Transformer help with 3-phase

[Keith Marshall]

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-Tapped-12-0-12-Transformer/productinfo/7845+TR/

 

[Phil Allison]

"Keith Marshall"

So it arrived today, I wired it in and powered the saw up and it worked
great... until I started smelling smoke!! :-(

** So you can smell smoke but not see any ??

Is any part of the tranny getting hot to the touch ?

( Just turn off the AC supply and run your fingers over it )


..... Phil

 

[Phil Allison]

"spamtrap1888"


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


** The *unloaded* secondary voltage will be 10 to 15% higher than the
specified and loaded value.



..... Phil

 

[Jeff Urban]

"is labeled as 240v but it's actually 2 phases giving
208v"

That is about a 16% drop in voltage, it should not cause smoke.
Question # 1 becomes, why did the old one go bad ? You say the
transformer is overkill on the current (you don't do that with
voltage) so whatever short that may have blown the fuse or thermal
fuse in the transformer may still exist. That's if it is a short.

If it is not a short, is it possible you screwed up on the primary
wiring ? Let's put it this way to just cut the trees down now - did
you actually measure the voltage at the secondary before applying it
to the circuit ?

Deal is this : If the voltage is correct you have a short. If the
voltage is 184% or double what it should be, you have a short NOW.
Some components fail very quickly with too much voltage applied and
others do not.

"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......"

I think it very possible you got the primary wiring wrong somehow, or
maybe that it was the wrong transformer. Luckily there is not alot of
silicon around but DO look for anything that looks like a diode. It
may be a Zener intended to protect the trigger switch from arcing.
These things usually short when subjected to excessive voltage so
therefore, it might be energized all the time, not working, and
releasing magic smoke whenever powered up. It makes sense.

Get in there with an ohmmeter, DVM or whatever. Those cheapo tool
places like Harbor Freight have meters for about six bucks. They are
not quite a Fluke but they are good enough for most everyday tasks so
no excuse. If you disconnect the secondary of the transformer and it
still smokes it is bad. You may have made it bad with too much
voltage, BUT, you could have gotten a mismarked part, or the specsheet/
data on it was faulty or for some other part.

If you are wondering about the difference in a two phase and three
phase, 208 is less than 240. That can't fry much. BUT -

Ummmm, you didn't connect the neutral by chance did you ? You don't do
that, it simply doesn't work. There is a possibility that if you
hooked up the neutral, with two legs feeding primaries, you only blew
the transformer. You can't do that unless the phases are 180 out, not
120 or anything else. If that's what you did, get another transformer
and leave the white wire alone

J

 

[Keith Marshall]

Thanks for all the quick replies!! Rather than respond to each reply I'll
try and answer an abbreviated version of everyone's questions here.

What killed the original?

I don't know at this point. In my limited experience it's been fairly
common to find a failed transformer with no apparent cause in older
equipment. I've replaced 3 or 4 in furnaces and other similar circuits in
the past and successfully fixed the problem so I haven't looked further on
this problem yet. I mostly wanted to know if 2 phases of 3 phase power
might introduce a problem I wasn't aware of. It seems to me that having 2
separate windings connected in series might be different from having one
winding but I still can't get that part clear in my head.

As for running it at 208v instead of 240v, the original transformer had a
terminal strip mounted on top of it with 2 wires going in labeled 0v and
240v and another 2 wires labeled 0v and 24v. I used that original terminal
strip and left all connections from the saw as original. On the transformer
I then connected a red wire and a black wire together and capped them off
and then connected the other black wire to the 0v in and the other red wire
to the 240v in as shown on the example diagram on the mpja.com page for 240v
operation. I then connected the 2 blue wires to the 0v and 24v output
terminals and capped off the unused yellow center tap wire. Sound right?

The saw is available in a 240v single phase version and the schematic of the
controls is identical using all the same part numbers. The difference is
that only the 2 legs of the 240v are tied to the 3-pole contactor so the
208v vs. 240v difference isn't what concerns me.

If anyone is interested in seeing the schematic it's on page 39 of the
manual here:

http://cdn0.grizzly.com/manuals/g0682_m.pdf

And the 240v version is shown on page 30 here:

http://cdn0.grizzly.com/manuals/g0665_m.pdf

It also has nothing to do with a neutral because the neutral isn't even
connected to this saw. It's not used on the 240v single phase version
either.

In answer to someone's question about smoke, I definitely do see smoke,
progressively getting worse the longer I leave the power to the saw on. At
this point I'm sure the new transformer is fried, I'm just trying to
determine how I fried it. :-( And more importantly how to keep from doing
it again!!

