Thanks for the great info Manfred. I didn't get your email until after
I sent my latest. I agree 1000%, I can't use it as it is. I think after a
few hours running even at idle it would get hot enough to burn a winding,
even with lots of air.
Your paragraph on the air gap & eddy currents jolted my memory- when I
unpacked this transformer the lower angle aluminum mounting stock was bent a
bit on one corner, as if it had been dropped on that corner. Everything
else looked fine otherwise & I straightened the corner of the angle stock
out easily with a large adjustable wrench. However, the bolt right next to
that corner is the one generating the 275 deg F heat, worse than the other 3
by almost 100 deg. Maybe when the bend occurred it dislocated the bolt,
bringing it into contact with the core.
I am going to pull the transformer out of the chassis & see what
hardware I need to replace the bolts, as well as what size teflon tube will
fit into the hole thru the core. I hope there isn't too much varnish in
there. I'd consider formvar tape but I have not been able to locate
anywhere to get it. I suppose teflon will hold up better anyway.
73 & thanks,
Dave N3DB
----- Original Message -----
From: "Manfred Mornhinweg" <mmornhin@gmx.net>
To: <amps@contesting.com>
Sent: Saturday, May 03, 2008 10:09 PM
Subject: Re: [Amps] HV transformer issue
> Hi Dave,
>
> interesting problem, with those bolts!
>
> Firstly, I would not believe that they can get so hot from hysteresis
> losses. They are not really taking much magnetic flux at all, because
> they are separated from the laminations by the air gap made by the
> clearance between the bolts and the holes. As a consequence, replacing
> these bolts with nonmagnetic ones, such as stainless steel, will not
> help, or at least not because of any magnetic phenomena.
>
> The heat must be caused by an enormous current flowing through them, or
> more likely, a quite large current flowing through the poor connections
> made by the nuts on the surface they sit on. So, the obvious solution is
> to insulate them, at least one end. You say that the bolts are thinner
> than the holes' diameter. That's great, as it might allow you to use
> these insulating washers that also have a sort of little tube attached.
> Sorry for the poor description, but I mean insulating washers such as
> those used for mounting transistors on heat sinks, using metal bolts.
> Complementing the in-hole insulation with a tube made of paper is also a
> good idea.
>
> Regarding material of the bolts, what might help is using a material
> with poorer conductivity. After all, the best reason why transformer
> laminations are made of silicon steel is that this alloy has much higher
> resistance than carbon steel! That reduces eddy currents. It is possible
> that your problem comes mostly from eddy currents in the bolts, and in
> that case the only thing that would help is using a material with higher
> resistance. But I don't think this is the case, because of the air gap
> you have there, which should keep magnetic flux and thus eddy currents
> in the bolts rather low.
>
> If all else fails, use some longer pieces of angle stock so that you can
> install the clamping bolts outside the laminations, to keep them free of
> strong fields and make them easy to insulate.
>
> I would not use the transformer like it is, because such extreme heating
> of the bolts can cause more problems, in addition to being unnecessary
> power loss.
>
> Manfred, XQ6FOD.
>
> ========================
> Visit my hobby homepage!
> http://ludens.cl
> ========================
>
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