Air Inductors

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Air Inductors

Postby K9ACT » Thu Sep 09, 2010 10:25 am

I am having trouble relating Steve's photo of an air inductor filter with the world I live in.

I see something on the order 12 and 15 turns on the two coils. Not sure of the diameter or wire size but it is hard to understand how we can get 10's of uH out of these.

My 40 uH coil has about 50 turns of #18 enamel wire on .1" centers.

His are close spaced but with plastic insulation so they would come out about the same spacing.

The pics make it look like why bother with expensive cores but the 90 uH air version gets pretty serious as I see it.

The pics are a mystery that I ignored but going to higher power means I need one for the out put coil as the core I am using is not going to hack it.

Is there any reason that I can not close space #18 enamel wire (or any other size) for this application, i.e. 200W carrier?

Jack
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Re: Air Inductors

Postby kf1z » Thu Sep 09, 2010 11:05 am

I can only assume the coils at the bottom of this page is what you are referring to ?

http://classeradio.com/newpwm.htm


That is a different design filter than what you are using...

There are 3 inductors in that filter.

The diameter is likely 4 1/2 inches ( 4" sched 40 pvc)
The wire is awg #6.



At 4.5" form diameter... 20 turns .. coil length of 3" ( #10 insulated wire) would yield 40.3uH
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Re: Air Inductors

Postby kf1z » Thu Sep 09, 2010 11:16 am

I would use at least #14 wire Jack for 200 watt rig.
Just to avoid voltage drop... as now you're talking a few feet of wire!

But I believe you could indeed close spaced enameled wire for the inductors....
Only talking 150 volts maximum here ...
I could be wrong though!
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Re: Air Inductors

Postby K9ACT » Thu Sep 09, 2010 11:52 am

Thanks for the explanation. It was not obvious that the form was 4.5", etc.

I just found a bomb that says my rig can't possibly work.

I am using a 43 1.4" core for the second coil. According to the calculator this will saturate at about 10V. Surely I am doing something wrong.

What freq am I supposed to use for this? 150kc or the audio freq?

Seems to work well enough.

js
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Re: Air Inductors

Postby kf1z » Thu Sep 09, 2010 12:06 pm

I believe the 150khz...

But I have done no calculations to do with saturation...

I have only used the high-flux cores Steve mentioned, and air core coils....
So I have not had the worry about it :-)

Bruce
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Re: Air Inductors

Postby steve_qix » Thu Sep 09, 2010 10:11 pm

K9ACT wrote:Thanks for the explanation. It was not obvious that the form was 4.5", etc.

I just found a bomb that says my rig can't possibly work.

I am using a 43 1.4" core for the second coil. According to the calculator this will saturate at about 10V. Surely I am doing something wrong.

What freq am I supposed to use for this? 150kc or the audio freq?

Seems to work well enough.

js


Wow, a type 43 core as a PWM filter - YIKES ! It probably "really" isn't working correctly. The problem with type 43 is that the permability changes a LOT with current, so the inductance will fluctuate all over the place - and this is not good.

When doing the saturation calculations, you take the highest voltage that the modulator will produce on a regular basis (maybe base it on 150% modulation) and divide by the load resistance. This will give the peak current. That is the design current of the inductor. I generally use between 2 and 2.5 times the carrier current.

Number 14 wire would be acceptable for a 200 watt transmitter. I've actually used it in 400 watt transmitters. For the 2nd inductor, you can layer the windings particularly if the insulation is fairly think (like #14 wire taken from romex cable). You don't want to introduce too much layer to layer capacitance, or the coil will be partially negated.

Regards,

Steve
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Re: Air Inductors

Postby K9ACT » Fri Sep 10, 2010 10:07 am

steve_qix wrote:Wow, a type 43 core as a PWM filter - YIKES ! It probably "really" isn't working correctly. The problem with type 43 is that the permability changes a LOT with current, so the inductance will fluctuate all over the place - and this is not good.



Does this look any better?


Image

While pondering all this I happened to look on a shelf and spotted that big coil that I made last year for some 160 meter project and never used. Would you believe that it is exactly 98 uH? I patched it in last night and held court for about an hour on 3885 working about 10 stations, getting nothing but glowing audio reports.

The thing next to the coil has nothing to do with this project. It is my tube exciter for my other rigs.


>When doing the saturation calculations, you take the highest voltage that the modulator will produce on a regular basis (maybe base it on 150% modulation) and divide by the load resistance. This will give the peak current. That is the design current of the inductor. I generally use between 2 and 2.5 times the carrier current.

The calculator that I use "MINIRING" only wants voltage and freq as the variables along with the inductance. What do I do with the current? How does your approach deal with frequency?

js


Steve
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Re: Air Inductors

Postby n4lta » Fri Sep 10, 2010 3:48 pm

I'd move that big coil away from all that aluminum. At least a diameter away to keep the Q from being lowered substantially.
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Re: Air Inductors

Postby steve_qix » Fri Sep 10, 2010 10:26 pm

[quote="K9ACT]

The calculator that I use "MINIRING" only wants voltage and freq as the variables along with the inductance. What do I do with the current? How does your approach deal with frequency?

[/quote]

Hmmmmmm.. Not familiar with Miniring, but to figure an inductor's ability to work in a DC environment (which a PWM filter is), the current must be part of the design equations for that inductor. Other calculations do not necessarily need the current (for instance, figuring the PWM filter values). Once you know the inductances and capacitances you need, the next task is to design the individual components - capacitors and inductors - to function properly in the environment in which they will be operated (current and voltage).

Anyway, that's where the current comes in.

Regards,

Steve
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