Dead transformer? 1

[tapiochre]

Hello folks,

I need to be guided if I am being really stupid or not...

I have a seemingly "never used" power transformer with dual primaries and a single secondary. The primaries are labelled (left to right) 0, 120, 0, 120, and the secondary is labelled 45V.2A (no apparent spaces between the V and the dot). See front face below

The rear of the transformer is marked T2052 D.E.

The requirement is for appx 45V out from 240V in, so I wired the primaries in series (0, 120-Link-0, 120) and put 240VAC across the two outside taps.

I used a 500mA fuse in line with the live 240V rail.

When I first applied power however the 500mA fuse blew. I double checked all connections and tried again but the second time the mains circuit trips at the fuse box (it didn't take the 500mA fuse out but it did tripped the earth leakage device at the consumer unit).

Am I wiring this thing incorrectly? Am I being really stupid or is this transformer stuffed?

If it helps, measuring across the 0-120 tabs I read appx 7.4 ohms on one winding (LHS) and appx 8.1 ohms on the other.

I measure appx 1 ohm across the secondary

All help gratefully received!

Regards

Chris

 

[dpc]

Try measuring the resistance of the windings to ground. Use a "Megger" or other high-voltage ohmmeter if available.

If you have variable voltage source, try energizing the transformer from a lower voltage. Or try energizing it at 120V when configured for 240V.

For "mystery" transformers, an old trick is to wire the transformer primary in series with light bulb in order to provide some limitation of current.

If you can get it energized at some lower voltage, you can measure primary and secondary voltages to check the apparent turns ratio.

 

[tapiochre]

Thanks dpc...

So you don't think I'm wiring up wrong?

I don't have a megger (only a low cost Fluke 7-300 meter) but might be able to put togther a variable AC input to see what happens.

I like the idea of the lamp in series... might try that too.

Chris

 

[rbulsara]

Its the fuse!! The 500mA fuse is too small to handle the inrush. Small control transformers can have inrush 25 to 40 times FLA.

Place 5A or so fuse in it. You are only protecting under short circuit. OL is protected by secondary side fuse.

 

[Skogsgurra]

And if nothing else helps. Consider the phasing. Try "0 120 link 120 0" instead.

.2 A secondary should correspond to about .05 A primary. And while it is true that the inrush can be high, it is not probable that it can blow a 500 mA fuse that usually has a bit of slow-blow characteristic. The inrush current for such a small transformer usually is no problem.

Yes, the marking says that you wired it correctly. But who guarantees that the marking is right?

 

[tapiochre]

Thanks to rbulsara and skogsgurra.

I just reviewed the booklet for the project and it does state that slo-blo fuses be used, so that's the first thing I'll apply. On order!

As for the reversal of phasing, it can't harm so I'll try that as well.

You guys are all fantastic. Thanks for being so patient with this dummy!

Chris

 

[tapiochre]

All,

RESOLUTION!

rbulsara may have been right after all.

Today I replaced the 500mA quick-blo with a 3A standard fuse and all is well. When the slo-blos arrive I'll be putting one inline and retesting.

AC out is 48.2V so thats cool.

Many thanks to all!

Chris

 

[Skogsgurra]

It is good to have your feed-back. We all learn from it.

 

[rbulsara]

Glad to be of help and yes, thanks for the feed back..