How Inductors Work
An inductor works by converting electrical energy into magnetic energy. When electric current flows through a coil of wire, a magnetic field is created around it. If the current changes, the magnetic field also changes, producing a voltage that opposes the change in current. This property, known as electromagnetic induction, makes inductors essential components in power supplies, filters, oscillators, transformers and many other electronic circuits.
Current Creates a Magnetic Field
Whenever electric current flows through a conductor, a magnetic field forms around it. Wrapping the conductor into a coil concentrates this magnetic field, making it much stronger than that produced by a straight wire.
Adding more turns or using a magnetic core such as iron or ferrite increases the strength of the magnetic field and therefore the inductance.
Electromagnetic Induction
If the current through the coil changes, the magnetic field changes as well. According to Faraday's Law of Electromagnetic Induction, this changing magnetic field induces a voltage in the coil.
The induced voltage always acts in a direction that opposes the change in current. This behaviour is described by Lenz's Law.
Back Electromotive Force (Back EMF)
When the current through an inductor increases, the induced voltage opposes the increase. When the current decreases, the collapsing magnetic field produces a voltage that attempts to keep the current flowing.
This induced voltage is often called back EMF or counter EMF.
Increasing Current ↓ Magnetic Field Increases ↓ Back EMF Opposes Increase
Energy Storage
Unlike a resistor, which converts electrical energy into heat, an ideal inductor stores energy in its magnetic field.
The amount of stored energy depends on the inductance and the current flowing through the coil.
Stored Energy = ½ × L × I²
| Symbol | Meaning |
|---|---|
| L | Inductance (H) |
| I | Current (A) |
Inductor Behaviour with DC
| Moment | Behaviour |
|---|---|
| Immediately after power is applied | Current increases gradually. |
| After the magnetic field stabilises | The inductor behaves almost like a short circuit. |
An ideal inductor has no resistance, although real inductors have a small resistance due to the copper wire.
Inductor Behaviour with AC
With alternating current, the magnetic field is constantly changing. Therefore, the inductor continuously produces an opposing voltage.
The opposition to AC is called inductive reactance.
XL = 2πfL
| Symbol | Meaning |
|---|---|
| XL | Inductive Reactance (Ω) |
| f | Frequency (Hz) |
| L | Inductance (H) |
As frequency increases, the inductive reactance also increases. This is why inductors are commonly used to block high-frequency signals while allowing low-frequency or DC currents to pass.
Factors Affecting Inductance
| Factor | Effect |
|---|---|
| More turns | Higher inductance. |
| Larger core permeability | Higher inductance. |
| Larger coil diameter | Higher inductance. |
| Shorter magnetic path | Higher inductance. |
| Air gap | Usually reduces inductance but increases saturation current. |
Practical Applications
- Buck and boost converters.
- Switch-mode power supplies.
- Audio crossover networks.
- Radio tuning circuits.
- EMI suppression filters.
- Motor drives.
- Oscillators.
- Transformers.
Real-World Examples
| Device | How the Inductor Works |
|---|---|
| Phone Charger | Stores energy between switching cycles. |
| LED Driver | Controls current supplied to LEDs. |
| Computer Power Supply | Smooths output current. |
| Audio Amplifier | Forms part of speaker crossover filters. |
| Car DC-DC Converter | Transfers energy efficiently between voltage levels. |
Common Misconceptions
| Myth | Reality |
|---|---|
| An inductor blocks DC. | It only opposes changes in current. After the current stabilises, DC flows with only the winding resistance causing voltage drop. |
| All inductors behave the same. | Core material, construction and frequency determine performance. |
| Bigger inductors always store more energy. | Stored energy depends on both inductance and current. |
Key Points
- Current flowing through a coil creates a magnetic field.
- A changing magnetic field induces a voltage.
- Inductors oppose changes in current through back EMF.
- Energy is stored in the magnetic field surrounding the coil.
- Inductors behave differently with DC and AC.
- They are fundamental components in power electronics, filters and communication systems.