How to Check Industrial Control Transformer for Shorts or Overload When Fuses Blow?

I had an issue with a control circuit. I tested EVERY component within the enclosure and found nothing defected which caused the two fuses on the primary side to blow, even the wires (no short to ground or phase to phase). While troubleshooting I decided to remove the two wires off the primary side of the industrial control transformer and powered up the unit. Fuses did not blow. I decided to check if i had voltage at the two terminals and i had 480VAC. After that I inserted the two wires back to it's proper terminals and as soon as I powered it up the fuses blew. 

I decided to remove the transformer from the enclosure and take it to the shop and wired it there. After wiring 480VAC to the primary side I tested the secondary side and had control voltage, but what threw me off was the voltage on the primary side was 139VAC, almost 20V more then what i needed. 

My question is how do you test a industrial control transformer? All I know is if you have a secondary voltage then the transformer is good, if you do not have secondary voltage then that mean it's bad. What can cause my secondary voltage to be 139VAC and not 120VAC (tranferformer was a few days old)? What are ways to troubleshoot/test a transformer with a digital multimeter? Plz help.

You don't say if the secondary was fully loaded when you measured the secondary voltage, a load will drag it down a bit, the regulation of a transformer does that. It is worse on smaller cores.
You can still have some secondary voltage with a shorted turn in the transformer as well.
What is the maximum inrush current on the transformer, it is common to use slow blow fuses to take the possible inrush current into account.
Cheers,
Richard Gabric

I had a typo. What I meant to say was when I took the transformer back to the shop for further testing, I had 139VAC on the secondary side of the transformer WITH NO LOAD WIRED TO THE SECONDARY SIDE. When the transformer was wired in the enclosure the only load I have on the secondary side was the two coils to the two contactor (motor starter), the rest was a small three position selector switch and two small indicator light, one for when motor 1 is running or when motor 2 is running. Both motors do not run at the same time.

What do you mean by "shorted turn in the transformer"? I didn't think the coil to can short out. I thought the coil is wounded around the core then coated on the exterior to prevent shorts.

I'm sorry, but I'm trying to have a better understand on troubleshooting transformers. All I know is that it changes voltage (step up or step down). Do you have a link you can give me that'll have a better explanation?

Hi Christopher,
I picked up on the typo straight away, so it was of no consequence.
With no load on the transformer, the voltage will be higher, the secondary voltage is usually specified for the designed load current, so it will drop once a load is applied.
Shorted turns come about when the internal copper wires insulation is damaged, and a turn (or turns, depending on where it occurs) can form a shorted turn internally. They normally result in rapid overheating and failure of the transformer, it would be obvious it it had occurred. Look up "shorted turns in transformers" on the web to get more info.It is not a common occurrence.
Cheers,
Richad

For your question, and  for the issue about industrial control transformer. I can give you a site: www.amperis.com for your information. Maybe Amperis engenieers have the solution. Cheers!

1. what is the max sec current of your  xmer , what is your max load currentyour load curent should not go above the sec  current of your transformer if exeeds it will blow the fuse.check your  fuse rating. for checking your transformer provide avariable  480v ac  using a dimmestat and with out load check the sec voltage after this load the sec step by step ie go on increasing theload the sec voltage should not go down when the sec voltage of xmer goes down that shows you are exceeding the current limit of your xmer.stop before this point and confirm the curent  rating of your xmer.  make sure that your sec your sec winding is having any short in the winding