I'm starting to think that the problem may lie in a short on the output
which is just the coil of a contactor, but the contactor does work so I
still have to wonder. If it is causing too much load then it may be that
the first time I pulled the trigger to test the saw the insulating coating
on the new transformer windings began to break down and now it's to the
point where it doesn’t need a load on the output for it to continue to
worsen. Since I have no way of knowing what the resistance of the
contactor coil should be I may have no choice but to purchase another
transformer, probably one with a single primary winding this time just to be
sure, and connect it but not actually pull the trigger until I know it's not
going to smoke. But then I still won't know for sure until I pull the
trigger and then if it DOES smoke I'll know it's the contactor. That's a
really dirty way to do it but I don't know what else to do so I'm certainly
open to suggestions...

"Keith Marshall" wrote in message news:jmlcpe$sl6$1@dont-email.me...

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-Tapped-12-0-12-Transformer/productinfo/7845+TR/

 

[spamtrap1888]

On Apr 17, 8:34 pm, "Keith Marshall" <tooladd...@progressivelogic.com>
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-Tapped-12-0-12-Transformer/producti...

 

[Phil Allison]

"Keith Marshall"

( having read all the preceding posts)


** OK - I'll go for a " left of field" scenario.

The 3-phase motor of your saw is capable of generating a large back EMF
(voltage spike) when it is switched off or the AC supply is disconnected.

As the small tranny is wired directly in parallel with this motor - it may
suffer damage from even one such event. Shorted turns in the primary being
the most likely, layer to layer when in series mode as you have it.

Fixes:

1. Wire a new, similar tranny in 120 volt mode and connect it to a single
phase supply via its *own* lead.

2. Fit spike voltage protection across the primary of the tranny, ie a 270
VAC varistor or a 4 uF, 275VAC rated capacitor.

3. Attach a permanent load to the small tranny, like a 50 watt halogen light
on a goose neck.



..... Phil

 

[Jeff Urban]

"t also has nothing to do with a neutral because the neutral isn't
even
connected to this saw"

Well that eliminates that. Now we are into the transformer itself or
the wiring thereof.

More later, it's late.

(now that sounds fucking logical lol)

 

[Michael A. Terrell]

Keith Marshall wrote:

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

http://www.mpja.com/24V-4A-Center-Tapped-12-0-12-Transformer/productinfo/7845+TR/

Did you wire the primary for 240 by connecting the inner Red & Black
wires together, as shown in the drawing?


--
You can't have a sense of humor, if you have no sense.

 

[Peter]

On Wed, 18 Apr 2012 17:36:04 +1000, "Phil Allison" <phil_a@tpg.com.au>
wrote:

"Keith Marshall"

I would first try the transformer just connected to the input volts,
and with absolutely no connections to the secondary, and see if it on
its own produces smell/smoke. That should be a start to determine if
the problem is the transformer at fault, or the circuit it is
connected to.

I assume you have checked the transformer to see there are no
insulation problems to ground from either winding, or between windings
?

peter

 

[Keith Marshall]

In thinking the problem over a bit more it occurred to me that the secondary
is protected by a 1 amp fuse so with this being a 4amp transformer a problem
on the output should not be the problem. That is of course provided the
owner of the saw didn't replace it with a larger fuse at some point and
failed to mention it to me so I'll have to verify that when I look at it
again this afternoon.

As for the suggestion that a large back EMF spike from the motor could cause
a problem, I'll keep that in mind but at this point I'm still hoping for
something easier to deal with.

I assume you have checked the transformer to see there are no
insulation problems to ground from either winding, or between windings

I haven't yet but I will today.

At this point the first thing I plan to do is contact mpja.com and see if
they'll replace the transformer. If so I'll wait until the replacement
arrives and try that. If not I'll order one with a single primary winding
from a different supplier. Either way it'll probably be a week or so before
I get to try a new one but I promise to report back here when I do. This is
not a newsgroup I frequent but I used to keep up with a couple of
metalworking/welding newsgroups and I always HATED it when someone asked
about a problem and then never bothered to come back and let us all know the
final outcome. Also if I learn anything helpful today I'll report that too.

Thanks again for all your suggestions and help!!

"Keith Marshall" wrote in message news:jmlnb6$57b$1@dont-email.me...

Thanks for all the quick replies!! Rather than respond to each reply I'll
try and answer an abbreviated version of everyone's questions here.

What killed the original?

I don't know at this point. In my limited experience it's been fairly
common to find a failed transformer with no apparent cause in older
equipment. I've replaced 3 or 4 in furnaces and other similar circuits in
the past and successfully fixed the problem so I haven't looked further on
this problem yet. I mostly wanted to know if 2 phases of 3 phase power
might introduce a problem I wasn't aware of. It seems to me that having 2
separate windings connected in series might be different from having one
winding but I still can't get that part clear in my head.

