# Is it 3-phase?

A colleague and I are having some disagreement on what configuration the power system we are working on. Kindly look at my (ugly) drawing at
http://img.photobucket.com/albums/v395/angkin/eestuff/power01.jpg
On the left is what we saw outside the building, two transformers connected as such. On the right is the power panel inside the building. The two ground symbols at the transformers are actually solidly connected and buried to the ground.
My colleague, Jimmy, said "It is single-phase because on the primary there is no 3rd phase line, just 2 phase lines and ground".
When I took voltage measurements on the power panel, on the breaker terminals, they read 231, 232, and 235 VAC. I also measured against the ground bus and read 231, 0, and 232. I said "It is a 3-phase, open-delta, common-grounded".
Jimmy later talked to a guy at the power company and was told it is single-phase and that a 3-phase motor won't work. He was also shown a diagram and now he says it is V-phase, not 3-phase, and 3-phase motor won't work. We both agree that V-phase is another name for open-delta, yet he insists motor won't work because the voltages are not 120 degrees apart.
I told Jimmy I am not convinced and I'm sticking to my guess. I told him that a delta 3-phase motor will work, but a Y-motor will only work if the neutral is not grounded.
Jimmy will bring a delta 3-phase motor on Tuesday to finally settle this.
So, experts .... is it 3-phase? or single phase as Jimmy initially asserted? Or a V-phase where motor won't work? Will Jimmy's motor rotate or not?
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| A colleague and I are having some disagreement on what configuration | the power system we are working on. Kindly look at my (ugly) drawing at | |
http://img.photobucket.com/albums/v395/angkin/eestuff/power01.jpg | | On the left is what we saw outside the building, two transformers | connected as such. On the right is the power panel inside the building. | The two ground symbols at the transformers are actually solidly | connected and buried to the ground. | | My colleague, Jimmy, said "It is single-phase because on the primary | there is no 3rd phase line, just 2 phase lines and ground". | | When I took voltage measurements on the power panel, on the breaker | terminals, they read 231, 232, and 235 VAC. I also measured against the | ground bus and read 231, 0, and 232. I said "It is a 3-phase, | open-delta, common-grounded". | | Jimmy later talked to a guy at the power company and was told it is | single-phase and that a 3-phase motor won't work. He was also shown a | diagram and now he says it is V-phase, not 3-phase, and 3-phase motor | won't work. We both agree that V-phase is another name for open-delta, | yet he insists motor won't work because the voltages are not 120 | degrees apart. | | I told Jimmy I am not convinced and I'm sticking to my guess. I told | him that a delta 3-phase motor will work, but a Y-motor will only work | if the neutral is not grounded. | | Jimmy will bring a delta 3-phase motor on Tuesday to finally settle | this. | | So, experts .... is it 3-phase? or single phase as Jimmy initially | asserted? Or a V-phase where motor won't work? Will Jimmy's motor | rotate or not?
It looks like corner grounded open delta to me, what you were probably labeling as V-phase. A three phase motor can work fine with that as long as you don't exceed the capacity, which is reduced over that of a closed delta.
You might be tempted to connect such a system through a single phase panel. Just be aware that if one line is drawing 100 amps and the other line is drawing 100 amps, the neutral will be drawing 173.2 amps due to the 60 degree phase angle. If the neutral has a 100 amp capacity, you can only use a motor that draws 57.7 amps per line since that would come up to 100 amps in the neutral (it isn't really very neutral).
You can power a normal (closed) three phase Delta-Wye or Delta-Delta transformer with this. The reduced capacity still has to be considered back upstream to the power source, but between such a transformer and and the load, it can then be figured as a normal wye or delta circuit.
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| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
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Where do you get this shit? 173 amps on the neutral after balancing 2 legs at 100 think about it
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I suggest that you check it out by drawing a phasor diagram - then make a comment- with backup reasoning. Open delta is 3 phase but capacity limited to 57% of what would be there with a 3rd transformer.
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Don Kelly @shawcross.ca
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My first thought walking into a situation you described with a three phase breaker and 2 transformers is the main incoming supply is being boosted to a higher voltage. I would think the supply is 120/208 three phase three wire "wye" connection. There is no neutral in this case. The two transformers are connected to boost the voltage by 24 volts. Standard for this application. Your voltage measurements appear to support this since 208+24#2. Your measurements are in the ballpark. If the nameplate on the transformers shows a 12/24 volts secondary, that is the case. This is considered open delta. Motors will run on this. You cannot get three phase by connecting this to a single phase system. I also cannot see any way you get phase to phase measurements at the breaker at the voltages you stated in any proper power installation that was not three phase.
