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Open Delta #44010 10/26/04 05:08 PM
Joined: May 2004
Posts: 697
Dave55 Offline OP
I have to confess that I don't know much of anything about what comes from the POCO. I have a potential customer who has had 3PH machines burn up. The POCO came and metered 265 volts where there should have been 240 volts. They told him that an open delta can vary by 10%. Their suggested solution for him is to bring in another type of transformer if someone will do the load calculations. Meanwhile he has a lot of buck/boost transformers on the machines to bring the voltage to 240.



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Re: Open Delta #44011 10/26/04 07:12 PM
Joined: Dec 2003
Posts: 751
earlydean Offline
I would wonder if this high voltage is constant. I would start with a recording voltmeter left in place for a week to determine how much voltage variation this customer has. (if any)
Also, measure the voltage at each machine before it starts, then with no load, then again under full load.
Load calculations will be necessary for every piece of equipment, and for the shop distribution layout.
Possible solutions require a lot more information than you have provided. Maybe an electrical engineer should have been required to lay out the entire building.
Is 480 volt service available?

Re: Open Delta #44012 10/26/04 11:54 PM
Joined: Dec 2003
Posts: 524
Attic Rat Offline
...I'm gonna show my ignorance here,gang,..."Open Delta" means no neutral,..or am I mistaken??,..Please advise..Thanx,

.."if it ain't fixed,don't break a Licensed Electrician"
Re: Open Delta #44013 10/27/04 02:19 AM
Joined: Sep 2003
Posts: 200
u2slow Offline
Is the POCO calling an ungrounded delta system "open delta"? Or are they pointing out the customer may have an "open delta load" (broken load phase) that is causing the higher voltage?

Is the building serviced with 240V 3-phase or is it transformed in-house from a higher voltage?

Re: Open Delta #44014 10/27/04 08:42 AM
Joined: Mar 2003
Posts: 849
Yoopersup Offline
Open Delta is a delta hook up with one side open. With an open Delta each secondary winding must suppy 100% of the current.Lots of times an open delta well be used where they feel larger furture loads are possible , They would then add the 3th transformer. Ifo out od Howard W. Sams & Company (Transformers & Motors)dated 1989

Re: Open Delta #44015 10/27/04 02:12 PM
Joined: Apr 2004
Posts: 349
Radar Offline
We one time closed up an open delta on a project - in order to increase overall system capacity (as the previous poster indicated). As I understand it, an open delta behaves exactly the same as a normal delta (secondary connection) except at only about 67% of nameplate KVA capacity - which makes sense because one of the three transformers is missing. External wiring is exactly the same.

Also, if I remember correctly, the POCO primary was not a delta, but a wye, which I could not ever seem to explain to the POCO personnel we encountered.


There are 10 types of people. Those who know binary, and those who don't.
Re: Open Delta #44016 10/27/04 06:21 PM
Joined: Oct 2000
Posts: 2,721
Scott35 Offline
Broom Pusher and

Also, if I remember correctly, the POCO primary was not a delta, but a wye, which I could not ever seem to explain to the POCO personnel we encountered.

Could the setup be an Open Wye?


...I'm gonna show my ignorance here,gang,..."Open Delta" means no neutral,..or am I mistaken??,..Please advise..Thanx,

The Open Delta could be a 3 Phase 3 Wire setup (Grounded or Ungrounded), or a 3 Phase 4 Wire Grounded setup, which would include a Grounded Neutral Conductor.

Not a bad question! Most Open Delta "Vee" Setups discussed are targeted towards Motor Loads, but there are a lot of them in areas with Residential/Commercial customers fed from the same bank of Transformers.
Those would be 4 Wire Delta setups.

All the Open Delta connections I see, and have seen for years, are of the "Vee" configuration. Have yet to see any 3 Phase 3 Wire Open Delta "Tee" configurations on Utility Poles (excluding the Ancient Port areas of Long Beach, San Pedro and such, where 2 Phase systems were still in use in the 1980s. These would be "Tee" configs.).


Scott " 35 " Thompson
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Re: Open Delta #44017 10/28/04 11:06 AM
Joined: Aug 2002
Posts: 141
elecbob Offline
[Linked Image from]

These are great for apartment houses with a 3 phase elevator motor. The local POCO requires all single phase loads be connected to the 120/240 volt transformer. Only the elevator is connected to the high leg (every third circuit breaker position in a 3 phase panel is the high leg).

[This message has been edited by elecbob (edited 10-28-2004).]

[This message has been edited by elecbob (edited 10-28-2004).]

Re: Open Delta #44018 10/28/04 08:18 PM
Joined: Apr 2004
Posts: 201
CharlieE Offline
Sorry but the voltage from the grounded point on the lighting (larger, center tapped) transformer to the B phase is 208 volts nominal, not 265 volts. This configuration is normally used as an open wye/open delta and each transformer serves 57.7% of the three-phase load. The lighter serves the entire single-phase load.

The voltages from A to ground to C are usually very stable and the voltage from the common end of the open delta to the high phase are also usually very stable. In both cases, there is a real transformer in the circuit. From the other end of the lighter to the high phase, the voltage is usually very stable as long as the motor load is not too great.

In this case, I am betting that either the bank is very close to a substation or a capacitor bank is on top of the installation (the utility's capacitor bank). Of course it could be both.

Most of the state public service commissions require the investor owned electric utilities to maintain their voltages to about 5% of nominal. It may be time to call them if you haven't been able to get through to the correct people to get their problem taken care of. I highly recommend that you get through to their power quality engineer or at least a distribution engineer first. [Linked Image]

- - - - - - - - - - - - - - - - - - - - - - - - - - -
Charlie Eldridge, Indianapolis, Utility Power Guy

Charlie Eldridge, Indianapolis Utility Power Guy
Re: Open Delta #44019 10/28/04 09:11 PM
Joined: May 2004
Posts: 697
Dave55 Offline OP
I thought the 10% variation sounded too high also. I have doubts about the POCO here going to any great lengths for customer service, though.

Is there any kind of UPS or other equiptment for 3PH 240 volt maybe around 20 amp where no mater what you put into it (within obvious limits), that it would stabalize voltage and frequency variations?

I have a cut sheet for the new machine and it's outlining dire consequences for problems with the power including electrical noise.


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