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hi guys,

I'm doing some troubleshooting on a Moeller motor starter and it's a bit out of my comfort zone.

After a power outage the transformer smoked really bad when I bypassed the fuse on sec. I guess there is a short somewhere, a wire touching the cabinet maybe... but really can't see it.

I'm confused why the transformer burnt when I bypassed the fuse as I can't see any short in my control box. Checked contactor and obviously there is power on L123 and nothing on Ts as the coil isnt energized.

what usually goes wrong after a power outage and possible a spike ?
Ok,

So I am understanding that a Control transformer smoked- this would be 120v correct?

I am thinking you have 230 or maybe 480 on the top side of the starter (L1, L2, L3) and that seems to be working because you have voltage.

When you bypassed the fuse on the secondary for the CONTROL transformer,
you smoked it. Try to never bypass a fuse, bad idea because you smoke things.;)

If I am understanding this right, what you need to do is this.

Kill power to the CONTROL transformer, remove the two wires coming from it, and start looking for a short to ground on the output side. This is what is driving power for your push buttons, starters, and selector switchs, maybe some sensors, etc.

Use your OHM meter, disconnect the motor starter at A1(we dont want to read through that yet) and watch your meter as you push buttons and generally try to bring everything that could be shorted into the circuit. If you dont find a short to ground, move onto the starter coil itself.

I have no idea what you should actually read there, so its best if you check it against a good one.

Measure between A1 and A2 on the starter in OHMs, from where I am sitting, I would expect to see maybe 50 to 250 OHMS (this is a best guess and not meant as a definite, so grain of salt time).

If you see 1 Ohm, or 10 Ohms (a very low reading), the starter coil is shorted.

If the starter coil is bad, ISOLATE the control transformer, and Megger it. It may still be good (but probably not). If you dont have a megger, dont chance it and replace the control transformer (say it happened during the spike;)), replace the fuse, and the motor starter.

Dont try it without a fuse again, thats ALWAYS a bad idea.

And thats only if I am understanding your problem correctly.:laughing:
 

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did you check upstream of the transformer?
A transformer isolates faults to one side or the other, because it induces a voltage- there is no physical connection between the primary and secondary side.

So if he is blowing a fuse on the secondary side, the fault is in the devices connected to the secondary side. Make sense?
 

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I still have the old OL and I was wondering how do you test it if it's good or not.
Karl,

When it comes to those types of things, I rarely trust the used ones. It may be fine, or it may last 3 minutes when you need it most.

The only way to really tell if it is good enough is to test it under load conditions. Meaning put it back in- and thats a gamble.

If it comes down to trying to save a dollar, I would save the O.L. block and just keep it for emergency. Or find a non critical place it works and just swap it in and see what happens.
 
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