Open Delta Transformer

[KP2]

I don't understand how phase A to B has 240 volts on this open Delta transformer.

I think I get the 208 part. (240 + 120) / 1.732 = 207.852

Am I getting warm???:-?

I found this picture in a post from iwire, hope its OK.

 

[KP2]

This is a better picture but the phase letters changed. In this picture the 240v is from B to C.
Pic from Dennis Alwon

 

[roger]

An open or closed delta doesn't automatically mean "High Leg", either variety can have a high leg if one winding is center tapped.

Roger

 

[KP2]

I'm just not clear on the open delta how there is 240 volts between B & C.

 

[LarryFine]

Because of the relative timing of the remaining phases. Let me see if I can paint a mental picture of why it works:

If you were to open one corner connection of a full Delta, there would (theoretically) be zero volts across the gap. Plus, when you parallel two equal-voltage sources, there would be no current between them.

An unloaded full Delta has (again, theoretically) zero current across any phase's winding, so it acts as if the actual winding is "in parallel" with the imaginary source formed by the other two phases.

Otherwise, there would be great circulating currents even when the bank is unloaded, as one would have with one transformer connected out-of-phase with the way it should be connected.

The 240v across the open side of an open Delta is less 'stable' than a full Delta because the missing actual winding results in a higher source impedance (the ability to not be affected by being loaded.

 

[Smart $]

I'm just not clear on the open delta how there is 240 volts between B & C.

Think of it this way...

IF there wasn't 240 volts there, what do you think would happen when you did put 240 volts there (i.e. closed the delta)?

 

[Smart $]

...

I think I get the 208 part. (240 + 120) / 1.732 = 207.852

...

It's just by coincidence that your formula arrives at the correct answer.

The 208 is a result of two out-of-phase voltages with a common connection. It is the voltages being out-of-phase by 60? that yields the vectorial sum of 208.

 

[mivey]

I'm just not clear on the open delta how there is 240 volts between B & C.

If it makes it easier, just think of it this way: The source holds B & C in a fixed position relative to each other.

Further back up the line, there is another transformer or generator with windings between B & C that are maintaining that voltage relationship. Just because we remove the coil on our end doesn't mean the voltage between B&C will collapse (or diverge) because the coils further up the line would have to be collapsed as well.

 

[roger]

Here is an illustration that may help.

Roger

 

[KP2]

Think of it this way...

IF there wasn't 240 volts there, what do you think would happen when you did put 240 volts there (i.e. closed the delta)?

So then the load closes the the gap???

Thanks for the 60 deg. that is helpful

 

[LarryFine]

So then the load closes the the gap???

Sort of. The load takes advantage of the fact that there is 240v across the open side, depsite the fact that there is not an actual secondary winding there.

However, there's no free power. It still comes from the two present secondary windings. Adding a third transformer will increase the entire Delta's ampacity.

 

[Smart $]

So then the load closes the the gap???

Not what I meant... besides the load is not a source of voltage.

I meant in comparison to a "closed" delta secondary, with all three windings. We know each winding is the source of 240 volts. If there wasn't 240 across the open terminals of two windings, putting the third 240 volt source in would likely let the smoke out of the windings... i.e. any difference in voltage between the open terminals of the open delta windings and the third winding by itself would, when all three are connected, generate circulating current through all three windings... but since the gap is 240 volts of the same phasing as the third winding, the kids all play nice together... and don't smoke

Incidentally, this configuration is also used at times to maintain power supply to a normally "closed" delta service when one of three transformer's in a bank goes bad. The POCO simply disconnects the bad one and the customer still has power, though not to full capacity. The bad transformer can be pulled and replaced later if there exists a higher priority issue elsewhere, such as during widespread natural disasters.

 

[mivey]

...We know each winding is the source of 240 volts...

I know what you meant. But to clarify for others, it is another 240 volt source not the only 240 source. Because of the configuration, the voltage from the coil we add also matches the voltage across the ends of the other two coils.

The source across the ends of the two coils is one power source. When we add the 3rd coil, we have an additional power source paralleled across the open coil source. The two sources have the same voltage and are in phase with each other.

That is why we must double-check polarities when adding the 3rd coil as we might attempt to connect a source that is 180 degrees out of phase with the existing open coil source.

 

[Smart $]

...
That is why we must double-check polarities when adding the 3rd coil as we might attempt to connect a source that is 180 degrees out of phase with the existing open coil source.

That'd be the smoking group

 

[mivey]

That'd be the smoking group

Definitely a good place for a "No Smoking" sign.

 

[Smart $]

I know what you meant. But to clarify for others, it is another 240 volt source not the only 240 source...

...and I did not sststtutter when I said each winding is the source of 240 volts... three windings, three sources of 240 volts.

 

[mivey]

...and I did not sststtutter when I said each winding is the source of 240 volts... three windings, three sources of 240 volts.

also two windings, three sources of 240 volts

Add: or three source voltages, anyway.

 

[Smart $]

also two windings, three sources of 240 volts

Add: or three source voltages, anyway.

Okay... if you're gonna get picky: three windings, three discrete sources of 240 volts

 

[KP2]

If you were to open one corner connection of a full Delta, there would (theoretically) be zero volts across the gap. Plus, when you parallel two equal-voltage sources, there would be no current between them.

Because they are the same point, or same potential voltage??

Further back up the line, there is another transformer or generator with windings between B & C that are maintaining that voltage relationship. Just because we remove the coil on our end doesn't mean the voltage between B&C will collapse (or diverge) because the coils further up the line would have to be collapsed as well.

The 240v across the open side of an open Delta is less 'stable' than a full Delta because the missing actual winding results in a higher source impedance (the ability to not be affected by being loaded.

Incidentally, this configuration is also used at times to maintain power supply to a normally "closed" delta service when one of three transformer's in a bank goes bad. The POCO simply disconnects the bad one and the customer still has power, though not to full capacity. The bad transformer can be pulled and replaced later if there exists a higher priority issue elsewhere, such as during widespread natural disasters.

Sort of. The load takes advantage of the fact that there is 240v across the open side, depsite the fact that there is not an actual secondary winding there.

However, there's no free power. It still comes from the two present secondary windings. Adding a third transformer will increase the entire Delta's ampacity.

In conclusion the open side has 240 volts for the same reason we get 208 volts at the center tap. Except Two windings of 240 volts each held 60 deg angles results in 240 volts.

I also understand now why it is unstable and mostly used for supply to buildings with mostly single phase loads and how it works in case one winding goes bad.

Thanks for helping me out.
Kevin

 

[LarryFine]

Because they are the same point, or same potential voltage??

Because they're the same potential. If they weren't, simply closing the Delta would cause abnormal currents to flow. If one used one wrong-voltage secondary in a full Delta, that would happen, too.

In conclusion the open side has 240 volts for the same reason we get 208 volts at the center tap. Except Two windings of 240 volts each held 60 deg angles results in 240 volts.

Start with looking at the center-tapped secondary all by itself. It is exactly like a typical 120/240v secondary in every way. In fact, the open Delta started as a 3ph modification to existing 1ph services.

I also understand now why it is unstable and mostly used for supply to buildings with mostly single phase loads and how it works in case one winding goes bad.

It's not all that "unstable", really. If it were, it wouldn't function. You can supply 240v loads across the open side. I've put 1ph and 3ph 240v loads in a panel without skipping spaces.

The main thing to remember is that, while the open Delta functions okay, high-starting-current loads can cause a bit of voltage sag as much as any transformer bank of similar capacity.

Nobody today would install a new open-Delta service, unless they were doing so to work with an existing such supply, which I've done several times, saving the customer a lot of POCO $.