Electronics > Beginners
ASK: Determining transformer's windings phase
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BravoV:
ArthurDent, thanks, I'm aware of that video.  :-+

Just want to know if there is a simple method without involving advanced tools like scope + sig-gen, even though I own it, its just I want to help friend that is at remote area, that has only DMM to test the transfomer on site, and the incandescent lamp method pointed by Ian.M is perfect for my case.

Gyro:
It's going to be hard to give a close figure, but with correctly phased primaries and open secondaries, all you're left with is magnetizing current. That will depend on the frame size and quality of the transformer - the steady state current ought to be in the 10's of mA range or less. A shorted turn would certainly make its presence known.

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.
Jwillis:
The general practice when transformers are wound with a Live ,Common and Tap on the primary side is a bifilar winding.Meaning 2 parallel wires are wound together at the same time.This method give a more uniform balanced coil with matching number turns. These turns start at the center and tap out on either side.
Even if the coils wound monofilar from  one side to center to other side, your center  taps  are always tied to together (series coils)for 240V.For 120V the center tap of coil 1 is tied to outside of coil 2 and center tap of coil 2 is tied to outside of coil 1 (parallel coils).
Regardless whether its a bifilar or monofilar windings, the coils are always wound the same direction. And the Live and Neutral lines are on the outside in both cases (120V or 240V). This is the proper procedure used when winding E core or Toroidal transformers.
basinstreetdesign:

--- Quote from: Ian.M on December 11, 2018, 08:16:10 am ---The incorrectly phased primaries on the right will act pretty much like a dead short (limited only by the lowish winding resistance), as their magnetic fields cancel out.   Result: one dead smoking transformer.

For first powerup of a mains transformer, if you aren't 100% certain of *all* winding phasing,  its a good idea to have a mains voltage incandescent* bulb (ideally of a wattage between 5% and 50% of the transformer's VA rating) in series with the supply, and to *not* connect the secondaries to any load till you've checked voltages and phasing.  If there is a  fault, the bulb will limit the current so its low enough not to burn out the primary.  If the bulb lights, you've got a problem . . .

* It must be a real tungsten filament bulb.  CCFL and LED bulbs are totally unsuitable, because you need the non-linear resistance characteristic of a heated metal filament.

--- End quote ---

Has anybody ever had this situation with the dual primaries wired in series but out of phase?  I have and the result is non-climactic.  With all secondaries open, if the primaries are wired out of phase their magnetic fields are in opposition and cancel within the core.  All secondaries then show next to zero voltage.  Except for magnetizing current, the turn-on transient is minimal.  Swapping the leads on one primary brings it right.  Testing this requires nothing more than a meter.

Carrying this further, I have used some of the low voltage secondaries wired in series with, and, in or out of phase with the primary(s) to adjust the voltage on other secondaries as needed.
The only time there may be smoke is if there is a shorted turn or three.
Jwillis:
I did make a mistake re winding primaries once and all it did was trip the breaker.On secondaries all you get is the difference .For example with to coils one at 12.5 volts and one at 12 volts all you get is around 0.5 volts.Your right its not very dramatic.Theirs a few rewind businesses locally so I just went down to a few to get some practical information for my experiments.But they showed me how to keep things in phase. It was really interesting and the winding machines are really cool.Especially the toroid winder.
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