3 phase questions

[Sam Beagle]

So I bought an old elks lodge which I’m turning into my new shop. I recently found out the building has 3 phase power in which I’ve been paying a monthly fee to have. How can I find out what is 3 phase and where’s it is coming I

[Rod Sheridan]

Hi, 3 phase services can have both 3 phase and single phase loads.

The only 3 phase loads you might have would be motors, air conditioners and large electric heaters.

You can look at the nameplates and determine whether they are single or 3 phase......Rod

[Mike Henderson]

Most woodworkers would love to have 3 phase in their shops - consider yourself lucky that you have it. Three phase motors are essentially maintenance free and extremely reliable.

Mike

[Jim Becker]

The only 3 phase loads you might have would be motors, air conditioners and large electric heaters.
This list also may include kitchen equipment in a place like a former lodge.

[Frank Pratt]

The downside to having 3 phase is that 240V electric heating loads will put out substantially less (roughly 75%) heat when run on 208V. Not likely to be an issue though. And a 240V single phase motor will be drawing more current to compensate for the lower voltage. Many motors are designed to be OK with that though. Pay close attention to voltage drop when you running circuits.

[Paul F Franklin]

In several shops I've worked at that had 3 phase, there was a separate meter and panel for the 3 phase, and then a meter and panel for the 240/140 service. That was the best of both worlds as there was no need to run 240 volt loads on 208. So see if you have multiple meters and panels. A three phase panel will have triple pole breakers.

[Sam Beagle]

Here’s what I know. There are currently like 7-8 air compressors hooked up that are 3 phase motors. Also the heating and AC units are using Big 3 phase squirl cage motors.
This is all new to me

[Sam Beagle]

Also there’s a big walk in cooler that I plan on turning into a bowl kiln for myself.

[Jim Becker]

I agree with the statement that having 3-phase available can be an advantage for your new shop. You might want to consider engaging with your "favorite" licensed electrician to do a walk-through to fully ascertain what you have and how things are setup so you can better plan for changes as you develop your shop plan. A hundred bucks could be well spent money from both the "as built" understanding and also the "what can/should you change" advise you will obtain. You may also want to consider how efficient the old HVAC system is vs replacement with a modern and economic to run MiniSplit system for four season comfort.

Nice idea for the walk-in...just make sure it has an inside un-latch for safety. Something really old might be wonky.

[Greg R Bradley]

Don't make any assumptions that you will be running 240v single phase loads at 208v. There are several varieties of 3 phase supply. That is just one variation.

[Frank Pratt]

Don't make any assumptions that you will be running 240v single phase loads at 208v. There are several varieties of 3 phase supply. That is just one variation.

Agreed. There are a few goofy, kludgy 3 phase configurations that have been tried over the ages, but at least around here (Alberta), they haven't been installed for probably 70 or 80 years. Anything that isn't absolutely ancient will be straight 120/208V. You haven't even been able to get a 3 phase delta service of any voltage installed for at least 40 years.

[Greg R Bradley]

Agreed. There are a few goofy, kludgy 3 phase configurations that have been tried over the ages, but at least around here (Alberta), they haven't been installed for probably 70 or 80 years. Anything that isn't absolutely ancient will be straight 120/208V. You haven't even been able to get a 3 phase delta service of any voltage installed for at least 40 years.
I've never seen 120/208v Wye in light industrial buildings around here. The norm on strip malls, apartment buildings, etc. where it is almost all 120v loads. Light industrial is 120/240v Delta and heavy industrial is 277/480v Wye.

[Andrew Seemann]

Agreed. There are a few goofy, kludgy 3 phase configurations that have been tried over the ages, but at least around here (Alberta), they haven't been installed for probably 70 or 80 years. Anything that isn't absolutely ancient will be straight 120/208V. You haven't even been able to get a 3 phase delta service of any voltage installed for at least 40 years.

Ah yes, the high leg. I remember that too well from a shop I worked in college, built originally by the WPA in 1938, roughly 80 years ago as a matter of fact. It had 120/240 delta. Since it was 1938, the hots were all tar-colored rag wire. You had to watch out for that 208 leg.

