Placing BB transformer before or after 1-ph to 3-ph conversion. Pros/cons.

I'm brand new to posting here, but I've been reading PM for awhile now. Thanks for sharing all your experience, and wisdom. I have a few questions related to this xfmr, phase conversion, etc, but I'll keep each one in a separate thread, as board sticky says.

I'm getting my new shop space ready for a Speedio, and generating 3-phase via a phase perfect converter. Utility power hovers around 240VAC, and Brother app engineer told me that it would be preferred to have voltage centered around 208VAC or so, even though the Speedio can accommodate up to 245. So, a buck autotransformer with 16/32V windings seems like it should do the trick. (Side-note: I'm aware that they need to be sized based only on the BB voltage, not the overall voltage). PP recommended the (made-in-USA) Dongan brand. Spoke to app guy there who suggested I buck the single phase, prior to 3-ph conversion at the PP. That was my plan, until I read a few folks on here talking about using dual BB xfmrs after conversion to modify the 3-ph power. Since PM guys generally do things for good reason, that made me want to better understand the pros and cons of each choice. Here's what comes to mind so far:

Buck 1-ph (before phase conversion):

1) Less expensive and easier to install (only need one BB xfmr)

2) Less flexibility in terms of different 3-ph voltages (only get 208VAC 3-ph, in my case)


Buck 3-ph (after phase conversion):

1) More expensive and harder to install (need two BB xfmrs, more complicated wiring)

2) More flexibility in terms of different 3-ph voltages (can access both 240VAC 3-ph and 208VAC 3-ph)


In my case, the flexibility of having multiple different 3-ph voltages is not particularly important. Am I missing any other pros? Eg, perhaps some benefits to power quality or the like? If not, or if they are not significant, I think I'll just continue to go ahead with the planned 1-ph buck.

Thanks so much for advice.

I have a PP and use BB transformers. My voltage is between 245 and 250. The PP is a current limited device - not power limited. So you get more kVA with higher line voltage. I use the BB transformers after the PP and end up with a higher maximum output current than I would if I did it the other way. For a 10 Hp PP, my input voltage and 208 out - it amounts to 8 extra amps. For me, worth the extra cost/hassle.

I have a 10hp PP also running either an Enshu or Kitamura drill/tap mill. I put my Dongan transformer before the PP per PP's suggestion. If the transformer is before the phase converter you can save a few hundered dollars on the transformer. Since I don't need the extra power, generally, it doesn't matter in my situation. Been this way since 2004.

Third approach, then, but not a "economy" move at all.

I'd "prefer" to put a Delta-Wye output transformer on at least one of my two 10 HP P-P.
With even ONE adjustable tap, the "right one" could cover modest adjustability.

More important to me is re-establishing a local "Neutral" from the Wye side such that any leg to Earth doesn't go above 133 VAC or thereabouts.

I'm not all that keen on distributing the P-P's "native" output directly, either as full-floating OR corner-grounded Delta.

FWIW-may-be-irrelevant-here dept:

- A VFD is usually a "good neighbour" upline, not always downline as to electrical noise & c.

- An RPC puts harmonic distortion upline. Can't be avoided.

- A DC Drive can be nasty-ass on BOTH input and output, is well-served with a "Drive Isolation" AND NOT "buck-boost" AKA autotransformer upline.

- A Phase-Perfect is about the most polite, both sides, as one can GET.

Delta-Wye issue aside, a P-P doesn't otherwise much care if a buck-boost Autotransformer OR a full-isolation transformer is utilized.

That said...

Sometimes.. a recycled full-isolation transformer can be cheapest - especially if within comfortable go-fetch range, rather than shelling out for palletizing and freight.

2CW

David: Thanks for confirming that you've had success over the past ~15 years with a scenario (using PP and Dongan BB xfmr) like the one I initially had in mind.

Bruce: Really interesting input! I never considered (though perhaps should have) that placement of BB xfmr could, in effect, "derate" the ampacity of the PP. I'm a bit surprised that the PP engineer didn't mention this over the phone when I sought advice. Instead, I got the somewhat proverbial "it'll be just fine on single-phase". Given that amp capacity is the overriding concern with sizing PP converters, his advice seems like it should have been paired with your important caveat on trade-offs.

