> From: Rich Measures <measures@vc.net>
> Subject: Re: [AMPS] SB-220 C4 failure
To: <amps@contesting.com>
> Date: Wed, 9 Jul 97 02:06:09 +0000
I wrote:
> >The current limits because of the components internal ESR, which was
> >a natural result of the low power levels (1 kW ICAS INPUT power) and
> >duty cycle (less than 50%) the 220 was designed for. Same with the
> >filament transformer, any properly sized transformer combined
> >with the resistance of the filament choke, wiring, power line
> >ESR, and so on would limit filament inrush. There was no special
> >design, it was a conventional transformer.
Rich replied:
> I disagree. The TL-922 has a conventional-core filament transformer that
> delivers 48A of filament inrush current to each 3-500Z. OTOH, the
> filament inrush current in a SB-220 is 23.2A per tube. This amount of
> current limiting can not be explained by the ESR in the filament circuit.
> (according to Eimac, the max. allowable filament inrush current for the
> 3-500Z is 29A)
Really? Doesn't the TL-922 use a series filament string??
> - The filament transformer in the SB-220 appears to be similar to the
> design of current-limiting transformers used in neon signs.
> Rich---
I worked for Heath, and was involved in modernization of the
SB-220 series. That made all engineering data and service history
available to me.
While your intentions are probably good, the claim the transformer
was "specially designed like a neon sign transformer" are nonsense.
Current limited transformers are designed with HIGH flux leakage. You
can find this in welders or microwave oven transformers, where a shunt
bar is included in the core BETWEEN the primary and secondary
windings.The secondary winding is REMOTE from the primary winding,
and the core flux is given an alternative magnetic path so the counter
counter magnetic force of the secondary winding can drive the primary flux
away from the secondary path, limiting the secondary excitation under
periods of high load currents WITHOUT causing undue primary current.
Many welders vary the gap of the shunt or the secondary path, while
most microwave oven transformers have a fixed shunt bar in the
transformer.
It might look like a neon sign to you Rich, but the fact is it is
just a conventional transformer with no special design specs except
it has to meet EIA and UL-CSA safety requirements, as all Heath power
line operated transformers did. If you look at the SB-220
transformer, you'll it is a conventional E-I lamination transformer
with a conventional winding style of layered primary and secondary.
It has:
1.) No shunt bar
2.) Layered windings with the secondary and primary in normal
locations.
Nice try, but no cigar. The current limiting is a natural
consequence of the small size and high ESR of the entire filament
system. Just as it is in the AL1200, AL1500, AL82 and other PA's
using properly sized transformers.
73, Tom W8JI
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|