductance actively interferes with the growth. Each turn of the wire is bent by a circular path. The generated magnetic field will cross the adjacent segment. If the turns are arranged one after another, the nature of the interaction will intensify. Considered is called its own thread coupling.

The nature of the process is as follows: the induced emf impedes field changes. The current is trying to grow rapidly, flux linkage pulls back. Instead of a step we see a smoothed protrusion. The energy of the magnetic field is spent to prevent the process created. The case of reactive power. The phase differs from the useful, harms. Ideal: vector direction perpendicular to the active component. It is understood that the resistance of the wire is zero( a fantastic alignment).

When the circuit is turned off, the process will be reversed. The current tends to instantly fall to zero, energy is stored in the magnetic field. Dam up the inductance, the transition will take place suddenly, the flux linkage gives the process a different color:

  1. A decrease in current causes a decrease in the magnetic field strength.
  2. The effected effect induces a counter-emf of turns.
  3. As a result, after the power supply is disconnected, the current continues to exist, gradually fading out.

Graphs of voltage, current, power

Reactive power is a kind of inertia link that is constantly delayed and interferes. The first question is: why do we need inductances? Oh, they have enough good qualities. Benefit makes put up with reactive power. A common positive effect is the work of electric motors. The transfer of energy goes through a magnetic flux. Between the turns of one coil, as shown above. The interaction affects a permanent magnet, a choke, everything that can capture an induction vector.

Cases can not be called descriptive comprehensive. Sometimes a clutch flow is used in the form shown for example. The principle is used by control gears of gas-discharge lamps. The choke is equipped with an innumerable number of turns: a voltage shutdown does not cause a smooth decrease in current, but a surge in a large amplitude of the opposite polarity. The inductance is great: the response is truly amazing. Exceeds the original 230 volts by an order of magnitude. It is enough for a spark to appear, the light is on.

Reactive power and capacitors

Reactive power is stored up by inductance magnetic field energy. And the capacitor? It is the source of the reactive component. Let us complete the review with the theory of vector addition. Understand the ordinary reader. Oscillatory processes are often used in the physics of electrical networks. Well-known 220 volts( now accepted 230) in a 50 Hz socket. A sinusoid whose amplitude is 315 volts. Analyzing the circuit, it is convenient to represent the vector rotating in a clockwise direction.

Analysis of circuits by the graphical method

. The calculation is simplified, the engineering representation of reactive power can be explained. The phase angle of the current is considered to be zero, and it is deposited to the right along the abscissa axis( see fig.).The reactive energy of the inductance coincides with the phase voltage UL, ahead of the current by 90 degrees. Perfect case. Practitioners have to consider winding resistance. Reactive on inductance will be part of the power( see fig.).The angle between the projections is important. The value is called the power factor. What does it mean in practice? Before answering the question, consider the concept of a resistance triangle.

Triangle of resistances and power factor

In order to make analysis of electrical circuits easier, physicists suggest using a triangle of resistances. The active part is deposited, like a current, to the right of the x-axis. Agreed, inductance to direct up, capacity - down. Calculating the impedance of the circuit, the values ​​are subtracted. The combined case is excluded. Two options are available: reactance is positive or negative.

Obtaining capacitive / inductive resistance, the parameters of the elements of the circuit are multiplied by the coefficient denoted by the Greek letter "omega".The circular frequency is the product of the frequency of the network by the double number Pi( 3.14).One more remark about the finding of reactive resistances let us indicate. If the inductance is simply multiplied by the indicated coefficient, the values ​​inverse for the product are taken for the capacitances. It is clear from the figure, where the indicated relations are given, which help to calculate stresses. After multiplication we take the algebraic sum of inductive, capacitive resistance. The first are considered positive values, the second - negative.

. Formulas of reactive components.

. Two components of resistance - active and imaginary - are projections of the impedance vector on the x-axis and ordinate. Angles are preserved when transferring abstractions to power. Active is deposited on the abscissa axis, reactive - along the soybean ordinate. Capacities and inductances are the underlying cause of negative effects on the network. It was shown above: without reactive elements it becomes impossible to build electrical devices.

