Using 3 Phase Transformer to Step Up Single Phase Voltage

I already have a 25HP RPC produces 240V and 440V that I use for my CNC lathe and VMC, but I'd like to avoid having to spin it up just to run smaller machines.

I have some VFDs that take 380V-500V input, 1ph or 3ph.

So, I'd like to step up my household 240V 1ph to 480V 1ph.

Single phase transformers seem to be more costly than 3 phase transformer on a $/KVA basis, so I was thinking of getting a 3ph transformer to use to transform single phase power.

I'm thinking a delta LV/delta HV transformer would be best, but would like to see arguments/discussion on this.

I've already done some ogling on the Internet and most of the results are not pertinent and talk about doing 1ph->3ph, RPCs, ...


I want: 1ph 240V -> (3 ph transformer) -> 1ph 480V


1 bajillion KVA, but I'll settle for 10+ KVA

Have a look here...

https://www.practicalmachinist.com/vb/attachments/f11/263341d1550124681-clausing-drill-press-480-240-a-xform-1-240v-480v.jpg

SAF said:

Have a look here...

https://www.practicalmachinist.com/vb/attachments/f11/263341d1550124681-clausing-drill-press-480-240-a-xform-1-240v-480v.jpg

Click to expand...

Thanks!
That’s for a single phase transformer though.
I suppose if both the low voltage and high voltage side of a 3ph transformer were multitapped, one could do a similar thing.

The point was you can get 2X the KVA capacity, better to get 1 bajillion KVA with.

SAF said:

The point was you can get 2X the KVA capacity, better to get 1 bajillion KVA with.

Click to expand...

Thanks! Yep.
That is/was my backup plan.
Good point that I only need a much more modest 0.5 bajillion KVA transformer... :-)

I tried the single phase into a 3 phase transformer. It worked sort of. You can only use one core. You would have to use a delta LV/ delta HV like you said. I had a delta/wye when i tried it.I put in 120 into one core and got out 480 on one core. So you have to unhook all the delta wire connections so you can isolate the one core. so you can to hook up to each side of the one core to get your 480 voltage. Cause if you look at a delta symbol each flat side is a core. To get 480 volts on a meter you connect to any two corners of the triangle. So that means each core is 480 volt to 120 volt for me for you with an older 240/480 delta/delta each core would be 240 to 480. Basically It's super sketchy and dangerous to do and really inefficient because you will still be energizing the other two cores with the magnetic flux even tho no wires are connected to them. Also the inrush currently when powering it up is huge because of the one core taking all the burden of stabilizing the magnetic flux in the whole transformer. The breaker will be huge so you'll have to add a proper rated slow blow fuse to actually protect the wire from overload. I'm not sure about hooking up the multiple cores in parallel to get more kva. The induction on each core could be slightly different and cause them to fight each other. Hope this helps but probably won't. Keep an eye open on eBay I got a 25 kva 480/240 transformer for $250 once I realized my efforts listed above where futile. Because I believe you have to derate the transformer by 2/3 because you are only using 1 core. So for 10kva of power you'd need a 30kva transformer I think. Not positive on that.
I am doing this right now for power untill I finish another RFC for myself. I keep getting requests to build RFC for friends and never got around to making my own.
Here is a video of my single phase 480 transformer going into a vfd and giving me 3 phase out.
YouTube

Thanks for your response!

I picked up a 15 KVA (30 KVA if wired as an auto transformer) 1ph 240V 480V transformer locally.
I have a couple of ABB 380V-500V VFDs that I intend to use with that.

I like your compressor setup!
I actually picked up an Ingersoll-Rand 2 stage compressor with a 3HP 3ph motor.
It’s mounted on isolation mounts in a cool sound reducing cabinet.
It is wired 230V, so I’m going to get a 230V 10HP VFD to drive it.

Can you elaborate on your wiring between the VFD and compressor?
Ie, how do you have the pressure switch wired? ESTOP?
Just wired in series?

Is that a [horse] barn?
I wish I had a lot more shop floor space!
A big barn with solid floors would be nice!

