Various aspects of home brew inverters

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Joined: 02/02/2017
Location: Australia
Posts: 1384
Posted: 03:16pm 03 Jun 2017      

Part 9: Air core inductor possible or am I on drugs..

Air cored inductors can not saturate and so I wondered is it feasible to make one for our home brew inverters.
From the wikipedia inductor page for a short air core coil we have
L = (r2 x N2)/(9r + 10l) uH
N = turns
r = radius of coil (inches)
l = height or length of coil (inches)
So I had some 1mm diameter single core wire laying about.
I made a coil of about 4" diameter, 13 turns, and about 0.3" high

The LCR meter measures it at 27.4uH
The formula gives (2 x 2 x 13 x 13)/(9 x 2 + 10 x 0.3) = 32 uH
Close enough for me.
The resistance of this coil is 0.35 Ohms and that may be a problem when itís passing primary winding currents for realistic loads.
It looks like this:

But it works fine, here are the two DSO captures at idle and under 440 W load.


440W load

The coil gets hot very quickly and it no wonder why. (pink trace is primary current, 41mV/Amp scale, so 1.44V = 35A)
35 Amps RMS through 0.35 Ohms = 429 W

I want to make the same coil with much ticker wire, 13 T of 4Ē diameter coils means about 4 meters of cable. This wont cost too much.

I am thinking that maybe I donít need anything other than an air core coil as the primary side choke. No more saturation worries!

wronger than a phone book full of wrong phone numbers