I've never really measured voltage of a transformer primary before (from microwave - 1000-1200w cooking power - power rating usage on unit says 1600w - the secondary burnt up and was removed), across the two primary inputs - it is not grounded (yet). At times I get .05-.06 (between the neg and neutral/positive) and when I measure one side and the other to the case, I get 7.6v and the other connector to the case I get the .05 - .06v. The transformer buzzes slightly (more than other's I've heard). When I plugged it in, a screw was pulled over to it, not very strongly, but there is a field around the lower part of the unit where the primary is.

I've used 12g wire w/5 turns to check V output of secondary to see how many windings I need, so IDK if having that little amount of secondary could be an part of this.

IDK if this is normal behaviour, but I've never had magnetism happen nor the buzz like this. I was wondering if there could have been damage to the primary, though it looks good and the secondary wasn't really that bad when I removed it.

 

I have no wish to insult but I have to ask: you do have your meter set to AC volts?

 

I have no wish to insult but I have to ask: you do have your meter set to AC volts?

Yes. The meter was set to AC on all the calculations when testing this transformer. I turned it off, and re-selected AC 2x after the first time I got these odd readings.

 

Are these measured voltages across the primary or your 5 turn secondary?

 

I get 7.6v and the other connector to the case I get the .05 - .06v.

Sounds like the primary may be shorted to the core. Check resistance from one lead to the iron core and see if you get any resistance at all. Should be infinite (no connection).

Also, if you're getting a voltage on a transformer - is it plugged into anything? Even if the power is switched off - you could be getting some capacitive leakage across the hot and neutral lines. Happened to me when I was moving the plug for my refrigerator. Using a "Non-Contact Voltage Tester" (NCVT) I wanted to make sure the circuit was off at the breaker box. When I switched off the breaker to that line I still detected a voltage. I ended up shutting off the entire house to move that plug. Apparently something in the wiring isn't 100% kosher (old house and wiring was touched by a previous owner who thought AC was just AC. Had no idea of HOT and NEUTRAL. AND he burned up one of my USB cables when I plugged a printer into the computer. The hot and neutral were reversed AND the white wire was also tied to the ground terminal on the outlet. With the hot reversed my computer was case hot instead of case ground. That precipitated a complete teardown of the basement finished ceiling until I found the black to white and white to black flying splice hidden behind the drywall.)

Measure your transformer for voltage with it not connected to anything. If you're still finding a voltage then somewhere you're being exposed to a very high and changing magnetic field. Are you close to transmission lines? The 50KVAC lines that carry power long distances?

 

As the transformer is buzzing and attracting things I assume it is connected to the mains. No?

 

Well, Al, I'm assuming he's turning it on and off. If he's had to wire in a 12 gauge wire then at some point it must have been off. IF he's unplugging it - that's fine. But if he has it plugged into a power strip with a switch then there might be other issues causing these odd readings. However, I'm still wondering about a possible short to the core.

 

I have two MOT that I have rewound secondary for low voltage, high current.
While powered on, I have approx. 7.5 volts on one terminal and approx. 1 volt on the other terminal, to the core.
Checking resistance from primary to core reads open, even on meg/ohm scale.
The humming or buzzing is because of the type of transformer used. You may be surprised at the current reading of the primary, even with an unloaded secondary.

 

If the core isn't connected to anything then a voltmeter will likely read voltages from the core to the secondary because of capacitance between the primary and secondary windings and the core. Because the voltmeter is high resistance you need very little capacitance to get that sort of reading when you start with 110V or 230V on the primary. Likewise a neon screwdriver may light up when touched to the core with your finger on the other end.

 

Originally the secondary had burnt up in the microwave. It was black and shorted inside. I removed it perfectly (no cutting of the steel core) and lined the core with a couple layers of electrical tape. I then used high strand 12g wire (meant for power cords) and wrapped it around 4-5 times inside the secondary opening so I could get an idea of what voltage each wrap/turn would be.

When I plugged it in, a screw about 1.5 - 2" was sucked right to it, the transformer started a low frequency hum and it smoked very slightly for about 3-4 seconds, then it stopped. This was the first time plugging it in since it was pulled from the unit.

The primary looks perfect, had I not known that the secondary had burnt, I'd never know by looking at it. The burning wasn't extreme like it caught fire or anything.

Now the Voltage and humming is when it is plugged into 120v mains. The voltage when touching one input to the other (those connected to the mains) reads about .05-.06 vac as well as touching one of the leads to the case/steel core. Touching the other lead to the steel core I get the 7.6v.

I let the transformer plugged info about 3 minutes and it didn't get hot at all nor did it seem to act oddly other than the readings I got.

I haven't had time to test other transformers to see if this phenomenon is normal with the V readings between terminals or to the case.

In the original, the secondary is grounded/terminated to the case which I don't do with the re-winding.

 

Now the Voltage and humming is when it is plugged into 120v mains. The voltage when touching one input to the other (those connected to the mains) reads about .05-.06 vac

Measuring the voltage across the primary, with the primary connected to the mains, reads 0.05V to 0.06V?
Have I understood that correctly?

