

Reducing 240 volts 3 phase to 210 volts 3 phase ?
Assuming the need of 30 kVa, anyway to reduce 240 to 210 or 200 volts without using a transformer ? In this case high/low are so close in voltage was wondering if large resistors could be used ? Or is the problem with that idea that the resistors would also reduce the current flow to unacceptable levels ?

The voltage drop across resistors will be a function of how much current your
load machine is drawing, and of course they will dissipate heat in operation.
For this reason buck transformers are the usual method of doing this. Depending
on the power required by the load, the buck transformers (two are typically
used) may well be smaller than the resistor solution.
Jim

Originally Posted by Milacron
Assuming the need of 30 kVa, anyway to reduce 240 to 210 or 200 volts without using a transformer ? In this case high/low are so close in voltage was wondering if large resistors could be used ? Or is the problem with that idea that the resistors would also reduce the current flow to unacceptable levels ?
I think the missing info here is that buck boost transformers can pass 1020 times their rates kVA when configured as an autotransformer. ie, the primary and secondary are linked together in some manner. This is how a 1.5kva transformer is capable of 1015kva. This is also how to use a very small transformer for something like this. The larger the voltage delta induced by the transformer, the lesser the kva rating will be.
The rating put on a buck boost transformer is a rating in an isolated configuration.
I am still trying to get the calculations for figuring the actual kva or amperage with a given voltage delta though. Seems most just use the tables provided by mfrs but those are not always handy.
I am also trying to get calcs for 12/24 and 16/32 transformers. Most list either 10% or 13% delta but you can usually configure them in other deltas but the kva rating of course will change too. Anyone have some good calcs for this??

"Assuming the need of 30 kVa, anyway to reduce 240 to 210 or 200 volts without using a transformer ?"
You would use two 240:24 3 KVA buck/boost transformers in buck mode.
Because of the 240:24 transformation ratio (10:1), you need 30 KVA / 10 = 3 KVA transformers.
One transformer from A to B, with the buck from A, giving A'.
One transformer from C to B, with the buck from C, giving C'.
(B remains B).
The alternative is a threephase transformer rated the full 30 KVA.
A 240:32 would be a better choice, if available, and would give you 208, but the transformation ratio would be 7.5:1, so you would need two transformers rated 30 KVA / 7.5 = 4 KVA. You may have to settle on 5 KVA transformers.
Buck/boost may seem to be a "black art", so get a licensed electrician to help you, should you feel the least bit unsure of the application.

Not trying to knock electricians here but all the ones I have talked to so far could not talk pasted a printed chart of transformer specs and have no idea how to properly size transformers. I indirectly worked for a commercial electrical contractor and they relied heavily on their suppliers to "tell" them what should be used...I then would talk to the suppliers and they would simply look up transformers in their OEM charts but would have no idea if it is indeed right. I honestly do not know how we ever kept things from burning down. And, you "don't" ask if something is right or not. of course it is....
I always end up with an OEM application engineer on the phone to confirm proper components and configuration.
Peter, would you happen to have some equations handy for buck/boost transformers? example, wanting to consider the rated kva for buck mode at all voltage delta configurations. I know you have more than some experience with transformers. All I have been able to grab is basic transformer equations. I would really like to be able to calculate kva for a buck/boost mode in the field from an existing transformer.