As for running it at 208v instead of 240v, the original transformer had a
terminal strip mounted on top of it with 2 wires going in labeled 0v and
240v and another 2 wires labeled 0v and 24v. I used that original terminal
strip and left all connections from the saw as original. On the transformer
I then connected a red wire and a black wire together and capped them off
and then connected the other black wire to the 0v in and the other red wire
to the 240v in as shown on the example diagram on the mpja.com page for 240v
operation. I then connected the 2 blue wires to the 0v and 24v output
terminals and capped off the unused yellow center tap wire. Sound right?

The saw is available in a 240v single phase version and the schematic of the
controls is identical using all the same part numbers. The difference is
that only the 2 legs of the 240v are tied to the 3-pole contactor so the
208v vs. 240v difference isn't what concerns me.

If anyone is interested in seeing the schematic it's on page 39 of the
manual here:

http://cdn0.grizzly.com/manuals/g0682_m.pdf

And the 240v version is shown on page 30 here:

http://cdn0.grizzly.com/manuals/g0665_m.pdf

It also has nothing to do with a neutral because the neutral isn't even
connected to this saw. It's not used on the 240v single phase version
either.

In answer to someone's question about smoke, I definitely do see smoke,
progressively getting worse the longer I leave the power to the saw on. At
this point I'm sure the new transformer is fried, I'm just trying to
determine how I fried it. :-( And more importantly how to keep from doing
it again!!

I'm starting to think that the problem may lie in a short on the output
which is just the coil of a contactor, but the contactor does work so I
still have to wonder. If it is causing too much load then it may be that
the first time I pulled the trigger to test the saw the insulating coating
on the new transformer windings began to break down and now it's to the
point where it doesn’t need a load on the output for it to continue to
worsen. Since I have no way of knowing what the resistance of the
contactor coil should be I may have no choice but to purchase another
transformer, probably one with a single primary winding this time just to be
sure, and connect it but not actually pull the trigger until I know it's not
going to smoke. But then I still won't know for sure until I pull the
trigger and then if it DOES smoke I'll know it's the contactor. That's a
really dirty way to do it but I don't know what else to do so I'm certainly
open to suggestions...

"Keith Marshall" wrote in message news:jmlcpe$sl6$1@dont-email.me...

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-Tapped-12-0-12-Transformer/productinfo/7845+TR/

 

[Keith Marshall]

Did you wire the primary for 240 by connecting the inner Red & Black
wires together, as shown in the drawing?

Yes I did.

"Michael A. Terrell" wrote in message
news:aLmdnduyBcmJWBPSnZ2dnUVZ_o6dnZ2d@earthlink.com...


Keith Marshall wrote:

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

http://www.mpja.com/24V-4A-Center-Tapped-12-0-12-Transformer/productinfo/7845+TR/

Did you wire the primary for 240 by connecting the inner Red & Black
wires together, as shown in the drawing?


--
You can't have a sense of humor, if you have no sense.

 

[spamtrap1888]

On Apr 17, 10:03 pm, "Phil Allison" <phi...@tpg.com.au> wrote:

"spamtrap1888"

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

** The *unloaded* secondary voltage will be 10 to 15%  higher than the
specified and loaded value.

Ah, thanks.

 

[Jon Elson]

Keith Marshall wrote:

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!! :-(

OK, first does the transformer have a 120/240 V rating on it anywhere?

Second, the two primary windings have to be connected with the right phase
when you user them both. I'd connect one winding to 120 V and see
if the other winding also produces 120 V. If not, then this is NOT the
typical split primary winding and can't be used in this application.
If it does, then connect one wire of the second winding to one wire of
the first one. Measure across the two other wires. You will get either
240 V or nearly zero volts. If nearly zero, switch which wire of the 2nd
winding is tied to the first. When you get 240 V, those are the two
wires of the 240 V primary input. Connect 240 V there and check that
you get 24 V AC on the secondary. When you get all this right,
supply 240 V and measure primary current. It should be quite small,
maybe just a couple hundred mA. If it is more than half an Amp,
I think the transformer is just bad.

Jon

 

[Jon Elson]

Keith Marshall wrote:

I mostly wanted to know if 2 phases of 3 phase power

might introduce a problem I wasn't aware of.
No, this is totally normal for control transformers

on a 3-phase supply. But, where does your 3 phase power come from?
Is this an industrial location with utility 3-phase, or are you using
a "phase converter" in your garage?

