Question on how to size Buck transformers or do I need an isolation transformer

I have a 20hp rotary phase converter I have been using for over a decade now on various vmc's and cnc lathes. I now have a supermax V56t with a Fanuc control hooked up to it and this control isn't liking the voltage it is seeing. I measure the voltage to be 240-245 but I am pretty sure the real trouble is the wild leg. The machine is rated for 220v 17kva. I need to install a buck transformer I think to drop the voltage. The machine has the most trouble on weekends and late nights when the businesses at the end of my road are not open and the line voltages are the highest. I talked to the electric company and they are running the voltage to the high side to supply the businesses at the far end of my line.

Can someone help me with the sizing of a buck transformer or would an isolation transformer help?

TIA

Funny you should ask - I just went through this exercise. This should help:


Other brands have similar online calculators.

It's surprising to me how small the transformer is to handle 22 kVA. It makes it a challenge to fit and connect the 21-kVA-size wiring in a 0.75-kVA-size box that does the bucking. My local utility power runs at up to 247 VAC single phase, which violates the specs of 230-V max that Fanuc says is the max. I am bucking single phase ahead of a Phase Perfect.

Thanks for the info, I must be missing something though. My machine is a 17kva is that not what I enter for the machine load? If so, the spreadsheet gives me a transformer that is $624 each for a single phase and does that mean I would need 2? Sorry for the newb questions, I am just getting started trying to understand this scenario.

A1CNC said:

Thanks for the info, I must be missing something though. My machine is a 17kva is that not what I enter for the machine load? If so, the spreadsheet gives me a transformer that is $624 each for a single phase and does that mean I would need 2? Sorry for the newb questions, I am just getting started trying to understand this scenario.

Click to expand...

Yes, if you want to buck or boost three phase, you need at least two single phase buck-busts in open delta arrangement or a three-phase buck-boost. (I’ve never looked at those.) That price seems like MSRP. I paid a little over $300 on Zoro for my one, with discount code.

Search Buck Boost transformers on ebay. I see some 1kva transformers on there now starting at 115. Can't figure if that's each or a pair. I've bought a bunch off ebay in the past. In the ebay add I saw, the guy had used them to buck his Mori lathes and other machines just like you want to.

17kva isn't that much. I don't have any of my tech sheets with me at home, but I'm betting something between 1 to 1.5kva will do. (The charts will tell you.) Remember the transformers don't see anything close to full current, as they're dealing with only a part of it. Although I'm boosting 208 up to 220 or so, I run a mid 90's YCI Supermax with what I'm thinking are a 1 or 1.5 kva transformer pair. The machine wiring is protected with a 60 amp breaker if that gives you any clue.

You might not have too much connection trouble at the smaller sizes, but there always seems to be one wire that runs straight through but needs one of the small leads attached along the way. What I've done is strip a section of larger wire without cutting it, and splice into it with the small one using one of the copper coated bolt on thingys. Get some of that electricians tape that stretches and molds to the connection. Awesome stuff for this kind of work.

I have a couple of American Rotary transformers on my machines.

Thank you all for the info, I really appreciate it. I think I am getting a handle on this. Seems like there are 2 ways I could go.

1. A single transformer before the 20hp rotary that would drop the voltage for all of my machines (up to 3 running at once)

2. (2) transformers for just the one trouble machine. This machine is 17kva 220v so if I figure it right, I would need (2)

I haven't had any voltage problems with any of the mitsubishi cnc machines (Have had (3) different ones) only the fanuc controlled ones.

A friend just told me he has (2) buck/boost tranformers he will let me try so as long as they are 1kva I will try that

Make sure to dig up info relating to your specific brand of transformer. There a dozens of wiring schemes that output a multitude of different before and after voltages. Also there are two types: 12/24 and 16/32. Only a pair of one type wired one way will get you where you want to go. The chance that your friends transformers are suitable will be a lucky break. The manufacturers literature and wiring diagrams are all available online. Get them.

I'm thinking with 20years of RPC experience you already know this, but just in case it got overlooked on your Fanuc machine. Make sure it is your two single phase lines that are feeding the machines control transformer. The manufactured RPC leg should only feed motors and drives.

Thanks 13engines, I do know about the two single phase lines should be feeding the machines control transformer.

The problem with this Fanuc is on the Fanuc 3 phase power supply, The 245-247 gives me a 345-350v on the DC bus side. The power supply is supposed to output a max of 325V. I am pretty certain the high dc bus voltage is why the control is faulting on this machine.

A1CNC said:

Thank you all for the info, I really appreciate it. I think I am getting a handle on this. Seems like there are 2 ways I could go.

