LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Divisible by three, as in even distribution among phases.
Last edited:
Best service type for EV charging?
- Thread starter pipe_bender
- Start date
pipe_bender
Member
- Location
- Boston
- Occupation
- Electrician
Thanks everyone for the info on these oddball systems, sorry i was busy with other work
A few questions on the two-phase or double split phase concept,
So three phase primary goes in and you get two 120/240 systems out?
The transformer supplier did not even flinch about two-phase he said standard single phase transformers can get wired together on the primary side and output two 120/240 split phase feeds on the secondary and that is not a Scott-T so what is this connection called?
Or you can order a customized transformer called a Scott-T with a 'common core'?
They make them to do something called a back to back T which is way over my head.
Presumably the two feeders have to be balanced at all times?
What if one feeder is fully loaded with chargers and the other is empty?
Would I need a 4-pole disconnect at the transformer or could it just go directly into two single phase 3R panelboards?
They said you need a 200% neutral, but the chargers do not have a neutral so would you run a 200% neutral to each panelboard? The chargers do not need a neutral, but they would see 120 to ground from each leg?
Would there be any other code issues?
Thanks in advance.
A few questions on the two-phase or double split phase concept,
So three phase primary goes in and you get two 120/240 systems out?
The transformer supplier did not even flinch about two-phase he said standard single phase transformers can get wired together on the primary side and output two 120/240 split phase feeds on the secondary and that is not a Scott-T so what is this connection called?
Or you can order a customized transformer called a Scott-T with a 'common core'?
They make them to do something called a back to back T which is way over my head.
Presumably the two feeders have to be balanced at all times?
What if one feeder is fully loaded with chargers and the other is empty?
Would I need a 4-pole disconnect at the transformer or could it just go directly into two single phase 3R panelboards?
They said you need a 200% neutral, but the chargers do not have a neutral so would you run a 200% neutral to each panelboard? The chargers do not need a neutral, but they would see 120 to ground from each leg?
Would there be any other code issues?
Thanks in advance.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Ok, the first issue is 'why do you even care'. You could just select a large single phase transformer to feed these chargers and be done.
The problem is that there is absolutely no way to take a single phase load and distribute that load evenly on all three phases using only transformers. (You _can_ balance a single phase load evenly on 3 phases if you have energy storage, but that is even more off in the world of physically permitted fantasy than using 2 phase power.) If your power supply system is large enough, then you might not care; how often does the power company just pick one primary line to supply single phase to a large residence. All of the discussion of 2 or 3 or 6 phases is in service of balancing 3 phase loading. But always ask question 0: is this even necessary.
Before thinking about 2 phase, start with a 'simpler' idea: you have 3 separate single phase transformers, each creating a separate 120/240V supply for a subset of the chargers. (Simpler only because 3 phase is common, 2 phase is an oddball.) You could connect the 3 separate primary circuits to different phases, and balance the three phase loading that way. You can see that if you have 3 transformers each feeding 8 chargers you could balance the 3 phase loading if the chargers were used evenly.
Now take those three separate single phase transformers and wind them on a single common 3 leg core. Still 3 separate 120/240V systems, on 3 separate phases, but you are sharing transformer iron, just like a 208/120 transformer feeding a bunch of 120V loads.
Think standard delta:wye transformer, except that you have 3 separate center tapped secondary coils, rather than the secondary coils connected in wye.
The above are the three phase analog to what the transformer distributor was describing.
It turns out that with a proper combination of transformers you can convert _any_ 'polyphase' (say 3 phase) to any other polyphase system, say 2 or 5 or 9 phases. The 'Scott-T' arrangement is a clever coil arrangement that converts between 3 phase and 2 phase. If you have a balanced 2 phase load, it will convert it into a balanced 3 phase load.
If you have a transformer with a Scott-T primary arrangement, you get a 2 phase magnetic field running on the transformer core. Put 2 separate 120/240 coils on each leg, and you get two separate 120/240V systems produced. (Actually, it would be called 2 phase 5 wire, described here: https://forums.mikeholt.com/threads/two-phase-systems.92486/post-1478246 but you are ignoring any voltage relationship between the phases and thinking of the secondary as 2 separate single phase systems.) This is simply the 2 phase version of having separate 120/240V coils on each phase, blended with the Scott-T arrangement to convert to 3 phase.
