Reactance is the opposition that inductors and capacitors present to alternating current (AC) due to energy storage in magnetic or electric fields, measured in ohms but dissipating no real power. In a real circuit or installation, reactance changes the phase relationship between voltage and current—causing the current to either lag or lead the voltage—which directly impacts your system's power factor, apparent power draw, and breaker sizing. Beginners most commonly confuse reactance with resistance (which opposes both AC and DC and burns energy as heat) and impedance (the total vector combination of both resistance and reactance).
The Core Mechanism: Inductive vs. Capacitive Reactance
Unlike a resistor, which simply turns electrical energy into heat, reactive components temporarily store energy and return it to the circuit. This storage mechanism creates a time delay between the applied AC voltage and the resulting current flow.
Inductive Reactance: XL = 2πfL (Increases with frequency and inductance)
Capacitive Reactance: XC = 1 / (2πfC) (Decreases with frequency and capacitance)
Where f = frequency in Hz, L = inductance in Henries, C = capacitance in Farads.
Inductive Reactance (XL): Inductors (coils, chokes, motor windings) store energy in a magnetic field. Think of an inductor like a heavy mechanical flywheel: it resists changes in rotational speed. In electrical terms, it resists changes in current. When AC voltage is applied, the collapsing and expanding magnetic field induces a back-EMF that fights the current flow, causing the current to lag the voltage by up to 90 degrees. As noted in standard AC inductance theory, this lagging current is the primary reason industrial facilities suffer from poor power factor.
Capacitive Reactance (XC): Capacitors store energy in an electric field between two conductive plates separated by a dielectric. They resist changes in voltage. Because current must flow to charge the plates before a voltage can develop across them, the current leads the voltage by up to 90 degrees. This leading effect is exactly why we use capacitor banks to cancel out the lagging current of heavy inductive motor loads.
Worked Numeric Example: Sizing a Motor Run Capacitor and Choke
Let's look at a real-world bench scenario: you are analyzing the auxiliary winding circuit of a 240V, 60Hz pool pump motor, and you need to verify the current flow through the run capacitor and an added EMI filter choke.
Scenario A: The 20µF Motor Run Capacitor
You have a standard CBB60 motor run capacitor rated at 20µF (0.00002 F) and 370VAC. The line frequency is 60Hz.
- Step 1: Calculate XC = 1 / (2 × π × 60 × 0.00002)
- Step 2: XC = 1 / 0.0075398 = 132.63 Ω
- Step 3: Calculate current (I = V / XC). I = 240V / 132.63Ω = 1.81 Amps.
This 1.81A is purely reactive current. It shifts the phase to keep the motor's main and auxiliary windings 90 degrees apart for maximum starting and running torque, but it does not add to the real wattage consumed by the pump.
Scenario B: The 50mH Line Choke
To suppress VFD (Variable Frequency Drive) noise on the same line, you install a 50mH (0.05 H) ferrite core common-mode choke in series.
- Step 1: Calculate XL = 2 × π × 60 × 0.05
- Step 2: XL = 18.85 Ω at the fundamental 60Hz frequency.
At 60Hz, the choke only drops about 34V (1.81A × 18.85Ω). However, if high-frequency VFD switching noise at 5,000Hz tries to pass through, the reactance skyrockets to 1,570 Ω, effectively choking the high-frequency noise while letting the 60Hz power pass. This frequency-dependent behavior is the superpower of AC reactance.
Where You Meet Reactance in Practice
You won't just see reactance in textbooks; it dictates component selection on real jobsites and workbenches.
- HVAC and Pool Pump Motors: The metal or plastic oval cans on the side of single-phase motors are run capacitors. They provide the exact capacitive reactance needed to create a rotating magnetic field. If the capacitor degrades and its microfarad value drops, XC increases, current drops, and the motor loses torque and overheats.