Also, since it was an engineering laboratory, and since engineers think they know everything (apologies to any engineers reading this) there were all kinds of uninspected mods done by unlicensed people. I remember one day, the foreman asked me what the ampacity of 3" conduit was. I asked why, and he said because the neutral between the second and 3rd floor was the conduit between the panels. Additionally, at some point I swear they got a deal on green wire, because someone in the 1960s used it for everything, hots, neutrals, and sometimes even grounds. Too bad they didn't use some as a neutral between those two panels.

[Frank Pratt]

I've never seen 120/208v Wye in light industrial buildings around here. The norm on strip malls, apartment buildings, etc. where it is almost all 120v loads. Light industrial is 120/240v Delta and heavy industrial is 277/480v Wye.

Here it's always 120/240V single phase for residential up to small row housing condos. Small commercial & light industrial is 120/208V 3 phase. Larger commercial & industrial is 347/600V 3 phase. In the 347/600V building there are transformers to handle the 120/208V stuff. 277/480V was phased out of new construction 30 or 40 years ago, but it still exists in lots of older buildings. Most of downtown Calgary (even the new stuff) is 277/480V cause it's all connected to the network distribution system. New industrial building can be supplied with 277/480V at the customer's request, but it is very rare.

[Lyndon Klassen]

Here it's always 120/240V single phase for residential up to small row housing condos. Small commercial & light industrial is 120/208V 3 phase. Larger commercial & industrial is 347/600V 3 phase. In the 347/600V building there are transformers to handle the 120/208V stuff. 277/480V was phased out of new construction 30 or 40 years ago, but it still exists in lots of older buildings. Most of downtown Calgary (even the new stuff) is 277/480V cause it's all connected to the network distribution system. New industrial building can be supplied with 277/480V at the customer's request, but it is very rare.

Here in rural Alberta 277/480 is very common. Mostly because about 90% of irrigation equipment is 480v. I installed a 600 to 480 step-down transformer in one place so they could use some second hand equipment that they got a deal on.

[Darcy Warner]

My new building has 240v corner tapped delta. I prefer it over 208Y, but I would rather have 480Y service.

With 240v delta, just install a couple sub panels for your 120/240 single phase needs and keep all 3 phase loads in same panels. I need to set a couple transformers, one to get 480v and one to get 600v.

[Julie Moriarty]

So I bought an old elks lodge which I’m turning into my new shop. I recently found out the building has 3 phase power in which I’ve been paying a monthly fee to have. How can I find out what is 3 phase and where’s it is coming I

The fact you have to pay a fee for your 3 phase service makes me think it's tapped from lines feeding mostly residential services. If so that means you have the option to keep the 3 phase service or change it over to single phase and drop the fee.

As far as what is 3 phase, I'm guessing you mean what in the building requires 3 phase to feed it. First thing I would do is look at the panel and see how many 3 pole breakers there are. If a panel directory exists and it is accurate and updated, that will tell you what the 3 phase loads are. I'm taking it you want to reuse as many of the existing feeds there are to power up your tools.

You asked "where’s it is coming I". Do you mean where is the feed outside?

[Martin Wasner]

My new building has 240v corner tapped delta. I prefer it over 208Y, but I would rather have 480Y service.

With 240v delta, just install a couple sub panels for your 120/240 single phase needs and keep all 3 phase loads in same panels. I need to set a couple transformers, one to get 480v and one to get 600v.

I've got a 40kva step up transformer I'd sell/trade. You're not going to be able to run anything huge, but I think that's in the ballpark of 45amps of 480

[Darcy Warner]

I've got a 40kva step up transformer I'd sell/trade. You're not going to be able to run anything huge, but I think that's in the ballpark of 45amps of 480

That may take care of section of machines. There is a 112.5 kva in there I am going to move, I have another 112.5kva I am moving with me.

[Alan Lightstone]

Sam:

Out of curiousity, what does your utility charge you as the monthly fee to have 3-phase electricity? I took a different route, as I really hate dealing with Dork Energy down here.

[Jon Snider]

My new building has 240v corner tapped delta. I prefer it over 208Y, but I would rather have 480Y service.

With 240v delta, just install a couple sub panels for your 120/240 single phase needs and keep all 3 phase loads in same panels. I need to set a couple transformers, one to get 480v and one to get 600v.

i have a Canadian Wadkin with a 600v 3 phase motor. Is it cheaper / better to rewind motor to 240v or buy a step up transformer (which don’t seem very common from some prelim sesrches).

