|
Home > Archive > Electrical Engineering > February 2008 > Single phase loads on Open Delta
You are viewing an archived Text-only version of the thread.
To view this thread in it's original format and/or if you want to reply to
this thread please [click here]
| Author |
Single phase loads on Open Delta
|
|
| berry.korsten@gmail.com 2008-02-19, 3:25 am |
| Hi everyone,
Im an electrical engineer in europe,
But now I have to design an Electrical installation where they have a
so called "Open Delta" connection.
I believe it is the same as our 3-phase delta system, exept that one
leg of the transormer is 'missing".
Theoretical I understand the principle, but how to use it in practice?
In the installation there are several pumps (3-phase electrical
motors) wich should not be any problem,
But there are also a lot of single phase consumers
Computers 100% duty cycle,
24VDC generators 100% duty cycle
Heaters (On/Off switching depending of temp demand)
coils
eetc.
How should I devide these consumers between the 3 phases.
a) equaly over all 3 phases
or
b) so that all single phase loads are wired the the phisical windings
(not using the missing leg of the open delta connection)
Can onyone help me with this situation,
Thanks alot in advance,
| |
| phil-news-nospam@ipal.net 2008-02-19, 9:25 am |
| On Tue, 19 Feb 2008 00:51:13 -0800 (PST) berry.korsten@gmail.com wrote:
| Hi everyone,
|
| Im an electrical engineer in europe,
| But now I have to design an Electrical installation where they have a
| so called "Open Delta" connection.
|
| I believe it is the same as our 3-phase delta system, exept that one
| leg of the transormer is 'missing".
| Theoretical I understand the principle, but how to use it in practice?
|
| In the installation there are several pumps (3-phase electrical
| motors) wich should not be any problem,
|
| But there are also a lot of single phase consumers
| Computers 100% duty cycle,
| 24VDC generators 100% duty cycle
| Heaters (On/Off switching depending of temp demand)
| coils
| eetc.
|
| How should I devide these consumers between the 3 phases.
| a) equaly over all 3 phases
| or
| b) so that all single phase loads are wired the the phisical windings
| (not using the missing leg of the open delta connection)
|
| Can onyone help me with this situation,
The open delta does have one side of the delta triangle missing. What that
means for loading is that any loads connected across the open side will use
current through both of the other two secondary windings that are present.
For a single phase load connected on the open end, you have to derate the
capacity of the transformer to one half because each winding can have only
that original current maximum, but you are only getting the regular voltage
while using two windings in "series".
For a three phase balanced load, you have to calculate what the current is
through the windings based on the vector sum of the three phases of load.
Normally, you would connect the single phase loads to only the sides that
have windings, and not to the open side. Three phase loads connect as if
the configuration were a closed delta. Now you just need to add up all of
the currents, using the correct vector formulas, for the two windings that
are present. Once you have the current in each winding, select the one
that is larger and double it to use two identical windings, and multiply
by the voltage (230 or 400 or whatever it is you will be using). Maybe
you will want to go back and rebalance the single phase loads to reduce
the current on the winding with the most.
You will also need to calculate the current on the common corner wire of
the system, which would be the vector difference of the current in the
two windings (which, because the vector angles are quite different, will
be a lot). With all three phase loads it will be the same as the other
two wires. With all single phase loads in balance (and not connected
across the open end), it will be twice as much as each end wire. The mix
of single phase and three phase loads will be somewhere in between.
Connecting single phase loads across the open end would normally not be
done. But if you have more transformer capacity and less neutral/common
wire capacity, connecting single phase loads across the open end would
be a way to lessen the loading on that wire, at the expense of excess
loading on the transformer.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-02-19-0756@ipal.net |
|------------------------------------/-------------------------------------|
| |
| gfretwell@aol.com 2008-02-19, 1:25 pm |
| On Tue, 19 Feb 2008 00:51:13 -0800 (PST), berry.korsten@gmail.com
wrote:
>Hi everyone,
>
>Im an electrical engineer in europe,
>But now I have to design an Electrical installation where they have a
>so called "Open Delta" connection.
>
>I believe it is the same as our 3-phase delta system, exept that one
>leg of the transormer is 'missing".
>Theoretical I understand the principle, but how to use it in practice?
>
>In the installation there are several pumps (3-phase electrical
>motors) wich should not be any problem,
>
>But there are also a lot of single phase consumers
>Computers 100% duty cycle,
>24VDC generators 100% duty cycle
>Heaters (On/Off switching depending of temp demand)
>coils
>eetc.
>
> How should I devide these consumers between the 3 phases.
>a) equaly over all 3 phases
>or
>b) so that all single phase loads are wired the the phisical windings
> (not using the missing leg of the open delta connection)
>
>Can onyone help me with this situation,
>
>Thanks alot in advance,
>
Open delta will be grounded on the centertaop of one of the
transformers, and this is usually larger than the other transformer.
All of your 120v single phase loads need to go on the center tapped
transformer. The "wild leg" will be 208v to ground/neutral.