The power factor is called the cosine of the angle between the full resistance vector and the horizontal axis. The parameter is ascribed to the importance as the useful part of the source energy is a fraction of the total waste. The fraction is calculated by multiplying the total power by a factor. If the voltage and current vectors coincide, the cosine of the angle is one. Power is lost by the load, volatilizing heat.

Told to believe! The average power of the period when connected to the source of pure reactance is zero. Half the time inductance takes energy, the second gives. The motor winding is indicated on the diagrams by adding an EMF source describing the energy transfer to the shaft.

Practical interpretation of the power factor

Many people notice the inconsistency in the case of practical consideration of reactive power. To reduce the coefficient, it is recommended to include large capacitors parallel to the motor windings. Inductive resistance balances capacitive, the current again coincides with the voltage phase. It is difficult to understand here for what reason:

  1. Suppose that the primary winding of a transformer is connected to an AC voltage source.
  2. Ideally, active resistance is zero. Power must be reactive. But this is bad: the angle between voltage and current tend to be zero!

Power Factor

The amount of energy stored by a field is determined by the size of the inductance or capacitance. Read in any textbook of physics for universities( Course of physics Zhdanov and Marandzhyan, vol. 2, p. 234), more precisely - is proportional to the square of the magnitude. The theory of reactive power assumes: a certain energy is stored each period by parasitic inductance, capacitance, then goes to the external circuit. It turns out a kind of circulation inside the oscillatory circuit. The connecting wires get very hot if the inductance is too far from the capacitance.

But! The oscillatory process is indifferent to the operation of motors, transformers. The theory of reactive power assumes: the whole energy oscillates. To the last drop. In the transformer, the motor from the field, there is an active “leakage” of energy to the performance of work, the induction of the current of the secondary winding. Energy cannot circulate between the source and the consumer.

The real chain makes it difficult to reconcile individual sections. For reinsurance, suppliers require that capacitors be installed parallel to the motor winding so that the energy circulates in the local segment and does not go out, heating the connecting wires. It is important to avoid overcompensation. If the capacitance is too large, the battery will cause an increase in power factor.

With regard to the phase shift occurs on the secondary winding of the transformer substation. The role is not this. The engine is running, some of the energy is not converted into useful work, is reflected back. The result is a power factor. The participating inductance component is a technological, structural defect. The part is not beneficial. We compensate by adding capacitor blocks.

Verification of the matching is carried out in the absence of a phase shift between the voltage and current of the operating motor. Excess energy circulates between the excess inductance of the windings installed by the capacitor unit. The goal of the event has been achieved - to avoid heating the conductors of the power supply network device.

What is being offered under the guise of saving electricity

The network offers to buy power saving devices. Reactive power compensators. It is important not to bend the stick. For example, it would be appropriate to look at the compensator next to the fridge compressor turned on, with the collector engine of the vacuum cleaner, to burden the apartment with measures when the incandescent lamps are working - a dubious enterprise. Before installation, take the trouble to find out the phase shift between voltage and current, according to the information, correctly calculate the volume of the capacitor bank. Otherwise, attempts to save in this way will fail, unless you accidentally manage to point your finger at the sky, to get to the point.

The second aspect of reactive power compensation is accounting. It is made for large enterprises, where there are powerful engines that create large angles of phase shift. Special meters of reactive power metering, paid according to the tariff, are being installed. For calculation of the payment coefficient, the assessment of heat losses of wires, deterioration of the cable network operation mode, some other factors are applied.

Prospects for the further study of reactive energy as the phenomenon

Reactive power is the phenomenon of energy reflection. Ideal chains of the phenomenon are devoid. Reactive power manifested by the heat released on the active resistance of cable lines, distorts the sinusoidal waveform. A separate topic of conversation. In case of deviations from the norm, the engines do not work so smoothly, transformers - disturbance.

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