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#128452 - 03/19/03 03:12 PM Open delta w/2 phase primary  
sabrown  Offline
Joined: Dec 2002
Posts: 302
Ogden, Utah, USA
I have just run acrossed a system where we have a site fed with a 2-phase primary and an open delta to feed a 3-phase corner grounded 480 vac pump. I wonder how this affects things?

It seems that I should get 480 on 2 lines and something in the range of 800 volts on the 3rd leg, but there must be other ways to connect the transformers to the system for the report that I received was that:
L1-grd NL (no Load)=501 vac, FL (under load conditions)=489 vac
L2-grd = 0 volts
L3-grd NL=450 vac, FL=446 vac
With load currents of:
L1=28.6 amps
L2=13.9 amps
L3=24.1 amps

How would these tranformers be connected to give this? What effect does this have on the load? What would be the phase angle relationship?

Thanks, Shane

Tools for Electricians:

#128453 - 03/19/03 06:18 PM Re: Open delta w/2 phase primary  
Bjarney  Offline
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
sa — Sounds like you are describing an open-wye primary, open-delta secondary transformer bank. Phase-to-neutral difference in the [wye] primary windings is 120°. It is a misnomer to call the arrangement “2 phase,” for 2-phase is sort of reserved for circuits with 90° phase angles. Phase-to-phase [delta] secondary windings have 60° displacement.

There is a secret website where transformer connections are revealed. [Please, don’t tell anyone.] See figure 21 in It is identical to the plain-vanilla grounded-wye/delta configuration, with one pot {1ø transformer} is missing. Compare the phasor representation of figure 21 to that in figure 20, but understand that secondary grounding is on Bø {and not at a center tap} in your 480V case. As your measurements show, balanced readings can be kinda’ sloppy with only 2 transformers. It’s more economical for the utility with smaller loads, especially for lower-HP single-motor cases.

If you want to delve deeper, get a copy of IEEE Std C57.12.70-2000 American National Standard Terminal Markings and Connections for Distribution and Power Transformers {There’s even six-phase, tee and zig-zag windings covered there.} Also, IEEE Std C57.105-1978 Guide for Application of Transformer Connections in Three-Phase Distribution Systems is interesting. [They are not free, though, unless you find a good electrical-engineering library.]

#128454 - 03/20/03 09:33 AM Re: Open delta w/2 phase primary  
sabrown  Offline
Joined: Dec 2002
Posts: 302
Ogden, Utah, USA
Thanks very much, from this I was able to figure out all my other questions last night. I will be going through the referenced material again this morning in hopes that it will stick. It also helped me understand the phase relationship. Sorry about the reference to 2-phase when I knew what that was and meant 2 phases of a 3-phase system (it was lunch time and my mind was in a hurry).


#128455 - 03/20/03 03:43 PM Re: Open delta w/2 phase primary  
Bjarney  Offline
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
A variation of the 2-transfomer open-wye/open-delta arrangement is open-delta connections on high and low sides. Where two phases and the distribution-line neutral is used with OY-OD; with OD-OD, three phases are connected {less the neutral} to the bank primaries. In overhead construction, open-wye uses two fused cutouts, where open-delta uses three. Generally, there is somewhat better secondary phase-to-phase voltage balance with the second configuration. This leads to cooler-running motors served by the secondary windings of a two-transformer bank.

Of course, voltage balance is best using a symmetrical bank, like the traditional delta-wye configuration.

#128456 - 03/21/03 07:04 AM Re: Open delta w/2 phase primary  
Scott35  Offline

Broom Pusher and
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA

Adding to Bjarney's excellent reply posts, I have some Questions to shoot at 'ya:

1: Does the Transformer Bank's connections resemble the ones in this schematic below:
[Linked Image]


Are the Transformers connected more like the way shown in this schematic below-as to the one on the left side:
[Linked Image]

2: Are the readings (testing voltage) with a DVM set on "X1" or readout for "000.00" VAC?
The readings might be a result of the Motor's contribution back into the supply, from unevenly loaded circuitry.