It is also bad practice to connect L/L loads to the wild leg on open
delta.
| |
| phil-news-nospam@ipal.net 2008-02-19, 1:25 pm |
| On Tue, 19 Feb 2008 12:36:59 -0500 gfretwell@aol.com wrote:
| On Tue, 19 Feb 2008 00:51:13 -0800 (PST), berry.korsten@gmail.com
| wrote:
|
|>Hi everyone,
|>
|>Im an electrical engineer in europe,
|>But now I have to design an Electrical installation where they have a
|>so called "Open Delta" connection.
|>
|>I believe it is the same as our 3-phase delta system, exept that one
|>leg of the transormer is 'missing".
|>Theoretical I understand the principle, but how to use it in practice?
|>
|>In the installation there are several pumps (3-phase electrical
|>motors) wich should not be any problem,
|>
|>But there are also a lot of single phase consumers
|>Computers 100% duty cycle,
|>24VDC generators 100% duty cycle
|>Heaters (On/Off switching depending of temp demand)
|>coils
|>eetc.
|>
|> How should I devide these consumers between the 3 phases.
|>a) equaly over all 3 phases
|>or
|>b) so that all single phase loads are wired the the phisical windings
|> (not using the missing leg of the open delta connection)
|>
|>Can onyone help me with this situation,
|>
|>Thanks alot in advance,
|>
|
| Open delta will be grounded on the centertaop of one of the
| transformers, and this is usually larger than the other transformer.
| All of your 120v single phase loads need to go on the center tapped
| transformer. The "wild leg" will be 208v to ground/neutral.
| It is also bad practice to connect L/L loads to the wild leg on open
| delta.
Given the OP is in Europe, it is unlikely to be a 208/120 setup. Maybe
it will be a 400/230 setup. But maybe it will just be 230 delta with
the corner grounded. The OP should have provided more details.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-02-19-1253@ipal.net |
|------------------------------------/-------------------------------------|
| |
|
| gfretwell@aol.com wrote:
> On Tue, 19 Feb 2008 00:51:13 -0800 (PST), berry.korsten@gmail.com
> wrote:
>
>
> Open delta will be grounded on the centertaop of one of the
> transformers, and this is usually larger than the other transformer.
> All of your 120v single phase loads need to go on the center tapped
> transformer. The "wild leg" will be 208v to ground/neutral.
> It is also bad practice to connect L/L loads to the wild leg on open
> delta.
..
Depending the current rating of the phases, I would think you would want
to balance the loads on different phases. Is it more acceptable to
connect L-L to the high leg with closed delta?
--
bud--
| |
| gfretwell@aol.com 2008-02-20, 1:25 pm |
| On Wed, 20 Feb 2008 01:49:57 -0600, bud-- <remove.budnews@isp.com>
wrote:
>.
>Depending the current rating of the phases, I would think you would want
>to balance the loads on different phases. Is it more acceptable to
>connect L-L to the high leg with closed delta?
>
>--
If you have 3 transformers it is just delta and all phases are
basically the same for L/L loads.
The only place I have seen it is when it "grew" from open delta but
these are utility decisions.
| |
| Don Kelly 2008-02-20, 9:25 pm |
| ----------------------------
"bud--" <remove.budnews@isp.com> wrote in message
news:e1127$47bbda1e$4213ea89$10556@DIALUPUSA.NET...
> gfretwell@aol.com wrote:
>
> .
> Depending the current rating of the phases, I would think you would want
> to balance the loads on different phases. Is it more acceptable to connect
> L-L to the high leg with closed delta?
>
> --
> bud--
Note that for a single phase load across the open leg, you are actually
loading both of the actual legs rather than a single leg. You will have the
same current magnitude in both of the actual legs. If you carry single phase
loads on only the actual legs- then you can handle twice the total load.
Draw a few diagrams and look at the currents.
This problem doesn't occur with closed delta.
--
Don Kelly dhky@shawcross.ca
remove the X to answer
| |
|
| Don Kelly wrote:
> ----------------------------
> "bud--" <remove.budnews@isp.com> wrote in message
> news:e1127$47bbda1e$4213ea89$10556@DIALUPUSA.NET...
>
> Note that for a single phase load across the open leg, you are actually
> loading both of the actual legs rather than a single leg. You will have the
> same current magnitude in both of the actual legs. If you carry single phase
> loads on only the actual legs- then you can handle twice the total load.
> Draw a few diagrams and look at the currents.
Open delta screws up the power factor of the supply transformers so they
have to be derated which I think is what you refer to. My recolleciton
is the derating is 87%, but that would be for equal loading all the way
around.
>
> This problem doesn't occur with closed delta.
..
The loading on the transformers is the problem of the utility. From the
users standpoint, I would think you would also want to load the high leg
within the rating of the service.
My recollection is that Xcel only provides open delta and voltages can
be further out of balance than Y. Heavier loading on the 120/240 side
tends to unbalance the delta. SquareD used to have a quite low
percentage of load on its delta transformers that could be used for
120V. I suspect that was also because of imbalance that could cause
circulating currents in the delta secondary.
