three phase panel - 240Y/139 instead of 240 delta

| snipped-for-privacy@ipal.net writes: | |>|>| snipped-for-privacy@ipal.net writes: |>| |>| Having the maximum voltage being 139V vs 208V doesn't seem to be much |>| of an advantage. Presumably all the 240V motors are all insulated |>| sufficiently for 240V (or at least 208V). Also load imbalance for |>| lighting off a center tapped phase shouldn't be an issue if the power |>| co. knows of it and installs proper transformers. I've seen a few |>| installations which appear to be 240V open delta with a large transformer |>| with a grounded center tap and a small one for the B phase. (Including |>| several in a residential neighborhood in Baton Rouge LA. Whole house |>| air conditioners using 3 phase?) | |>If going from 139V to 208V (line to ground) is "not much", then going up |>to 277V (line to ground) would be "not much more". So why not run 480Y/277 |>everywhere that three phase is needed. Europeans already use 400Y/231 which |>is in between these. | | That's different. Going from 240 delta to 240Y/139 doesn't change the | voltage the device sees, while 480Y/277 certainly does! The only real | differences are line-to-ground voltages.
But in terms of safety, 480Y/277 is 277 to ground, while 240D-CT is 208 to ground on one phase. The ratio between them is 1.333333 while the ratio between 208 and 139 is 1.5 (using the exact derived values).
|>Sure, proper transformers can allow more lighting power. But then it's not |>a simple delta anymore. But the power company doesn't like this extra cost |>even if it's a small teaser for the B phase. | | Are 139V transformers an off-the-shelf item?
I have seen them in the form of dual winding 277x139. As to whether they are off-the-shelf, I cannot say. If utilities wanted to do 240Y/139 more, they certainly would become so. And it might not be that hard to make a dual-tapped dual-winding (120-139)x(240-277) transformer that could do a wide range of configurations in a single stock (per kVA rating) model.
| (Also if an idiot electrician causes problems if he wires something to | the stinger phase in a 240V delta, consider the same idiot wiring outlets | to a 139Y/240 board. Everything would probably work OK...except | incandescent bulbs would burn out really quickly...and occasionally some | random device will burst into flames...)
There would have to be separate panel boards for 240Y/139 and 208Y/120. If properly marked, I think the error rates would be no worse and might even be less with 240Y/139. An electrician would have to be blind to make a mistake with 240Y/139. But with 240D-CT, he'd have to only be color blind to make a mistake.
They do have a similar issue in Japan. 100 volts is the nominal there. But three phase is 200Y/115. I'm sure they are not mixing things up.
Reply to
phil-news-nospam
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| "Greg" wrote... |> Why "wye"? |> |> I know this might be a good EE test question but it doesn't make much sense | in |> the real world. If you don't have any 139v loads why not delta? | | Once had a situation where 18 high-vacuum cold traps furnished with 230V | 1?-PSC hermetic compressors needed power. Facility 208V was not acceptable for | serving the 230V hermetic compressors. | | Three {one per phase} 120-16V, 3kVA transformers boosted a 208Y120 feeder to | 236Y/136V 3? in a 225A panelboard, which served the 230V compressors in the | cold traps via 2-pole circuit breakers on the rather unique panelboard 3? | bus..
Sounds like an acceptable, and probably common, solution. It would be cheaper than running three larger 208V to 240V isolation transformers.
You could have hit the voltage even closer to either 230 or 240, depending on which you really wanted, with various boost transformers connected to other phases. You'd end up with a "bent tip wye".
------------------------------------------------------------------------ For 228.9 volts:
120V @ 0deg + 13.85640646V @ 30deg = 132.181693134V @ 3.004491599deg 132.181693134 * 1.732050807568877 = 228.94540833856618
B/B is wired for 240 to 16 (can be a 240 to 256 autotransformer): To boost phase A, B/B transformer primary is connected to A-B. To boost phase B, B/B transformer primary is connected to B-C. To boost phase C, B/B transformer primary is connected to C-A. Note that the 240 volt winding is really getting 208 volts so the secondary voltage is reduced accordingly.
------------------------------------------------------------------------ For 231.4 volts:
120V @ 0deg + 24V @ 60deg = 133.626344708V @ 8.948275565deg 133.626344708 * 1.732050807568877 = 231.44761826396854
B/B is wired for 120 to 24 (isolation only): To boost phase A, B/B transformer primary is connected to B-N. To boost phase B, B/B transformer primary is connected to C-N. To boost phase C, B/B transformer primary is connected to A-N.