Something doesn't sound right on the above. As I described, you could have 3 separate standard single phase transformers to connect 3 single phase systems to a 3 phase system in a balanced fashion, but I don't think you can do this with 2 standard transformers to get the balanced loading. Maybe the supplier is just describing putting two separate single phase transformers on two phases.
Not sure on the 4-pole disconnect.
The arrangement describe would give you proper 120/240V single phase, 120V to each leg.
Other than maybe needing a 4 pole OCPD for secondary protection, I think it would otherwise be a standard transformer installation.
Personally I'd go with standard transformers; 3 separate circuits to 3 separate single phase transformers used in the standard fashion. That is if 1 standard transformer and unbalanced 3 phase loading was not acceptable.
The problem is that there is absolutely no way to take a single phase load and distribute that load evenly on all three phases using only transformers. (You _can_ balance a single phase load evenly on 3 phases if you have energy storage, but that is even more off in the world of physically permitted fantasy than using 2 phase power.) If your power supply system is large enough, then you might not care; how often does the power company just pick one primary line to supply single phase to a large residence. All of the discussion of 2 or 3 or 6 phases is in service of balancing 3 phase loading. But always ask question 0: is this even necessary.
Before thinking about 2 phase, start with a 'simpler' idea: you have 3 separate single phase transformers, each creating a separate 120/240V supply for a subset of the chargers. (Simpler only because 3 phase is common, 2 phase is an oddball.) You could connect the 3 separate primary circuits to different phases, and balance the three phase loading that way. You can see that if you have 3 transformers each feeding 8 chargers you could balance the 3 phase loading if the chargers were used evenly.
Now take those three separate single phase transformers and wind them on a single common 3 leg core. Still 3 separate 120/240V systems, on 3 separate phases, but you are sharing transformer iron, just like a 208/120 transformer feeding a bunch of 120V loads.
Think standard delta:wye transformer, except that you have 3 separate center tapped secondary coils, rather than the secondary coils connected in wye.
The above are the three phase analog to what the transformer distributor was describing.
It turns out that with a proper combination of transformers you can convert _any_ 'polyphase' (say 3 phase) to any other polyphase system, say 2 or 5 or 9 phases. The 'Scott-T' arrangement is a clever coil arrangement that converts between 3 phase and 2 phase. If you have a balanced 2 phase load, it will convert it into a balanced 3 phase load.
If you have a transformer with a Scott-T primary arrangement, you get a 2 phase magnetic field running on the transformer core. Put 2 separate 120/240 coils on each leg, and you get two separate 120/240V systems produced. (Actually, it would be called 2 phase 5 wire, described here: https://forums.mikeholt.com/threads/two-phase-systems.92486/post-1478246 but you are ignoring any voltage relationship between the phases and thinking of the secondary as 2 separate single phase systems.) This is simply the 2 phase version of having separate 120/240V coils on each phase, blended with the Scott-T arrangement to convert to 3 phase.
Something doesn't sound right on the above. As I described, you could have 3 separate standard single phase transformers to connect 3 single phase systems to a 3 phase system in a balanced fashion, but I don't think you can do this with 2 standard transformers to get the balanced loading. Maybe the supplier is just describing putting two separate single phase transformers on two phases.
A Scott-T with a 'common core' or single phase transformers custom tapping arranged in a Scott-T fashion would do the 3 to 2 phase conversion.
This is just a type of 3 phase to 3 phase transformer, that saves money for small transformers by using 2 coils in the Scott-T arrangement. They are going from a 3 phase supply, to a 3 phase load, but via 2 phase magnetic flux. The goal is to save money and be confusing.
As mentioned before, if you have a single phase load, no combination of transformers will convert it into balanced 3 phase loading. So if one single phase feeder is fully loaded and the other empty, then the three phase supply will be unevenly loaded. Remember the question: do you care? In this example, when both feeders are fully loaded, the 3 phase supply will be balanced. If one feeder is fully loaded and the other empty, then the 3 phase supply will not be balanced, but at 50% load; since you are not fully loaded maybe you don't care.
Not sure on the 4-pole disconnect.
The arrangement describe would give you proper 120/240V single phase, 120V to each leg.
Other than maybe needing a 4 pole OCPD for secondary protection, I think it would otherwise be a standard transformer installation.
Personally I'd go with standard transformers; 3 separate circuits to 3 separate single phase transformers used in the standard fashion. That is if 1 standard transformer and unbalanced 3 phase loading was not acceptable.