Green machinist said:

I tried the single phase into a 3 phase transformer. It worked sort of. You can only use one core. You would have to use a delta LV/ delta HV like you said. I had a delta/wye when i tried it.I put in 120 into one core and got out 480 on one core. So you have to unhook all the delta wire connections so you can isolate the one core. so you can to hook up to each side of the one core to get your 480 voltage. Cause if you look at a delta symbol each flat side is a core. To get 480 volts on a meter you connect to any two corners of the triangle. So that means each core is 480 volt to 120 volt for me for you with an older 240/480 delta/delta each core would be 240 to 480. Basically It's super sketchy and dangerous to do and really inefficient because you will still be energizing the other two cores with the magnetic flux even tho no wires are connected to them. Also the inrush currently when powering it up is huge because of the one core taking all the burden of stabilizing the magnetic flux in the whole transformer. The breaker will be huge so you'll have to add a proper rated slow blow fuse to actually protect the wire from overload. I'm not sure about hooking up the multiple cores in parallel to get more kva. The induction on each core could be slightly different and cause them to fight each other. Hope this helps but probably won't. Keep an eye open on eBay I got a 25 kva 480/240 transformer for $250 once I realized my efforts listed above where futile. Because I believe you have to derate the transformer by 2/3 because you are only using 1 core. So for 10kva of power you'd need a 30kva transformer I think. Not positive on that.
I am doing this right now for power untill I finish another RFC for myself. I keep getting requests to build RFC for friends and never got around to making my own.
Here is a video of my single phase 480 transformer going into a vfd and giving me 3 phase out.
YouTube

Click to expand...

Depending on the exact VFD, it's probably possible to rewire the input stage to the bus capacitors - I've done this to get from European single-phase 230v to European 3-phase 415v, pretty simple mod (lathe's 3-speed motor's not rewireable to 240, hideously complicated and expensive to have rewound).

Bearing in mind that European single-phase is 230v line, 0v neutral, not sure how this would work on USAnian 110-0-110! Saying that, the VFD bus voltage is isolated from control voltages etc. but it would be sensible to pay particular attention to the HV wiring and insulation from earth / machine casings etc. but here goes:

The neutral (or one leg of USAnian split 220) is fed to the common between the two banks of reservoir capacitors, the line (other leg of split 220) to two of the 3-phase inputs. This turns the rectifier and capacitors into a voltage doubler, alternate half-cycles charge one or the other bank, developing twice the line voltage across the pair. The capacitor life is shortened somewhat (an undervoltage alarm will tell you when it's time to change 'em) as they're only charged half the time and discharged more deeply,so more ripple current, and it's wise to use a VFD rated for the next grade up (so for a 3HP motor use a 4 or 5 HP VFD - my motor's rated FLA is 6A, VFD 9A plus a 50% over-current allowance I've not hit yet).

This works on most older VFDs, mine's an ABB ACS-300 series from the 90's!

For the mill (3 separate 3-phase motors, painful in the wallet to convert to 3 VFDs) I use a 240-415v transformer, an old oil-cooled welder rated 9 KVA continuous with the welding winding stripped off and a new 415v secondary added, into a native 415v RPC - it has some "sequencing" interlocks using contactor aux. contacts so the transformer's powered before starting the idler before connecting to load, to reduce the peak surge current.

Dave H. (the other one)

Dave H. (the other one0

Hopefuldave said:

Depending on the exact VFD, it's probably possible to rewire the input stage to the bus capacitors - I've done this to get from European single-phase 230v to European 3-phase 415v, pretty simple mod (lathe's 3-speed motor's not rewireable to 240, hideously complicated and expensive to have rewound).

Bearing in mind that European single-phase is 230v line, 0v neutral, not sure how this would work on USAnian 110-0-110! Saying that, the VFD bus voltage is isolated from control voltages etc. but it would be sensible to pay particular attention to the HV wiring and insulation from earth / machine casings etc. but here goes:

The neutral (or one leg of USAnian split 220) is fed to the common between the two banks of reservoir capacitors, the line (other leg of split 220) to two of the 3-phase inputs. This turns the rectifier and capacitors into a voltage doubler, alternate half-cycles charge one or the other bank, developing twice the line voltage across the pair. The capacitor life is shortened somewhat (an undervoltage alarm will tell you when it's time to change 'em) as they're only charged half the time and discharged more deeply,so more ripple current, and it's wise to use a VFD rated for the next grade up (so for a 3HP motor use a 4 or 5 HP VFD - my motor's rated FLA is 6A, VFD 9A plus a 50% over-current allowance I've not hit yet).

This works on most older VFDs, mine's an ABB ACS-300 series from the 90's!

For the mill (3 separate 3-phase motors, painful in the wallet to convert to 3 VFDs) I use a 240-415v transformer, an old oil-cooled welder rated 9 KVA continuous with the welding winding stripped off and a new 415v secondary added, into a native 415v RPC - it has some "sequencing" interlocks using contactor aux. contacts so the transformer's powered before starting the idler before connecting to load, to reduce the peak surge current.

Dave H. (the other one)

Dave H. (the other one0

Click to expand...