Thx

jon

[Rollie Meyers]

I've got a 40kva step up transformer I'd sell/trade. You're not going to be able to run anything huge, but I think that's in the ballpark of 45amps of 480


A transformer by code can only be used to step up the voltage if the manufacturer says it can be done with that transformer, that requirement was added to the 2014 NEC. 450.11(B)

[Martin Wasner]

A transformer by code can only be used to step up the voltage if the manufacturer says it can be done with that transformer, that requirement was added to the 2014 NEC. 450.11(B)

Manufacturer of what? The transformer, or what is being plugged into it?

[Frank Pratt]

Manufacturer of what? The transformer, or what is being plugged into it?

The transformer manufacturer. We have the same rule in the CEC. It was adopted in the last release, or maybe the one before. Functionally, a transformer will work as a step up or step down, depending on which windings you connect to the line. But inspection authorities require that the line be connected to what the label identifies as the primary windings & the load to the secondary windings. The only explanation I've been able to find for this somewhat bizarre rule is that the transformer was tested & approved by the certification agency as per the label, and connecting it in reverse is an off label application therefore illegal.

I am a believer in sticking to the code and the vast majority of its requirements are justified. But unless there is something I don't know, I think this one is a pretty silly. At some point, rational thought and logic have to be allowed to be used.

[Rod Sheridan]

Frank, although the transformer will indeed work backwards, a standard Delta primary, Wye secondary transformer when connected backwards yields a non ground referenced secondary which can be dangerous.

Regards, Rod

[Mike Henderson]

Frank, although the transformer will indeed work backwards, a standard Delta primary, Wye secondary transformer when connected backwards yields a non ground referenced secondary which can be dangerous.

Regards, Rod

Three phase power is certainly not my area of expertise, but wouldn't any delta connected three phase power have no ground reference? How would you connect a ground reference with a delta connection?

Mike

[And if you have the secondary of a transformer and that secondary has no ground, current can't flow between a line and ground. The danger, to my mind, is that a ground occurs on one line (which no one notices) and that provides a path for current to flow from one of the other lines (perhaps through a person) to ground. But I'm not at all an expert on power systems.]

[Martin Wasner]

It is a step up transformer.

I got it with a piece of equipment where the guy didn't have have 480, and the machine couldn't be switched over to 208

[Bill Dufour]

Of course the Op may be on the other side of the world. This could make shipping a heavy transformer expensive. I do not think he is in Zimbabwe is he?
Bill D
USA

[Matt Mattingley]

Frank, although the transformer will indeed work backwards, a standard Delta primary, Wye secondary transformer when connected backwards yields a non ground referenced secondary which can be dangerous.

Regards, Rod

Is this dangerous because of possible short transmission connected a to common Ground Backfeeding?

Rod, so what is your position on autotransformers. They can step up or step down. The manufacture can state it either way.

With a ground fault breaker adequately in place, Wouldn’t this alleviate stepping up using a auto transformer ground issue?

I’m not a EE but this is how my system has been set up.

[I’m fully aware of the distribution “clause” with anything more than 150 V to ground permanent hook up.]

Six of my machines have autotransformers built in. 2 of my machines use step up auto transformers before.

One has two transformers, one stepping up to 600 V, and one stepping down to 120 V from 240 V three-phase.

[Rod Sheridan]

Three phase power is certainly not my area of expertise, but wouldn't any delta connected three phase power have no ground reference? How would you connect a ground reference with a delta connection?

Mike

[And if you have the secondary of a transformer and that secondary has no ground, current can't flow between a line and ground. The danger, to my mind, is that a ground occurs on one line (which no one notices) and that provides a path for current to flow from one of the other lines (perhaps through a person) to ground. But I'm not at all an expert on power systems.]

Hi Mike, you’re correct.

True ungrounded delta systems are very rare, I’ve worked in large places with them, and they had voltage monitoring protection to determine if one phase was grounded through a fault.

In a 600/347 system the line to ground voltage would rise to 600 from 347 on the ungrounded phases, this could reduce insulation life and increase shock hazard for personnel.

Many systems that people think are delta, are simply grounded wye systems where the neutral isn’t carried beyond the secondary, which is solidly grounded or resistivels or reactively grounded.

The other issue with a non grounded system is that voltages can rise well beyond rated voltage to ground via transients, very risky.

