Table of Contents
What Is Inductive Reactance?
Inductive reactance (XL) is the opposition that an inductor presents to alternating current (AC). Unlike resistance, which dissipates energy as heat, reactance stores energy in a magnetic field and returns it to the circuit. Inductive reactance is measured in ohms and increases with both frequency and inductance.
When AC flows through an inductor, the changing current creates a changing magnetic field, which in turn induces a back-EMF (electromotive force) that opposes the change in current. This back-EMF is what creates the reactance. The current through a pure inductor lags the voltage by exactly 90 degrees, meaning the voltage peaks one quarter-cycle before the current.
Reactance Formula
Where XL is inductive reactance in ohms, f is frequency in hertz, L is inductance in henries, and ω (omega) is the angular frequency in radians per second.
Frequency Dependence
| Frequency | XL (100 mH) | Current (at 120V) |
|---|---|---|
| 50 Hz | 31.4 Ω | 3.82 A |
| 60 Hz | 37.7 Ω | 3.18 A |
| 400 Hz | 251.3 Ω | 0.48 A |
| 1 kHz | 628.3 Ω | 0.19 A |
| 10 kHz | 6283.2 Ω | 0.019 A |
Applications
- Low-pass filters: Inductors block high-frequency signals while passing low frequencies, used in audio crossovers and power supplies.
- Power factor correction: Inductors are used in combination with capacitors to manage reactive power in AC power systems.
- RF chokes: Inductors with high reactance at radio frequencies prevent RF signals from entering DC power lines.
- Current limiting: Fluorescent light ballasts use inductive reactance to limit current through the lamp tube.
Frequently Asked Questions
What is the difference between reactance and impedance?
Reactance (X) is the opposition to AC due to inductance or capacitance alone, and does not dissipate power. Impedance (Z) is the total opposition to AC, combining both resistance (R) and reactance (X): Z = sqrt(R² + X²). For a pure inductor with no resistance, impedance equals the inductive reactance.
Why does inductive reactance increase with frequency?
At higher frequencies, the current changes more rapidly, which creates a stronger changing magnetic field and a larger back-EMF that opposes the current. This increasing opposition is why inductors are used as low-pass filters -- they progressively block higher frequencies.
Does an inductor have zero reactance at DC?
Yes. At DC (0 Hz), inductive reactance is zero because XL = 2 x pi x 0 x L = 0. An ideal inductor acts as a short circuit at DC. In practice, real inductors have some wire resistance, which limits DC current. This is why inductors pass DC but block high-frequency AC.