--
bud--
| |
| phil-news-nospam@ipal.net 2008-02-22, 8:25 pm |
| On Fri, 22 Feb 2008 14:16:38 -0600 bud-- <remove.budnews@isp.com> wrote:
| Open delta screws up the power factor of the supply transformers so they
| have to be derated which I think is what you refer to. My recolleciton
| is the derating is 87%, but that would be for equal loading all the way
| around.
Yes, that is the derating factor for a balanced three phase load. Actually
closer to 86.6%. For single phase loads on the 2 real legs, no derating of
the transformer is needed. However, the neutral wire needs to be derated.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-02-22-1722@ipal.net |
|------------------------------------/-------------------------------------|
| |
|
| phil-news-nospam@ipal.net wrote:
> On Fri, 22 Feb 2008 14:16:38 -0600 bud-- <remove.budnews@isp.com> wrote:
>
> | Open delta screws up the power factor of the supply transformers so they
> | have to be derated which I think is what you refer to. My recolleciton
> | is the derating is 87%, but that would be for equal loading all the way
> | around.
>
> Yes, that is the derating factor for a balanced three phase load. Actually
> closer to 86.6%.
..
It is even closer to 86.60254%. Obviously it was rounded.
> For single phase loads on the 2 real legs, no derating of
> the transformer is needed.
> However, the neutral wire needs to be derated.
>
Because?
--
bud--
| |
| phil-news-nospam@ipal.net 2008-02-23, 9:27 am |
| On Sat, 23 Feb 2008 02:22:22 -0600 bud-- <remove.budnews@isp.com> wrote:
| phil-news-nospam@ipal.net wrote:
|> On Fri, 22 Feb 2008 14:16:38 -0600 bud-- <remove.budnews@isp.com> wrote:
|>
|> | Open delta screws up the power factor of the supply transformers so they
|> | have to be derated which I think is what you refer to. My recolleciton
|> | is the derating is 87%, but that would be for equal loading all the way
|> | around.
|>
|> Yes, that is the derating factor for a balanced three phase load. Actually
|> closer to 86.6%.
| .
| It is even closer to 86.60254%. Obviously it was rounded.
|
|> For single phase loads on the 2 real legs, no derating of
|> the transformer is needed.
|
|> However, the neutral wire needs to be derated.
|>
| Because?
OK, I finally did the math and it needs to be derated 57.735%.
Consider a 3-wire circuit with a variable phase angle. The line to neutral
voltage is the same on each. The loads are purely resistive and equal.
This results in an expected specific current on each line wire. Consider
a load and voltage such that the line current on each phase is 1.0. The
current on the neutral at an angle of 180 degrees is 0.0. The current on
the neutral at an angle of 0 degrees (that's equivalent to two circuits
on the same phase trying to share a neutral) is 2.0. At angles from 120
to 0, the neutral current will at or more than the line currents. See
the 60 degree difference angle line below:
difference neutral current
angle amps angle
180 0.000000000 ---
175 0.087238775 87.5
170 0.174311485 85.0
165 0.261052384 82.5
160 0.347296355 80.0
155 0.432879228 77.5
150 0.517638090 75.0
145 0.601411599 72.5
140 0.684040287 70.0
135 0.765366865 67.5
130 0.845236523 65.0
125 0.923497226 62.5
120 1.000000000 60.0 see NEC rule 310.15(B)(4)(b)
115 1.074599217 57.5
110 1.147152873 55.0
105 1.217522858 52.5
100 1.285575219 50.0
95 1.351180415 47.5
90 1.414213562 45.0
85 1.474554674 42.5
80 1.532088886 40.0
75 1.586706681 37.5
70 1.638304089 35.0
65 1.686782892 32.5
60 1.732050808 30.0 derate 57.735%
55 1.774021666 27.5
50 1.812615574 25.0
45 1.847759065 22.5
40 1.879385242 20.0
35 1.907433901 17.5
30 1.931851653 15.0
25 1.952592014 12.5
20 1.969615506 10.0
15 1.982889723 7.5
10 1.992389396 5.0
5 1.998096443 2.5
0 2.000000000 0.0
For extreme precision trivia:
57.735026918962576450914878050195745564760175127012687601860232648397767 %
86.602540378443864676372317075293618347140262690519031402790348972596650 %
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-02-23-0416@ipal.net |
|------------------------------------/-------------------------------------|
| |
|
| phil-news-nospam@ipal.net wrote:
> On Sat, 23 Feb 2008 02:22:22 -0600 bud-- <remove.budnews@isp.com> wrote:
> | phil-news-nospam@ipal.net wrote:
> |> On Fri, 22 Feb 2008 14:16:38 -0600 bud-- <remove.budnews@isp.com> wrote:
> |>
> |> | Open delta screws up the power factor of the supply transformers so they
> |> | have to be derated which I think is what you refer to. My recolleciton
> |> | is the derating is 87%, but that would be for equal loading all the way
> |> | around.
> |>
> |> Yes, that is the derating factor for a balanced three phase load. Actually
> |> closer to 86.6%.
> | .
> | It is even closer to 86.60254%. Obviously it was rounded.
