Phasing unknown split primary winding terminals on a mains transformer? - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: April 14, 2014, 11:05:40 am
content: On an unknown colour coded toroidial mains transformer that I *do* know has two split 120V primary windings of red / brown on one winding, and black / blue on the other, how do I safely find the correct phasing to run on 240V? I have a Variac and a an ammeter. Thanks.

author: Anks
author_rank: Frequent Contributor
timestamp: April 14, 2014, 12:25:18 pm
content: A few ways but the easiest way it to put a ac signal 50-60Hz in to the two windings and test the AC voltage on the secondary. Then reverse the phase on one of the primary's and if the secondary voltage as dropped it was in phase in the first test. If it as increased then it is in phase for the second part of the test.

author: Chris Wilson
author_rank: Super Contributor
timestamp: April 14, 2014, 01:53:37 pm
content: OK, thanks for that Anks, much appreciated, I will test it and mark the wires accordingly. Cheers.

author: Tube_Dude
author_rank: Regular Contributor
timestamp: April 26, 2014, 01:52:28 am
content: Quote from: Chris Wilson on April 14, 2014, 11:05:40 amConnect a analog multimeter in DC Volts to one of the primaries. Touch a 1,5 Volts battery briefly to the secondary widding, you will see the analog needle to make a brief wiggle, in one direction when you connect the battery, and in the opposite when you disconnect. Now you have the polarity of one of the primaries. Repeat with the multimeter in the other primary, to have the same reaction and you will get the other's polarity. Now you can safely put them in series or parallel....

author: retrolefty
author_rank: Super Contributor
timestamp: April 26, 2014, 03:09:30 am
content: Quote from: Tube_Dude on April 26, 2014, 01:52:28 amSame trick as using the battery to check phasing of loudspeakers to see if cone moves in or out with a given polarity input.

author: Tube_Dude
author_rank: Regular Contributor
timestamp: April 26, 2014, 03:22:58 am
content: Quote from: retrolefty on April 26, 2014, 03:09:30 amYes, it is! Remark that I have edited my post...

author: SeanB
author_rank: Super Contributor
timestamp: April 26, 2014, 06:12:49 am
content: Simpleat is to have another transformer say 12VAC. Connect the 2 primaries in series and connect the 12VAC to the one so that the centre tap is common. Then measure AC volts across the outer sides and see if it is 24VAC. If so then they are correctly phased. If it is 0V or very close ( depends on the winding imbalance of the 2 windings, ideally they should be identical but often are a few dozen turns different) then switch off and reverse one of the primary coil windings and try again, where you will get 24VAC.Low voltage to test means you will not damage the transformer. You can also use this and measure the secondary side to get a rough idea of turns ratio and secondary voltages with an unmarked transformer.

author: Chris Wilson
author_rank: Super Contributor
timestamp: April 26, 2014, 06:41:31 am
content: I sorted the toroidal one, but some more great tips there, thanks for the ideas again!