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:

You are not allowed to disassemble the generator. The generator is essentially a black box.

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.

You are free to model each winding as resistor (pic), inductor (pic), resistor + inductor (pic), or any other model of your fancy.

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.

You do not know the resistance or inductance of the windings. You only know that they’re all the same.

All windings are isolated from ground. If it is a Y, there is no neutral connection.

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.

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.

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.

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.

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.

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.

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.

Yep, this was the first thing another engineer tried. Doesn’t work.

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.

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.

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.