> |
> |> For single phase loads on the 2 real legs, no derating of
> |> the transformer is needed.
> |
> |> However, the neutral wire needs to be derated.
> |>
> | Because?
>
> OK, I finally did the math and it needs to be derated 57.735%.
>
> Consider a 3-wire circuit with a variable phase angle.
What are you talking about. This is high leg delta. The neutral is with
a 120/240 single phase circuit and caries the unbalanced hot load. Or
are you trying to feed a 208V load to the high leg?
--
bud--
| |
| Richard Fry 2008-02-24, 8:25 pm |
| While this isn't an exact response to the question asked, the link below
leads to a short paper that might be of some interest and application toward
the subject of open delta vs. closed delta and WYE configurations.
It was written by a former colleague of mine at Harris Broadcast Division
(USA).
http://rfry.org/Software%20Download...figurations.pdf
RF
| |
| phil-news-nospam@ipal.net 2008-02-25, 3:25 am |
| On Sun, 24 Feb 2008 02:10:17 -0600 bud-- <remove.budnews@isp.com> wrote:
| phil-news-nospam@ipal.net wrote:
|> On Sat, 23 Feb 2008 02:22:22 -0600 bud-- <remove.budnews@isp.com> wrote:
|> | phil-news-nospam@ipal.net wrote:
|> |> On Fri, 22 Feb 2008 14:16:38 -0600 bud-- <remove.budnews@isp.com> wrote:
|> |>
|> |> | Open delta screws up the power factor of the supply transformers so they
|> |> | have to be derated which I think is what you refer to. My recolleciton
|> |> | is the derating is 87%, but that would be for equal loading all the way
|> |> | around.
|> |>
|> |> Yes, that is the derating factor for a balanced three phase load. Actually
|> |> closer to 86.6%.
|> | .
|> | It is even closer to 86.60254%. Obviously it was rounded.
|> |
|> |> For single phase loads on the 2 real legs, no derating of
|> |> the transformer is needed.
|> |
|> |> However, the neutral wire needs to be derated.
|> |>
|> | Because?
|>
|> OK, I finally did the math and it needs to be derated 57.735%.
|>
|> Consider a 3-wire circuit with a variable phase angle.
|
| What are you talking about. This is high leg delta. The neutral is with
| a 120/240 single phase circuit and caries the unbalanced hot load. Or
| are you trying to feed a 208V load to the high leg?
I thought it had shifted topic to a 2 leg delta.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-02-24-2215@ipal.net |
|------------------------------------/-------------------------------------|
| |
|
| phil-news-nospam@ipal.net wrote:
> On Sun, 24 Feb 2008 02:10:17 -0600 bud-- <remove.budnews@isp.com> wrote:
> | phil-news-nospam@ipal.net wrote:
> |> On Sat, 23 Feb 2008 02:22:22 -0600 bud-- <remove.budnews@isp.com> wrote:
> |> | phil-news-nospam@ipal.net wrote:
> |> |> On Fri, 22 Feb 2008 14:16:38 -0600 bud-- <remove.budnews@isp.com> wrote:
> |> |>
> |> |> | Open delta screws up the power factor of the supply transformers so they
> |> |> | have to be derated which I think is what you refer to. My recolleciton
> |> |> | is the derating is 87%, but that would be for equal loading all the way
> |> |> | around.
> |> |>
> |> |> Yes, that is the derating factor for a balanced three phase load. Actually
> |> |> closer to 86.6%.
> |> | .
> |> | It is even closer to 86.60254%. Obviously it was rounded.
> |> |
> |> |> For single phase loads on the 2 real legs, no derating of
> |> |> the transformer is needed.
> |> |
> |> |> However, the neutral wire needs to be derated.
> |> |>
> |> | Because?
> |>
> |> OK, I finally did the math and it needs to be derated 57.735%.
> |>
> |> Consider a 3-wire circuit with a variable phase angle.
> |
> | What are you talking about. This is high leg delta. The neutral is with
> | a 120/240 single phase circuit and caries the unbalanced hot load. Or
> | are you trying to feed a 208V load to the high leg?
>
> I thought it had shifted topic to a 2 leg delta.
>
I should have said open delta (2 leg) in a high leg delta configuration
(neutral at the centertap of one of the transformers). I don't think
there is properly a neutral in a corner grounded delta. I don't know
what else you are talking about.
