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Old 13th January 2002, 03:31 PM   #11
PassFan is offline PassFan  United States
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Yes, a transformer has a polarity. We generally don't run into problems with this with single winding primarys or secondarys. once you move to dual windings or three phase its a different story. Transformers induce voltage from primary to secondary through a magnetic field and that field has an electrical orientation. In a dual primary it is possible to wire these two fields to oppose one another much the same as shoving two magnets together - to -. The magnets will force themselves apart, the windings in the transformer cannot. Large amounts of current will flow when this happens. If you don't have a fuse in the primary I would assume that the end result would not be pretty. I realize that I may have not represented the correct terminology in my description here, I tried to put this in laymans terms so everybody could understand this phenomenon. So, go easy on me guys.
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Old 13th January 2002, 04:20 PM   #12
paulb is offline paulb  Canada
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"Polarity" for a transformer is called phase. You gotta get your phasing right.
This is an unfortunately overloading of this word because it could lead to all kinds of puns. However, it's time to phase the music and get our terminology straight.
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Old 13th January 2002, 09:51 PM   #13
PassFan is offline PassFan  United States
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Default Transformer phasing

Actually, the book "Alternating Current Fundamentals" from Delmar Press refers to the leads as poles and goes on to discuss the differrence between additive polarity and subtractive polarity transformers. It lists three requirements for paralleling transformers which are:
1) secondary terminal voltages must match

2) polarities must be the same

3)must have the same percent impedance

The section goes on to describe test procedures for determining which type you have if the leads are unknown.
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Old 14th January 2002, 12:54 AM   #14
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PassFan, think you could tell me what that book of yours says about my problem? please..........
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Old 14th January 2002, 01:40 AM   #15
PassFan is offline PassFan  United States
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Default What the Book Says

Only at DIY can I learn something and help other people at the same time. I love it. Actually subwo 1 has the right idea although this is a secondary test it will tell you if you have the polarity crossed. First determine your matching pairs from primary to secondary by powering up the primarys one set at a time. Once you know which secondarys match which primarys guess at the polarities of the primarys and hook them up in parallel. I would at this time do two things 1) make sure I have a FUSE installed on the primarys (probably around 2 amps) and 2) power up with the secondarys open and make sure the fuse doesn't blow. This will probably tell you what you want to know as far as the primarys are concerned ,but the rest of the test will ensure exact parallel operation. Tie one wire from one secondary to one wire from the other secondary together. You should now have both primarys hooked up in parallel, one wire from each secondary dangling, and one wire from each secondary hooked up. Now using a volt meter, turn on the voltage to the primary and measure the voltage between the two dangling secondary wires. If the voltage is twice the secondary rating you have the polarities crossed. If the voltage is 0 you have the polarities correct. Bear in mind that I crossed my primarys up on a toroid and drew more than 10 amps of current so please use a fuse on your primarys. I have deduced from my misfortune that the opposing magnetic fields bucked each other and caused large amounts of current to flow so be careful. Once you have the polarity correct then you will know the proper phase direction for series operation. I hope this helps.FUSE FUSE
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Old 14th January 2002, 02:13 AM   #16
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Default Re: What the Book Says

Quote:
Originally posted by PassFan
Only at DIY can I learn something and help other people at the same time. I love it. Actually subwo 1 has the right idea. First determine your matching pairs from primary to secondary by hooking the primaries together one set at a time. Once you know which secondaries match which primaries guess at the polarities of the primaries and hook them up. Tie one wire from one secondary to one wire from the other secondary together. You should now have both primaries hooked up in parallel, one wire from each secondary dangling, and one wire from each secondary hooked up. Now using a volt meter turn on the voltage to the primary and measure the voltage between the two dangling secondary wires. If the voltage is twice the secondary rating you have the polarities crossed. If the voltage is 0 you have the polarities correct. I hope this helps.
What do you mean 'find which primaries match which secondaries'?
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Old 14th January 2002, 02:42 AM   #17
PassFan is offline PassFan  United States
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My mistake. There are two x-former threads going here and I'm stuck with info from both of them in my head at the same time. You have a multi tapped secondary not a parallel one, sorry. I have edited the post you quoted from though it won't do you any good. The book doesn't give a rundown on all types of transformers just single winding ones. If you fuse your primarys and connect them in parallel it should tell you what you want to know. If the fuse blows you have the polarity crossed. If your correct, the fuse holds and you know what the proper phase direction is for each winding. This happened to me with a toroid. What I deduced from my misfortune was that having the polarity crossed I caused two opposing magnetic fields to buck each other which drew a lot of current (enough to blow a 5 amp fuse). If you connect them in parallel and blow a fuse you've got them backwards. I hope this helps as it's all I have right now. I will keep looking
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Old 15th January 2002, 03:53 AM   #18
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Quote:
Originally posted by blmn
AudioFreak,
I think you can put the secondaries (identicals) in paralel, applying a 60Hz signal from an generator and, with an scope, see the phase of the signal at the primaries. When tey are the same you can use it has a relative "beginnings" and "ends" for both sides of the transformers. Make it at opposite way works too, but, It's hard to setup and you must use more voltage.
regards
This works too; use a signal generator on the secondaries and measure the voltages through the primaries... don't use more than 10VAC, otherwise you could easily get some VERY high voltages on the primary (not nice )
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Old 19th January 2002, 01:40 AM   #19
PassFan is offline PassFan  United States
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Audiofreak did you figure it out? I spoke with an engineer and he recomended powering it up and reading the secondary. When your out of phase on the primary the voltage will be double on the secondary.
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Old 19th January 2002, 01:52 AM   #20
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I've not had much time for it but i spent yesterday afternoon playing with it and i've almost got it figured out.... i blew 3 fuses .... when the primaries were out of phase, the secondaries werent twice the voltage.... the fuse blew when the secondaries were out of phase i got 0V when in parallel or series but i got it working so now i've got to check for an approx VA rating.... I'll post all my findings later.
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