This is why you need either an auto-transformer or a delta secondary so you can preserve the ground reference or establish a new one by grounding the neutral point.

Regards, Rod

[Rod Sheridan]

Matt, auto-transformers continue the ground reference, they are not isolating transformers.

That is why they are used for that function.

The alternative is wye secondary with the neutral point grounded.....Rod

[Chip Lindley]

Please allow me to jump in with a question about my 3-phase. I have run a home shop for years off of a 240v rotary converter Now I have acquired a nice Delta 16-inch (long arm) radial arm saw ( but it is 200 volt/not 240 volt) I hesitate to consider running the 200 on 240. Just my luck it will cook the windings.

A 240 volt, 5-hp 3-phase motor pulls 12 amps. The 200-volt 5-hp pulls 15 amps. What kind of money am I looking at for a step-up transformer with sufficent amperage for a 5-hp 3-phase motor? All input is appreciated.

[Matt Mattingley]

Please allow me to jump in with a question about my 3-phase. I have run a home shop for years off of a 240v rotary converter Now I have acquired a nice Delta 16-inch (long arm) radial arm saw ( but it is 200 volt/not 240 volt) I hesitate to consider running the 200 on 240. Just my luck it will cook the windings.

A 240 volt, 5-hp 3-phase motor pulls 12 amps. The 200-volt 5-hp pulls 15 amps. What kind of money am I looking at for a step-up transformer with sufficent amperage for a 5-hp 3-phase motor? All input is appreciated.I’m not exactly sure. I’ve never seen a name plate that has 240 V three-phase.

I have seen 208-230 and I’ve also seen 208, I’ve also seen 200, i’ve also seen 220 and 230. But never 240V. If you already have an RPC, my thoughts are are you will want to have a step down transformer. Do you mind sharing with us what your actual voltages are coming off your RPC during this one machine load? What size is your RPC? And how many amps is feeding it a single phase? If you use the VFD, you can restrict its out put voltage in a linear form through pulse wave modulation.

[Julie Moriarty]

Chip, I suggest taking your question to Mike Holt's forum. If they can't definitively answer your questions, no one can.

[Jeff Bartley]

Except that Mike Holts forum is exclusive to electrical contractors! I read this thread, and have the same question as Chip, so I registered on mike holts forum. First post I put up got locked and I was sent a link stating they only support those in the electrical industry.

Kind of a bummer! Reminds me of bad jobsites where all the different trades are uncooperative with one another! I don't like jobsites like that.



Chip, I suggest taking your question to Mike Holt's forum. If they can't definitively answer your questions, no one can.

[Darcy Warner]

I’m not exactly sure. I’ve never seen a name plate that has 240 V three-phase.

I have seen 208-230 and I’ve also seen 208, I’ve also seen 200, i’ve also seen 220 and 230. But never 240V. If you already have an RPC, my thoughts are are you will want to have a step down transformer. Do you mind sharing with us what your actual voltages are coming off your RPC during this one machine load? What size is your RPC? And how many amps is feeding it a single phase? If you use the VFD, you can restrict its out put voltage in a linear form through pulse wave modulation.

Any motor made in the last 10 years is so that is dual voltage will usually say 240/480. Also suitable for use on 208. 200v motors are basically made for 208v services.


That motor on 240v will probably last a long time with moderate use, 6+ hours a day, not so much.

[Chip Lindley]

Thank you all for your interest in my question about my 200v 3-phase Delta radial arm saw being run in the home shop with commercial ARCO Type R, 7.5hp RPC. The RPC is on a dicatated 40A single-phase circuit. I am not right at my shop to measure the amp draw, but my largest 3ph machine is 5 hp (12 amps) on that curcuit. I hesitate to run the 200v saw on 240v since the nameplate does not state any voltage other than 200v at 15 amps. (sure, I could run it, but it would be in the overheating range, I am sure) Like my Daddy used to say, "It doesn't take much more to go 1st Class."

404384

I need a suitable 3 to 5 KVA buck-boost transformer to reduce 240v 3 phase to 200v (208v probably) 3- phase. I know they are available new, in the $400+ range. Hopefully I can do a lot better than that . Thanks for any help out there.

