Bear with me as I am not a power systems engineer nor an electrician. I know just enough to be both dangerous and also cautious.
I was given a 3 phase autotransformer and it’s been collecting dust in my basement. I’ve got a project in mind and I think it might be useful but first I want to be sure I understand what I have.
It looks like I can’t post pictures so I’ll describe. It’s a marelco wye autotransformer. 23KVA. Primary taps labeled 480, 440, 380, 240 and secondary taps labeled 220.
I believe it’s a 3 leg core, and as I understand it, it’s intended for stepping down any of those primary voltages (L-L) to 220v (L-L)
Meaning the primary is rated for 480V (L-L) in a wye connection, which would put the insulation rating of any one winding at 240V max. I’m not 100% on how I would determine the per winding current ratings. But if my quick math holds I’m guessing it’s something like 10-15 amps per winding.
So first question, is what I just wrote an accurate understanding of what I’m describing?
Now for the unusual application. I have some of chinas finest 12Vdc to 120V inverters on hand. When the power goes out I typically fire up an 8KW generator, lockout and tagout my main, and then connect to / back feed a 2 pole 20a breaker with 3 wire 10awg wiring, no N-G bond other than what’s in the panel. I use a couple of amp clamps to check the balance between the two legs, and I have intentionally setup all of my computing/office equipment on 240v PDUs such that my biggest constant loads are already balanced. This of course means I can’t use my 120V inverter as a back feed source for a lot of my house. It also means the generator sees whatever imbalance I do have.
So what I’d like to do is run another 20A 2pole breaker to an autotransformer, to balance the loads when running on generator, and to derive the second 120V leg and thus 240v when I use the inverter to back feed instead of the generator (loud).
I have 1.5KVA transformer on hand (240x480 to 120/240) but that’s a little undersized for my goal.
It seems like there has got to be a way to use the aforementioned 23KVA 3ph autotransformer as a very inefficient and oversized solution.
Simplest answer seems to be determining if I can access the Wye point / neutral, and if I can, use one of the windings as a single phase center tap auotransformer. I’d use the 480v primary tap and the wye point as my two hots connections, with the 240v tap as the center tap/neutral. One half of that winding would be sourcing power from the inverter, and the other half of the winding would be deriving the second leg. Or in the case of the generator it would simply be balancing the legs.
Is that accurate?
What’s more intriguing to me are just how many ways you could hook something like this up? How many would work? How many require access the the wye point?
For example, if I modify the design parameters to only step up the voltage from 120v to 240v with no need for split phase usage, just powering 240v loads exclusively, I believe the wye point is no longer needed? Could I then use two of the windings? Like connected my 120V to two of the 220V secondary taps and then connect the 440V taps to my 240V loads? I guess what I’m really going to need to study up on is the dot notation of windings, and the interaction of flux in the 3 leg cores. Seems to me that if they polarities line up, an unwanted 3 phase transformer might have some use as an overbuilt single phase transformer?
And for any one shaking their head picturing my house burning down after reading this, there’s a reason I’m posting here and not just down in my basement with wire nuts and romex going to town. I respect the danger involved in this topic from both an electrocution as well as a combustion stand point. The pragmatic answer here is to buy a Sol-Ark 15K and never look back. This post is more about my interest in the myriad of ways transformers can be wired up than it is an intent to actually build what I’m describing.
I was given a 3 phase autotransformer and it’s been collecting dust in my basement. I’ve got a project in mind and I think it might be useful but first I want to be sure I understand what I have.
It looks like I can’t post pictures so I’ll describe. It’s a marelco wye autotransformer. 23KVA. Primary taps labeled 480, 440, 380, 240 and secondary taps labeled 220.
I believe it’s a 3 leg core, and as I understand it, it’s intended for stepping down any of those primary voltages (L-L) to 220v (L-L)
Meaning the primary is rated for 480V (L-L) in a wye connection, which would put the insulation rating of any one winding at 240V max. I’m not 100% on how I would determine the per winding current ratings. But if my quick math holds I’m guessing it’s something like 10-15 amps per winding.
So first question, is what I just wrote an accurate understanding of what I’m describing?
Now for the unusual application. I have some of chinas finest 12Vdc to 120V inverters on hand. When the power goes out I typically fire up an 8KW generator, lockout and tagout my main, and then connect to / back feed a 2 pole 20a breaker with 3 wire 10awg wiring, no N-G bond other than what’s in the panel. I use a couple of amp clamps to check the balance between the two legs, and I have intentionally setup all of my computing/office equipment on 240v PDUs such that my biggest constant loads are already balanced. This of course means I can’t use my 120V inverter as a back feed source for a lot of my house. It also means the generator sees whatever imbalance I do have.
So what I’d like to do is run another 20A 2pole breaker to an autotransformer, to balance the loads when running on generator, and to derive the second 120V leg and thus 240v when I use the inverter to back feed instead of the generator (loud).
I have 1.5KVA transformer on hand (240x480 to 120/240) but that’s a little undersized for my goal.
It seems like there has got to be a way to use the aforementioned 23KVA 3ph autotransformer as a very inefficient and oversized solution.
Simplest answer seems to be determining if I can access the Wye point / neutral, and if I can, use one of the windings as a single phase center tap auotransformer. I’d use the 480v primary tap and the wye point as my two hots connections, with the 240v tap as the center tap/neutral. One half of that winding would be sourcing power from the inverter, and the other half of the winding would be deriving the second leg. Or in the case of the generator it would simply be balancing the legs.
Is that accurate?
What’s more intriguing to me are just how many ways you could hook something like this up? How many would work? How many require access the the wye point?
For example, if I modify the design parameters to only step up the voltage from 120v to 240v with no need for split phase usage, just powering 240v loads exclusively, I believe the wye point is no longer needed? Could I then use two of the windings? Like connected my 120V to two of the 220V secondary taps and then connect the 440V taps to my 240V loads? I guess what I’m really going to need to study up on is the dot notation of windings, and the interaction of flux in the 3 leg cores. Seems to me that if they polarities line up, an unwanted 3 phase transformer might have some use as an overbuilt single phase transformer?
And for any one shaking their head picturing my house burning down after reading this, there’s a reason I’m posting here and not just down in my basement with wire nuts and romex going to town. I respect the danger involved in this topic from both an electrocution as well as a combustion stand point. The pragmatic answer here is to buy a Sol-Ark 15K and never look back. This post is more about my interest in the myriad of ways transformers can be wired up than it is an intent to actually build what I’m describing.