"Peter, would you happen to have some equations handy for buck/boost transformers? example, wanting to consider the rated kva for buck mode at all voltage delta configurations. I know you have more than some experience with transformers. All I have been able to grab is basic transformer equations. I would really like to be able to calculate kva for a buck/boost mode in the field from an existing transformer."
The primary of a transformer is employed to excite the core, which, in turn, is employed to excite the secondary.
The secondary is capable of supplying, in amperes, the rated secondary voltage, whether buck or boost does not matter, as these amperes will either be adding to or subtracting from the composite rating of the supply (presumed to be infinite, but certainly limited by the service, service equipment, feeder and feeder overcurrent protection).
So, the entire set of information necessary for field use is:
1) the transformation ratio, usually listed as primary voltage and secondary voltage, and
2) the rating in KVA,
both of which are specified in the transformer's nameplate.
For, if the transformation is 240 to 24 volts, then the transformation ratio is 10:1, or, simply, 10.
If that transformer was operated at 230 or even 220 volts, then the ratio is still 10, as 230 would be transformed to 23, and 220 would be transformed to 22.
So, the critical number is the ratio, not the voltage.
The wire sizes on a 10:1 transformer would also be sized to that ratio.
If the transformer was 1 KVA (1000 voltamperes) and the primary and secondary were 240 and 24, respectively, then the primary amperes would be 1000 / 240 = 4.167 amps (small AWG, but certainly at least 14 AWG for the useraccessible conductors) and the secondary amperes would be 1000 / 24 = 41.667 amps (most probably #8 AWG, but possibly #10 AWG with higher temp insulation), respectively.
By reducing buck/boost to just the most basic rules, above, a table which derives every possible application can be made.
I'll leave that exercise to others, but, in closing, I would suggest using a spreadsheet program, such as Excel or NeoOffice or whatever, wherein the cells contain the relevant formulae, which may automatically derive the specifications for a particular application by copying and pasting the cells to other cells.

Here ...
Yet another 3ph buck/boost transformer thread
... is an example, with a schematic.
If the input voltages were balanced, and were 240, then the output voltages would be 240  24 = 216, and would be slightly imbalanced by reason of using two transformers.
If 240:32 transformers were available, then the output voltages would be 240  32 = 208.

Maybe we can work through a problem to try and get my questions answered??
lets say we want to buck 240 to say 208V. We would want to use a 240/32V transformer.
The ratio would be 7.5:1 so 240/7.5 = 32 So 240V  32v = 208V.
Let's say we need to achieve 15kVA from the transformer. We would take 15kVA/7.5 (step ratio) to get 2kVA for the supply transformer. Do I have that right so far??
One of the things I can never find is that a 240/32 transformer often has the secondaries broke down to create other ratios. I want to say it is a function of the name plate step so maybe a 7.5:1 is also capable of 30:1 and 15:1 by changing some taps around.
This would say take the 240/32 setup with the 15:1 ratio and we would have 240V/15 = 16V
So 240V  16v = 224V I would want to take the 15kVA/30 and get a target 3kVA transformer. I know the transformer demand should be smaller if I am to have a smaller delta V. What am I missing here?
Am I incorrect in the multi taps on the secondaries?

"Maybe we can work through a problem to try and get my questions answered??
"lets say we want to buck 240 to say 208V. We would want to use a 240/32V transformer.
"The ratio would be 7.5:1 so 240/7.5 = 32 So 240V  32v = 208V.
"Let's say we need to achieve 15kVA from the transformer. We would take 15kVA/7.5 (step ratio) to get 2kVA for the supply transformer. Do I have that right so far??"
Yes.
"One of the things I can never find is that a 240/32 transformer often has the secondaries broke down to create other ratios."
Most transformers do.
Such as ...
240/480:16/32
... meaning the primary may be connected for a 240 or a 480 source, and for a 16 or a 32 load.
You still apply the ratio to the nameplate KVA, so there can be these possible connections:
240 to 16,
240 to 32,
480 to 16, and
480 to 32
The ratios are as follows ...
240 to 16 (15),
240 to 32 (7.5),
480 to 16 (30), and
480 to 32 (15).
You would still apply the ratio to the KVA, and if that transformer was rated 3 KVA, then the KVA of the composite would be ...
240 to 16 (15 * 3 = 45),
240 to 32 (7.5 * 3 = 22.5),
480 to 16 (30 * 3 = 90), and
480 to 32 (15 * 3 = 45).
"I want to say it is a function of the name plate step so maybe a 7.5:1 is also capable of 30:1 and 15:1 by changing some taps around."
Yes, as stated above.
"This would say take the 240/32 setup with the 15:1 ratio and we would have 240V/15 = 16V"
240:32 is a 7.5 ratio, not a 15 ratio.
240:16 is a 15 ratio.
"So 240V  16v = 224V I would want to take the 15kVA/30 and get a target 3kVA transformer. I know the transformer demand should be smaller if I am to have a smaller delta V. What am I missing here?
"Am I incorrect in the multi taps on the secondaries?"
Your error was in miscalculating the ratio.
For 240  16 = 224, you would use a 15 ratio (240 / 16 = 15).
15 KVA / 15 = 1 KVA.

