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Reactance Properties
Reactance is the opposition exhibited to the movement of alternating current in an inductor or capacitor due to inductance or capacitance. Reactance is like an alternating current equivalent of direct current resistance.
Resistance stores and disperses energy as heat when there is reactance to the movement of alternating current in an inductor, capacitor or conductor, an electric or magnetic field.
There are two types of structures of reactance. The reactance is inductive if the field is magnetic. The reactance is capacitive if the field is electric.
Similar to resistance, Ohm is used to measure reactance. Positive values indicate the inductive structure of reactance, and negative values indicate the capacitive structure of reactance. The symbol X represents it.
An inductor or capacitor and resistor are ideal if they have zero resistance.
Types of reactance
Inductive reactance and capacitive reactance are the two types of reactance.
Inductive reactance
When there is resistance to the movement of alternating current in an inductor, it is known as inductive reactance.
The inductive reactance is directly proportional to the frequency of the current and the inductance. Therefore, the formula of inductive reactance is XL = ωL, where ω is the frequency and L is the self impedance of the inductor.
The dimensional structure of inductive reactance is the same as that of resistance, and its SI unit is Ohm (W).
How does inductive reactance work?
The current in a purely inductive AC circuit lags the applied voltage by 90o, or (π/2 rads). This is because the applied voltage and the current cannot remain in phase due to the opposing force of the counter emf.
The applied AC voltage continually increases in the first quarter cycle (0o to 90o), the applied AC voltage. It can be determined that a circuit has inductive resistance if the current does not increase during this cycle. This is because the inductive reactance would resist any flow in the current.
The voltage goes back to zero in the next quarter-cycle (90o to 180o). Current starts flowing in the circuit, and as the voltage reaches zero, the current reaches a maximum value.
In the third quarter cycle (180o to 270o), the applied voltage rises to its highest value in the reverse direction. The current falls to zero as a result. At the end of the third quarter cycle, when the voltage reaches its maximum again, the voltage and current values are precisely inverse to what they were during the first half-cycle. The applied voltage exceeds the resulting current by one quarter-cycle or 90o.
As the cycle reaches 360o, the current is maximum as the voltage falls to zero. The amplitude and direction of both the voltage and current always keep varying as long as the applied voltage is not removed.
Capacitive Reactance
When there is resistance to the movement of alternating current in a capacitor, it is known as capacitive reactance.
The capacitive reactance is directly proportional to the frequency of the current and the capacitance. The formula of capacitive reactance is therefore, Xc = 1/ωC. Where ω is the frequency and C is the self capacitance of the inductor.
The dimensional structure of capacitive reactance is the same as that of resistance, and its SI unit is Ohm (W).
How does capacitive reactance work?
When compared to an inductive circuit, it is seen that the current-voltage relationship in a capacitive circuit is the exact opposite. The capacitor’s current is ahead of the voltage across the capacitor by 90o or π/2 rads.
Assume that the AC voltage is acting on a capacitor.
At the start of the first quarter-cycle (0o to 90o), the voltage increases positively. The change in voltage is at its greatest when it is at 0o. Since the charge of a capacitor directly corresponds to the voltage, the change in charge of the capacitor is highest at 0o. In simple terms, the movement of electrons from one plate to another is the greatest.
When the voltage increases towards its maximum at 90o, the current across the capacitor moves towards zero as the rate of change of the voltage decreases. The capacitor is fully charged at 90o, and the movement of electrons from one plate to another stops.
At the start of the second quarter-cycle (90o to 180o), the alternating voltage falls. As the voltage decreases, the capacitor plate that gained excess electrons starts losing them—the direction of the flow of current reverses. The voltage falls to zero at 180o. At this point, the two plates hold an equal number of electrons, and the current is maximum as the rate of change of voltage is at its highest.
At the start of the third quarter-cycle (180o to 270o), the polarity of the voltage gets reversed. The voltage reaches its negative maximum at 270o. During this cycle, the charge slowly increases to a maximum as the change in voltage decreases. At 270o, the capacitor is fully charged, which means that the current is zero as there is no transfer of electrons. Apart from the reversed polarity, the conditions are identical to the end of the first quarter-cycle (90o).
At the start of the last quarter cycle (270o to 360o), there is a decrease in the voltage, and the capacitor starts losing electrons. The capacitor continues discharging during the last cycle until the voltage falls to zero. Due to this charging and discharging action, the current is 90o ahead of the voltage.
Conclusion
Reactance is the opposition exhibited to the movement of alternating current in an inductor or capacitor due to inductance or capacitance. There are two types of reactance depending on the type of field. When there is resistance to the movement of alternating current in an inductor it is known as inductive reactance. The current in a purely inductive AC circuit lags the applied voltage by 90o, or (π/2 rads). This is because the applied voltage and the current cannot remain in phase because of the opposing force of the counter emf. When there is resistance to the movement of alternating current in a capacitor it is known as capacitive reactance. Due to this charging and discharging of the capacitor action the current is 90o ahead of the voltage in a completely capacitive circuit.
