Guru
Joined: 09/08/2007
Location: AustraliaPosts: 4406
| Posted: 03:53pm 03 Jun 2017 |
Air cored coils can be very useful, but there is one huge problem.
You need a lot of copper wire length and a very great number of turns to reach any reasonable amount of inductance.
That increases the resistive copper losses beyond anything acceptable for most power magnetics.
As soon as you start using thicker wire, that spaces the individual turns further apart which lowers the inductance. The most efficient proportions for designing an air cored coil are the "Brookes Coil", and there are some software calculators on the internet for designing Brookes coils, or any air cored coil.
http://www.nessengr.com/techdata/brooks/brooks.html
A magnetic core offers a much more efficient and cost effective solution.
Chokes are about the most difficult of magnetic components to design properly, and there is a lot more to it than what follows.
But the simplest practical nuts and bolts approach for something fairly ordinary would go something like this:
First determine your required wire size from the resistive heating point of view.
Fit as many turns as possible onto the prospective magnetic core.
Adjust the air gap sufficiently wide so it has an adequate saturation margin.
If it then has enough inductance remaining after gapping, the job is done.
Or go to a larger core and try again if you still require more inductance.
Cheers, Tony.