Help Needed Understanding/Identifying this Buck Boost Transformer

[dalmatiangirl61]

Somedays you just have to take a leap of faith and try it. I wired the transformer in pic 3 as shown in pic 2 (post 1), inline with an extension cord, pressed start button for rpc, it started just fine, transformer did not go up in smoke. I plugged in a machine that has not wanted to run for more than 5-10 minutes on the rpc, it has now been running for over an hour without tripping out, I'm calling this a win.
Voltages before transformer
L1-L2 235v
L1-L3 250v
L2-L3 275v

Voltages after transformer
L1-L2 235v
L1-L3 250v
L2-L3 217v

So it dropped voltage on the L2-L3 circuit by 58 volts. Do I understand it any better, no. Is it perfect, no. Does it work, the answer for now appears to be yes. I do have an eye out for a 20hp motor to just build an rpc with.

 

[JST]

JST
The generated leg runs high, yes there are capacitors, in another thread a year or so ago I experimented with removing some of them, I see no reason to repeat those failed experiments. Pic 1 in post 1 above is the mfr recommended fix to the problem, a transformer with only 3 taps, labeled 238V/208V/0. I want to figure out exactly what that transformer is, and where to find one, besides ebay where the sellers cannot provide specifications for them.

oooookay.......

That is probably not the best way to go about it, which is why I suggested a different way. (see end).

But, the buck-down is done as I mentioned.

So.......

What voltage are you starting with that needs to be reduced?

What voltage is coming to the RPC?

What amperage does the largest load (or combination) draw?

As for other things, in no particular order:

The transformer does not need to be large..... It only carries load current on the low voltage portion, the rest of the winding is lower current in proportion to the difference in voltage rating between the windings. So whoever suggested a honkin 3 phase transformer is off-base.

You have ONE high line, and that is the only line that needs reduction. Again, you do not need a big 3 phase beast.

It does not make a big spark because it is best thought of as really just one winding, with a tap. The fact that the low voltage part is made with bigger wire does not change that.

The voltages are such that each input or output "sees" the voltage it needs, so there is no voltage difference to make a problem or draw large fault currents.

Because it is really just one large winding, the voltages are always in phase all through the winding. Only the actual voltage is different along the winding, phase is maintained.

It's just like a variac with the slider permanently in one position. Those do not cause issues, and neither will the transformer.


Now, with a two winding transformer having 4 wire connections, you DO need to pay attention to phase....which secondary wire you connect where. In one connection, the output is less than the applied voltage, in the reverse connection the output is higher than the applied voltage.

Your results....

You have those voltages most likely due to the capacitors connected. I'd bet there are different capacitor values L1-L3 and L2-L3. That is typical of the usual internet plans.

L1-L3 and L2-L3 should be about the same with no capacitors. Essentially, L3 is low in voltage with respect to both.

That is correctable by boosting it using no capacitors and one boost autotransformer. But if you can deal with what you have, that's fine.

 

[dalmatiangirl61]

But if you can deal with what you have, that's fine.

For now I can live with it, I'd prefer 120v line to line on all, but I'll take what I can get today. Line to line voltages are in post 21. I just finished running the Gorton mill for 2 hours without the heater/overload relay tripping, I have not been able to get more than an hour out of it at a time without that happening. Right now, only running one machine at a time, I'm pulling 10 amps max, but I have a few more slightly larger machines still in the que. I seriously doubt I'll ever be running more than 2 machines at once, the only way it would be 3 is if I get the big air compressor going.

 

[awander]

For now I can live with it, I'd prefer 120v line to line on all, but I'll take what I can get today. Line to line voltages are in post 21. I just finished running the Gorton mill for 2 hours without the heater/overload relay tripping, I have not been able to get more than an hour out of it at a time without that happening. Right now, only running one machine at a time, I'm pulling 10 amps max, but I have a few more slightly larger machines still in the que. I seriously doubt I'll ever be running more than 2 machines at once, the only way it would be 3 is if I get the big air compressor going.

120V line-to-line?

 

[JST]

For now I can live with it, I'd prefer 120v line to line on all, but I'll take what I can get today. Line to line voltages are in post 21. I just finished running the Gorton mill for 2 hours without the heater/overload relay tripping, I have not been able to get more than an hour out of it at a time without that happening. Right now, only running one machine at a time, I'm pulling 10 amps max, but I have a few more slightly larger machines still in the que. I seriously doubt I'll ever be running more than 2 machines at once, the only way it would be 3 is if I get the big air compressor going.

OK, we don't seem to know anything about the capacity of the transformers.... The autotransformers may be rated 10A, or they might not be. If what you are using did not get stupidly hot, then it is probably Ok at the load you had on it.

I would probably seriously question "the big air compressor" being OK with it as-is.

You need to estimate the power (or amps) for the max load you will use, derive the amperage (if you didn't use that directly) , and make sure your buck transformer can handle it.

 

[dalmatiangirl61]

120V line-to-line?

Brain fart, 230-240 line to line. A nice balanced voltage across all line to line readings.

 

[dalmatiangirl61]

If what you are using did not get stupidly hot, then it is probably Ok at the load you had on it.

After about 4 hours of running today it was warm to the touch, I could hold my hand on it without worry of getting burned. I'll keep an eye on it when I run more or larger machines.

