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Pure inductance does not consume electric energy, it only exchanges energy with power supply continuously.




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The average power of the inductor is:




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Calculation formula of average power of inductance



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If the average power is zero, it only means that the inductor does not consume active power, and it does not mean that there is no power in the inductor, and there is energy exchange between the inductor and the power supply, so the instantaneous power is not equal to zero. The maximum value of instantaneous power, i.e. the product of UI, is called reactive power, which is represented by symbol QL. It reflects the maximum scale of energy exchange between inductance and power supply.




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Calculation formula of inductive reactive power




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In engineering, transformers and motors work by converting electric energy and magnetic energy between inductance coil and power supply, but reactive power must be provided by power supply, so reactive power is the necessary power when inductance element works, which cannot be understood as useless power.




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Title: the inductance of the inductance coil is L = 1H, connected to the power supply with the voltage of u = 220 ¡Ô 2Sin (314T + 60 ¢X) V, and the current I and reactive power QL flowing through the inductance are calculated.




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Solution: the effective value phasor of voltage is:




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Phasor representation




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Calculated inductance circuit




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AC circuit of capacitor


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In the AC circuit as shown in the figure below, capacitor element C is considered as an ideal linear element, and its reference direction of current and voltage has been marked in the figure.




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AC circuit of capacitor




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Relationship between current and voltage


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When the voltage applied to both ends of the capacitor is u = umsin £s t = u ¡Ô 2Sin £s T, and u is taken as the reference quantity, the following formula is given in the previous passive components in the circuit: resistance, capacitance and inductance:




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Relationship between current and capacitance




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The current of the available capacitance circuit is:




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Current formula of capacitor circuit




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It can be seen from the above formula that the current and voltage on the capacitor are sinusoidal quantities of the same frequency; the relationship between the current and voltage is


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