# Is it possible to determine if an AC generator is a "Y" or "delta" from electrical measurements? **URL:** \ **Category:** Factual Questions\ **Created:** [September 18, 2016, 3:04pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255 "2016-09-18T15:04:29Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [September 18, 2016, 3:04pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/1 "2016-09-18T15:04:29Z")
You are given a small, three-phase, permanent-magnet generator. There are three windings inside the generator spaced 120° apart. Each winding has identical characteristics, i.e. each winding has the exact same resistance and exact same inductance. There are three wires coming out of one side of the generator labeled A, B, and C. These wires connect to the windings inside the generator. On the other side is a shaft. When the shaft is rotated, a three-phase AC voltage is generated at the wires. Your challenge: \*\*determine if the windings inside the generator are wired as a “Y” or a “delta.” \*\* Here are the rules: 1. You are not allowed to disassemble the generator. The generator is essentially a black box. 2. You can connect any electrical/electronic equipment & components to the three wires coming from the generator. This includes power supplies, current sources, ohmmeters, voltmeters, ammeters, resistors, capacitors, inductors, transistors, etc. You can also use wires to short across the windings during the measurements. 3. You are free to model each winding as resistor ([pic](https://www.dropbox.com/s/knpn11xqh4zl6ss/Windings%20as%20R.JPG?dl=0)), inductor ([pic](https://www.dropbox.com/s/rs3z93ih9v8rsmh/Windings%20as%20L.JPG?dl=0)), resistor + inductor ([pic](https://www.dropbox.com/s/tzdlepp622kxrin/Windings%20as%20R%20and%20L.JPG?dl=0)), or any other model of your fancy. 4. As stated above, each of the three windings has the exact same resistance and exact same inductance. This is true if it is a Y or delta. 5. You do not know the resistance or inductance of the windings. You only know that they’re all the same. 6. All windings are isolated from ground. If it is a Y, there is no neutral connection. 7. You have no other generators to compare it to. You have one generator and one generator only. I am some other EEs at work have been trying to come up with way to do it. So far… nothing. ☹ Since it would appear there is equivalency between a Y and delta, it may not be possible to determine if it’s a Y or delta strictly through electrical measurements. ---
**Author:** ![friedo](https://avatars.discourse-cdn.com/v4/letter/f/8edcca/32.png) [@friedo](https://boards.straightdope.com/u/friedo)\ **Post date:** [September 18, 2016, 3:17pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/2 "2016-09-18T15:17:10Z")
Without a center tap it’s tough. I can’t think of a way to do it offhand, but maybe it’s a good question for the [EE Stack Exchange](http://electronics.stackexchange.com). ---
**Author:** ![Me\_Billy](https://avatars.discourse-cdn.com/v4/letter/m/e480ec/32.png) [@Me\_Billy](https://boards.straightdope.com/u/Me_Billy)\ **Post date:** [September 18, 2016, 3:36pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/3 "2016-09-18T15:36:41Z")
Connect a 3 phase electric motor wired for wye/delta, and note the HP… > **[We need a universal "UL" mark ???](https://forums.mikeholt.com/threads/we-need-a-universal-ul-mark.98320/)** > > We are going thru UL testing with someone other than UL labs. Now after the fact (we have a signed contract) we come to find out IF we obtain our UL that certain wholesalers have the right to refuse our product strictly on the fact they want to see... [http://ivanbennett.com/forum/index.php?topic=26.0](http://ivanbennett.com/forum/index.php?topic=26.0) ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [September 18, 2016, 3:38pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/4 "2016-09-18T15:38:50Z")
This seems simple enough for a real generator with resistance in the windings. Call the wires coming out XYZ. Measure the voltage with no load across all legs. With the generator running, put a heavy load on XY and then measure the voltage across YZ and ZX. If the voltage falls on YZ and ZX when XY is loaded, then it’s a delta connection. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [September 18, 2016, 3:53pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/5 "2016-09-18T15:53:21Z")