--
bud--
| |
| phil-news-nospam@ipal.net 2008-02-25, 9:25 am |
| On Mon, 25 Feb 2008 00:02:39 -0600 bud-- <remove.budnews@isp.com> wrote:
| phil-news-nospam@ipal.net wrote:
|> On Sun, 24 Feb 2008 02:10:17 -0600 bud-- <remove.budnews@isp.com> wrote:
|> | phil-news-nospam@ipal.net wrote:
|> |> On Sat, 23 Feb 2008 02:22:22 -0600 bud-- <remove.budnews@isp.com> wrote:
|> |> | phil-news-nospam@ipal.net wrote:
|> |> |> On Fri, 22 Feb 2008 14:16:38 -0600 bud-- <remove.budnews@isp.com> wrote:
|> |> |>
|> |> |> | Open delta screws up the power factor of the supply transformers so they
|> |> |> | have to be derated which I think is what you refer to. My recolleciton
|> |> |> | is the derating is 87%, but that would be for equal loading all the way
|> |> |> | around.
|> |> |>
|> |> |> Yes, that is the derating factor for a balanced three phase load. Actually
|> |> |> closer to 86.6%.
|> |> | .
|> |> | It is even closer to 86.60254%. Obviously it was rounded.
|> |> |
|> |> |> For single phase loads on the 2 real legs, no derating of
|> |> |> the transformer is needed.
|> |> |
|> |> |> However, the neutral wire needs to be derated.
|> |> |>
|> |> | Because?
|> |>
|> |> OK, I finally did the math and it needs to be derated 57.735%.
|> |>
|> |> Consider a 3-wire circuit with a variable phase angle.
|> |
|> | What are you talking about. This is high leg delta. The neutral is with
|> | a 120/240 single phase circuit and caries the unbalanced hot load. Or
|> | are you trying to feed a 208V load to the high leg?
|>
|> I thought it had shifted topic to a 2 leg delta.
|>
| I should have said open delta (2 leg) in a high leg delta configuration
| (neutral at the centertap of one of the transformers). I don't think
| there is properly a neutral in a corner grounded delta. I don't know
| what else you are talking about.
A corner grounded delta identifies the grounded corner at least as the
grounded conductor. That can therefore be termed the neutral, as well,
even though the meaning of "neutrality" is less applicable. The list
of conductor currents I gave applies to that configuration, regardless
of whether the transformer arrangement is open delta or closed delta.
It applies to the corner conductor even if it is not the grounded one.
So if you have a 240 delta with a center tap grounded, and put single
phase 240 volt loads on just TWO of the sides (for example, A-B and B-C)
then the table applies to the corner conductor (B for this example).
Since the delta has 60 degree corners, you use the 60 degree line. If
the angle is different, the calculation is different. For example, for
2 wires and center of wye, it is 120 degrees, so use the 120 degree line.
If you have an old 2-phase system at 90 degrees, use the 90 degree line.
Single phase would use the 180 degree or 0 degree line depending on how
you connect the loads.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-02-25-0823@ipal.net |
|------------------------------------/-------------------------------------|
| |
|
| phil-news-nospam@ipal.net wrote:
> On Mon, 25 Feb 2008 00:02:39 -0600 bud-- <remove.budnews@isp.com> wrote:
> | phil-news-nospam@ipal.net wrote:
> |> On Sun, 24 Feb 2008 02:10:17 -0600 bud-- <remove.budnews@isp.com> wrote:
> |> | phil-news-nospam@ipal.net wrote:
> |> |> On Sat, 23 Feb 2008 02:22:22 -0600 bud-- <remove.budnews@isp.com> wrote:
> |> |> | phil-news-nospam@ipal.net wrote:
> |> |> |> On Fri, 22 Feb 2008 14:16:38 -0600 bud-- <remove.budnews@isp.com> wrote:
> |> |> |>
> |> |> |> | Open delta screws up the power factor of the supply transformers so they
> |> |> |> | have to be derated which I think is what you refer to. My recolleciton
> |> |> |> | is the derating is 87%, but that would be for equal loading all the way
> |> |> |> | around.
> |> |> |>
> |> |> |> Yes, that is the derating factor for a balanced three phase load. Actually
> |> |> |> closer to 86.6%.
> |> |> | .
> |> |> | It is even closer to 86.60254%. Obviously it was rounded.
> |> |> |
> |> |> |> For single phase loads on the 2 real legs, no derating of
> |> |> |> the transformer is needed.
> |> |> |
> |> |> |> However, the neutral wire needs to be derated.
> |> |> |>
> |> |> | Because?
> |> |>
> |> |> OK, I finally did the math and it needs to be derated 57.735%.
> |> |>
> |> |> Consider a 3-wire circuit with a variable phase angle.
> |> |
> |> | What are you talking about. This is high leg delta. The neutral is with
> |> | a 120/240 single phase circuit and caries the unbalanced hot load. Or
> |> | are you trying to feed a 208V load to the high leg?
> |>
> |> I thought it had shifted topic to a 2 leg delta.
> |>
> | I should have said open delta (2 leg) in a high leg delta configuration
> | (neutral at the centertap of one of the transformers). I don't think
> | there is properly a neutral in a corner grounded delta. I don't know
> | what else you are talking about.
>
> A corner grounded delta identifies the grounded corner at least as the
> grounded conductor. That can therefore be termed the neutral, as well,
> even though the meaning of "neutrality" is less applicable.
..
I don't see how grounding a phase conductor makes it a neutral.
310.15(B)(4)(b)applies to wye systems.
I haven't seen where a neutral wire has to be derated.
--
bud--
| |
| gfretwell@aol.com 2008-02-25, 5:25 pm |
| On Mon, 25 Feb 2008 11:56:22 -0600, bud-- <remove.budnews@isp.com>
wrote:
>I don't see how grounding a phase conductor makes it a neutral.