 

[BT Fabrication]

L1 and L2 are fine, only L3 is high. Again, I'm just copying what the mfr sells/recommends, and its not a massive 3ph transformer. It appears to me they are simply reducing the voltage on the generated leg.

the problem with all the little transformers is that they are only good for a small amount of amps. the ones you hold in your hand size only work for 1-5 amps, maybe 10 if big enough.
KVA matters if its powering a machine.
and just pulling down one leg is a bad idea if it needs all 3 phases on a RPC.
L1 and L2 are line incoming voltage, so L3 is always high. unless you play around with capacitors to suit a particular load. as the L3 will pull down really far to 160V also when starting then if its backfeeding it will spike.
Only way I found was to be running some other 3 phase motor to take up the excess energy. they are much more forgiving as an added inductive load that will also dissipate the excess energy fed back.

 

[BT Fabrication]

from hammond on a buck boost transformer on 3 phase needs 2 of them. at about $400 a piece.

 

[JST]

the problem with all the little transformers is that they are only good for a small amount of amps. the ones you hold in your hand size only work for 1-5 amps, maybe 10 if big enough.
KVA matters if its powering a machine.
and just pulling down one leg is a bad idea if it needs all 3 phases on a RPC.
L1 and L2 are line incoming voltage, so L3 is always high. unless you play around with capacitors to suit a particular load. as the L3 will pull down really far to 160V also when starting then if its backfeeding it will spike.
Only way I found was to be running some other 3 phase motor to take up the excess energy. they are much more forgiving as an added inductive load that will also dissipate the excess energy fed back.

A 0.5 kVA transformer with a 32V low side will handle up to about 15A, and while not baseball sized (as I assume you mean) is not actually very large.

The kVA needed is what is added by the boost transformer when boosting. But that is the VA supplied by the low voltage, so the transformer need only supply that smaller amount, it need not handle the total kVA going to the load.

With an RPC the typical case is that the generated leg is low. That is the reason for all the kerfuffle with capacitors. The idler inherently produces correct phase, the capacitors are not for that. They actually use a resonance effect to boost voltage. Unfortunately, that boost is not very stable under heavy load.

If the generated leg is high with respect to the others, then the direct and correct thing to do is to lower it's voltage without affecting the other lines. If all were lowered together, the imbalance would still exist, just at a different voltage level.

Similarly, a low voltage on the generated leg can be boosted. Same method, just wiring for a boost, not a buck.

For "generic delta 3 phase", you would need 2 transformers. For a wye supply, one would do it.

But for an RPC, supplied with domestic 240V center-tapped power, there is a "cheat". This exploits part of the "Scott-T" connection. The center-tapped voltage is already present, so only the voltage from that center-tap to the third (generated) leg is needed, and one transformer is enough. Look up the "Scott-T" to understand how and why this works.

Connecting an additional induction motor is equivalent to having a larger RPC idler. That will always stabilize the generated leg to the extent that the net idler size is increased. The added idler supplies it's back EMF in parallel to the RPC idler, increasing the total capacity to provide current to the generated leg, and lowering the total impedance of the generated leg, making it hold up voltage better under a starting load as well as continuous loads.

 

[dkmc]

Somedays you just have to take a leap of faith and try it. I wired the transformer in pic 3 as shown in pic 2 (post 1), inline with an extension cord, pressed start button for rpc, it started just fine, transformer did not go up in smoke. I plugged in a machine that has not wanted to run for more than 5-10 minutes on the rpc, it has now been running for over an hour without tripping out, I'm calling this a win.

It passes my electrical inspection...which is:No smoke, sparks, fire in 2 days, PASS.

 

[BT Fabrication]

more or less the last line is the most important

 

[dalmatiangirl61]

Back to the original question, where can I find transformers like the ones pictured in Post 1? With only 3 taps labeled 230v/200v/com?

 

[SomeoneSomewhere]

It's quite possibly custom. Transformers. auto-transformers, and inductors are really easily customised, and unless it's pretty commonly used you might not find something off-the-shelf that's suitable, especially with the move away from magnetics to solid state equipment. They look all but identical unless you pay close attention to the number, size, and arrangement of conductors and windings. Then you choose to stick it in a box, or not. Most transformers of this type seem to be specified for installation by electricians outside equipment, so they get their own NEMA rated box. You want an 'open' type TX.

These folks have a lot listed, but their website is borked. Nothing seems to match up perfectly, though. There's a "P/N 18357 208:240 33A 8kVA Single Phase" that might be close

 

[dalmatiangirl61]

It's quite possibly custom. Transformers. auto-transformers, and inductors are really easily customised, and unless it's pretty commonly used you might not find something off-the-shelf that's suitable, especially with the move away from magnetics to solid state equipment. They look all but identical unless you pay close attention to the number, size, and arrangement of conductors and windings. Then you choose to stick it in a box, or not. Most transformers of this type seem to be specified for installation by electricians outside equipment, so they get their own NEMA rated box. You want an 'open' type TX.

These folks have a lot listed, but their website is borked. Nothing seems to match up perfectly, though. There's a "P/N 18357 208:240 33A 8kVA Single Phase" that might be close

Thank you for that link! It confirms my suspicion that this is an off the shelf item, albeit not a common item as they are scarce on the surplus market. Interesting that they differentiate them as being either buck transformers or boost transformers vs the more common buck/boost transformers. Considering the application it was removed from, I'd guess what I have is a boost transformer, and no idea what the ramifications are of wiring it as a buck transformer.