[David Kumm]

Chip, I think the size of the buck transformers can be smaller. I used two 1 kva single phase transformers to boost my 240 to 275 for my 7.5 hp Robinson ET/E. They were even a tad oversized. I think I paid less than $100 total for both. Dave

[Frank Pratt]

Chip, I think the size of the buck transformers can be smaller. I used two 1 kva single phase transformers to boost my 240 to 275 for my 7.5 hp Robinson ET/E. They were even a tad oversized. I think I paid less than $100 total for both. Dave

Both buck and boost transformers need to be sized to the load. There is no reduction for a buck transformer. Whether increasing or decreasing the voltage, the transformer still needs to be able to handle the load of the motor.

[David Kumm]

Two single phase Buck Boost transformers wired open delta need to be sized .5 kw or to be extra safe at .75kw to handle a 15-18 amp 208v motor. They do not need to be sized at 3-5 kw. Dave

[Frank Pratt]

Two single phase Buck Boost transformers wired open delta need to be sized .5 kw or to be extra safe at .75kw to handle a 15-18 amp 208v motor. They do not need to be sized at 3-5 kw. Dave

Dave, I must be missing something here. His RAS motor uses a 5190 VA at full load, so event a 5 kVA buck transformer is technically too small, but the saw is not going to be running continuously so he could get away with it. Although when doing a really tough cut there may be voltage drop issues at full load. How do you calculate a .5 kVA?

[David Kumm]

Frank, I used the calculator supplied by various buck boost transformer manufacturers. The RPC supplies 240v delta so two transformers only need to be sized to reduce the voltage to 208 which reduces the size needed. I'm above my pay grade here but I got the same info from others when I needed to boost voltage for my 7.5 hp motor. The sizing depends on how large the voltage adjustment is and whether you use two or three transformers. I calculated using two and wiring them open delta. Using three would reduce the size so .5 kva each would be more than enough. Dave

[Frank Pratt]

Frank, I used the calculator supplied by various buck boost transformer manufacturers. The RPC supplies 240v delta so two transformers only need to be sized to reduce the voltage to 208 which reduces the size needed. I'm above my pay grade here but I got the same info from others when I needed to boost voltage for my 7.5 hp motor. The sizing depends on how large the voltage adjustment is and whether you use two or three transformers. I calculated using two and wiring them open delta. Using three would reduce the size so .5 kva each would be more than enough. Dave

I'm an electrician, though I may be forgetting something (that seems to happen more frequently lately), I'm confident I've got this right. If using single phase transformers, you can go with an open delta configuration, but the sum of the capacity for the 2 transformers still must at least equal the total motor load. So for a 5 kVA motor, you could use 2 transformers of 2.5 kVA each, or 3 transformers of 1.67 kVA each. Or use a 3 phase transformer of 5 kVA. The bottom line is that the VA capacity still needs to at least match the motor.

How much the voltage is being change has no effect on sizing. Even if you needed to change only 1V, you still need the same capacity. Changing up or down has no bearing on this. If you try running a 7.5 HP motor on 2 transformers of .75 kVA each, The motor will only develop about 2 HP maximum. That's still enough to cut, buy nowhere near 7.5 HP

When sizing the transformers, the kVA rating the calculators show may vary depending on the voltage change, but that's just a function of the transformer models available.

The laws of physics dictates that if your load needs 5 kVA to do its job, then the transformer feeding the load must have a capacity of at least 5 kVA. No getting around that.

Autotransformers are the thing to use for this because they are cheaper and physically smaller than an isolation type transformer.

[David Kumm]

I deleted my prior post so posterity doesn't rely on my info. Dave

[David L Morse]

Frank, hopefully someone will chime in....

Ok, I'll try. Let's keep it simple and just look at a single phase autotransformer. Also, I'll use nice round numbers to keep the arithmetic from being a distraction.

Consider the following:

404470

An autotransformer has it's primary and secondary connected in series. For stepdown the input is applied to the series connection and the output taken from the secondary. In this example our turns ratio is 1:5 so an applied 240V divides to 40V on the primary and 200V on the secondary (1:5 = 40:200).

For a load of 2.4kVA we have to have 2.4kVA in (neglecting transformer losses). So 2.4kVA/240V = 10A in. The 10A flows into the primary so the primary sees 10A x 40V = 0.4kVA.

The secondary output current is one fifth of the primary current (turn ratio) or 2A. Secondary kVA = 2A x 200V = 0.4kVA.