I would question your schematic. I see a grounded conductor connected to the breaker. Doesn't legitimately happen. Otherwise, I would consider the drawing to be a single phase series connected dual primary/series connected dual secondary transformer.
When you say "working on" do you mean this is an existing system that has shown problems and you are troubleshooting?
My gut feeling is 3 phase with improperly installed boost transformers.
Jeff

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What they drew, and described by measureing voltages, is an open wye/ open delta (corner grounded) connection. It is quite common. It is normally used for customers who will have little single phase load.
Charles Perry P.E.
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After doing phasor analysis, the result shows you are correct Charles. The secondary could have been wired open-wye (by swapping X1 and X2 connections on any one transformer) but it would give readings of 230, 230, & 398-ish. As it turned out, the power company wired it to open-delta (corner-grounded) configuration and not realizing they produced a proper 3-phase voltage wave for the building.
So now my guess is backed by phasor analysis and certainly would expect Jimmy's motor to turn smoothly. Jimmy expects the motor to judder insisting that the voltages are not 120 degrees apart, saying it is V-phase (which now I believe he is referring to open-wye case).
For backgrounder, the customer is a 50-bed hospital facility. All loads are single-phase 230VAC/60Hz. We were given the task of converting the power supply into 3-phase for expansion purposes. The transformers on the pole are 2 x 25kVA units.
We were told that they also tend to experience undervoltages. Their power bills showed an average energy consumption at 9,000kWH a month, which translates to an average 12.5kW running continuously, hence, their undervoltage problem is probably due to grossly unbalanced load distribution and not from an under-rated transformer.
I'll give you guys an update on the motor experiment. Thanks for your replies.
Frank R.
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| After doing phasor analysis, the result shows you are correct Charles. | The secondary could have been wired open-wye (by swapping X1 and X2 | connections on any one transformer) but it would give readings of 230, | 230, & 398-ish. As it turned out, the power company wired it to | open-delta (corner-grounded) configuration and not realizing they | produced a proper 3-phase voltage wave for the building. | | So now my guess is backed by phasor analysis and certainly would expect | Jimmy's motor to turn smoothly. Jimmy expects the motor to judder | insisting that the voltages are not 120 degrees apart, saying it is | V-phase (which now I believe he is referring to open-wye case). | | For backgrounder, the customer is a 50-bed hospital facility. All loads | are single-phase 230VAC/60Hz. We were given the task of converting the | power supply into 3-phase for expansion purposes. The transformers on | the pole are 2 x 25kVA units.
When all the loads are single phase and the 2 phases are at 60 degrees, that is when you get 1.732 times as much current on the neutral (with an equal amount on each phase). At 120 degrees it would be just 1.0 times as much. At 180 degrees it would be 0 (making it a legitimate neutral, I suppose).
The point being, corner grounded open delta (or closed delta for that matter), does NOT give a balance of current flow under conditions of two equal single phase loads. Here's a table showing the common wire current at various phase angles, assuming 100 amp single phase loads:
0 degrees -> 200.000000 common amps 15 degrees -> 198.288972 common amps 30 degrees -> 193.185165 common amps 45 degrees -> 184.775907 common amps 60 degrees -> 173.205081 common amps 75 degrees -> 158.670668 common amps 90 degrees -> 141.421356 common amps 105 degrees -> 121.752286 common amps 120 degrees -> 100.000000 common amps 135 degrees -> 76.536686 common amps 150 degrees -> 51.763809 common amps 165 degrees -> 26.105238 common amps 180 degrees -> 0.000000 common amps
I would suggest, if you want to ready a single phase building wiring for three phase service, you go with the normal 400/230 volt start (wye) system, and divide the building into 3 segments. Feed segment 1 with phases A and B. Feed segment 2 with phases B and C. Feed segment 3 with phases C and A. Wire the 3 phase needs separately and use 400 volt loads. If you have anything that needs 230 volt delta, use a separate transformer to get that voltage. This way all the phase angles will be 120 degrees and will not overload single phase wiring.
| We were told that they also tend to experience undervoltages. Their | power bills showed an average energy consumption at 9,000kWH a month, | which translates to an average 12.5kW running continuously, hence, | their undervoltage problem is probably due to grossly unbalanced load | distribution and not from an under-rated transformer.