Other approach to this, read the section in the McMaster Carr catalog where they
say, "you want to buck or boost to this level, with this much load current total, buy
*this* part number."
Worked for me.
The fact that nobody has mentioned how to size resistors for this application
might possibly reduce the 'fume factor' from Milacron. The only way resistors
would work well is if the application provided for a fairly constant current
draw.
Jim

Thanks a bunch Peter for clearing that up. I have jotted down those simple equations for future reference. BIG help!

OH, I also wanted to ask about clearing up ampacity ratings.
As I understand it, the kVA rating on a transformer relates to it's capacity only in an isolated state. This just means a 1.5kVA transformer would be able to handle 6.25A at 240V on the primary and 62.5A at 24V on the secondary?
Now if we configure as an autotransformer, we would use the calculated kVA rating to determine amps on either side of the fence? Example, bucking at 10:1 with a 1.5kVA transformer, 15000VA/240V primary = 62.5A on the primary side. 15000VA/216V = 69.4A on the secondary side.
Is this correct?

"As I understand it, the kVA rating on a transformer relates to it's capacity only in an isolated state. This just means a 1.5kVA transformer would be able to handle 6.25A at 240V on the primary and 62.5A at 24V on the secondary?"
Correct.
"Now if we configure as an autotransformer, we would use the calculated kVA rating to determine amps on either side of the fence? Example, bucking at 10:1 with a 1.5kVA transformer, 15000VA/240V primary = 62.5A on the primary side. 15000VA/216V = 69.4A on the secondary side."
The primary is used to excite the secondary, as previously described.
The current flowing into the primary would be 6.25 amps.
The current flowing through the secondary would be about ten times that, when connected for a factor of 10.

Seems both are nearly the same as per your description, the primary really does not see much amperage. The "flow through" to the secondary is more what we are looking at in terms of larger value amps?
Do we still divert back to using the calculated kVA then to calculate the amps at a given voltage?

"Do we still divert back to using the calculated kVA then to calculate the amps at a given voltage?"
This is a valid question, for which I have no ready answer.
I would have to defer to a transformer applications engineer on this one.
However, one would surely be safe in limiting the load to the rated current of the secondary, which would be 62.5 amperes in the instant case.
One thing to consider when using buck/boost: should the primary fail as an open, which is the usual case, then there would be no excitation of the transformer, and the primary voltage would simply be passed through. So, instead of the anticipated 240 + 32 = 272 [ * ] or 240  32 = 208, 240 would be observed when in a failed state.
So, taking the worst case (transformer failure), I would have to say that 62.5 is the correct value, and not 69.4.
As is customary, the transformer leads are made of fine stranded wire, and is insulated at a higher than normal temperature.
This gives the transformer's pigtail a higher ampicity, perhaps a one AWG size advantage, possibly 40 amps for a #10 pigtail.
The source and load feeders still have to be rated at their customary ampicity, #8 for 40 amps.
[ * ] 240:32 is a particularly good transformer to have on hand, should a 277 piece of equipment be installed.

Peter, I have a question. We were recently sent the wrong transformer for a configuration. it is wired I guess 208 primary and 120/240V secondary and is 1.5kVA. We are needing a buck/boost transformer really with either a 12/24 or 16/32 configuration.
Do you see a way to configure this transformer to buck voltage 13.3%? IIRC, these do not have taps at the right points to connect as buck/boost and only made as isolation transformers. We would want to run this one backwards and feed the secondary 240V for the 208V output.
A typical 16/32 1.5 kVA works out to 11.25kVA bucking 240 to 208.
The unit is an Acme Model T2S531411S

Don,
Just last night I was reading a VFD manual and noticed that some can output any given percentage of the input... ie... set the value to 90whatever percent of 240 gives you 210 or whatever. It's not a transformer function and what I'll probably do to get my KT 2D running, which has nothing but 208v motors.
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