I'm wondering if you could explain where you get the value of 8A (with PT-330 and 250V->208V buck), as the difference in max current from the PP, between the two configurations?

Before answering, let me see if I can figure it out. According to its specs, the PT-330 is rated at 30/36A (one is nominal, one is max, but PP told me that's just some weirdness about UL language; for all intents and purposes, they told me to treat it as a 36A output, as it can do so with 100% duty cycle). So, with 36A output on 3-ph side, it would require 62A on 1-ph input (36*sqrt(3)=62), in line with the PP's requirement of a 60-70A input breaker. These figures correspond to a power of 15.5kVA (=62*250). Now, if we buck the input to bring it down to 208VAC, the PP only has 12.9kVA (=62*208) of input power to use to generate 3-ph output. That power then corresponds to ... ... 36A of current output (=12900/208/sqrt(3)).

Uh oh, I failed to find that missing 8A. Could you help me out, please?

Thanks!

Monarchist said:

Third approach, then, but not a "economy" move at all.

I'd "prefer" to put a Delta-Wye output transformer on at least one of my two 10 HP P-P.
With even ONE adjustable tap, the "right one" could cover modest adjustability.

More important to me is re-establishing a local "Neutral" from the Wye side such that any leg to Earth doesn't go above 133 VAC or thereabouts.
~~
Sometimes.. a recycled full-isolation transformer can be cheapest - especially if within comfortable go-fetch range, rather than shelling out for palletizing and freight.

2CW

Click to expand...

Finding one of those, 230 delta to 230Y used, would be like searching for hens teeth. 460 to 230 are out there used, but not 230/230, at least I've never seen one.

Three 1Φ units used, wired up into a Y would be more likely available used.

Your luck may be better than mine...

SAF Ω

I prefer transformers at each machine wired after the breaker/switch on the machine. Then the transformer isn't on all the time. If you're only running one machine on your PP then it doesn't matter.

240-208 Delta/Wye transformers are nearly as common used as 480/208.

Monarchist: Great points you add. Thank you.

I had given some thought to using an isolation transformer after the PP, instead of the BB transformer (either before or after), specifically to get Wye 3-Ph.

My main rationale (and perhaps others', as well) was that some shop equipment wouldn't accept the Delta 3-ph coming out of the PP. For example, all of the Kaiser air compressors I looked at included strong warnings that use of Delta 3-ph could cause damage. Kaiser also offers them in 1-ph (at least in the smaller capacities I looked at), though, so that was an alternative even without the isolation transformer. (Side note: I chose not to go the Kaiser route.)

What ended up turning me away from the full isolation transformer approach, was its size more than its expense (which I could be okay with, even for new). My shop space is very limited, and the BB transformer can mount on a wall, without using any extra floor-space, while all the isolation transformers (capable of handling 30-40A 3-ph) that I've found so far are about 3x3 feet (~10sf). I'd imagine there's not much chance of finding smaller / lighter ones, given simply by the need for such larger windings, sized to handle ampacity, thermal dissipation, etc. Is there, in fact, anything smaller? (I think I'll look further...)

Incidentally, if I were to go the route of the isolation transformer, re-establish a neutral, and end up with Wye 3-ph, how does one typically use that neutral with a machine like the Speedio that only takes 3-conductors (L1, L2, L3) + ground as inputs (vs 4-wire + G)?

A couple other follow-up questions to clarify for my sake:

1) I really appreciated your rundown of the impact of these devices (VFD, RPC, DC Drive, and PP) on the overall power quality of electrical distribution. On that note, though, where are some typical places we might find DC drives in a machine shop? Manual mills / lathes? Grinding machines? Bandsaws? Vibratory tumblers?

2) I'm guessing that the servo drives in modern CNC equipment are "hidden" or "protected" by all the control mechanisms of the mill / lathe, and thus don't contribute to making for a nasty power-quality environment, like the naked DC drives... Could you clarify, please?

Thanks.

The BB transformer supplies the extra current. So you get the rated current from the PP and the low voltage winding on the BB adds current capacity. Said another way, the current flowing in the PP generated leg coming turns out to be less than the current flowing in the corresponding bucked leg. This is because the BB is a step down transformer for voltage, but a step up transformer for current. For this to work out, you use two BB transformers. Their common connection is the generated leg.