Transformer help with 3-phase

[Keith Marshall]

"Jon Elson" wrote in message
news:d-6dneRp97BMjhLSnZ2dnUVZ_r-dnZ2d@giganews.com...

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

It doesn't say 240v on it anywhere. It shows 2 120v windings on the primary
and mpja.com sells it as 120v/240v

Second, the two primary windings have to be connected with the right phase
when you user them both.

I have it connected as per the instructions provided by mpja.com here:

http://www.mpja.com/ShowAdditionalImg.asp?number=7845 TR&picnum=1#

And with 213vac (close enough to 208v I would think) showing on he primary
I'm getting 24vac on the secondary with no load so I assume I have it
connected correctly.

I may yet check the primary current but at this point I wouldn't trust it
because if the transformer has gotten hot enough to smoke it's probably
already ruined with the windings starting to short out. :-(


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

 

[Keith Marshall]

In answer to your earlier question, there is a 1-amp fuse in line with the
secondary and the new transformer is supposed to rated at 4-amps. There are
no specs or part numbers or any other markings on the original transformer
but a terminal strip that was mounted on top just shows 240v in and 24v out.

You bring up some interesting points about the contactor and especially the
possibility of a diode so I have something else to check. Thanks!!

"Jamie" wrote in message news:QCGjr.472$Jf7.425@newsfe21.iad...

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

 

[Jamie]

Keith Marshall wrote:

"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.


something smells funny here. If You disconnected the load on the

secondary side and you are still getting heat on the secondary side?
Something is not correct.

Going by memory, you stated you had 2 primaries that allows you to
wire for 120 or 240 on the input, that being the case you should have an
order of H1 H2 H3 H4 in your case, L1 = H1, H2+H3, L2/L3 = H4; in other
words, you use both primary windings but in series. If that was a 120
volt source it would be L1 = H1+H3, L2/L3= H2+H4

Also, it may sound stupid however, make sure you do have the ground
as the ground on the plug coming in and no legs are grounded in the shop.

If you knew how many times this kind of stupid error happen with
extension cords and equipment cords being missed wired like this, hot
attached to ground etc., you'd be shocked.


Jamie

 

[Keith Marshall]

"Jamie" wrote in message news:q1Ljr.10877$mL3.1453@newsfe23.iad...

something smells funny here. If You disconnected the load on the secondary
side and you are still getting heat on the secondary side? Something is
not correct.

No, I don't actually know which side the heat is on. I see smoke coming
from the transformer but it takes it 20 seconds or more to start and I can't
tell which winding it's coming from. The way it's setup, there is nothing
actually loading the transformer until you pull the trigger to start the
saw. When I first turned power on with the new transformer I measured input
and output voltages on the transformer and all looked good so I pulled the
trigger and the saw came on. I ran it 5 seconds or so and released the
trigger. A few seconds later I noticed a smell so I shut it down and looked
for obvious problems. I turned power back on and didn't pull the trigger
but the smoke came back after 20 to 30 seconds.

Going by memory, you stated you had 2 primaries that allows you to wire
for 120 or 240 on the input, that being the case you should have an
order of H1 H2 H3 H4 in your case, L1 = H1, H2+H3, L2/L3 = H4; in other
words, you use both primary windings but in series. If that was a 120 volt
source it would be L1 = H1+H3, L2/L3= H2+H4

Correct, it's wired in series and the output is correct so I'm fairly
certain I have it wired correctly.

Also, it may sound stupid however, make sure you do have the ground as the
ground on the plug coming in and no legs are grounded in the shop.

I'll check it but the saw has been working in the shop for over a year
without any problems until this incident.

If you knew how many times this kind of stupid error happen with
extension cords and equipment cords being missed wired like this, hot
attached to ground etc., you'd be shocked.

Oh no I wouldn't. :-D I've seen plenty of it myself. In fact, my home had
some outlets with the neutral on the wrong prong and a few other very
strange quirks when I moved in. The previous owner worked for a large,
national home improvement company and thought he knew how to wire, among
other things! You wouldn't believe some of the simple things he screwed
up!! He even ran 2 wires overhead to feed power from the house to the
garage and swapped neutral and ground on the feed lines so that everything
in the garage was backwards. And didn't even run a ground because most of
the outlets in the house are old 2-wire.

 

[Phil Allison]

"Keith Marshall"

No, I don't actually know which side the heat is on. I see smoke coming
from the transformer but it takes it 20 seconds or more to start and I
can't tell which winding it's coming from.

** FFS do a finger test !!!

The secondary is not covered so check that - if it is cool then the primary
must be the culprit.



..... Phil

 

[Keith Marshall]

"Phil Allison" wrote in message news:9v9h11F23kU1@mid.individual.net...

** FFS do a finger test !!!
The secondary is not covered so check that - if it is cool then the
primary must be the culprit.
Thanks, I'll give it a try.

Thanks, I'll give it a try. Unfortunately it'll probably be next week
before I get a chance to look at it again.

 

[Cydrome Leader]

Keith Marshall <tooladdict@progressivelogic.com> wrote:

"Phil Allison" wrote in message news:9v9h11F23kU1@mid.individual.net...
** FFS do a finger test !!!
The secondary is not covered so check that - if it is cool then the
primary must be the culprit.
Thanks, I'll give it a try.

Thanks, I'll give it a try. Unfortunately it'll probably be next week
before I get a chance to look at it again.

have you measured the actual voltage going to the primary yet?

 

[Keith Marshall]

have you measured the actual voltage going to the primary yet?

Yes, it's 212-213v. Sorry to take so long to reply but I have back problems
and have been out of commission for a few days so I haven't thought much
about the transformer.

Anyway, mpja.com is sending a new transformer and I'll do some more
troubleshooting when it arrives, assuming my back is up to it. :-/

"Cydrome Leader" wrote in message news:jn29g0$1r1$1@reader1.panix.com...

Keith Marshall <tooladdict@progressivelogic.com> wrote:

"Phil Allison" wrote in message news:9v9h11F23kU1@mid.individual.net...
** FFS do a finger test !!!
The secondary is not covered so check that - if it is cool then the
primary must be the culprit.
Thanks, I'll give it a try.

Thanks, I'll give it a try. Unfortunately it'll probably be next week
before I get a chance to look at it again.

have you measured the actual voltage going to the primary yet?

 

[Keith Marshall]

I received the new transformer and installed it yesterday and it worked
great with no smoke so the first transformer was faulty. Of course I
checked quite a few things before powering the new one up and left the
output completely disconnected for the initial power-on but it turned out to
be unnecessary.

Thanks for all the input and help!!

"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/

 

[David Lesher]

I can't follow the mishmash here, but clearing the air isn't hard.

a) 240V is 240V. This is a single-phase transformer. If you
correctly strap it for 240 in, that's what matters.

If you have 208wye 3phase, you'll get 208. BFD for this case.
You'll get ~21V out, not 24.

b) If it's smoking, I'll bet you screwed up. I don't know how.

But pull it out, put it on the bench, and CAREFULLY feed it 240 with
the secondaries floating. The best way to do is is to put a 20W or so
120v lamp [The kind Edison made; NOT a CF/LED or whatever....] in
each leg of the power.

If you draw too much current, the lamps illuminate.

For CAREFULLY, I recommend a safety man ready to kill power
while you measure the secondary voltage with ONE hand.