The output current is the sum of the input current and the secondary current (because of the series connection) or 12A. 12A x 200V = 2.4kVA.

In general, the required transformer kVA is the output kVA times the ratio of the difference voltage to the series applied voltage, in this case 40V/240V x 2.4kVA = 0.4kVA.

And that's why:


Autotransformers are the thing to use for this because they are cheaper and physically smaller than an isolation type transformer.

[David Kumm]

David ( and Frank), is my analysis correct? I don't want to screw the OP up for his machine. If the input 240v amperage needs to be 14-15 ( 4kw ) and the BB transformer is 240-32 ( more common than a 40v change ), it will take two .75kw single phase BB transformers wired open delta to provide about 15 amps of 208v three phase power to the motor. Dave

[Frank Pratt]

Dave, your concept of power, voltage and energy is incomplete. Let's explore.

VA is a unit that quantifies the rate energy consumption. It is the product of voltage (the 'pressure' that drives electrons through the wire) multiplied by amps (the quantity of electrons flowing past a point in the wire per second)

We'll look at a 200 V, 25 A motor as an example. To keep the math a little simpler, a single phase motor. But the principal is identical to 3 phase. For purposes of this discussion, we'll assume the transformer is 100% efficient. Again, to keep the math simple.

First, some facts and conventions that must be kept in mind:
- The primary side is always connected to the supply
- The secondary side is always connected to the load
- V x A = VA
- VA/A = V
- VA/V = A
- VA and W are the same in a resistive circuit. In an inductive or capacitive circuit, the V and A will be slightly out of phase resulting in a VA value that is higher than the W value would be . The theory and math get much more complicated so we'll just stick with using VA because this is and inductive load.
- VA of the primary side will always equal VA of the secondary side (ignoring the inefficiencies)
- Whatever load is being run, the transformer must have the capacity to feed that load. Think of a crane. If you need to lift 1000 Lbs, then a 500 Lb crane is not going to do.

The motor, at full power, is using 5000VA (200V x 25A = 5000VA). But we have a problem because our circuit that the motor will be run on is 240V. The solution is a buck transformer. The secondary will have 200V output and from that, our motor will draw 25A. The primary will be connected to 240V, so let's figure out what the primary current will be. 5000VA / 240V = 20.83A. So the current on the primary side is lower because it there are more volts available to keep the VA at 5000.

So, as you can see, if you're boosting the voltage, the current on the secondary will go down. Conversely, if the voltage is being reduced, the secondary current will increase. Remember, the VA on both side must be the same. Whatever the case may be, the one thing that cannot change is that you need that transformer to be able to deliver the VA load that is needed for the motor.

Now I'll try to address your math above. You need to drop the 240 V supply voltage to 208V (by 32 V) which is a reduction of 13.33% requiring a 13.33% increase in current on the secondary. I'm not following your math at all because you're mixing units and multiplying things that cannot be multiplied. 240 V / 32 V = 7.5 is true, but that has nothing at all to do with VA. The voltage is being bucked from 240 V to 208 V, which is a reduction of 32 V: the voltage is not being reduced to 32V.

Your statement "The BB are not from 240-208 but reducing from 240-32 which is why smaller units are OK." is completely wrong. The buck is from 240V to 208V. If it did reduce the voltage to 32V, your motor would not even start, but would sit the & hum until it overheated.

The last sentence "Another way to size is output volts x 32/1000 or 16x32/1000= .52 kva for each rounded up to .75 for capacity." makes no sense to me at all. I read this as: 208 V x 32 V/1000 is not an equation that can be calculated. You can't multiply volts x volts. I can't figure out what is meant by 16x32/1000.

I'm not great at explaining things, but I hope this makes things more clear. I wonder if you can check the BB transformer labels & post the VA ratings here?

[David Kumm]

Frank, I applaud your diplomacy and I am happy to not spread crap on the web- probably too late. I was attempting to translate the info from the Schneider Electric transformer sizing table calculator I googled. I tried to follow their formula and am sure I screwed it up. The calculator does lead me to the two .75 kva transformers for the application but if possible, I will delete the post that balls everything up. Dav

[Frank Pratt]

Dave, I just read David's post & think I see where some of your confusion is originating from.