True, this is one if not the only time when the grounded conductor is
not a neutral (carrying the unbalanced current) although "neutral" is
never really defined in the NEC.
Corner delta is not seen a lot but it does show up in sewer lift
stations and other places where you only need 3 phase.
It has the advantage of being able to use simpler equipment although,
to the initiated it might trick you into thinking you were looking at
single phase. You will see 2 ungrounded conductors, a 2 pole breaker
and a white one going to the ground bus. The first tip off would be no
bare grounding conductor from the utility.
| |
| phil-news-nospam@ipal.net 2008-02-25, 9:25 pm |
| On Mon, 25 Feb 2008 11:56:22 -0600 bud-- <remove.budnews@isp.com> wrote:
| phil-news-nospam@ipal.net wrote:
|> On Mon, 25 Feb 2008 00:02:39 -0600 bud-- <remove.budnews@isp.com> wrote:
|> | phil-news-nospam@ipal.net wrote:
|> |> On Sun, 24 Feb 2008 02:10:17 -0600 bud-- <remove.budnews@isp.com> wrote:
|> |> | phil-news-nospam@ipal.net wrote:
|> |> |> On Sat, 23 Feb 2008 02:22:22 -0600 bud-- <remove.budnews@isp.com> wrote:
|> |> |> | phil-news-nospam@ipal.net wrote:
|> |> |> |> On Fri, 22 Feb 2008 14:16:38 -0600 bud-- <remove.budnews@isp.com> wrote:
|> |> |> |>
|> |> |> |> | Open delta screws up the power factor of the supply transformers so they
|> |> |> |> | have to be derated which I think is what you refer to. My recolleciton
|> |> |> |> | is the derating is 87%, but that would be for equal loading all the way
|> |> |> |> | around.
|> |> |> |>
|> |> |> |> Yes, that is the derating factor for a balanced three phase load. Actually
|> |> |> |> closer to 86.6%.
|> |> |> | .
|> |> |> | It is even closer to 86.60254%. Obviously it was rounded.
|> |> |> |
|> |> |> |> For single phase loads on the 2 real legs, no derating of
|> |> |> |> the transformer is needed.
|> |> |> |
|> |> |> |> However, the neutral wire needs to be derated.
|> |> |> |>
|> |> |> | Because?
|> |> |>
|> |> |> OK, I finally did the math and it needs to be derated 57.735%.
|> |> |>
|> |> |> Consider a 3-wire circuit with a variable phase angle.
|> |> |
|> |> | What are you talking about. This is high leg delta. The neutral is with
|> |> | a 120/240 single phase circuit and caries the unbalanced hot load. Or
|> |> | are you trying to feed a 208V load to the high leg?
|> |>
|> |> I thought it had shifted topic to a 2 leg delta.
|> |>
|> | I should have said open delta (2 leg) in a high leg delta configuration
|> | (neutral at the centertap of one of the transformers). I don't think
|> | there is properly a neutral in a corner grounded delta. I don't know
|> | what else you are talking about.
|>
|> A corner grounded delta identifies the grounded corner at least as the
|> grounded conductor. That can therefore be termed the neutral, as well,
|> even though the meaning of "neutrality" is less applicable.
| .
| I don't see how grounding a phase conductor makes it a neutral.
|
| 310.15(B)(4)(b)applies to wye systems.
|
| I haven't seen where a neutral wire has to be derated.
You will when it burns up. I've explained the theory. I don't see why I
should re-explain it.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-02-25-2105@ipal.net |
|------------------------------------/-------------------------------------|
| |
| phil-news-nospam@ipal.net 2008-02-26, 3:25 am |
| On Mon, 25 Feb 2008 15:18:29 -0500 gfretwell@aol.com wrote:
| On Mon, 25 Feb 2008 11:56:22 -0600, bud-- <remove.budnews@isp.com>
| wrote:
|
|>I don't see how grounding a phase conductor makes it a neutral.
|
| True, this is one if not the only time when the grounded conductor is
| not a neutral (carrying the unbalanced current) although "neutral" is
| never really defined in the NEC.
|
| Corner delta is not seen a lot but it does show up in sewer lift
| stations and other places where you only need 3 phase.
| It has the advantage of being able to use simpler equipment although,
| to the initiated it might trick you into thinking you were looking at
| single phase. You will see 2 ungrounded conductors, a 2 pole breaker
| and a white one going to the ground bus. The first tip off would be no
| bare grounding conductor from the utility.
I've noted that special breakers are required, even though 2-pole, for
corner grounded delta. I suppose that is probably because of 240 volts
between line and ground. NEC 240.85 spells this out without reasons.
I also note that the short circuit interruption ratings are lower for
CGD. It is not specified whether it matters if the system is open delta
or closed delta.
As long as the load is balanced three phase pulling the same current on
each wire, there is no need for a derating. But if the load is simply
two single phase loads, each pulling 100 amps, the conductor in common
will have a current of 173.2 amps. It sure isn't very neutral and any
labeling as such would be misleading. But it is still common to label
the grounded conductor also as the neutral.
