Transformer Markings

[electrogram]

I am considering buying a transformer marked 20-10-0-120-240v primary. Does this mean the 20-10 sections are bucking windings?

[SteveCG]

I find that description ambiguous. The 20-10 windings could add or subract from the main 120 and 240 primary windings. If I had it to hand an AC voltmeter would quickly resolve matters.

[broadgage]

It is indeed a bit ambiguous, but I strongly suspect that the 10 volt and 20 volt tappings are subtractive from the selected voltage.

If selecting the 120 volt input, one might well want 10 volts less, as 110 volts was common in years gone by. The chance of needing 10 volts more for 130 volt input is less likely, and the likelihood of needing 120 volts plus 20 volts for 140 even less likely.

If selecting the 240 volt input then one might well want 10 volts or 20 volts less, as both 230 volt and 220 volt supplies are common.
The chances of needing 240 volt plus 10 or 20 volts, for 250 or 260 volts supply is remote.

Odd voltages including 127 (close enough to 130), and 250 volts certainly existed but were rare in comparison to 110, 120, 220,230,and 240 volts.

[Herald1360]

In that order, I would go for 120, 130, 140, 240, 250, 260. But that doesn't make sense of likely mains voltages as noted.

A better order for the logical voltages would be 0-10-20-120-240 or perhaps
20-10-0-100-220.

[trobbins]

Winding resistance measurement should remove the ambiguity.

[Nicklyons2]

The resistance of 2 sets of windings in series would be the same regardless of the phasing; the voltages would however differ dependent on the phasing -
phase coherent = additive, 'bucking'/anti-phase = subtractive.

I admit that it seems ambiguous, IMHO a more sensible manufacturer would have all 'in-phase' windings and have them labelled something like 20-10-5-0-110-220.

[Herald1360]

If you simply want to use the TX and the secondary voltage(s) are what you want, just buy it. It has 240V explicitly marked whatever the rest mean and you can enjoy poking at it to find out exactly what the manufacturer was thinking of! We can all enjoy tutting about "what were they thinking" when you report back with the details

[short wave]

Hi electrogram
If you have baught your transformer and want to test if the 10 volt / 20 volt windings are additive /subtractive is to apply “mains” to the “0” and “240” windings. Measure the voltage at these terminals then move the “0 volt” probe to the “10” volt winding (still keeping the other probe on the “240 “ terminal) if the voltage goes up say 10 volts or so its additive ...or down subtractive This is using the primary like an “auto transformer”
Regards S-W

[electrogram]

Thanks for all your replies gents. I suppose what I was really interested in was is there a convention for such markings - are they additive or subtractive - it would appear not! The only way to be certain is to measure,not possible if buying online.I'm afraid the transformer mentioned and a couple of others were being considered for a nefarious purpose - ahem - an output transformer on a well known 1950s 'hi-fi' record reproducer.