Energy Storage in Inductors
One of the most important characteristics of an inductor is its ability to store energy in a magnetic field. Unlike a capacitor, which stores energy in an electric field, an inductor stores energy whenever current flows through its winding. This stored energy can later be released back into the circuit, making inductors essential components in switch-mode power supplies (SMPS), DC-DC converters, motor controllers, relay circuits and many other electronic systems.
How an Inductor Stores Energy
When current begins flowing through an inductor, a magnetic field forms around its winding. As the current increases, the magnetic field becomes stronger, storing more energy. When the current decreases or is interrupted, the magnetic field collapses and the stored energy is released back into the circuit.
Stored Energy Formula
The energy stored in an inductor depends on its inductance and the current flowing through it.
Energy = ½ × L × I²
| Symbol | Description | Unit |
|---|---|---|
| E | Stored Energy | Joules (J) |
| L | Inductance | Henries (H) |
| I | Current | Amperes (A) |
Notice that the stored energy is proportional to the square of the current. Doubling the current increases the stored energy by a factor of four.
Worked Example
An inductor has:
Inductance = 100 mH Current = 2 A
Calculate the stored energy.
L = 0.1 H E = ½ × L × I² E = ½ × 0.1 × 2² E = 0.2 J
The inductor stores 0.2 joules of magnetic energy.
Factors Affecting Stored Energy
| Factor | Effect |
|---|---|
| Higher Inductance | Stores more energy. |
| Higher Current | Greatly increases stored energy. |
| Core Material | Affects magnetic flux and saturation. |
| Saturation | Limits the maximum usable stored energy. |
| Temperature | May affect core characteristics and losses. |
Charging and Discharging
| Current Increasing | Current Decreasing |
|---|---|
| Magnetic field builds. | Magnetic field collapses. |
| Energy is stored. | Energy is released. |
| Current rises gradually. | Current falls gradually. |
| Back EMF opposes the increase in current. | Back EMF attempts to maintain current flow. |
Applications
| Application | Purpose |
|---|---|
| Buck Converters | Temporary energy storage. |
| Boost Converters | Voltage step-up. |
| Flyback Converters | Energy transfer through magnetic fields. |
| Motor Controllers | Current regulation. |
| Relay Coils | Magnetic actuation. |
| Electromagnets | Magnetic field generation. |
Design Considerations
- Select an inductance suitable for the application.
- Ensure the current rating exceeds the maximum operating current.
- Choose a saturation current above the peak current.
- Use low DCR to minimise copper losses.
- Select an appropriate core material for the operating frequency.
Testing Stored Energy Performance
- Measure inductance using an LCR meter.
- Measure operating current.
- Check for excessive heating.
- Inspect the magnetic core for cracks or damage.
- Observe switching waveforms using an oscilloscope.
Common Faults
| Fault | Possible Cause |
|---|---|
| Reduced Stored Energy | Incorrect inductance. |
| Core Saturation | Excessive current. |
| Overheating | High DCR or overload. |
| Voltage Spikes | No discharge path for stored energy. |
| Open Winding | Broken wire. |
Interesting Facts
- Capacitors store energy in electric fields, while inductors store energy in magnetic fields.
- Stored energy increases with the square of the current.
- Most modern switch-mode power supplies depend on inductors storing and releasing energy thousands of times every second.
- Large industrial inductors can store several joules of energy.
- The magnetic field disappears when the stored energy has been released.
Key Points
- Inductors store energy in magnetic fields.
- The stored energy depends on inductance and current.
- Higher current greatly increases stored energy.
- The stored energy is released when current decreases.
- Energy storage is fundamental to the operation of SMPS, DC-DC converters and many electromagnetic devices.