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New VMC, Phase Perfect and bucking questions


Cast Iron
Nov 22, 2006
south west michigan
Have a new machine landing in a couple of weeks and am getting the rest of the ducks in a row. I'm waiting to see more detailed info from the manual so I'm not absolutely certain on everything quite yet.
This will be a 220(ish) install. One thing I do know for sure is that my dealer will not fire it up unless my voltage is less than 235. I believe this is phase to phase and phase to ground.
For now the plan is a PT030 and bucking the voltage somehow. Last voltage check was 243 in the evening. I'm going to do a couple more checks before ordering transformers.
I've spent the last few days reading everything I can find on transformers and my brain is almost full.
From what I gather the best thing to feed the new machine would be wye with delta configuration being a second in terms of voltage balance.
To get my voltage in range I could buck after the PP and get a 240D to 208Y xformer. This might leave some of the other equipment I have a little low on power. I would prefer to get to 220 and run everything from one new panel.
The other choice would be to drop the voltage before the PP but this would leave me with high leg delta service.
I found one catalog listing for a 240D to 220Y transformer but list was north of 8K. I'd like to stay below that if possible.
Seems 240 to 208 3 phase are pretty common in the wild. How far can one go off of "target" while tapping the xformer to boost the output voltage up a little?
240 to 220 single phase in the Kva required look to be more expensive than a 240D to 208Y 3 phase.
Are there options I'm overlooking?
For my VMC that required <230V with my service being ~247V, I bucked the PhasePerfect input. That was far cheaper, lighter, and more compact than adjusting the 3-phase output. I have high-leg delta coming out of my Phase Perfect, and have no issues with it. The only other machine on the PP is a 575-V mill from Canada. For that I have an open-delta transformer setup that takes the 230V 3-phase up to 575V 3-phase, corner grounded. Before the VMC came along, I fed the PP with 247V and chose the open delta transformer taps to give me 575V. When the PP output dropped down to 230V, I just changed the open delta transformer taps.
I also have a PT030 and was in the same position as you. My Kitamura wanted 220+/- with utility voltage north of 240. I am also in the position that in the next handful of years I will be relocating all of the equipment, so I wanted to make sure whatever I ended up with could reasonably accommodate that. Bucking the voltage on the input side is definitely cheaper (if purchasing new). I opted for a used 45kVA 240D to 208Y. I confirmed it was multi-tap before purchase, and I have it tapped for ~225V to my panel. I found 480D to 208Y transformers to be much more common on the secondary market, but there are plenty of the 240's around. I found mine at a used machine tool dealer. Price ended up slightly less than a new single-phase transformer of a similar rating.

The Phase Perfect won't care about the transformer placement (before, after, or both) as long as you maintain the supply side above 187V. The PT030 may struggle to energize a transformer larger than 45kVA on the output. Transformer mode will help, but it may not muscle through energizing the next size up, which is typically 60kVA.
With 240VAC high leg, the voltage to ground is well under the limits. Around 208VAC. With 220V it is 190V, and with 208 it is 180V

So bucking the input of the PP should not cause any issues at all if the limit is 243VAC to ground... And the transformer is much cheaper for bucking than for carrying 100% of the power.
No question but that the wye is probably going to avoid all possible issues and objections.

Remember that the secondary is a "separately derived system", and you will need to bond the secondary neutral to a source of "ground" . That can be the system it is connected to, or building steel close by the transformer, etc. No obligation to connect that neutral to source ground (no objection either)

Also, the green wire for the load needs to be connected to that secondary side neutral. No harm if that neutral is connected to the primary side grounding, but the secondary "equipment grounding conductor" has to return to the secondary side neutral/ground.
JST, I'm going to run the full voltage through the PP and go with an isolation 240 multi tap delta nput to 220 wye output.
Spendy but crossing my fingers that it gives me the best/cleanest power to the machines.
Running full voltage single phase power to the PP also gives you maximum available kW the PP can deliver downstream. Bucking the voltage after the PP is more costly but you end up with the most available power.
Running full voltage single phase power to the PP also gives you maximum available kW the PP can deliver downstream. Bucking the voltage after the PP is more costly but you end up with the most available power.
it's rather a technical distinction, but regarding the PP as CURRENT LIMITED, which it is, that is correct. You get more power at a given current with a higher voltage.

There is a loss in the transformer, however, which does affect the total available power. Depending on the amount of voltage change, that may still be a "win", or not.

If you are operating the PP that close to the limit, so that you really need that extra power, you probably really need to bump up to the next larger size.
I’ve bought a few of these in 15kva/240-208v auto transformers for my Brother mills ( link below). (edit- I put in the am rotary link instead of the ebay link)
They have them in a few sizes. Since they’re auto transformers they’re considerably smaller and cheaper than an isolation transformer. I thought this was a nicer solution than an open delta bucking transformer pair. They’ve worked well. Last year i found a used 45 kva 240delta-208y isolation transformer that I am now using for the whole system. The auto transformers were per machine as needed. This works well too but i can’t say it’s better.

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