Inductance is the property of a circuit whereby a change in current induces an electromotive force that opposes that change, storing energy in a magnetic field. It is measured in henries, where one henry produces one volt for a current change of one ampere per second.
Learning objectives
- Define inductance and the henry
- Calculate inductance from turns, flux and current
- Calculate the back-EMF from a rate of change of current
- Calculate the energy stored in an inductor
- Combine inductors in series and parallel
Progressive explanation
Start with the plain-language idea, then move into the engineering detail. Each level stands on its own, so you can stop at the depth you need.
Beginner The core idea
The energy is stored in the magnetic field around the coil. The more turns and the better the magnetic core, the greater the inductance.
Intermediate How it behaves in a circuit
Inductors in series add: Ls = L1 + L2, assuming no mutual coupling. In parallel they combine as 1/Lp = 1/L1 + 1/L2. Mutual inductance couples two coils and can either add to or subtract from the total, depending on the winding direction.
Advanced Limits, assumptions, engineering detail
Formulas and variables
Units of measurement
| Quantity | Symbol | Unit |
|---|---|---|
| Inductance | L | henry (H) |
| Magnetic flux | Φ | weber (Wb) |
| Current | I | ampere (A) |
| Energy | E | joule (J) |
Worked examples
Inductance of a coil
Problem. A 500-turn coil carries 2 A and links a flux of 0.4 mWb per turn. What is its inductance?
Solution. L = NΦ/I = 500 × 0.0004/2.
Back-EMF during current rise
Problem. The current in a 0.5 H inductor rises by 2 A in 10 ms. What average voltage is induced?
Solution. di/dt = 2/0.010 = 200 A/s. e = L × di/dt = 0.5 × 200.
Stored energy
Problem. How much energy is stored in a 0.5 H inductor carrying 2 A?
Solution. E = ½ × L × I² = 0.5 × 0.5 × 2².
Practical applications
- LC and EMI filters
- Switch-mode power supplies
- Relays, solenoids and contactors
- Ignition and pulse generation
- Transformers and wireless power
- Energy storage in converters
Automotive applications
Ignition coils, injector solenoids, relays and DC-DC converters all use inductance. The coil stores energy and releases it quickly, which is why a freewheel diode or suppression is needed to protect the switching device and limit voltage spikes.
Industrial applications
Chokes and reactors smooth current, limit inrush and reduce harmonics in variable-frequency drives, while inductors in switch-mode supplies set the ripple and energy transfer. Contactors and solenoids rely on magnetic energy to actuate, and their coils need suppression.
Measurement procedure
- Use an LCR meter at a specified frequency; the displayed inductance depends on the test conditions.
- Estimate inductance from the L/R time constant by observing the current rise.
- Measure winding resistance separately; a real inductor is not pure inductance.
- Check saturation behaviour by increasing current and observing the fall in inductance.
Common mistakes
- Measuring inductance with a DC ohmmeter, which reads only winding resistance.
- Assuming inductance is constant regardless of current and temperature.
- Ignoring mutual coupling when combining nearby coils.
- Opening an inductive circuit without suppression and damaging the switch.
Safety considerations
- An inductor produces a high voltage when its current is interrupted; provide a freewheel path or suppression.
- Magnetic cores can store significant energy; allow discharge and follow lockout procedures.
- Keep strong magnetic fields away from sensitive devices and implants.