# 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:** 2
**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, 7:27pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/21 "2016-09-19T19:27:55Z")
> [@Dr.Strangelove](#): > > 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. Uh, nevermind. A-C will show slightly higher resistance even with a delta. Not sure if it comes out exactly equal or not, but I’d give it better than odds that it does. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 19, 2016, 10:51pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/22 "2016-09-19T22:51:53Z")
**am77494** ’s post reminds me of another old (but simpler) puzzle: You have a black box with two contacts, which contains either a 1-amp constant current supply in parallel with a 1-ohm resistor, or a 1-volt constant voltage supply in series with a 1-ohm resistor, but you don’t know which. Using any measurement equipment outside of the box, how can you tell the difference? ---
**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, 11:28pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/23 "2016-09-19T23:28:15Z")
[spoiler]Well–I’d say “short it, and if the box heats up then it’s a CV supply.” But if this is really an idealized black box, then we can’t necessarily guarantee that. If it has some kind of super-battery, and an internal heat reservoir, then it can keep the external heat flux to whatever it wants by making the internal reservoir hotter. Alternatively, and more practically, you could have a variable heater in both boxes that keeps both of them emitting a constant flux. 'Course, you didn’t say the boxes had this other stuff… but you also didn’t specify the power supply or anything else, so there’s a lot that has to be inferred.[/spoiler] ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 20, 2016, 1:25am UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/24 "2016-09-20T01:25:18Z")
[spoiler]You don’t even need to short it, because the constant-current box is already heating up at 1 W even without the short (and will stop heating when you short it). In any event, it’s a clear proof that Thevenin doesn’t cover _everything_, so you might be able to tell the difference from the heating.[/spoiler] And it feels like the OP’s problem should be solvable using an oscilloscope, but I’m too far out of practice on AC to figure out the details. ---
**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 20, 2016, 11:00am UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/25 "2016-09-20T11:00:24Z")
> [@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. Here is something I tried: 1. At room temperature, measure resistance between leads A & B, A & C, and B & C. 2. Stick a voltage source between A and B. (Leave C disconnected.) Leave it hooked up for a few minutes. 3. Repeat step 1. Calculate ratios. If it is a Y generator, two of the leads will heat up. The third will not. If it is a delta generator, all three leads will heat up (though not equally). I modeled both the Y and delta generators in Excel. I assumed a certain temperature rise with the tempco of copper at 0.393%/degree C. The Y and delta generators had the same ratios. ☹ ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 20, 2016, 11:06am UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/26 "2016-09-20T11:06:20Z")
Ah, yes, I see… The trouble is that, on a Y generator, one winding won’t heat up, but you can’t measure the resistance of a single winding. You _should_, in principle, be able to tell the difference using calorimetry (the delta will produce more total heat), but that would be very difficult to do in practice. ---
**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 20, 2016, 11:07am UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/27 "2016-09-20T11:07:28Z")
> [@Chronos](#): > > [spoiler]You don’t even need to short it, because the constant-current box is already heating up at 1 W even without the short (and will stop heating when you short it). > > In any event, it’s a clear proof that Thevenin doesn’t cover _everything_, so you might be able to tell the difference from the heating.[/spoiler] > > And it feels like the OP’s problem should be solvable using an oscilloscope, but I’m too far out of practice on AC to figure out the details. A coworker has been trying to come up with all kinds of clever schemes using external power supplies, switches, diodes, etc. And each time, my analysis shows that the approach cannot differentiate between a delta and Y configuration. And again, this assumes you do not know the winding resistance or winding inductance. You only know that all three windings are the same. (If the winding resistance or inductance is known, the problem becomes trivial.) I am beginning to think it is not possible; there is no solution. ---