In an autotransformer, the line between primary & secondary appear somewhat blurred. There really is just 1 winding that has 2 ends and 1 or more tap(s) in between. I'll try to clear up the terminology here. The primary side includes all the windings between where the 2 supply wires are connected. The secondary side includes all the windings between where the 2 load lines are connected. So if it is a boost transformer, the primary only includes the large part of the winding, whereas the secondary includes all the windings. The reverse holds true for a buck transformer. To borrow David's sketch:

https://sawmillcreek.org/attachment.php?attachmentid=404470&d=1551140419


Where the 240V side is the primary & the 200V side is the secondary: The small portion of the winding at the top is not the primary, but the entire winding, by definition, is the primary (primary being the winding that is across the supply voltage). The lower portion of the winding is the secondary. An autotransformer VA rating is for the entire assembly, not for just a part of a winding. If you calculate the VA of just that small portion of the winding then you will have an undersized transformer.

So if you do have 2 - .75 kVA transformers running that saw, then you have it severely throttled. The good news is that if you put properly sized transformers in you'll get yourself a massive power boost.

David explains thusly:
"For a load of 2.4kVA we have to have 2.4kVA in (neglecting transformer losses). So 2.4kVA/240V = 10A in. The 10A flows into the primary so the primary sees 10A x 40V = 0.4kVA.

The secondary output current is one fifth of the primary current (turn ratio) or 2A. Secondary kVA = 2A x 200V = 0.4kVA.

The output current is the sum of the input current and the secondary current (because of the series connection) or 12A. 12A x 200V = 2.4kVA.

In general, the required transformer kVA is the output kVA times the ratio of the difference voltage to the series applied voltage, in this case 40V/240V x 2.4kVA = 0.4kVA."

This is correct, except for the issue of terminology & looking at where the ratings are. The portion of the winding that handles the 40V difference is indeed .4kVA, but that's not how a transformer is rated. As I said earlier, the VA rating is for the entire transformer.

[David Kumm]

I'm really mixing up KW and KVA and finally realized that part. My saw doesn't show problems but a 7.5 hp 275 v motor that is running on something less probably would need to run a long time to show issues. Can you explain how the various sizing charts work? I've plugged in the input amperage for the OP motor, three phase, % of buck or boost, and output amperage and every chart I've used shows I need two .75KVA transformers. If I understand your posts, I would need two transformers, each sized at 5.5kva for my saw or 4 kva for the OPs.

The OP is looking to buck to 208 while my saw needed boost. I have two 1kva transformers on mine. Thanks for the discussion. Dave

[Frank Pratt]

Frank, I applaud your diplomacy and I am happy to not spread crap on the web- probably too late. I was attempting to translate the info from the Schneider Electric transformer sizing table calculator I googled. I tried to follow their formula and am sure I screwed it up. The calculator does lead me to the two .75 kva transformers for the application but if possible, I will delete the post that balls everything up. Dav

No worries Dave, this is a complex subject & I certainly had to put my own brain in high gear to keep things straight.

But now there's a new twist. I went to the Schneider site to have a look & they have a different way of rating the transformers than is done here in the great white north. The rating they give an auto transformer is what it would be if used as an isolation transformer (which is the rating of the little winding that makes up the voltage differential. So you were indeed correct when you chose the .75 kVA transformers.

Sorry for all the confusion I've created.

[David Kumm]

Confusion always precedes knowledge and I'm happy to learn. I think Acme, and my others are all rated the same. I didn't know there were differences in ratings. Dave

[Rollie Meyers]

Except that Mike Holts forum is exclusive to electrical contractors! I read this thread, and have the same question as Chip, so I registered on mike holts forum. First post I put up got locked and I was sent a link stating they only support those in the electrical industry.

Kind of a bummer! Reminds me of bad jobsites where all the different trades are uncooperative with one another! I don't like jobsites like that.

I was going to mention that DIY questions are not allowed at the Mike Holt forum, they are immediately locked when it's been deemed DIY. Since it's been a while since was here last too little too late. BTW, Electrician Talk has the same policy.

[Frank Pratt]

I was going to mention that DIY questions are not allowed at the Mike Holt forum, they are immediately locked when it's been deemed DIY. Since it's been a while since was here last too little too late. BTW, Electrician Talk has the same policy.

Electrician Talk does have a DIY section though.

[Rollie Meyers]

Electrician Talk does have a DIY section though.

They lock a DIY question & suggest DIY chatroom, same ownership but different forum.