Envelope-to: dave@picks.force9.co.uk
Delivery-date: Fri, 03 Feb 2006 15:32:49 +0000
From: Warren K2ORS/WD2XGJ <k2ors@comcast.net>
To: rsgb_lf_group@blacksheep.org,  rsgb_lf_group@blacksheep.org
Cc: "james moritz" <j.r.moritz@herts.ac.uk>
Date: Fri, 03 Feb 2006 15:31:08 +0000
Message-Id: <020320061531.1550.43E3773B000EF12C0000060E22073007930B97010D0A020E06979D0E03@comcast.net>
Subject: Re: LF: RE: Keeping the smoke in
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Content-type: text/plain; charset=windows-1252; format=flowed
MIME-Version: 1.0
Content-transfer-encoding: 8bit


Hi Jim,

     Thanks for the analysis. The transformer is in a closed weathertight box which I'm sure contributes to the heat build up.
The high voltage on the transformer secondary (loop or low impedance side) comes from the fact that it is in a resonant circuit.
The voltage will be E=I*X, in this case the I is about 30 amps and X is near to 200 ohms so E= 6000 volts. I have seen actual arcing when the spacing or insulation was inadequate.

--
73 Warren K2ORS/WD2XGJ
FN42hi
http://www.w4dex.com/wd2xgj.htm

 -------------- Original message ----------------------
From: "james moritz" <j.r.moritz@herts.ac.uk>
> Dear Warren, LF Group,
> 
> Sorry to hear about your problem. The transformer core seems more than
> adequate for the power level you are using - the flux density in the core at
> 1.2kW should be about 40mT, which is very low, so core size is more than
> ample; certainly I am using smaller cores at the same power level without
> heating problems. At that flux density, magnetic losses in the core would be
> something like 5 or 6W, according to the Fair-Rite graphs for 77 material.
> You might lose a similar amount in the winding, making about 12W loss
> overall. I guess if this was thermally very well insulated it could get very
> hot - this might happen if the box has very low thermal conductivity, and
> there are other heat-producing components inside as well. 
> 
> What mystifies me is why high voltages should also be present around the
> transformer. 1.2kW into 50R is about 245V RMS, which is nothing in corona
> terms. The low impedance secondary should only have 46V across it. A high
> voltage between primary and secondary windings could result from having the
> transformer connected into the loop at a high potential point, but it isn't
> obvious to me why that would be done. This could also result in increased
> core losses due to dielectric loss in the ferrite.
> 
> Cheers, Jim Moritz
> 73 de M0BMU
> 
> -----Original Message-----
> From: owner-rsgb_lf_group@blacksheep.org
> [mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Warren K2ORS/WD2XGJ
> Sent: 02 February 2006 22:41
> To: rsgb lf reflector
> Cc: LWCA LF Reflector
> Subject: LF: Keeping the smoke in
> 
> Hello the list,
> 
>       I've heard that the secret to keeping things going in electronics is
> to make sure you don't let the smoke out (that's what makes it all work
> apparently).
> 
> ....
>     I guess I need a bigger core than the 3 stacked FT-290-77s any
> suggestions out there?
> 
> --
> 73 Warren K2ORS/WD2XGJ
> FN42hi
> http://www.w4dex.com/wd2xgj.htm
> 
> 
> 
>