I could argue that in a system based on 2 phases connected from a 3 phase
wye system (as one might see for single phase services to apartments and
condos in a large residential building), the grounded conductor there is
also not validly labeled a neutral. NEC 310.15(B)(4)(b) refers to the
"common conductor" and to "line-to-neutral loads", suggesting that they
don't want to call it a neutral in this case, but acknowledge that the
term gets used. A 100 amp load on each of the two phases in that system
results in 100 amps of current on the common conductor, requiring that it
be counted in multi conductor derating where a neutral in a true single
phase system would not need to be.
There is not a lot of code in the NEC related to a corner grounded delta
or even delta in general, other than mid-point grounded delta. None even
considers single phase loads at all. Presumably no one would conenct
such loads? But you never know. Someone could be fooled by CGD being
wired through what is essentially a 2-pole panel with 2-pole breakers.
Even if they knew it was CGD, they might assume it can't be worse than
the 2"-phase wye" situation (even though it is worse).
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-02-25-2111@ipal.net |
|------------------------------------/-------------------------------------|
| |
|
| phil-news-nospam@ipal.net wrote:
> On Mon, 25 Feb 2008 11:56:22 -0600 bud-- <remove.budnews@isp.com> wrote:
> | phil-news-nospam@ipal.net wrote:
> |> On Mon, 25 Feb 2008 00:02:39 -0600 bud-- <remove.budnews@isp.com> wrote:
> |> | phil-news-nospam@ipal.net wrote:
> |> |> On Sun, 24 Feb 2008 02:10:17 -0600 bud-- <remove.budnews@isp.com> wrote:
> |> |> | phil-news-nospam@ipal.net wrote:
> |> |> |> On Sat, 23 Feb 2008 02:22:22 -0600 bud-- <remove.budnews@isp.com> wrote:
> |> |> |> | phil-news-nospam@ipal.net wrote:
> |> |> |> |> On Fri, 22 Feb 2008 14:16:38 -0600 bud-- <remove.budnews@isp.com> wrote:
> |> |> |> |>
> |> |> |> |> | Open delta screws up the power factor of the supply transformers so they
> |> |> |> |> | have to be derated which I think is what you refer to. My recolleciton
> |> |> |> |> | is the derating is 87%, but that would be for equal loading all the way
> |> |> |> |> | around.
> |> |> |> |>
> |> |> |> |> Yes, that is the derating factor for a balanced three phase load. Actually
> |> |> |> |> closer to 86.6%.
> |> |> |> | .
> |> |> |> | It is even closer to 86.60254%. Obviously it was rounded.
> |> |> |> |
> |> |> |> |> For single phase loads on the 2 real legs, no derating of
> |> |> |> |> the transformer is needed.
> |> |> |> |
> |> |> |> |> However, the neutral wire needs to be derated.
> |> |> |> |>
> |> |> |> | Because?
> |> |> |>
> |> |> |> OK, I finally did the math and it needs to be derated 57.735%.
> |> |> |>
> |> |> |> Consider a 3-wire circuit with a variable phase angle.
> |> |> |
> |> |> | What are you talking about. This is high leg delta. The neutral is with
> |> |> | a 120/240 single phase circuit and caries the unbalanced hot load. Or
> |> |> | are you trying to feed a 208V load to the high leg?
> |> |>
> |> |> I thought it had shifted topic to a 2 leg delta.
> |> |>
> |> | I should have said open delta (2 leg) in a high leg delta configuration
> |> | (neutral at the centertap of one of the transformers). I don't think
> |> | there is properly a neutral in a corner grounded delta. I don't know
> |> | what else you are talking about.
> |>
> |> A corner grounded delta identifies the grounded corner at least as the
> |> grounded conductor. That can therefore be termed the neutral, as well,
> |> even though the meaning of "neutrality" is less applicable.
> | .
> | I don't see how grounding a phase conductor makes it a neutral.
> |
> | 310.15(B)(4)(b)applies to wye systems.
> |
> | I haven't seen where a neutral wire has to be derated.
>
> You will when it burns up. I've explained the theory. I don't see why I
> should re-explain it.
..
If you are talking about corner grounded delta:
- it has no "neutral"
- as gfretwell wrote it normally powers 3 phase loads; the grounded and
phase conductors are the same size
- if A is grounded and only A-B and A-C are supplied, the *grounded*
conductor is sized larger than the phases to match the current it
caries; the wire is not derated
Wires have their current rating derated for temperature.
Transformers (like open delta) have their power rating derated for power
factor.
You appear to have a strange use for "derated". Your intent would have
been clearer without the table. It is well known how to add 3 phase
currents and voltages.
