Electronics > Beginners

ASK: Determining transformer's windings phase

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Ian.M:
Miswired smaller transformers may not
draw enough primary current to trip the breaker, as the fine wire of the primary can fuse at the leadout or an internal thermal/overcurrent fuse can blow before the breaker trips.   

I don't have any decent sized mains transformers hanging around at the moment so I grabbed my 250VA portable isolating transformer (toroidial 240V, 1A), took the plug off it and put a batten lamp holder in series.  A 100W bulb didn't glow at all.  A 40W bulb flashed dimly with the initial surge current then didn't glow.  Unfortunately I don't have any lower wattage mains incandescent bulbs.

On a cheap DMM's 10A range the surge flicked up to several hundred mA then the magnetizing current very rapidly dropped away below 10mA, the limit of its resolution.  If I'd tried a lower current range the surge would have almost certainly blown the multimeter fuse, so I *DO* *NOT* recommend attempting to measure the magnetizing current in the field with a DMM. 

If you want to do it on the bench, either use a 5 digit or better meter on its 10A range and don't use a transformer over 250VA, 240V or 120VA, 120V (allowing for an x10 surge at switchon, unless you have an essentially free source of meter fuses!  >:D ) or rig a shorting switch across the ammeter that you can close before power on, and when changing meter ranges, only opening it to take a reading.   

Scoping the magnetising current is a whole different ballgame.  There's little point of doing so unless you are investigating transformer design and core saturation.  You'll need an isolated current probe and an isolated HV differential voltage probe to see the phasing with respect to the supply voltage.  Trying to cheap-skate it without the proper probes is likely to result in the death of your scope, and yourself if careless.  An expert engineer can build suitable probes but their design is not really a fit topic for this forum as due to the Dunning–Kruger effect, presenting such a design is an attractive risk to all the idiots who shouldn't be trusted to even wire a plug.

Ian.M:
@basinstreetdesign,
I question your results with antiphase series primaries - IMHO without current limiting the result will at best be a blown fuse.

Consider a pair of wires side by side, joined at one end, then wound together onto a core.  As their magnetic fields cancel out at any significant distance from the wires, the presence of the core has no significant effect on the impedance seen at the free ends so if you apply voltage across the two wires at their free ends the current will be no less than it was before they were wound on the core.  This is the principle of the bifilar choke which has negligible effect on the load current, as it cancels out, but presents a high impedance to common mode conducted EMI.

ledtester:
Here's a youtube video which covers the topic along with the dangers of connecting the primary side windings in series with opposite phasing.

Discussion of the test procedure starts around 5:50.



BravoV:

--- Quote from: Gyro on December 12, 2018, 08:42:00 am ---A big NOTE though... At switch-on the current will peak very high - worst case, the transformer will get switched on at a voltage zero crossing and the core may well saturate in the first half cycle, peak current then will be AMPS. With separate A and mA jacks on the meter, that's going to be impossible to handle.

Difficult to arrange, but if you really want to measure quiescent current, you need to bypass the meter with a switch or something during power-up. Difficult to arrange safely - especially when trying to tutor a friend at a distance! You may well be advised to stick with the dim bulb tester - you can switch a low wattage mains bulb.
--- End quote ---

Well, my friend works as technician, developed a technique at the field, that he used for years on skipping the initial big inrush using his meter, he works mainly on AC motors.

What he told and taught me with this technique, when he forgot to bring his AC clamp meter, is to use an ordinary DMM, and connect it like this illustration below, using a sacrificial wire and a pair of safe isolated wire cutter.



His DMM has those expensive Hirschmann cable piercing clips and bite into the sacrificial wire touching the copper, and on the initial power on at AC motor, especially big one will hum and slowly starting to rotate, he will wait until the motor's spin stabilized, and then cut the wire in the middle to measure the current.

Of course this is not totally fool proof, but he claimed he never blown any fuse, as he knows & aware of the load. He used this method only occasionally and with great care as he aware of the dangerous.

He will be using this technique.  :P


IanMacdonald:
Might be worth clarifying that transformers (at least, mains transformers) do not cause any phase change. It is simply a question of the winding polarity (as in, having righthand or lefthand turns as seen from the same end) being the same or reversed.

The word phase tends to get bandied-about, and it confuses beginners into thinking there is some kind of resonance or time delay going on.  No, it's just the same as batteries in series. Except with AC.

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