How to determine phase of transformer secondaries?

[Onvinyl]

...without unwinding the transformer or opening it
Rüdiger

[richie00boy]

If you have a scope, connect windings in series and connect scope probe ground to this centre point. Now connect to each free wire in turn and observe the wave polarity. Better still with a dual channel scope look at both at once.

[Onvinyl]

great, thanks!
Rüdiger

[AndrewT]

Hi On,
do you have a DVM to measure Vac?

connect one wire from each secondary together to form a temporary centre tap.

Measure the voltage from one free end to centre.
Measure the voltage from the other free end to centre.
Now measure the voltage between the two free ends.
The two free ends should equal the sum of the first two measurements.

If it reads near zero volts you have to swap one secondary around. Disconnect your temporary centre tap and swap one secondary tapping for the other.
Now recheck the voltage between the free ends.

if the voltages are as expected, then proceed.

Note your colours or mark with indelible ink as follows.
First free end = 0Vac, same winding at centre tap = xVac, other winding at centre tap =0Vac, final free end =yVac.

The two 0Vac and the two Vac are in phase.

As I posted in each of the last two days be SAFE use a terminal block to isolate all your free ends and mains connections to avoid accidental shorting and/or electrocution.

[gmphadte]

Now that the phase of secondary is sorted out, the second method cannot be used in case u are going to use it to fire a gate of a thyristor

This is just for additional info

Gajanan phadte

[AndrewT]

Hi gmp,
what's wrong with the second method for identifying the phase of multiple secondaries?

[gmphadte]

There is nothing wrong, but there is something ...

The secondary phase relation in your solution is good for dc supplies like for amps and so on. Here the relation to primary is not considered. Whereas the trafo manufacturers' 0V will mean in-phase windings. As explained, the mains waveform will be followed by the secondary.

your method determines the relation between two or multiple secondaries, and may not relate to primary. It does not matter in amps, when u don't need to be in phase with the mains i.e. primary.

There are applications where u need to know this.

This relationship can be found without an oscilloscope.

Here it is...

Let all the wires be secured on a strip
Find out secondary phases with your method.
Now short one of the secondary lead to one of the primary lead.
Power the primary and measure voltage at the single primary end and single secondary end.
If u measure Vprimary+Vsecondary, U are in phase.

That means the shorted connection is a OV of one winding and Vx of other winding.

Now relate your secondaries to the primary.

Gajanan Phadte

Edit- In the first method also, the primary relation is not found out. So both the methods do not relate the phase to primary. Do not connect the scope to mains unless u know what u are doing

[gmphadte]

Your is to be edited for both the methods explained. Too much to rephrase.
Anyhow, I hope u understood what it is

Gajanan

[AndrewT]

Hi Gmp,
thanks I followed that precisely.

Besides mains thyristor firing, what other uses are there? or is the list too long?

[gmphadte]

I don't recollect any... but u never know

Gajanan