230/115 transformer windings is phase important?

[razorrick1293]

Hi there,
Does it matter which way the primary windings of a power transformer are connected on a 2x 115VAC winding? if one is backwards would it make a difference with anything? It seems a stupid question but testing an amp ive been using a variac so i can power up slowly. but with 40VAC in there is nothing at all on the HT winding (450vCT) ive checked for shorts and found nothing, and the mains fuse sems to be blowing (800mA fuse) at around 60VAC could this just be a faulty transformer?
Transformer is Hammond 260E.

Any help would be greatly appreciated!

[Huffler]

Yes primary winding phase IS important - it sounds like you have yours wired incorrectly.

Normally the windings have colour coded wires or are marked 0 - 115 0 - 115 or similar.

You connect the middle ones together and feed the mains to the outer 2.

If you connect 115 to 115 and feed mains to 0 and 0 the windings work against each other and cancel each other out - hence no output.

I hope this helps.

[Lazybutt]

Originally Posted by razorrick1293

Hi there,
Does it matter which way the primary windings of a power transformer are connected on a 2x 115VAC winding? if one is backwards would it make a difference with anything? It seems a stupid question but testing an amp ive been using a variac so i can power up slowly. but with 40VAC in there is nothing at all on the HT winding (450vCT) ive checked for shorts and found nothing, and the mains fuse sems to be blowing (800mA fuse) at around 60VAC could this just be a faulty transformer?
Transformer is Hammond 260E.

Any help would be greatly appreciated!

Of course it matters. What you have on the primary is 2 windings which generate a magnetic field.
The magnetic field is proportional to the tension applied
If you tie them in parallel, each winding will see the sector tension, (220v in the uk) if you tie them in series, each will see half the sector (110v)
If you invert the direction of one of the half primary, its "sign" will we inverted vs the other onr..... (it will be positive while the other is negative) and the magnetic fields generated by each "half" primary winding cancel each other, instead of adding up....so its normal you dont have tension on the secondary.
Also, Since there is no magnetic field, each winding behave as a wire (not a "self" inductance) so its close to a short circuit.. Thats why the fuse is blowing
Try inverting one of the half primary winding connection
Even at 10v you should see a tension on the secondary

Please dont take it bad, but if you are not clear with that sort of question, it probably means you should pay extra attention while working with HT..
Knowing and understanding how things work is IMHO a prerequisite for working with high voltage. (and unfortunately not a sufficient condition to avoid accidents) Before doing anything you are not totally confident with, do not hesitate to ask if you are not sure.

Cheers
Fred

[AndrewT]

Lay aside your Variac for the moment.

Use a bulb tester in the mains feed to the primary.

When you connect the primaries out of phase the bulb lights up drawing ~160mAac when the 40W bulb is upto temperature.
The initial current into the wrongly wired primary is limited to ~500mAac when the bulb filament is cold (cold resistance ~ 1/3rd of hot resistance)..
This current limiting action of the bulb tester prevents fuses blowing and prevents transformer from damage.

If the primary is correctly wired the bulb goes out and almost full mains voltage appears across the transformer. Whereas, with the faulty (out of phase dual primaries), the transformer will see ~1 to 5Vac across the primary.

To mimic the bulb tester's protection would require a bit of extra instrumentation and quite a bit of "on the fly" manual intervention.

Once you have your tranformer and rectifier and smoothing capacitor correctly wired then the Variac allows Testing/Measuring that cannot easily be achieved any other way.

[AJT]

you will blow a fuse if you get the phase wrong...yes a light-bulb test is helpful...

[razorrick1293]

Hmm ive never tried the light bulb test before, and you say 40W would be enough?

[AndrewT]

25W is enough to test the wiring of a transformer.

You only need to use a higher wattage bulb as you hang more load on the transformer.
A 60W bulb will usually start up a pair of chipamps with a PSU @ +-38Vdc and +-30mF of smoothing.