Wiring buck boost for balanced single phase

Ok single phase power from the street is 125/250 vac I have to run a RPC to get 3 phase My machines want 245 vac3 max. So I bought a single phase 12/24 buck boost to put before the RPC. If I wire it according to the diagram on the Buck/boost it will only drop one line. So I will have one leg at 101vac to ground and one leg at 125vac to ground. As far as the 3 phase delta goes the RPC should balance out close enough line to line.
But I'm not fond of having the single phase imbalance. I talked to the supplier I bought the buck boost from about buying a second and putting one on each line. He suggested that I wire it according to this diagram. He also said he has never seen it done and non of the manufacturers show it as an option but it should work. According to my understanding it looks like it should work. Each winding is carrying the same current, and both legs come out with the same reduction. I have wired it up this way just to test and voltages are good 113/225 on out put.

So am I missing something? The only problem I can think of would be an increase in heat but because current is the same it should be fine.

"So am I missing something?"

You never need to feed a neutral (groundED) conductor to an RPC system. A groundING conductor, of course you do.

As you are only needing two ungroundED conductors, and no groundED conductors, but a groundING conductor, you only need one buck/boost transformer.

Incidentally, in many three-phase installations, you only need two buck/boost transformers.

Yes the Neutral never goes to the RPC it just goes to the panel after the RPC for the single phase equipment that may be ran from that panel. The RPC dose have a groundING conductor.

So If I understand correctly I need to remove the neutral from the B/B so I am not creating an anchor point (or groundED conductor) for the delta 3 phase?

Forgive me if I'm confused I think I have read myself into confusion on what should be a simple subject.

Single-phase, whether one or two ungrounded conductors, uses one B/B transformer * .

Three-phase, which has three ungrounded conductors may use two B/B transformers, but in rare cases three B/B transformers.

Here, I am assuming the three-phase is not the now obsolete "corner grounded" delta three-phase system (240 or 480) which implicitly has two ungrounded conductors, but is indeed three-phase.

Three-phase delta (not "corner grounded" delta) almost always uses two B/B transformers.

In special cases, three-phase (wye) uses three B/B transformers.

[ * ] A possible rare exception would be the so-called "technical" variation of 120, which is really 60/120 ... 60 volts each side of the neutral, 120 volts line-to-line ... which is intended only for special purpose audio circuits, and may not be used for conventional loads.

Why not condition the single phase input so that your final 3 phase voltage is where it needs to be?

Bob
WB8NQW

The buck boost is going on the single phase input so the 3 phase voltage will be right.

I am also only using one buck boost transformer but I am wiring it so that it bucks 12 volts off each conductor instead of 24 from one which would be according to the standard wiring diagram. Hear is a different diagram that might be clearer.

The recommended connections for a buck boost would be. High voltage Power in to H1 and X1. Low voltage out of H1 and H4 the other sets would be jumperd together. (X2,X3) (X4,H4) (H2,H3)
This would give a 24 volt drop to only one conductor.

The way I am asking about wiring it would be to put high voltage power in at X1 and X4. The Low voltage out at H1 and H4 with the rest jumperd as follows (X2,H1) (X3,H4) (H2,H3) I would not connect the neutral to (H2,H3) like shown in my first diagram. I have already wired it this way and taken voltage measurement and it works really well.

What I don't understand is why do the manufactures wire it so you only drop one line instead of 2. I realize that with 240 single it dosent matter if they are balanced. But if I want to use any leg to ground for 120vac single I would have one at 120vac and the other at 100vac

I would also think that if the single phase is balanced to start with that the 3 phase would have better balance.

I hope I explained better what I am doing this time

"But if I want to use any leg to ground for 120vac single I would have one at 120vac and the other at 100vac "

Your rotary convert will not use the neutral. Do not connect the neutral to your converter.
The output of the rotary will in no way be improper based on using one buck-boost for the
input to the rotary.

If you want 120 volts at the machine, DO NOT USE THE manufactured power from the rotary for
this. Simply install a convenience outlet from your regular service.

I also question the need for a buck transformer at all. Most equipment (what are you running
with the rotary?) allows a ten percent variation so your 240 volt input (have you actually
measured this voltage at the point where the converter will be installed?) and this suggests the
service could be used as-is.

Hi Jim

The only place the neutral comes in is the breaker panel after the RPC. Then I run 3 machines off that panel.
Ignore the neutral on the first drawing I posted it will not be hooked up

Their may be a need for single phase to neutral from this panel and on one of the machines. I am just trying to have only one wild leg instead of two.

