[Amps] Need help understanding an old Transformer

[Edward Gable]

I concur with interesting comments on this thread.  No one has mentioned 
that a full wave bridge with an ungrounded center tap is not an option using 
these old transformers.  The transformers were designed and built to have 
the CT grounded and the insulation often will not withstand the high voltage 
at the ungrounded terminal.  Some old timers used to mount the transformers 
off ground on porcelain insulators to keep the potential difference between 
the case and the CT at a safer level.  Maybe that's why we don't have as 
many old timers - hihi.

73,  /Ed Gable  k2mp

----- Original Message ----- 
From: "W5CUL" <w5cul at sbcglobal.net>
To: <amps at contesting.com>
Sent: Monday, May 26, 2008 12:18 PM
Subject: [Amps] Need help understanding an old Transformer


> Hi All,
>
>
>
> I need some help in determining the capability of an old transformer that 
> I
> ran across.  It is an old Thordarson CHT Series built some time in the 
> late
> 30's early 40's.  The model number is T-15P21 and it weighs about 129lbs,
> and was designed for continuous commercial service (CCS) at its rated
> output.  What has me confused is how to determine its overall rated
> capability.  It is an old style with a Center Tap on the secondary's, and
> plenty of Sec taps for various voltages.  I actually found an old 
> Thordarson
> catalog which gives somewhat of a clue, but I suspect that I may not be
> reading it right.  Here is what it says in the catalog for the T-15P21:
>
>
>
> Sec A.C                        D.C                      D.C
> Pri.              Pri.
>
> Load Volts                    Volts                    M.A.
> Volts           Watts
>
>
>
> 3440/3440                    3000                     500
> 115-230         1690
>
> 2980/2980                    2500
>
> 2340/2340                    2000
>
> 1815/1815                    1500
>
>
>
> So what I am thinking, and please correct me if I am wrong, I would see 
> 3440
> Vac between either secondary leg and CT, or if across both 3440 secondary
> legs, I would see 6880Vac.  The "3000" D.C Volts shown in the catalog is
> only for one leg to CT and is rated low due to an older method of
> rectifying; basically using a smoothing choke with one or two diodes.  The
> .5mA at 3000Vdc rating is across only one of the 3440 secondary legs to 
> CT,
> with the other 3440 secondary leg to CT capable of the same amperage
> simultaneously. Or, if across both 3440 secondary legs with no center tap,
> the transformer could supply .5mA CCS at 6000Vdc, given a smoothing choke.
> I also suspect that the 1690 Watts primary is in keeping with the one 3440
> secondary leg to CT, even though it is under the Primary column, thus 
> giving
> a full rating across the primaries if both 3440 secondary legs were to be
> utilized simultaneously to that of 3380 Watts.
>
>
>
> Let me know if I am on the right track or if I am missing something here.
>
>
>
> Thank you for the help!!
>
>
>
> 73,
>
>
>
> Mike
>
> W5CUL
>
>
>
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