Could be.
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| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
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OK agreed, but show me the neutral in a open delta. only time i ever found "neutral" imbalances like this involved harmonics
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As others have pointed out, it is common practice in some places to ground one point of the open delta and this is the <so-called> "neutral" You will have balanced 3 phase voltages but unbalanced currents in the line and "neutral" even for balanced loads. The system could be float with respect to ground but as there are normal 120V single phase loads present, the "neutral" tied to ground makes sense.
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Don Kelly @shawcross.ca
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| |> remove the X to answer |> ---------------------------- |> | OK agreed, but show me the neutral in a open delta. | only time i ever found "neutral" imbalances like this | involved harmonics
The neutral, or common, wire will have a current equal to the vector sum of the currents of the loads attached to it. If these currents are equal in amperage, and 180 degrees of opposite phase, the vector sum is zero and the neutral will have no current flowing. That is the typical single phase setup in most places in North America.
If the currents are unequal, there will be some neutral current. We say it is the difference current because the opposing 180 degree phase angles really give the current components their opposite polarities. But the correct way to think about how this happens is the summation of currents at their respective phase angles ... the vector sum.
If the currents are equal, but the phase angle is less than 180 degrees, then there will be some current flowing due to the fact that at any instant, these currents do not cancel out. Just how much current there will be depends on the actual phase angle of these two equal amperage currents.
Here is a list of the common/neutral currents given a 100 amp load on each of two phases. If wires A B C have B grounded, then 100 amps on A-B and 100 amps on B-C would give these currents on wire B for these phase angles.
0 degrees -> 200.000000 common amps 15 degrees -> 198.288972 common amps 30 degrees -> 193.185165 common amps 45 degrees -> 184.775907 common amps 60 degrees -> 173.205081 common amps 75 degrees -> 158.670668 common amps 90 degrees -> 141.421356 common amps 105 degrees -> 121.752286 common amps 120 degrees -> 100.000000 common amps 135 degrees -> 76.536686 common amps 150 degrees -> 51.763809 common amps 165 degrees -> 26.105238 common amps 180 degrees -> 0.000000 common amps
Notice that with a phase angle of 0, the current is the exact sum of both loads. This would the the equivalent of wiring both sides of a two pole single phase (as found in the USA) on the same phase instead opposite phases. The neutral bus would have to carry the sum of the loads (instead of the difference), and thus be overloaded.
In some places in USA and Canada, and in many places in Mexico and Central America, it is common to wire a "single phase service" using 2 out of 3 phases from a three phase transformer (bank). The voltage combination is typically 208/120 or 220/127. In these cases the phase angle is 120 degrees. The neutral current will be equal to the 2 phase currents when they are both drawing the same current. This is why there is a rule in the National Electrical Code (NEC) that requires the neutral wire be counted when figuring wiring deratings for cable assemblies and conduits when wired to this kind of service. Otherwise with true single phase at 180 degrees, a 3-wire (2 poles plus neutral) circuit can be counted as 2 wires for conduit fill ratings since the actual dissipation will never be greater than what 2 wires would dissipate. But any angle smaller than 120 degrees poses a danger because the neutral current will be greater than either of the poles/phases, all the way up to twice the value for the extreme case of 0 degrees.
With a corner grounded delta you will have a 60 degree angle, and thus the common neutral wire will have about 1.732 times the current of balanced loads from 2 phases. Only loads connected directly between the 2 phases (and not connected to the neutral at all) will avoid drawing current on the neutral. But that's not the usual practice when supplying ordinary single phase loads. Supplying circuits intended for single phase with corner grounded delta is therefore a serieous hazard (mitigated only be very uncommon and expensive wiring practice).
FYI, the 1.732 value is the square root of 3, which is closer to: 1.7320508075688772935274463415058723669428052538103806280558069794519330169 or if you prefer a rational number, is fairly close to: 539095242162880600113350770354161026 / 311246783181585603728705429114242815
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| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
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I promised to give you guys an update on the motor experiment but it won't happen. The power company finally realized that the system is open-delta on the secondary, hence, a proper 3-phase 230V line-to-line entering the building. In this case, 120 degrees will apply. If the two lines carry 100A each, the grounded-corner, being actually the 3rd phase line, will also carry 100A.
The imbalance would occur, however, at the primary. The power company now knows it is their problem. Their neutral (at the primary) will carry 1.73 times line current whenever the secondary is carrying a balanced 3-phase load.
Thanks.