240 3ph ->480 Xfmr

[templdl]

The secondary of the two transformers would still be open delta. IMO running that through the vfd would negate the problem with open delta.

But why use two transformers if you can get by with one?

It would be an open delta is you used (2) 1Ph step-ups I suppose. But you could add a 3rd 1ph transformer and have a closed delta out and gain some reliablity whould one transforemr fail and you could keep running on two. If one out of 2 transformers fail your a left with one. If you have 3 and loose one your still have 2.
I'm concerned about if you loose one transformer with an open delta you're down. Then what is the labor and the transformer cost on possibly an emergancy basis to get a replacement trnasformer? What are your thoughts?

 

[Open Neutral]

1. The problem the open delta may present is imbalance, per the Franklin AIM page 43, & mentions here and elsewhere.
3. I'm trying to model in my head if given my open delta 240V PoCo supply; using open delta 240->480 may exacerbate the imbalance issue vs a conventional 3∅ 240->480 xfmr.
5. Worst case, a 3ph 240delta->480wye is far less than replacing the well pump [say $5K]....
7. If I buy two single-phase 240->480 transformers, and they have multiple taps on one side or the other [i.e. ~5% change], I can change those to help balance the current in the three motor phases, if needed. Looks like I can get NOS transformers in the $150+ each range.
9. I'd SWAG that a bigger single-phase 240->480, a VFD & a load reactor would be in the same ballpark as the other solutions, and have other issues.
11. Yes, losing one of the open delta xfmrs would disable the well pump. But the house will have 20,000 gals of gravity-feed tanked water. Further, this is the required legal but operationally secondary water source; there's a different well with a Grundfos pump that is solar-panel powered; it has less iron.
13. The EPS-200 panel that is in place now {..with no power source..} provides overload, open phase & phase reversal protection.
15. I have no problems with corner grounding; given your recommendations. As I said, it's something I've only read of, and thought it was obsolete & depreciated in new work.

Some backstory: We had planned to get 480 wye from well-known NorCal Po&GasCo, and bought & installed the pump. [You must show the well in action on a generator for 24H to get the building permit *application* looked at.] But then the PoCo $$ escalated out the ying-yang. [$65k]. They said we could get 208-wye but equally [non-polite word here] unaffordable. So we, after much discussion, reluctantly signed for single-phase. Only after much work was done did the PoCo say "Oh, by the way, you could have ordered open delta on a resi account....." More [n p w h] came from my mouth; the general contractor thought I was about to go postal. The owner/client/builder is of far calmer demeanor but also entertained visions of things such as waterboarding the PoCo CEO.

 

[kwired]

Couple questions.

1. It is my understanding this 2 hp submersible is the only three phase load, why wasn't a single phase submersible installed in the first place? Doesn't sound like there is any need for 480 volt three phase and 2 hp is like having pennies in your pocket on a 480 volt system of almost any capacity worth building. I know they make them in at least 3 HP single phase. Put it on a mono drive and you will probably never pull the pump for an electrical problem.


Since we have the three phase issue - why are we concerned about unbalanced current? The only unbalanced current is on the supply side of the transformer on the two phases used to supply the open delta. And again 2hp is almost nothing for the POCO, and on your step up - the two transformers will need be sized larger than if there were three with full delta, but again 2 hp is not going to require very big transformers either way. If properly designed the motor should still be very close to balanced current whether powered by open delta, full delta or wye.

 

[Open Neutral]

Couple questions.

1. It is my understanding this 2 hp submersible is the only three phase load, why wasn't a single phase submersible installed in the first place? Doesn't sound like there is any need for 480 volt three phase and 2 hp is like having pennies in your pocket on a 480 volt system of almost any capacity worth building. I know they make them in at least 3 HP single phase. Put it on a mono drive and you will probably never pull the pump for an electrical problem.

The swimming pool/lap pool/water slide/water fall gear will also be 3∅. Ditto a large deluge pump for soaking down the roof/sidewalls during wildfires. Also the ground source heat pumps. Neighbors with fair-sized single-phase loads suffer from light-blinking; we hope to minimize that by having bigger loads 3-phase.

The 480 vs 208 was because we're a long way from the pole line, and also from the solar array field we're installing; and wanted to keep losses down.

Since we have the three phase issue - why are we concerned about unbalanced current? The only unbalanced current is on the supply side of the transformer on the two phases used to supply the open delta. And again 2hp is almost nothing for the POCO, and on your step up - the two transformers will need be sized larger than if there were three with full delta, but again 2 hp is not going to require very big transformers either way. If properly designed the motor should still be very close to balanced current whether powered by open delta, full delta or wye.

I'm concerned because motor manufacturers are. They mention PoCo Open Delta voltage disparities may well be an issue.
From the Franklin AIM:

A full three-phase supply is recommended for all threephase
motors, consisting of three individual transformers
or one three-phase transformer. So-called ?open? delta
or Wye connections using only two transformers can be
used, but are more likely to cause problems, such as
poor performance, overload tripping or early motor failure
due to current unbalance.

Current unbalance should not exceed 5% at service
factor load or 10% at rated input load. If the unbalance
cannot be corrected by rolling leads, the source of
the unbalance must be located and corrected. If, on
the three possible hookups, the leg farthest from the
average stays on the same power lead, most of the
unbalance is coming from the ?power side? of the
system. If the reading farthest from average moves
with the same motor lead, the primary source of
unbalance is on the ?motor side? of the starter. In this
instance, consider a damaged cable, leaking splice,
poor connection, or faulty motor winding.

 

[kwired]

The swimming pool/lap pool/water slide/water fall gear will also be 3∅. Ditto a large deluge pump for soaking down the roof/sidewalls during wildfires. Also the ground source heat pumps. Neighbors with fair-sized single-phase loads suffer from light-blinking; we hope to minimize that by having bigger loads 3-phase.

