Inductors store energy in the form of a magnetic field.
The unit of Inductance is the Henry.
Inductors are coils of wire. They are also electromagnets. Adding an iron or ferrite core increases the inductance.
When the current in an inductor changes, a back EMF is produced. This is a voltage that opposes the current change. Sudden changes in current produce a large back EMF.
If the rate of change of current in a circuit is one ampere per second and the resulting BACK EMF is one volt, then the inductance of the circuit is one Henry. (EMF = electromotive force)
XL = 2 π f L
- XL is the inductive reactance measured in Ohms.
- f is the frequency in Hertz.
- L is the inductance in Henries.
Inductors are
- used in tuned circuits
- used in transformers
- used as relay actuators
- used in motors
- used to pass DC but block AC (the opposite of what capacitors do).
- used in filters (pass DC and low frequencies but block high frequencies).
- alternating current lags behind the applied voltage (is phase shifted by 90o)
Using a flywheel analogy, the current is the wheel rotation and the voltage is the force needed to get the wheel moving. A large force (voltage) is needed to accelerate the wheel. After a time delay, the wheel speeds up (current). Once the wheel is spinning, the driving force (voltage) can be removed and the flywheel will keep spinning (for a long time). The voltage leads the current.
If a spanner is thrown into the flywheel, the wheel will stop suddenly, destroying the spanner, wheel bearings, and the wheel. (A big flywheel would take out the entire building!). This is just like back EMF.
Adding an iron core to an inductor greatly increases the amount of energy that can be stored magnetically so the inductance also increases.
Solid iron is not an ideal material for a core because eddy currents are induced in the iron wasting energy in the form of heat. Thin slices of iron insulated from each other (a laminated iron core) prevent these currents and greatly improve the efficiency of the inductor.
Another useful core material is ferrite. This is iron dust glued and molded into a solid shape. The glue insulates the iron particles from each other preventing eddy currents.
Preset variable inductors can be made by winding a coil and placing a ferrite core down the middle that can be screwed in or out. Threaded plastic coil formers with a screw thread and molded ferrite slugs are available. Once adjusted, the slug can be secured with a drop of molten wax or nail varnish.
It takes a lot of energy to build up the magnetic field surrounding the inductor. It takes yet more energy to change the magnetic field. If you try to stop or change the current in the inductor suddenly, enormous voltages are produced (possibly destroying neighbouring components). This is called back EMF.
Screened Inductor used for an Intermediate Frequency Tuned Circuit
Variable Inductor tuned by moving the Ferrite Slug
Two Tiny Radio Frequency Chokes
These pass DC and low frequencies but block high radio frequencies.
