Author Topic: Phasing unknown split primary winding terminals on a mains transformer?  (Read 2532 times)

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Offline Chris Wilson

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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.
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                 Chris Wilson.
 

Offline Anks

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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.
 

Offline Chris Wilson

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OK, thanks for that Anks, much appreciated, I will test it and mark the wires accordingly. Cheers.
Best regards,

                 Chris Wilson.
 

Offline Tube_Dude

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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.

Connect 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....
« Last Edit: April 26, 2014, 01:20:35 pm by Tube_Dude »
Jorge
 

Offline retrolefty

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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.

Connect 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 secondary, to have the same reaction and you will get the other's polarity. Now you can safely put them in series....

 Same trick as using the battery to check phasing of loudspeakers to see if cone moves in or out with a given polarity input.

 

Offline Tube_Dude

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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.

Connect 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 secondary, to have the same reaction and you will get the other's polarity. Now you can safely put them in series....

 Same trick as using the battery to check phasing of loudspeakers to see if cone moves in or out with a given polarity input.

Yes, it is! Remark that I have edited my post...  ;)
Jorge
 

Offline SeanB

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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.
 

Offline Chris Wilson

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I sorted the toroidal one, but some more great tips there, thanks for the ideas again!
Best regards,

                 Chris Wilson.
 


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