 

So I did some more testing and I replaced the batteries in the voltmeter to make sure this isn't a problem. I also used a wire brush on my dremel to clean the contacts on the Primary (extremely well) as well as a spot on the steel case right next to the primary coil.

Before doing the cleaning, I measured V between primaries and got 2.3-2.46v for about 30 seconds before it jumped to 120v. I really scrapped and pushed hard with the probe points and I think it broke through corrosion/oxidation. The secondary read a stable 4.65v with 3.75 turns. After this is when I did the cleaning with dremel brush.

IDK how to describe the primary wires I call the White and Black. Black is the winding that starts at the bottom closest to the core, white is the outer most part that comes back down to the bottom to sit next to the black, beginning wire.

First test on primaries:
White wire to case - 1.78v
Black to case - 33.48v
Black to white testing between 2 primaries - 8.78
Test 2
primary White wire to case - 43.4v
Primary Black to case - 32.25

secondary tested at 4.66v again.

3rd round
Using only the black (com) wire on the volt meter touched to the case where it was ground down to steel - 3.03v. Touched both volt meter leads together and got 0.00 reading then tested same spot again getting 2.51v. Power cycled meter tested same spot got 2.302v, then touched both meter leads together to zero then tested same spot and got 2.26v

Tested using meter and black wire and brushed spot on case. Would "zero" unit by touching both meter wires together or powering off
3.03v - Black "COM wire touched to brushed spot on case
2.51vv = Black meter wire after zeroing by touching red meter wire. - then tested for the 2.51v
2.302v - powered off meter then used black wire to test bare case spot
2.26v - touched both meter probes together to zero then tested case spot with black wire


Now I did the same thing using the red wire from the meter to case
2.02v = Red wire (from meter) to ground case spot
3.53v = red wire after touching black meter wire to zero out - then tested for the 3.53v
3.412v - powered off meter then used red wire to test bare case spot
3.02v - touched both meter probes together to zero then tested case spot with red wire

Zero'd meter by touching wires then connected to primaries - read 5.8v for at least 15 seconds then meter read OL then 99v, 92, 118v & stabilized at 120v (took about 4 seconds after the OL reading)

2nd try - zero'd meter. 5.3v for about 20-30 seconds then read OL, then jmped to 100v, 119, then 120 and stabilized (the 100 - 120 took about 2 seconds)

I noticed that the unit was pretty hot, I would say maybe 85-100 degrees after testing for about 15 minutes, and after writing this it is still about 85-90 (30 mins plugged in). The buzz never re-appeared after the first night, though it is magnetic and I can get a voltage touching the nail stuck to the case.).

Well, that's what I've come up with with this strange transformer. IDK if this is normal behaviour at all, but I suspect that it is not.

 

If you really had to press that hard (before cleaning) to get 120 VAC then you have an open coil. It's possible you're getting some voltage to transfer, maybe via corrosion or something I can't imagine what.

Here's what I'd suggest: Test the outlet where you're plugging into. It should be 120 VAC (or thereabouts). Then check your cord at the transformer. Plugged into a good outlet you should get 120 VAC. I don't understand all your descriptions of voltage tests. Simply put, you should see 120 VAC on the primary if you're plugged into a 120 VAC source. Doesn't matter which wires from the meter go where OR what color they happen to be. Color is only for reference to the observer. If ALL your test leads were black then you'd have to expend extra effort to make sure which lead was plugged into which port on the meter.

So far I haven't seen any resistance reports from you. WITH THE TRANSFORMER UNPLUGGED - let me say that again; With the transformer UNPLUGGED - check for continuity between the two leads of the transformer. If you have to push really hard then I'd suspect you have a bad connection. OR maybe a broken connection somewhere within the primary winding; and when you press really hard you're forcing the core wires to touch and complete a circuit. Actually, I'm rethinking that. You should get 120 VAC no matter what condition the core is in (aside from a dead short). So I'm wrong on that.

You say you changed the battery in the meter. What about the meter leads? Do they go to nearly zero ohms when you touch the leads together? It's possible the reason for all your erratic test data may be due to bad leads. OR it could be a bad solder joint inside the meter - I've seen that before. Plugging and unplugging the leads stresses the solder joint(s) and they become unreliable. Could be dirty plugs where the leads plug into.

Bottom line, you need to be certain your test equipment is giving you reliable information. Once you're sure you have good test equipment then you should check for resistance between the primary leads and then test each primary lead to the core metal. Primary lead to lead should give you a resistance. Primary to core metal should NOT give you a resistance (should be infinite ohms - or OL).

Then when you take your readings again, draw a picture and list what points you're seeing what voltages. It's easier to decipher than to read what color lead you put where because you know what you know and we don't know what you know. You should get a voltage reading for the input (primary) of around 120 VAC and whatever voltage your secondary is giving you. That's two voltage readings. Then you should do a resistance check as described above. Resistance between the two primaries and resistance for each primary lead to case metal. Three more readings. Not ( I counted ) 34 readings. We REALLY can't tell what's going on that way. Please keep it simple. At least for me.