Thanks. But I would rather not turn the shaft and run it as a generator. I am looking for a way to determine if it’s a Y or delta strictly through electrical measurements with the shaft not turning. I should have been more clear about this. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [September 18, 2016, 3:54pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/6 "2016-09-18T15:54:43Z")
> [@Me\_Billy](#): > > Connect a 3 phase electric motor wired for wye/delta, and note the HP… > > [http://forums.mikeholt.com/showthread.php?t=98320](http://forums.mikeholt.com/showthread.php?t=98320) > > [http://ivanbennett.com/forum/index.php?topic=26.0](http://ivanbennett.com/forum/index.php?topic=26.0) I don’t think this will work. I have one generator, and one generator only; I have nothing to compare it to. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [September 18, 2016, 4:04pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/7 "2016-09-18T16:04:45Z")
nm ---
**Author:** ![Me\_Billy](https://avatars.discourse-cdn.com/v4/letter/m/e480ec/32.png) [@Me\_Billy](https://boards.straightdope.com/u/Me_Billy)\ **Post date:** [September 18, 2016, 4:05pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/8 "2016-09-18T16:05:04Z")
> [@Crafter\_Man](#): > > I don’t think this will work. I have one generator, and one generator only; I have nothing to compare it to. No… Three phase electric motors have the option of being configured/wired for delta or wye! They will operate more efficiently as designed if wired for the electric supply being provided. The comparison is with wiring the electric motor differently and noting the results of the electric motor performance. ---
**Author:** ![Me\_Billy](https://avatars.discourse-cdn.com/v4/letter/m/e480ec/32.png) [@Me\_Billy](https://boards.straightdope.com/u/Me_Billy)\ **Post date:** [September 18, 2016, 5:32pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/9 "2016-09-18T17:32:33Z")
P.S. If you have an actual generator there, then you could look at the nameplate on it and probably see the specifications. Or note the model number and google it. Also I googled the words… -portable generator 3 phase delta and got 248,000 results -portable generator 3 phase wye and got 452,000 results Based on that, it is more likely a wye generator! ---
**Author:** ![K2500](https://avatars.discourse-cdn.com/v4/letter/k/f6c823/32.png) [@K2500](https://boards.straightdope.com/u/K2500)\ **Post date:** [September 18, 2016, 5:43pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/10 "2016-09-18T17:43:21Z")
I take a stab at it. I measure resistance of winding AB, then short wires BC, then measure resistance from wire A to wire BC. If the value is half my initial measurement it’s delta, if it’s the same it’s a wye. Eta: if it’s a wye the measurement would be 3/4 of the i initial resistance value. ---
**Author:** ![mixdenny](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mixdenny/32/2962_2.png) [@mixdenny](https://boards.straightdope.com/u/mixdenny)\ **Post date:** [September 18, 2016, 6:25pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/11 "2016-09-18T18:25:17Z")
Yes, if this is an actual case, not just a quiz, the serial number may tell everything. If it is by chance an Onan, go here: [http://www.smokstak.com/forum/forumdisplay.php?f=1](http://www.smokstak.com/forum/forumdisplay.php?f=1) That is under a higher forum that covers other generators. Dennis ---
**Author:** ![friedo](https://avatars.discourse-cdn.com/v4/letter/f/8edcca/32.png) [@friedo](https://boards.straightdope.com/u/friedo)\ **Post date:** [September 18, 2016, 6:32pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/12 "2016-09-18T18:32:42Z")
Come on, did you even read the OP? It’s clearly a hypothetical exercise. In the real world, people know how to google model numbers. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [September 18, 2016, 6:35pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/13 "2016-09-18T18:35:39Z")
