Transformer help with 3-phase

"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"

** 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

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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.

I'm

up

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

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

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

ne

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

"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

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.

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:

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

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...

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:

"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.

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

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

.... Phil

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.

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

Ah, thanks.

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 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!!

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.

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:

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

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...

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:

Yes I did.

Keith Marshall wrote:

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.

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

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

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?

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

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

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"

** BTW

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

.... Phil

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"

** BTW

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

.... Phil

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

It's an industrial situation, a welding & fabrication business with real

3-phase power.

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.

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.

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

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

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:

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