**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 20, 2016, 11:12am UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/28 "2016-09-20T11:12:53Z")
> [@Chronos](#): > > You _should_, in principle, be able to tell the difference using calorimetry (the delta will produce more total heat), but that would be very difficult to do in practice. That is only true if I know the resistance of the windings. But I don’t. A Y will easily produce more total heat than a delta if its winding resistance is very low compare to the delta (and you use a voltage source). However, on a Y, one winding would remain at room temperature. On a delta, none of the windings would remain at room temperature. So if you could peer inside and take temperature measurements, it might be doable… ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [September 20, 2016, 11:48am UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/29 "2016-09-20T11:48:25Z")
So far I think all that has been shown is that the classic star-delta transformation will always stifle anything that tries to measure the simple impedances present. The point being that there is a transform that exactly transforms a delta to a Y. Even one with unbalanced legs. (It might have been that the transformation with unbalanced legs left you with a different result, but it doesn’t ☹ ) Whether there is some second order effect that allows you to tell, I’m not sure. But it isn’t going to be something that you can tell looking for an asymmetry between delta and Y, because there isn’t one. ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [September 20, 2016, 2:55pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/30 "2016-09-20T14:55:24Z")
> [@Crafter\_Man](#): > > So if you could peer inside and take temperature measurements, it might be doable… Why do you need to peer inside ? The terminals themselves get hot. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [September 20, 2016, 5:04pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/31 "2016-09-20T17:04:42Z")
> [@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. Lurker here, sort of understanding, learning: Is **friedo** correct or not? ---
**Author:** ![newme](https://avatars.discourse-cdn.com/v4/letter/n/e0b2c6/32.png) [@newme](https://boards.straightdope.com/u/newme)\ **Post date:** [September 20, 2016, 6:40pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/32 "2016-09-20T18:40:27Z")
> [@Crafter\_Man](#): > > Here is something I tried: > > 1. At room temperature, measure resistance between leads A & B, A & C, and B & C. > > 2. Stick a voltage source between A and B. (Leave C disconnected.) Leave it hooked up for a few minutes. > > 3. Repeat step 1. Calculate ratios. > > If it is a Y generator, two of the leads will heat up. The third will not. > If it is a delta generator, all three leads will heat up (though not equally). > > ☹ I assume the separate windings are thermally isolated, at least in the very short term? Quickly pass a large current between terminals A and B. After shutting off the current, immediately measure the resistance between A and B, and the resistance between A and C. By my calculations, the _change_ in resistance between A and B should be exactly twice the _change_ in resistance between A and C for a Y generator. For a Delta generator, it is more complicated. For small temperature changes, you must assume one coil (the one between A and B) will receive twice the current of the other two coils, and heat up twice as much, and consequently its resistance will change twice as much. My calculations show that in this circumstance the _change_ in resistance between A and B will be approximately 1.4 times the _change_ in resistance between A and C. ---
**Author:** ![newme](https://avatars.discourse-cdn.com/v4/letter/n/e0b2c6/32.png) [@newme](https://boards.straightdope.com/u/newme)\ **Post date:** [September 20, 2016, 8:55pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/33 "2016-09-20T20:55:41Z")
> [@newme](#): > > For a Delta generator, it is more complicated. For small temperature changes, you must assume one coil (the one between A and B) will receive twice the current of the other two coils, and heat up twice as much, and consequently its resistance will change twice as much. My calculations show that in this circumstance the _change_ in resistance between A and B will be approximately 1.4 times the _change_ in resistance between A and C. Just realized my error. For a Delta, one coil will receive twice the current, but will receive 4 times the power of each of the other two coils, so temperature and resistance of that one coil will increase by a factor of 4 also. It appears the overall resistance change will match that of a Y generator. You would have to exploit the non-linearity of the wire’s resistance versus temperature somehow. ---
**Author:** ![swampspruce](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/swampspruce/32/261_2.png) [@swampspruce](https://boards.straightdope.com/u/swampspruce)\ **Post date:** [September 20, 2016, 9:49pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/34 "2016-09-20T21:49:09Z")
nm ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [September 20, 2016, 11:52pm UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/35 "2016-09-20T23:52:33Z")