--
bud--
| |
| phil-news-nospam@ipal.net 2008-02-26, 9:25 am |
| On Tue, 26 Feb 2008 02:56:32 -0600 bud-- <remove.budnews@isp.com> wrote:
| If you are talking about corner grounded delta:
| - it has no "neutral"
But the grounded conductor:
1. still gets called neutral, anyway
2. can be wired as simply as single phase
3. can be confused for single phase by how it is wired
| - as gfretwell wrote it normally powers 3 phase loads; the grounded and
| phase conductors are the same size
See #3 above.
| - if A is grounded and only A-B and A-C are supplied, the *grounded*
| conductor is sized larger than the phases to match the current it
| caries; the wire is not derated
|
| Wires have their current rating derated for temperature.
| Transformers (like open delta) have their power rating derated for power
| factor.
Even for unity power factor?
| You appear to have a strange use for "derated". Your intent would have
| been clearer without the table. It is well known how to add 3 phase
| currents and voltages.
For any phase angle? It seems some people don't know the phase angles.
The term "derating" seems to be rather misplaced here. When you have a
higher current load on certain transformer windings, you call it "derating"
to correct it, but when you have a higher current load on a wire you do not?
Go back and look at the table I gave. There are 5 common cases where the
current adds differently resulting in different currents on the common
conductor. The common phase answers are 180, 120, 90, 60, and 0. I say
common even though for 90 degrees, you aren't going to see much in the
way of 2-phase/4-phase power anymore (it is common in the sense that the
other phase angles probably were never used outside of research labs).
And the 0 angle is a case of usage of the same phase as two separate line
conductors and a single common conductor (certainly not very neutral).
Now as you consider all these phase angles, and the variation in current
on the common conductor, you want to vary the terms such as whether neutral
is used, or derating is used?
Actually the whole term "derating" is misused, anyway. But that's another
whole argument thread ... and unlikely to ever change anything since bad
usage, once established, never goes away.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-02-26-0749@ipal.net |
|------------------------------------/-------------------------------------|
| |
|
| phil-news-nospam@ipal.net wrote:
> On Tue, 26 Feb 2008 02:56:32 -0600 bud-- <remove.budnews@isp.com> wrote:
>
> | If you are talking about corner grounded delta:
> | - it has no "neutral"
>
> But the grounded conductor:
> 1. still gets called neutral, anyway
> 2. can be wired as simply as single phase
> 3. can be confused for single phase by how it is wired
>
> | - as gfretwell wrote it normally powers 3 phase loads; the grounded and
> | phase conductors are the same size
>
> See #3 above.
..
Corner grounded delta is not common and likely to only be seen by
competent people. I doubt the people who use it
1. call the grounded conductor a neutral
3. are confused what it is
..
> | - if A is grounded and only A-B and A-C are supplied, the *grounded*
> | conductor is sized larger than the phases to match the current it
> | caries; the wire is not derated
> |
> | Wires have their current rating derated for temperature.
>
>
> | Transformers (like open delta) have their power rating derated for power
> | factor.
>
> Even for unity power factor?
..
Dumb questions don’t help.
..
>
> | You appear to have a strange use for "derated". Your intent would have
> | been clearer without the table. It is well known how to add 3 phase
> | currents and voltages.
>
> For any phase angle? It seems some people don't know the phase angles.
..
This is an electrical engineering newsgroup.
What you were saying would have been clearer if you just said the 2
resistive loads produced a current of 1.73x on the common lead. This is
familiar to anyone who is minimally competent with 3 phase.
..
>
> Actually the whole term "derating" is misused, anyway. But that's another
> whole argument thread ... and unlikely to ever change anything since bad
> usage, once established, never goes away.
..
Derating of conductors is a central issue for wiring design in the NEC.
Derating of transformers for power factor is well understood. You seem
to be the only one misusing “derating”.
--
bud--
| |
| phil-news-nospam@ipal.net 2008-02-26, 9:25 pm |
| On Tue, 26 Feb 2008 13:32:41 -0600 bud-- <remove.budnews@isp.com> wrote:
|> For any phase angle? It seems some people don't know the phase angles.
| .
| This is an electrical engineering newsgroup.
Last I checked, EE schools expected students to be able to do math.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-02-26-2029@ipal.net |
|------------------------------------/-------------------------------------|
| |
|
| phil-news-nospam@ipal.net wrote:
> On Tue, 26 Feb 2008 13:32:41 -0600 bud-- <remove.budnews@isp.com> wrote:
>
> |> For any phase angle? It seems some people don't know the phase angles.
> | .
> | This is an electrical engineering newsgroup.
>
> Last I checked, EE schools expected students to be able to do math.
>
That is my point. People here can do the math.
--
bud--
| |
| phil-news-nospam@ipal.net 2008-02-27, 9:25 am |
| On Wed, 27 Feb 2008 03:10:32 -0600 bud-- <remove.budnews@isp.com> wrote:
| phil-news-nospam@ipal.net wrote:
|> On Tue, 26 Feb 2008 13:32:41 -0600 bud-- <remove.budnews@isp.com> wrote:
|>
|> |> For any phase angle? It seems some people don't know the phase angles.
|> | .
|> | This is an electrical engineering newsgroup.
|>
|> Last I checked, EE schools expected students to be able to do math.
|>
| That is my point. People here can do the math.
I guess some can, and some can't.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-02-27-0858@ipal.net |
|------------------------------------/-------------------------------------|
|
|
|
|
|