Keith Marshall wrote:

I need help and this seems the best place to ask for it. If not I'd
greatly appreciate it if someone would point me in the right direction.

I'm repairing a metal-cutting cold saw that is powered by 3-phase with
each leg being 120 volts. It has a transformer that has gone bad and
the input to the transformer is labeled as 240v but it's actually 2
phases giving 208v. The output is labeled as 24v and is dead so I went
looking for a suitable replacement.

I found a transformer from mpja.com for $12.95 that can handle 4amps
which is way overkill because the only thing this transformer powers is
a 3 pole contactor to switch on power to the motor when you pull the
trigger. It's a bit oversized but the price was right and there's
plenty of extra room for it. So it arrived today, I wired it in and
powered the saw up and it worked great... until I started smelling
smoke!! :-(

The output isn't even connected to anything unless you're pulling the
trigger but the smoke kept getting worse even without the saw running so
I'm trying to figure out if they sent me a bad transformer or if I've
screwed up when it comes to 3-phase vs split phase and that's where I
need the help.

The transformer actually has 2 input windings and if you wire them in
parallel you can connect it to 120v or you can connect them in series
for 240v but will that actually work for 2 phases of 3 phase? I mean
I'm getting 24v at the output so it SEEMS ok but it's kind of hard to
ignore the smoke which I expect will not stop until the transformer
windings short together and melt into an ugly mass. :-(

There's also a center tap on the output which I'm not using but I can't
imagine that being a concern.

Is there a problem wiring a transformer this way because of the 2 phases
being 120 deg. out instead of 180 deg?

I've just never worked on anything where this might matter before so I'd
appreciate any help!! Is it a bad transformer or will I have to find
one with a single winding on the input? Or is there some other detail
I'm missing entirely????

Here's a link to the transformer which has schematic on the page if it
helps:

http://www.mpja.com/24V-4A-Center-Ta...tinfo/7845+TR/


To add to this..

Some relay coils (contactors) have integrated diodes in them. There
are two things to go wrong here, first off, the diode shorts, the coil
gets AC and it operates hot!. In many cases, it will still pull in the
contacts, but not well.

In the event as above, this can cause burning contacts and at some
point, the coil can deform and thus not fully pull in.

In the case that you don't have an integrated diode coil and it is
fully AC, then better check to make sure the relay coil isn't burned.

Either way, you'll get some stink!

The original transformer could of had voltage issues before it failed
and caused the relay to hang part way, there by, getting the coil over
heated due to lack of core material in its way. A common problem in
brown outs, Or in your case, if you have a severely sagging service
running that saw due to insufficient cord gauge or just not getting the
juice you need.

Jamie