Non linearity is the key. The star-delta transform guarantees that any effect that is linear won’t work. One that does occur to me is to try to use the hysteresis or saturation curve curve of the magnet cores. You should be able to see this with an oscilloscope, but it means driving quite a bit of current. Measured across two terminals a Y should give a single bump as you start to saturate the two coils in series, whereas a delta should show two bumps, as the current through the single coil will be twice that through the pair, it will saturate first, and the series pair saturates at double the current. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [September 21, 2016, 12:16am UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/36 "2016-09-21T00:16:45Z")
> [@Leo\_Bloom](#): > > Lurker here, sort of understanding, learning: > > Is **friedo** correct or not? The OP explains the provenance of the problem in post #14. ---
**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 21, 2016, 12:36am UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/37 "2016-09-21T00:36:38Z")
> [@newme](#): > > Just realized my error. For a Delta, one coil will receive twice the current, but will receive 4 times the power of each of the other two coils, so temperature and resistance of that one coil will increase by a factor of 4 also. It appears the overall resistance change will match that of a Y generator. > > You would have to exploit the non-linearity of the wire’s resistance versus temperature somehow. I made a spreadsheet model. * * * For the **Y** I did the following: Resistance of winding between A and center (R[sub]A[/sub]) at 25 °C = 1 Ω R[sub]B[/sub] at 25 °C = 1 Ω R[sub]C[/sub] at 25 °C = 1 Ω And thus the resistance _measurements_ would be R[sub]AB[/sub] = 2 Ω R[sub]AC[/sub] = 2 Ω R[sub]BC[/sub] = 2 Ω I apply a voltage between A and C and wait a few minutes. I then assume Temperature of R[sub]A[/sub]= 60 °C Temperature of R[sub]B[/sub]= 25 °C (assume no conduction here) Temperature of R[sub]C[/sub]= 60 °C Given the tempco of copper is 0.393%/°C, R[sub]A[/sub] = 1.13755 Ω R[sub]B[/sub] = 1 Ω R[sub]C[/sub] = 1.13755 Ω R[sub]AB[/sub] = 2.13755 Ω ( **6.8775% increase** ) R[sub]AC[/sub] = 2.2751 Ω ( **13.775% increase** ) R[sub]BC[/sub] = 2.13755 Ω ( **6.8775% increase** ) **13.775/6.8775 = 2** * * * For the **delta** I did the following: Resistance of winding between A and B (R[sub]AB[/sub]) at 25 °C = 3 Ω R[sub]AC[/sub] at 25 °C = 3 Ω R[sub]BC[/sub] at 25 °C = 3 Ω And thus the resistance _measurements_ would be R[sub]AB\_meas[/sub] = 2 Ω R[sub]AC\_meas[/sub] = 2 Ω R[sub]BC\_meas[/sub] = 2 Ω I apply a voltage between A and C and wait a few minutes. I then assume Temperature of R[sub]AB[/sub]= 38.75 °C Temperature of R[sub]AC[/sub]= 80 °C Temperature of R[sub]BC[/sub]= 38.75 °C (I assumed 4X the temperature rise for R[sub]AC[/sub] compared to R[sub]AB[/sub] and R[sub]BC[/sub]. This is because there’s twice as much current in R[sub]AC[/sub] compared to R[sub]AB[/sub] and R[sub]BC[/sub].) Given the tempco of copper is 0.393%/°C, R[sub]AB[/sub] = 3.2 Ω R[sub]AC[/sub] = 3.65 Ω R[sub]BC[/sub] = 3.16 Ω R[sub]AB\_meas[/sub] = 2.15948 Ω ( **7.97% increase** ) R[sub]AC\_meas[/sub] = 2.31368 Ω ( **15.68% increase** ) R[sub]BC\_meas[/sub] = 2.15948 Ω ( **7.97% increase** ) **15.68/7.97 = 1.97** Same as Y!!! ☹ ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [September 21, 2016, 12:53am UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/38 "2016-09-21T00:53:51Z")
> [@](#): > > Given the tempco of copper is 0.393%/°C That is the point you are screwed. That is a linear assumption. I don’t think anything will work until you find a non-linear relationship. Which is why I suggested the IB curve of the magnets. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [September 21, 2016, 8:54am UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/39 "2016-09-21T08:54:43Z")
Just to add, here is a theoretical view of a method I think will work. … And edited, because I found a flaw. ☹ ---
**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 21, 2016, 9:22am UTC](https://boards.straightdope.com/t/is-it-possible-to-determine-if-an-ac-generator-is-a-y-or-delta-from-electrical-measurements/766255/40 "2016-09-21T09:22:09Z")
Well, here’s a non-linear effect: Run a current through two terminals until one of the coils overheats and fails. If it’s a wye, then the current will drop to zero. If it’s a delta, then the low-resistance path will fail first, leaving the high-resistance path–so the current will not drop to zero. What? You just said I can’t disassemble it… [Previous 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=1) [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=3)