I'm starting to think that the problem may lie in a short on the output
which is just the coil of a contactor, but the contactor does work so I
still have to wonder. If it is causing too much load then it may be that
the first time I pulled the trigger to test the saw the insulating coating
on the new transformer windings began to break down and now it's to the
point where it doesn’t need a load on the output for it to continue to
worsen. Since I have no way of knowing what the resistance of the
contactor coil should be I may have no choice but to purchase another
transformer, probably one with a single primary winding this time just to
be sure, and connect it but not actually pull the trigger until I know
it's not
going to smoke.
You can measure the DC resistance of the contactor coil, but that does not

indicate the AC load. Due to inductance, the current draw will be less than
the DC resistance might indicate, and when the contactor closes, the
inductance goes up further and the current decreases even more.
Since the thing still works, connect a meter in the secondary circuit and
read the current with the contactor turned on. Then, get a transformer to
suit.

Jon

 

[Jamie]

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-Tapped-12-0-12-Transformer/productinfo/7845+TR/


you have something over loading it. Most likely your contactor coil has

a problem which more than likely shorted the other transformer. Don't
you have a current limiting fuse in line for that transformer?

Jamie

 

[Jamie]

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-Tapped-12-0-12-Transformer/productinfo/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

 

[Phil Allison]

"Phil Allison"

"Keith Marshall"

( having read all the preceding posts)


** OK - I'll go for a " left of field" scenario.

The 3-phase motor of your saw is capable of generating a large back EMF
(voltage spike) when it is switched off or the AC supply is disconnected.

As the small tranny is wired directly in parallel with this motor - it
may suffer damage from even one such event. Shorted turns in the primary
being the most likely, layer to layer when in series mode as you have it.

Fixes:

1. Wire a new, similar tranny in 120 volt mode and connect it to a single
phase supply via its *own* lead.

2. Fit spike voltage protection across the primary of the tranny, ie a 270
VAC varistor or a 4 uF, 275VAC rated capacitor.

3. Attach a permanent load to the small tranny, like a 50 watt halogen
light on a goose neck.

** BTW

The 3 suggested fixes are alternatives - you will not need to do all of
them.


..... Phil

 

[Keith Marshall]

Thanks, I understood that. the first alternative, a separate 110v
connection would be my choice but I don't think there currently is another
source available anywhere near the machine at this point. Hopefully I won't
need either of the fixes though.

"Phil Allison" wrote in message news:9v95srFu68U1@mid.individual.net...


"Phil Allison"

"Keith Marshall"

( having read all the preceding posts)


** OK - I'll go for a " left of field" scenario.

The 3-phase motor of your saw is capable of generating a large back EMF
(voltage spike) when it is switched off or the AC supply is disconnected.

As the small tranny is wired directly in parallel with this motor - it
may suffer damage from even one such event. Shorted turns in the primary
being the most likely, layer to layer when in series mode as you have it.

Fixes:

1. Wire a new, similar tranny in 120 volt mode and connect it to a single
phase supply via its *own* lead.

2. Fit spike voltage protection across the primary of the tranny, ie a 270
VAC varistor or a 4 uF, 275VAC rated capacitor.

3. Attach a permanent load to the small tranny, like a 50 watt halogen
light on a goose neck.

** BTW

The 3 suggested fixes are alternatives - you will not need to do all of
them.


..... Phil

 

[Keith Marshall]

"Jon Elson" wrote in message
news:G7OdnVcaD_qriBLSnZ2dnUVZ_judnZ2d@giganews.com...

No, this is totally normal for control transformers
on a 3-phase supply.

Thanks!! That's really what I was asking about. It's just not a situation
I've needed to work on before.

But, where does your 3 phase power come from?
Is this an industrial location with utility 3-phase, or are you using
a "phase converter" in your garage?

It's an industrial situation, a welding & fabrication business with real
3-phase power.

You can measure the DC resistance of the contactor coil, but that does not
indicate the AC load. Due to inductance,

I understand that, but often a resistance value is good enough to determine
whether the contactor's coil might be shorted. If I knew the expected
resistance I could at least have a starting point for troubleshooting since
resistance is much easier to test.

Since the thing still works, connect a meter in the secondary circuit and
read the current with the contactor turned on. Then, get a transformer to
suit.

The secondary is fused at 1-amp and this is supposed to be a 4-amp
transformer so I’m not too concerned about that part of it.

I'm more concerned with the primary current because the primary of the
original (bad) transformer is open. I didn't check that before and was
expecting the secondary to be open instead. The problem with the new
transformer seems to be on the primary since it's happening even without a
load connected to the secondary. At this point I'm pretty much convinced
the new transformer was faulty.

I wish I'd measured the current draw on the primary but at this point I'm
figuring it's too late. Since it's smoking it's probably already somewhat
shorted and drawing far more current that it normally would.
At any rate, my next move is to contact mpja.com and see if they'll replace
it. If not I'll get one from a local appliance parts dealer which I
probably should have done in the first place. They show a 35VA transformer
for $18.95 and a 40VA version for $25.50. Either way I'll be sure to check
several things before I pull the trigger next time.

I may still try and check the secondary current first though.