- Power Factor Correction (PFC) Banks: In commercial panels, large inductive loads (like banks of fluorescent ballasts or heavy HVAC compressors) cause the current to lag the voltage. Utilities penalize this. Electricians install switched capacitor banks to inject leading reactive current, canceling the lag and bringing the power factor closer to 1.0. See Fluke's guide on Power Factor for utility penalty thresholds.
- Audio Crossover Networks: In speaker building, passive crossovers use inductors and capacitors to route frequencies. A series capacitor blocks low-frequency bass (high XC at low Hz) from reaching a fragile tweeter, while passing high-frequency treble (low XC at high Hz).
- LED Driver Flicker Fixes: Cheap LED bulbs often flicker on dimmer switches because the dimmer's triac misfires on the highly capacitive or inductive reactance of the LED driver's internal rectifier. Adding a reactive bypass (usually a specialized RC snubber) stabilizes the impedance curve.
Reactance vs. Resistance vs. Impedance
It is critical to separate these three terms when troubleshooting AC circuits or sizing conductors.
| Property | Symbol | Opposes | Energy Result | Phase Shift | Frequency Dependent? |
|---|---|---|---|---|---|
| Resistance | R | AC and DC current | Dissipates as Heat (Watts) | None (In-phase) | No |
| Inductive Reactance | XL | AC current changes | Stored in Magnetic Field (VARs) | Current Lags Voltage | Yes (Increases with f) |
| Capacitive Reactance | XC | AC voltage changes | Stored in Electric Field (VARs) | Current Leads Voltage | Yes (Decreases with f) |
| Impedance | Z | Total AC opposition | Combination of Heat and Storage | Depends on R vs X ratio | Yes |
Note: Impedance (Z) is not a simple addition. Because resistance and reactance are 90 degrees out of phase, you must use vector addition: Z = √(R² + (XL - XC)²).
Frequently Asked Questions About AC Reactance
Does AC reactance cause heat in wires and components?
In pure theory, no. Ideal reactance stores and returns energy, resulting in zero real power (Watts) dissipation. However, in the real world, every inductor has wire resistance (DCR) and every capacitor has Equivalent Series Resistance (ESR). When reactive current flows through a motor run capacitor, the ESR generates heat. This is why a failing capacitor with high ESR will physically bulge or vent its dielectric oil, even though the reactance itself isn't what's generating the heat.
Why does inductive reactance increase with frequency?
Inductive reactance is governed by Faraday's law of induction: the faster the current changes (higher frequency), the stronger the induced back-EMF that opposes that change. At DC (0 Hz), the current isn't changing, so the back-EMF is zero, and an inductor acts as a dead short (limited only by its copper wire resistance). At 60Hz, it offers moderate opposition. At the 20kHz switching frequency of a modern GaN inverter, that same coil presents massive reactance, effectively blocking the AC signal.
Can I measure AC reactance with a standard digital multimeter?
No. If you put a standard DMM (like a Fluke 87V) in Ohms mode and touch it to a capacitor or inductor, you will get an error (OL) or a reading of the internal DC resistance, not the AC reactance. Reactance only exists when alternating current is flowing. To measure the actual inductance or capacitance to calculate reactance, you must use an LCR meter (like a Keysight U1733C or a budget DE-5000), which applies a specific AC test frequency (usually 100Hz or 1kHz) to measure the component's reactive properties.
What happens if I use a DC-rated capacitor in an AC reactance circuit?
This is a common and dangerous bench mistake. Electrolytic capacitors (which offer high capacitance in small packages) are polarized. If you apply AC voltage across a DC-rated electrolytic capacitor, the reverse voltage cycles will strip the internal oxide dielectric layer. This causes a dead short, rapid internal heating, and catastrophic failure (often an explosion of electrolyte). For AC reactance applications like motor running or audio crossovers, you must use non-polarized film capacitors (like CBB60, CBB61, or polyester film) rated specifically with an AC voltage rating (e.g., 250VAC or 370VAC), which is fundamentally different from a DC voltage rating.
