Xformer delta-delta with high leg situation, (irregular voltage)

[LarryFine]

Additional thoughts:

As this is a step-down unit in reverse, the output voltages won't be standard anyway,

The HV side has taps, which might not be set on the best taps for the purpose.

 

[Burdaneta]

The transformer does not care which is the high leg, only your service does.

The only reason the transformer has a high leg terminal is because it has a neutral terminal.

Since you're using the high-leg secondary as your primary, ignore which line terminal is which.

I changed wires but the variation is just 2v from last readings , i suppose i can use this values between 490 and 500 on the 480v side

 

[jim dungar]

You expect non-load voltages to be high. Can you load the transformer?

 

[Burdaneta]

Additional thoughts:

As this is a step-down unit in reverse, the output voltages won't be standard anyway,

The HV side has taps, which might not be set on the best taps for the purpose.

Taps are set on 480v position 3 on this case

 

[Burdaneta]

You expect non-load voltages to be high. Can you load the transformer?

I really dont have anything to make a load, i can connect to an ATS and it just gonna turn on the screen

 

[LarryFine]

Taps are set on 480v position 3 on this case

You may be able to lower the voltages enough by changing them to taps 4 or 5.

 

[Burdaneta]

You may be able to lower the voltages enough by changing them to taps 4 or 5.

I was thinking on do that, may i know your opinion on why is the voltage on the electrical panel coming soo high on the high leg? In reading the value C to neutral and i got 224v instead of 208v.

this panel is fed from the main meter soo nothing between i can change

 

[LarryFine]

may i know your opinion on why is the voltage on the electrical panel coming soo high on the high leg? In reading the value C to neutral and i got 224v instead of 208v.

Presuming your meter is accurate, that's something that only the POCO can do anything about.

 

[jim dungar]

You may be able to lower the voltages enough by changing them to taps 4 or 5.

Remember, the transformer is being run backwards, so the taps will also be backwards, so they may not match the transformer nameplate.

 

[Burdaneta]

Presuming your meter is accurate, that's something that only the POCO can do anything about.

Thanks i will change the taps and post final results.

 

[MD Automation]

You may be able to lower the voltages enough by changing them to taps 4 or 5.

Please correct me if I'm wrong, but I think to lower your secondary output here, with a back-fed xfmr and "high" primary voltage, the taps 1&2 will lower the secondary 480 VAC.

Everything is backwards, like Jim mentioned above.

Tapping the secondary at 4 or 5 will raise the output voltage even higher.

 

[LarryFine]

Remember, the transformer is being run backwards, so the taps will also be backwards.

Yes, I almost mentioned that, but I'm looking at from the point of view of the energized secondary.

Normally, changing taps works by applying a given voltage to a different quantity of turns, which alters the volts-per-turns ratio.

In this case, the volts-per-turns ratio is set by the incoming voltage and a fixed quantity of turns, so changing taps works "normally."

 

[LarryFine]

Tapping the secondary at 4 or 5 will raise the output voltage even higher.

No, because the volts-per-turns ratio is fixed, and the transformer is backwards-wired, the output voltage is directly related to the quantity of secondary turns tapped, or actually used.

 

[don_resqcapt19]

Maybe I explained wrong , the main issue that I have is on the H side I had those irregular voltages

You have an ungrounded system. Voltages to ground on the high side have no meaning.
To comply with code you will have to make the high side a corner grounded system or install ground detectors.
Not sure you can even get 480 out of the high side since you are supplying it with 208 and not 240. Give that the tap 1 ratio is 504/240 or 2.1, I think the high side voltage will only be around 436.

 

[jim dungar]

Yes, I almost mentioned that, but I'm looking at from the point of view of the energized secondary.

Normally, changing taps works by applying a given voltage to a different quantity of turns, which alters the volts-per-turns ratio.

In this case, the volts-per-turns ratio is set by the incoming voltage and a fixed quantity of turns, so changing taps works "normally."

Yes, taps work by changing the number of turns thereby affecting the turns ratios, so 468:240 should also be 240:468. However transformers are actually built with some amount of extra turns on the designed primary in order to compensate for losses in the transformer. When run backward these compensating windings are on the wrong side which slightly affects the actual turns ratio and the resultant tap voltage.

 

[LarryFine]

Not sure you can even get 480 out of the high side since you are supplying it with 208 and not 240.

No, he has a 240v high-leg delta. He keeps bringing up the 208v high leg.

 

[LarryFine]

Yes, taps work by changing the number of turns thereby affecting the turns ratios, so 468:240 should also be 240:468. However transformers are actually built with some amount of extra turns on the designed primary in order to compensate for losses in the transformer. When run backward these compensating winnings are on the wrong side which slightly affects the actual turns ratio and the resultant tap voltage.

I'm not suggesting that the tap voltage numbers are accurate, just that to lower the output voltage, he would need to use a tap of fewer turns, i.e., farther away from the end of the winding.

The same tap that would be used with a transformer being used in the right direction (as a step-down) that has been set to compensate for a lower incoming voltage.

 

[MD Automation]

No, because the volts-per-turns ratio is fixed, and the transformer is backwards-wired, the output voltage is directly related to the quantity of secondary turns tapped, or actually used.

Larry, you are correct. And I was not!

With no taps on the back wired primary, the magnetic flux is "fixed" and non-adjustable. And like you pointed out, the only way now to lower the HV output is to have fewer turns involved. And moving taps towards 4,5,6 and 7 does that.

It's just you are using the "higher" taps to "lower" the output - that's where I got crossed up.

Thx again and 1000 pardons.

 

[Burdaneta]

You have an ungrounded system. Voltages to ground on the high side have no meaning.
To comply with code you will have to make the high side a corner grounded system or install ground detectors.
Not sure you can even get 480 out of the high side since you are supplying it with 208 and not 240. Give that the tap 1 ratio is 504/240 or 2.1, I think the high side voltage will only be around 436.

In getting 501v on 2 of 3 phases and 480 on the third

 

[Burdaneta]

Yes, I almost mentioned that, but I'm looking at from the point of view of the energized secondary.

Normally, changing taps works by applying a given voltage to a different quantity of turns, which alters the volts-per-turns ratio.

In this case, the volts-per-turns ratio is set by the incoming voltage and a fixed quantity of turns, so changing taps works "normally."

Great explanation, will definitely try that tomorrow, in out of time for today, ill keep you guys posted, thanks so much