On Apr 17, 8:34*pm, "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!! :-(

Did you check it for shorts before you powered it up? Connect it to a
wall outlet? $12.95 might be some sort of closeout or surplus deal of
untested parts.


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

Why OK? If you're applying 10% less voltage to the primary, you should
get 10% less voltage at the secondary.

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. :-(

Smoke suggests a short, yet you are getting more or less the right
voltage on the secondary.


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?

Powering a low voltage transformer from the difference between two
phases seems normal for a motor drive circuit:

http://www.exman.com/ims.html


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????

Could be a bad transformer -- but what killed the transformer you're
replacing?


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

http://www.mpja.com/24V-4A-Center-Ta...ormer/producti...