[ARC5] Slightly OT: Transformers

[Roy Morgan]

On Nov 27, 2013, at 6:10 PM, Eric Mynes <kc8wzb at gmail.com> wrote:
> 
> 
> Currently I have a transformer on my bench ...
> It appears to have 3 windings.  A black set (I presume is the primary), a
> red set with CT, and a green set with CT.

Eric,

The normal color code for transformers has been mentioned. I would expect  yours to be:
Blk - Blk  primary
Red-CT-Red  high voltage
Grn - CT - Grn  filament.
With no Yellow secondary (usually rectifier filament) I would expect your unit is meant for use with solid state rectifiers.

Here is a good web page that explains more:
http://www.radioremembered.org/xfmr.htm

A web search for “transformer color code” will bring many more.

> I'm kind of skittish at just applying 110v to the black wires and seeing
> what the others have for voltages.

A very reasonable caution.  Another email suggests applying 12 volts (one tenth the likely primary voltage) to the black leads to see what the other winding voltages are.  This is a very good technique.  If you have a variac and a 12 volt transformer available, it makes even more sense - start at zero with the voltmeter attached to a secondary and see what happens.  The posted advice of using a light bulb, and not connecting your meter before switch-on is good.

> ... don't want to waste too much of the list's bandwidth on something that might be determined as an elementary
> question

This is a very fine question and well worth our time on the list. Asking questions before trying things will keep you safe, informed, and learning!

The table at the website:
http://wb0nni.dakotamade.com/xfmrpwr.html
Is very useful.  (the left most power column should be 0.1 not 1.) The abbreviation IVS (Intermittent Voice Service) is likely equivalent to the term ICAS (Intermittent commercial and amateur service) used in tube ratings.

The ARRL Handbooks of a long time ago offered a graph version of this table, plotting power rating vs. cross section core area.  Either will get you started.  (Count the power used in all secondaries.)  For filament windings that bring the actual winding wire out for connection, you can estimate current rating by wire size.  I think this is done less seldom than it used to be: modern transformers seem to use all flexible leads.  

I have not seen advice or tables that make a correction for potted transformers.  Oil filled military transformers are rare, but we do sometimes find steel cased, tar-filled commercial transformers.  I suggest using a number from the table, and under-loading the transformer you have by a modest amount.

Having a collection of transformer company catalogs is very useful.  www.bunkerofdoom has many such catalogs.

You’ll have a success with all the advice from the list, I’m sure.

Roy


Roy Morgan
RoyMorgan at alum.mit.edu
K1LKY Since 1958