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Old 25th February 2005, 05:51 AM   #1
sbear is offline sbear  United States
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Default Finding phase of dual primary tranny

I just got two surplus transformers for my Aleph-x project, but I can't find any phase markings on the them. They are 500VA EI with dual primaries and dual 17V secondaries and I need to wire the primaries in parallel. I had the thought of hooking up one primary to a AA battery and holding a magnet or compass next to the windings and note the magnetic effect on the magnet or needle, then repeating the procedure with the other primary. I just wonder if the DC from the battery will harm the transformer any.(I'm guessing it won't, since it will be a very small current) Has anyone tried this or have any other tricks to find the phase of transformer windings?

Just to make sure I don't blow anything up, the phase relationship of the secondaries is moot since they are being hooked straight in to the rectifiers, right? (I am using seperate rectifiers for each secondary)
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Old 25th February 2005, 02:26 PM   #2
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So you have four secondary wires, right? Twist two of them together and measure voltage from the two that are left. I put the tranny on a power strip and power up quickly and power down. If you get your 34VAC then the connection is correct. If you get -0- VAC then swap a wire on the connection. In other words, on a four wire connection, tie wires 1 and 3. If no voltage, then tie wires 1 & 4. Fuse your primary before you start ~ 4A? 5A?

I don't know what happens if you decide to go for lunch when and leave the tranny running "out of phase". Heat is my guess.

People on the board generally state admonitions regarding high voltage. Word is that it's lethal. So, my suggestion is that you not perform this experiment in the bath tub.
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Old 26th February 2005, 01:45 AM   #3
Gaucho is offline Gaucho  Canada
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Quote:
Just to make sure I don't blow anything up, the phase relationship of the secondaries is moot since they are being hooked straight in to the rectifiers, right? (I am using separate rectifiers for each secondary
Right, if separate rectifiers are used on each secondary the absolute phase feeding the different rectifiers is irrelevant.
You do however need to make sure you parallel the correct primary wires. Here's an easy non-destructive method:

In order to use numbers for clarity I will assume the primary windings are 120V. Secondaries are 17V.

Pick one primary winding and one secondary winding. Label one of the secondary wires A and the other B. Now connect wire B to either one of the primary wires and connect this to Neutral. Connect the other primary wire to Hot and measure the voltage from the primary 'Hot wire' to secondary wire A. You will either meaure the primary voltage plus secondary voltage (approx 137 V) or primary voltage minus secondary voltage (approx 103V). Record the voltage and flag the primary wire that was connected to Neutral.

This part is critical your success: Now do the procedure again with the other primary winding and the SAME secondary winding, keeping the SAME secondary wire (B) connected to Neutral. If you get the same end voltage (again either 103V or 137V) as the first time around flag the primary wire connected to Neutral, if the voltage is different flag the primary wire that is connected to Hot.

The two primary windings are paralleld for 120 V operation by tying the flagged wires together and the unflagged wires together.

Please don't die doing this! If at all possible use a GFCI receptacle or breaker to provide the power required for your testing. If you have a low voltage AC supply (like a cheap Radio Shack transformer) use that instead of line voltage.
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Old 26th February 2005, 02:27 AM   #4
sbear is offline sbear  United States
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Gaucho, thanks. thats exactly the kind of info I needed.
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Old 26th February 2005, 08:11 AM   #5
Gaucho is offline Gaucho  Canada
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Originally posted by sbear
Gaucho, thanks. thats exactly the kind of info I needed.
Glad I could help. Good luck with your Aleph-X.
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