> [@K2500](#): > > I take a stab at it. I measure resistance of winding AB, then short wires BC, then measure resistance from wire A to wire BC. If the value is half my initial measurement it’s delta, if it’s the same it’s a wye. > > Eta: if it’s a wye the measurement would be 3/4 of the i initial resistance value. No, that doesn’t work. In the Wye case, the initial resistance is 2R, the shorted resistance is 3/2R In the Delta case the initial resistance is 2/3R, the shorted resistance is 1/2R The initial vs. shorted resistance is 2/3 / 1/2 in both cases. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [September 18, 2016, 9:31pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/14 "2016-09-18T21:31:46Z")
> [@friedo](#): > > Come on, did you even read the OP? It’s clearly a hypothetical exercise. In the real world, people know how to google model numbers. It actually started when we received an actual generator at work. There was no nameplate or documentation. Our team lead asked, “Out of curiosity, is there a way to tell if it’s a delta or a Y”? At first we thought, “No problem - we can do some electrical measurements and determine if it’s a delta or Y.” We came up with lots of schemes on paper, and all were mathematically proven not to work. At this point, we really don’t care if the generator we received is a delta or Y. But the problem has intrigued from an academic/hypothetical POV. > [@beowulff](#): > > No, that doesn’t work. > > In the Wye case, the initial resistance is 2R, the shorted resistance is 3/2R > In the Delta case the initial resistance is 2/3R, the shorted resistance is 1/2R > > The initial vs. shorted resistance is 2/3 / 1/2 in both cases. Yep, this was the first thing another engineer tried. Doesn’t work. ---
**Author:** ![K2500](https://avatars.discourse-cdn.com/v4/letter/k/f6c823/32.png) [@K2500](https://boards.straightdope.com/u/K2500)\ **Post date:** [September 18, 2016, 9:35pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/15 "2016-09-18T21:35:51Z")
> [@beowulff](#): > > No, that doesn’t work. > > In the Wye case, the initial resistance is 2R, the shorted resistance is 3/2R > In the Delta case the initial resistance is 2/3R, the shorted resistance is 1/2R > > The initial vs. shorted resistance is 2/3 / 1/2 in both cases. Thank you, I should have tried to run the numbers before jumping to a conclusion. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [September 18, 2016, 9:38pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/16 "2016-09-18T21:38:24Z")
I have a feeling it can’t be done. I can sort of hear Léon Thévenin and Gustav Kirchhoff laughing at us right now… 😛 ---
**Author:** ![K2500](https://avatars.discourse-cdn.com/v4/letter/k/f6c823/32.png) [@K2500](https://boards.straightdope.com/u/K2500)\ **Post date:** [September 18, 2016, 10:13pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/17 "2016-09-18T22:13:54Z")
What if you were to apply a DC voltage to two lines and measure voltage and polarity to the third line? It’s easy for me to see what would show up on a wye, but on a delta I’m having trouble. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [September 19, 2016, 1:34am UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/18 "2016-09-19T01:34:19Z")
> [@K2500](#): > > What if you were to apply a DC voltage to two lines and measure voltage and polarity to the third line? It’s easy for me to see what would show up on a wye, but on a delta I’m having trouble. They Y and delta would have the same ratios. ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [September 19, 2016, 5:26pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/19 "2016-09-19T17:26:23Z")
Crafter - I am not sure if the math works out, but applying successively higher currents to one phase and measuring the resistance increase on the other phases (as the windings heat up) could give a clue. The insulation can be a give away too but I see you do not list a megger as one of your test methods. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [September 19, 2016, 7:19pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/20 "2016-09-19T19:19:38Z")
> [@am77494](#): > > Crafter - I am not sure if the math works out, but applying successively higher currents to one phase and measuring the resistance increase on the other phases (as the windings heat up) could give a clue. I was thinking the same thing–seems like it would work. Run a heavy current through A-B, then measure the resistance across A-C. If it’s a delta, there should be no change. If it’s a wye, then half of the electrical path is hot and it should show a slightly higher resistance. [Next page](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255.md?page=2)