The 480 vs 208 was because we're a long way from the pole line, and also from the solar array field we're installing; and wanted to keep losses down.





I'm concerned because motor manufacturers are. They mention PoCo Open Delta voltage disparities may well be an issue.
From the Franklin AIM:

A full three-phase supply is recommended for all threephase
motors, consisting of three individual transformers
or one three-phase transformer. So-called ?open? delta
or Wye connections using only two transformers can be
used, but are more likely to cause problems, such as
poor performance, overload tripping or early motor failure
due to current unbalance.

Current unbalance should not exceed 5% at service
factor load or 10% at rated input load. If the unbalance
cannot be corrected by rolling leads, the source of
the unbalance must be located and corrected. If, on
the three possible hookups, the leg farthest from the
average stays on the same power lead, most of the
unbalance is coming from the ?power side? of the
system. If the reading farthest from average moves
with the same motor lead, the primary source of
unbalance is on the ?motor side? of the starter. In this
instance, consider a damaged cable, leaking splice,
poor connection, or faulty motor winding.

I have seen much more imbalance from phase converters than from any open delta system. I have never seen unbalance from an open delta that was significant enough to be concerned about anything. There are a lot of open delta systems around here, both 240 and 480 volt.

 

[Open Neutral]

I have seen much more imbalance from phase converters than from any open delta system. I have never seen unbalance from an open delta that was significant enough to be concerned about anything. There are a lot of open delta systems around here, both 240 and 480 volt.

OK, thanks. That's reassuring.

 

[Open Neutral]

Just had a killer idea re: cake & eating it, too.

I'd just get 3 single-phase 2 kva xfmrs with 277 secondaries and create 480volt wye.

But of course, compared to 240-480 ones, there's little supply of those around.....

All my best ideas keep running headfirst into reality, sigh...

 

[Open Neutral]

I think it's been discussed here previously and I have no technical data to back what I'm saying so you can take it with a grain of salt, but, submersible pumps don't seen to like open delta supplies.

This older thread mentions that issue....

 

[kwired]

Just had a killer idea re: cake & eating it, too.

I'd just get 3 single-phase 2 kva xfmrs with 277 secondaries and create 480volt wye.

But of course, compared to 240-480 ones, there's little supply of those around.....

All my best ideas keep running headfirst into reality, sigh...

Those will be harder to find and will probably have a higher price because they are not common. Besides a three phase unit with one core will likely cost less.

2 HP should only need around 3 kVA total. I would think 3 - 1 kVA units would work, but you seem to be against using delta configuration. If only using 2 transformers they will need sized larger - Not quite sure how much larger the 2 kVA units may work.

If it were me I would have to make a decision between either single unit transformer or open delta made from two separate 1 phase units, OR pull pump and replace with correct voltage. All are going to be in about same price range.

I do like the derive 480 volts single phase and use a VFD. The VFD likely requires repair or replacement much more often than a transformer so that makes it a decision based on longer time period. Transformers could last well beyond 30 years if not undersized. But by then they may be obsolete who knows? We might have something better for power transformation.

 

[Open Neutral]

Those will be harder to find and will probably have a higher price because they are not common. Besides a three phase unit with one core will likely cost less.

New 3∅, yes. But I'm finding good selection/pricing on 480->240 1∅, NOS, etc. We're willing to risk non shiny-new-in-box iron.

2 HP should only need around 3 kVA total. I would think 3 - 1 kVA units would work, but you seem to be against using delta configuration. If only using 2 transformers they will need sized larger - Not quite sure how much larger the 2 kVA units may work.

The AIM Table 4 tells me I need 4 KVA total; two @ 2KVA, or three @ 1.5 KVA. But I'm finding 2KVA units are out there; I'll get 3. I'd like to have tapped (not center-tapped) but we'll see.

If it were me I would have to make a decision between either single unit transformer or open delta made from two separate 1 phase units, OR pull pump and replace with correct voltage. All are going to be in about same price range.

My back of the envelope makes three single-phase 2KVA units to be far cheaper; the difference from two to three is not a killer.

I'm OK with corner-ground delta given your reassurances. As I said, my reluctance is just one of no experience with corner-grounded systems. First glance says the existing motor controller will work fine.

Here is a code question. The 240->480 transformers will be in a building ~100 ft away from the wellhead panel. Will we be required to have overcurrent protection on/at the xfmr secondaries, or only the 240V primaries?

I do like the derive 480 volts single phase and use a VFD. The VFD likely requires repair or replacement much more often than a transformer so that makes it a decision based on longer time period. Transformers could last well beyond 30 years if not undersized.

This to me looks less desirable. While it avoids the phase-imbalance issue, it may create RFI and will put unequal-over-a-cycle load on the house grid. Not sure how the solar grid tie inverters and other electronics will like that. [I can relate a major tale of woe re: that under another Subject...]

But by then they may be obsolete who knows? We might have something better for power transformation.

I keep wondering if/when we will see switcher supplies as pole-pigs & such. I recall reading that several percent of the total grid is wasted in distribution transformer losses. From what we've seen in laptop supplies/wall-warts, if you make enough switchers, the price falls dramatically. Maybe someday soon, the TCO lines will cross....


ps: found this PGE note on power imbalance.

 

[mike_kilroy]

The AIM Table 4 tells me I need 4 KVA total; two @ 2KVA, or three @ 1.5 KVA.

I am just curious where this AIM table 4 can be found and who wrote it?

I wonder how someone can justify saying a 2hp motor requires 4kva transformer rather than 2kva?

Someone please provide link to this. thanks.

 

[ptonsparky]

try http://www.franklin-electric.com/media/documents/M1311_60_Hz_AIM_Catalog.pdf