1. A single transformer before the 20hp rotary that would drop the voltage for all of my machines (up to 3 running at once)

2. (2) transformers for just the one trouble machine. This machine is 17kva 220v so if I figure it right, I would need (2)

Click to expand...

The way I have always done it was three (3) boost transformers (208 to 240). I have seen wiring diagrams for doing two but they were strange and required larger transformers, so I always went with three.

The other possible advantage with three is, if you have one high leg, you can leave that one alone (in the boost direction, anyhow. You're bucking so probably no use to you).

As I remember it cost less to go with the smaller transformers. Not that that would affect Mr Cheapskate here but .... ahem

A1CNC said:

Thanks 13engines, I do know about the two single phase lines should be feeding the machines control transformer.

The problem with this Fanuc is on the Fanuc 3 phase power supply, The 245-247 gives me a 345-350v on the DC bus side. The power supply is supposed to output a max of 325V. I am pretty certain the high dc bus voltage is why the control is faulting on this machine.

Click to expand...

before doing any of these things i suggest you put an amp clamp and a volt meter on each leg of the fanuc machine during the normal voltage times to see which leg is drawing the most current as well as the highest voltage
normally the generated leg is the lowest voltage (thus requiring caps or a booster to raise the voltage)
if you are using caps to boost your generated leg, you can try removing one or two

as far as using split bolts and rubber tape .... check out these alternatives


yes these are way too big but they come in all sizes
#4 thru #14 usually being the smallest less than 20.00 at any local electrical supply house
ebay is probably less
i prefer burndy (they seem to take up the least room) there is also polaris brand
these two brands are carried by my supply house and i have used them hundreds of times, they are re-usable.
as a pro electrician the most expensive part of any job is time
one of these at the cost of 1/4 hour can save 3/4 hour of time
by the time you buy a split bolt and a roll of rubber tape plus a roll of good vinyl tape to cover the rubber tape
you are already close to the cost of an insulated terminal
not to mention that split bolts are difficult to tape properly and take some practice to get it right and avoid wearing thru the tape from vibration
you may also find that the bucked voltage is too low during 8-5, M-F
using these connectors you can quickly bypass the txr on the one leg that you bucked
an easier alternative would be a bypass contactor wired in parallel with the BB

if you only have one machine that requires lower voltage i strongly recommend that you buck it by itself
for two reasons : the other machines will not be happy with low voltage during the week
and the cost of the txrs goes up dramatically with increased amp loads.
if you are certain that you only need to buck the wild leg
then you can do it single phase before the machine and after the rpc,
this will allow you to size by the current going thru the wild leg only

If your rated input to the machine is 220V, and you have 245V coming to the RPC, you may be better off to buck the entire input to the RPC, as long as whatever else you have on the RPC can deal OK with the lower voltage. The voltage from the pass-through legs will be already 11% high vs the spec. That leaves nothing for line variations.

The wild leg is likely to have even larger variations in voltage, and may be your main issue.

If you just need to buck the wild leg voltage down, it may be a good idea to remove all or some of the "balance" capacitors on the wild leg. They raise voltage, and make it less stable, neither of which you want.

That might be all you need. Normally, without the "balance" capacitors, the wild leg will be lower in voltage than the input, because it is derived from the normal back EMF of the motor. (Some RPCs add windings so as to boost it back up, but those may not use any "balance" capacitors.)

For bucking just the wild leg, it is not "required" to use two transformers. You can use one, connected similarly to a Scott-T, between wild leg and RPC source neutral.

The voltage is lower than 240V so the boost or buck will be slightly less in voltage than the usual 240V transformer is marked for. But it will work. (It will also work in cases where you need to boost.)

You want to use a transformer where the low voltage is rated for at least as much current as the load will need. Better you use an oversized transformer, so as to avoid excessive voltage drop in the transformer itself.

It seems my voltage from the street is on high side to start, This morning I am measuring 240-243 into the RPC, and 241,241,245 out.
The machine I never have a problem is running fine as is right now. My balance looks good to me.

So, if I were to buck just the one machine, Do I need 2 or 3 transformers?

A1CNC said:

So, if I were to buck just the one machine, Do I need 2 or 3 transformers?

Click to expand...

You can do either way, probably depends on which s cheaper and more available for you. I always went with three because it was simpler to figure out and they were lighter to hang on the wall. But you only have to figure it out once, so maybe that's not a great way to decide.

A1CNC said:

So, if I were to buck just the one machine, Do I need 2 or 3 transformers?

Click to expand...

I've bucked and boosted many times with only two single phase transformers.

I just checked out my 17kva machine today. I'm using a pair of 1kva transformers to boost 208 to 220 or 230. Can't remember which. They're smallish and mounted right inside the magnetics cabinet on a big open wall.