There are some methods to "Ballast" the input of Volt Meters, which might be a good topic for discussion here! (hinting towards Bjarney! He can describe much better what I mean and how to do it).

If the readings were taken at the Transformer's Secondary terminals, this might be a normal situation (Secondary voltage is read as + 104.5% rated output voltage during no-load situations - as a result of the Primary-to-Secondary %Z and Transformer's Regulation at idle).
The Voltage read when a moderate load is applied, brings it down to nominal rating area.

Looks like ØB / L-2 is the Grounded Conductor of this Corner Grounded Delta configuration.

Voltage readings of (apx.) 480 VAC from L-1 to L-2, L-2 to L-3, L-1 to L-3, L-1 to ground and L-3 to ground would be normal. L-2 to ground would be near zero Volts.

Refer to the above attached Schematics to find out why these Voltage readings would be normal.
For more information or Schematics, check the list of material posted at the Technical Reference area, by using the Menu For Technical Reference Section , and selecting a link of interest.
FYI: If you right-click the link and open that item in a new window, you can keep the menu open/minimized for further selections!

Feel free to ask more questions.


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#128457 - 03/24/03 09:58 AM Re: Open delta w/2 phase primary  
sabrown  Offline
Joined: Dec 2002
Posts: 302
Ogden, Utah, USA
I wish I could tell you how the transformers are connected, but the site is several 100's of miles away. The information that I have been recieving has been over the phone. The power co. engineer was frustrating to talk to because I was improperly wording my questions to him, in otherwords, he never answered one of the questions that I was trying to ask.

The most important thing that I got out of him was that the secondary service that I wanted 480/277 wye was available. Now to sit down and write up instructions for an electrician to convert the existing pump controls from the corner tapped delta system (the pump about 2000' down a well requires no conversion). I just need to make sure that the electrician fully understands what I am refering to because the last thing anyone needs in this case is a literal bolted fault.

Thank you both for your time and replies and I am again sorry that I could not supply the information requested.


#128458 - 03/24/03 03:42 PM Re: Open delta w/2 phase primary  
Bjarney  Offline
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
sa -- Short of complete replacement, there is no practical way to convert the two-transformer configuration described into a 4-wire wye system, and very little would likley be gained by doing so. Is this a new installation, or has it been in normal/acceptable service for a time?

Can you respond with the actual motor-nameplate voltage, rated full-load current, horsepower and service factor? (Are you confident that the field-tested operating voltage and current readings are accurate?)

Relog the operating {motor-running} voltages at L1-L2, L2-L3 and L3-L1, neglecting any “to-ground” readings, and taken at the load-side connections of the starter. Then, record L1, L2, and L3 currents.

The reported currents are wildly unbalanced, and if accurate, indicate a severely crippled motor with very little ability to deliver mechanical horsepower to its shaft. Extreme overheating of the stator windings and consequent near-term failure should be expected.

There is limited online information available for submersible water-pump motors, but I don’t think it will help much.

Another listing of typical service-transformer/voltage relationships is

#128459 - 03/25/03 09:53 AM Re: Open delta w/2 phase primary  
sabrown  Offline
Joined: Dec 2002
Posts: 302
Ogden, Utah, USA
I am not sure of the readings or even when they were taken. These were reported to me and may be taken from old records and the motor since replaced. I am fairly well out of the loop as the installation is out of my region, I just was requested to expand this installation to service an entire location. So yes this will be an expanded service.

And yes, we will be completely replacing the transformer bank at a higher cost than it would be to just leave this service in and supplying a new single phase service for the rest of the loads. The power company does not want to continue the service in this way either as there current allowable largest motor load connected in this manner is 7.5 hp.

Yes, the service has been operating for many years, though I have heard of problems and a recent upgrade to the service.

The motor is a standard Franklin 20 h.p., 460 vac, 1.15 s.f., 26.9 fla, 30.3 s.f. amps, kva code J.


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