Inductance
Inductance is the property of an electrical conductor or coil that opposes changes in current by storing energy in a magnetic field. Whenever the current through an inductor changes, a voltage is induced that acts against the change. This phenomenon is known as electromagnetic induction. Inductance is one of the fundamental electrical properties used in power electronics, radio-frequency circuits, filters, transformers and motor control systems.
What Is Inductance?
Inductance describes how effectively a conductor stores magnetic energy when current flows through it. A coil with a higher inductance produces a stronger opposition to changes in current than a coil with lower inductance.
The greater the inductance, the more slowly the current changes when a voltage is applied.
Unit of Inductance
Inductance is measured in henries (H), named after the American scientist Joseph Henry.
| Unit | Symbol | Equivalent |
|---|---|---|
| Henry | H | 1 H |
| Millihenry | mH | 0.001 H |
| Microhenry | µH | 0.000001 H |
| Nanohenry | nH | 0.000000001 H |
| Picohenry | pH | 0.000000000001 H |
Formula for Induced Voltage
The voltage induced across an inductor is directly proportional to its inductance and the rate of change of current.
V = L × (dI / dt)
| Symbol | Meaning |
|---|---|
| V | Induced voltage (volts) |
| L | Inductance (henries) |
| dI/dt | Rate of change of current (A/s) |
Factors Affecting Inductance
| Factor | Effect |
|---|---|
| Number of Turns | More turns increase inductance. |
| Core Material | Ferrite or iron cores greatly increase inductance. |
| Core Size | Larger cores usually increase inductance. |
| Coil Diameter | Larger diameters generally increase inductance. |
| Magnetic Path Length | A shorter magnetic path generally increases inductance. |
| Air Gap | Usually reduces inductance but increases energy storage capability. |
Energy Stored in an Inductor
An inductor stores energy in its magnetic field while current flows through it.
Energy = ½ × L × I²
| Symbol | Meaning |
|---|---|
| L | Inductance (H) |
| I | Current (A) |
Increasing either the inductance or the current increases the amount of stored energy.
Typical Inductance Values
| Application | Typical Value |
|---|---|
| RF Matching Network | 10–500 nH |
| FM Radio | 100 nH–10 µH |
| Buck Converter | 1–100 µH |
| Power Supply Filter | 100 µH–10 mH |
| Audio Crossover | 0.1–10 mH |
| Industrial Power Filter | 10 mH–10 H |
How Inductance Is Measured
- LCR meter.
- Impedance analyser.
- Vector network analyser (RF applications).
- Oscilloscope using known test circuits.
- Dedicated inductance meter.
Applications of Inductance
| Application | Purpose |
|---|---|
| Power Supplies | Energy storage and filtering. |
| Transformers | Magnetic coupling. |
| RF Circuits | Tuning and impedance matching. |
| Motor Controllers | Current smoothing. |
| Audio Crossovers | Frequency filtering. |
| EMI Filters | Noise suppression. |
Inductance vs Resistance vs Capacitance
| Property | Inductance | Resistance | Capacitance |
|---|---|---|---|
| Stores Energy In | Magnetic field | Does not store energy | Electric field |
| Opposes | Changes in current | Current flow | Changes in voltage |
| Unit | Henry (H) | Ohm (Ω) | Farad (F) |
Interesting Facts
- The unit "henry" is named after Joseph Henry.
- Doubling the current increases stored energy by four times because energy depends on the square of the current.
- Many high-power inductors include an air gap to improve energy storage.
- Even a straight wire has a small amount of inductance.
- PCB tracks also have inductance, which becomes important at high frequencies.
Key Points
- Inductance is the ability of a conductor or coil to oppose changes in current.
- It is measured in henries (H).
- Inductance depends on the coil geometry and core material.
- Inductors store energy in a magnetic field.
- Inductance is essential in power electronics, filters, RF circuits and transformers.