------------------------------------------------------------------------ For 239.7 volts:
120V @ 0deg + 20.78460969V @ 30deg = 138.390751135V @ 4.306619095deg 138.390751135 * 1.732050807568877 = 239.69981226344021
B/B is wired for 240 to 24 (can be a 240 to 264 autotransformer): To boost phase A, B/B transformer primary is connected to A-B. To boost phase B, B/B transformer primary is connected to B-C. To boost phase C, B/B transformer primary is connected to C-A. Note that the 240 volt winding is really getting 208 volts so the secondary voltage is reduced accordingly.
------------------------------------------------------------------------ For 240.4 volts:
120V @ 0deg + 32V @ 60deg = 138.794812583V @ 11.517515882deg 138.794812583V * 1.732050807568877 = 240.3996672207561
B/B is wired for 120 to 32 (isolation only): To boost phase A, B/B transformer primary is connected to B-N. To boost phase B, B/B transformer primary is connected to C-N. To boost phase C, B/B transformer primary is connected to A-N.
------------------------------------------------------------------------
Reply to
phil-news-nospam
|>There are pluses and minuses for either wye or delta. The big plus for wye |>is that the line to ground voltage is never higher than 139 volts. | | All the more reason why that should just be 120v (208 wye) | Any designer who really wants wye would design for 208. The computer systems I | worked with could be tapped either way with no change in expected reliability.
Adding to my previous followup ...
The load might really be lots of 240 volt single phase loads which would normally be evenly spread over the phases. Not many single phase devices are designed for 208 volts.
I'm sure a computer switched to 240 volt would work fine on 208 volts. But I've definitely seen motors designed for 240 volts burn out when doing a lot of work with only 208 volts.
Reply to
phil-news-nospam
| Why "wye"? | | I know this might be a good EE test question but it doesn't make much sense in | the real world. If you don't have any 139v loads why not delta?
Adding to my previous followup ...
A delta secondary can result in backfeeding the primary when one phase is lost. Other loads on that primary line would then draw from this delta secondary transformer.
Reply to
phil-news-nospam
This was when "computers" had 3 phase motors. They were one of the things that didn't get tapped when we went from 208 to 240.
Reply to
Greg
connection
To produce a delta secondary, the options are a Y-D or D-D. If the area is already Y primaries for single phase 240V, then the same transformers can be used for Y-D 240V. Less types of transformer to stock.
Reply to
Matthew Beasley
|> So why are they Y-D? | To produce a delta secondary, the options are a Y-D or D-D. If the area | is already Y primaries for single phase 240V, then the same transformers | can be used for Y-D 240V. Less types of transformer to stock.
So why are they *-D? E.g. why is having a delta secondary the goal?
Reply to
phil-news-nospam
transformers
Well, I have an answer, but I am not sure it's a good one.... I believe it is so that standard 240V transformers can be used instead of 139V transformers. It also allows 120V single phase loads on the same system, but in many of the pump setups I have seen, the 120V is only used for the contactor coil. In smaller dairies, the 240V red leg delta is used to feed conventional 120V/240V edison three wire single phase plus 240V three phase for motors. Larger diaries use 480V with stepdowns for 120V.
Reply to
Matthew Beasley
|> |> |> |> So why are they Y-D? |> | To produce a delta secondary, the options are a Y-D or D-D. If the | area |> | is already Y primaries for single phase 240V, then the same | transformers |> | can be used for Y-D 240V. Less types of transformer to stock. |> |> So why are they *-D? E.g. why is having a delta secondary the goal? | | Well, I have an answer, but I am not sure it's a good one.... | I believe it is so that standard 240V transformers can be used instead | of 139V transformers. It also allows 120V single phase loads on the | same system, but in many of the pump setups I have seen, the 120V is | only used for the contactor coil. In smaller dairies, the 240V red leg | delta is used to feed conventional 120V/240V edison three wire single | phase plus 240V three phase for motors. Larger diaries use 480V with | stepdowns for 120V.
I can understand the answer. They do make 208 volt motors (but mostly for three phase since availability of 208 on single phase is low), so you could have used 208Y/120. Since 277 volt transformers are common, another option would be 480Y/277 (but the utility could charge more).
If the connection wiring for the motors allows wiring each winding totally separately, one option is to wire the motor with each winding branching off a neutral (e.g. requires WYE for power), and wire the transformer bank as 416Y/240. Same motor, same transformers, but no more backfeeding except what the motors themselves do.
Reply to
phil-news-nospam
I've had 240/138Y specified a few times. Every one was filled with 2P breakers feeding a bank of Level 2 EV chargers, which charge faster if fed 240V as opposed to 208V.
Reply to
Steve
I've had 240/138Y specified a few times. Every one was filled with 2P breakers feeding a bank of Level 2 EV chargers, which charge faster if fed 240V as opposed to 208V.
Reply to
Steve

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