As far as actual voltage measurements the voltage coming in is any were from 245vac to 258vac depending on the day. The machines are only rated at 220vac +_ 10% which would max out at 242 vac if voltage exceeds 245 the lathe throws an error that states voltage is too high. The RPC is also rated for 220 +-10% and has a warning not to exceed 250 vac. Which I have not done. At this point I have only turned things on when voltage was low enough for setting the shop up but I can't operate that way.

"As far as actual voltage measurements the voltage coming in is any were from 245vac to 258vac depending on the day."

Pretty typical for residences or for commercial premises at or towards the end of a lightly-loaded distribution line (the voltage rises as one gets towards the end of such a line).

230 is really 220 (pre-WW-II), 230 (immediately post-WW-II) and 240 (present day).

Most modern motors are rated 208-240/480, but some are rated 200-240/480 as Japan uses 200 and not 240 (and uses 100 and not 120 for wall receptacles).

Rmkscrambler said:

Ok single phase power from the street is 125/250 vac I have to run a RPC to get 3 phase My machines want 245 vac3 max. So I bought a single phase 12/24 buck boost to put before the RPC. If I wire it according to the diagram on the Buck/boost it will only drop one line. So I will have one leg at 101vac to ground and one leg at 125vac to ground. As far as the 3 phase delta goes the RPC should balance out close enough line to line.
But I'm not fond of having the single phase imbalance. I talked to the supplier I bought the buck boost from about buying a second and putting one on each line. He suggested that I wire it according to this diagram. He also said he has never seen it done and non of the manufacturers show it as an option but it should work. According to my understanding it looks like it should work. Each winding is carrying the same current, and both legs come out with the same reduction. I have wired it up this way just to test and voltages are good 113/225 on out put.

So am I missing something? The only problem I can think of would be an increase in heat but because current is the same it should be fine.

View attachment 80285

Click to expand...

I am dealing with this very same issue. The buck boost came in today. How is this working out for you? Did you end up staying with the second diagram or just buck one leg down like you had it originally?

Hi mike

The second Diagram has worked great. I have only ran a few parts so far without issue My CNC lathe doesn't show any voltage warnings like before. The manual mill also runs good. My cnc mill isn't running yet I'm waiting on parts for it.
I have been checking power regularly and I have a good 110-115 VAC on each single phase line to ground and 225 VAC leg to leg. The balance on these legs match what is coming in from the street which has been around +- 2 volt of each other. The leg to leg balance after the rotary phase converter has been within a 1% balance no load and light load, I haven't had a chance to check it under heavy load yet.

One Warning The first diagram I posted shows a neutral tied to H2 and H3 This should not be hooked up the Neutral needs to by pass the buck boost with H2 and H3 only tied to each other.

Rmkscrambler said:

Hi mike

The second Diagram has worked great. I have only ran a few parts so far without issue My CNC lathe doesn't show any voltage warnings like before. The manual mill also runs good. My cnc mill isn't running yet I'm waiting on parts for it.
I have been checking power regularly and I have a good 110-115 VAC on each single phase line to ground and 225 VAC leg to leg. The balance on these legs match what is coming in from the street which has been around +- 2 volt of each other. The leg to leg balance after the rotary phase converter has been within a 1% balance no load and light load, I haven't had a chance to check it under heavy load yet.

One Warning The first diagram I posted shows a neutral tied to H2 and H3 This should not be hooked up the Neutral needs to by pass the buck boost with H2 and H3 only tied to each other.

Click to expand...

Excellent. Thanks

Hi Mike
Did you get things hooked up. How did things work for you

Rmkscrambler,
I know my question is many years past the origination of the thread. How did this work for you long term? Did the buck transformer you initially ordered have any longevity? Any heat issues? Just wired one and am unhappy with the imbalance also. I appreciate finding your posts...thank you.

JDWZ28 said:

Rmkscrambler,
I know my question is many years past the origination of the thread. How did this work for you long term? Did the buck transformer you initially ordered have any longevity? Any heat issues? Just wired one and am unhappy with the imbalance also. I appreciate finding your posts...thank you.

Click to expand...

Still running strong no issues. I have since moved locations, however am still using the buck boost it is now wired in between the meter and the main panel to buck the power for the entire shop works great lines are balanced within 1 to 2 volts.