That's what you have to do to convert for European power maybe I don't know but in USA I just use the 50% method for sizing and just hook up the 2 hot legs we have available and your done. Yes it has a bit more ripple but oh well the vfd doesn't care output carrier frequency is 4,000 so it averages out just fine and being you can find these big 480v vfds on eBay for $200-300 they are great for the price

Hopefuldave said:

Depending on the exact VFD, it's probably possible to rewire the input stage to the bus capacitors - I've done this to get from European single-phase 230v to European 3-phase 415v, pretty simple mod (lathe's 3-speed motor's not rewireable to 240, hideously complicated and expensive to have rewound).

Bearing in mind that European single-phase is 230v line, 0v neutral, not sure how this would work on USAnian 110-0-110! Saying that, the VFD bus voltage is isolated from control voltages etc. but it would be sensible to pay particular attention to the HV wiring and insulation from earth / machine casings etc. but here goes:

The neutral (or one leg of USAnian split 220) is fed to the common between the two banks of reservoir capacitors, the line (other leg of split 220) to two of the 3-phase inputs. This turns the rectifier and capacitors into a voltage doubler, alternate half-cycles charge one or the other bank, developing twice the line voltage across the pair. The capacitor life is shortened somewhat (an undervoltage alarm will tell you when it's time to change 'em) as they're only charged half the time and discharged more deeply,so more ripple current, and it's wise to use a VFD rated for the next grade up (so for a 3HP motor use a 4 or 5 HP VFD - my motor's rated FLA is 6A, VFD 9A plus a 50% over-current allowance I've not hit yet).

This works on most older VFDs, mine's an ABB ACS-300 series from the 90's!

For the mill (3 separate 3-phase motors, painful in the wallet to convert to 3 VFDs) I use a 240-415v transformer, an old oil-cooled welder rated 9 KVA continuous with the welding winding stripped off and a new 415v secondary added, into a native 415v RPC - it has some "sequencing" interlocks using contactor aux. contacts so the transformer's powered before starting the idler before connecting to load, to reduce the peak surge current.

Dave H. (the other one)

Dave H. (the other one0

Click to expand...

rpseguin said:

Thanks for your response!

I picked up a 15 KVA (30 KVA if wired as an auto transformer) 1ph 240V 480V transformer locally.
I have a couple of ABB 380V-500V VFDs that I intend to use with that.

I like your compressor setup!
I actually picked up an Ingersoll-Rand 2 stage compressor with a 3HP 3ph motor.
It’s mounted on isolation mounts in a cool sound reducing cabinet.
It is wired 230V, so I’m going to get a 230V 10HP VFD to drive it.

Can you elaborate on your wiring between the VFD and compressor?
Ie, how do you have the pressure switch wired? ESTOP?
Just wired in series?

Is that a [horse] barn?
I wish I had a lot more shop floor space!
A big barn with solid floors would be nice!

Click to expand...

Thanks.
I had a hard time finding a vfd over 5hp at 230v that's why I went to 480v. Theres a lot of single to 3 phase vfd for 3hp. So you don't need to oversize the vfd for your application. Then after I got the comp going on the vfd. Which is wired just using the vfd supplied 24 volt output and the yaskawa has a few analog and a few digital inputs. From the display i set one up as an e-stop and one up as a trigger. So they share a common 24 volt signal but are two different channel inputs. Then I found out that my 3 phase military surplus AC units I want to use in the barn won't work with the vfd because they have voltage/phase monitors in them and they don't like carrier frequency. So I looked on YouTube for how to build a RFC and I found this. YouTube
He explains it ok.. the PDF that he attaches shows in detail but it's not quite right either. It says when you add caps to L1-L3 you will see the biggest voltage increase in L2-L3. But then I decided I can build that. Found a 30hp motor local on eBay for $150 NIB. Bought some caps and contactors and gauges and an enclosure and started going at it. Then I was talking to a friend and he told me he needed a 15hhp unit so I got ahold of the same local guy got a 15hp motor for $50. Screaming deals! Built my friend one then his grandpa said he needed a 20 hp unit for his 10 hp lathe so I just finished his building his yesterday and that's what I'm using now untill he gets back from Arizona for the summer. Then I'll build my 30hp unit for myself finally. Yes it is a horse barn but over this last year and a half I have been converting it over to my work shop by tearing out stalls, Installing LVL's to get a wider span between vertical posts and installed a 200 amp service before all that. In the last few months I've been buying equipment. Lathe, vertical and horizontal saws, drill press, surface grinder etc. And putting all of it on the new concrete.
Here what I've got so far in a slideshow.
YouTube