208 through an isolation transformer

[raithrovers1]

Hopefully someone can help me understand the following:
We have a UPS that was installed in the 80's. The input is from the A and B phase of a 3 phase 480/208 delta/star transformer at 208VAC. The UPS has an internal output isolation transformer on it. The plant take the A phase to neutral (120VAC)from the input and put it through a make before break external bypass switch. They also only use the A phase to neutral (120VAC) on the UPS output which also goes to the bypass switch.
I have always understood that you cannot use a normal 208VAC single phase in and out UPS with a make before break bypass switch. This is confirmed at this site as there is a 60VAC difference between the A phase input and the A phase output on this UPS while on static bypass.
What I don't quite understand is why the phase differnce is there. I have always thought that there is no noticable phase shift on the primary and secondary of a single phase isolation transformer. I realize that the input to this transformer is 2 phases of a 3 phase 208VAC supply but the potential across the primary winding is still 208VAC single phase. Therefore I would have thought that the secondary 208VAC would be in phase with the input. Why is this not the case?
I understand that the 120VAC will be out of phase because they are artificially created by winding taps on the secondary of the transformer rather than the A phase and B phase being 120 degrees apart and the neutral being created by a star center point.
I also understand that the fix to this situation is to install a 208VAc to 240VAC transformer on the input and install a 240VAC to 240VAC UPS. This will allow the use of an external make before break bypass switch.
Could someone help me understand the 208 to 208 phase shift?

UPS Service Manager

http://www.nolanpower.com

 

[sibeen]

hey also only use the A phase to neutral (120VAC) on the UPS output which also goes to the bypass switch.


You're confusing me a bit here. Is this a single phase output UPS?

 

[sibeen]

I think I've worked out what you are on about. You need to look at the vectors.

Draw an equilateral triangle, each leg of length 208. Points at A-B-C.

Now draw three lines from the corners into the centre point, you will note that each line to the centre (neutral) will be 120 units long.

Now draw a line from the centre (neutral point) to the mid pint of one of the legs of the triangle. That is the difference between the neutral and the A-B (or any other corners) point.

Hopefully you get 60 units.

I hope that makes sense....I've had quite a few beers whilst watching The Ashes.

 

[ScottyUK]

Your 208V is across two lines. Your 120V is across line-neutral. Draw out the vectors of the three phases with the neutral point at the centre. There is no way that a line-neutral voltage can be in phase with a line-line voltage regardless of the combination you pick.

 

[raithrovers1]

Thanks for the replies.
As usual, I have read my thread again and found that I missed out an important detail!!!
The UPS is 208 in to 208 out. They have just chosen to use only one leg of the 120VAC out (A phase to neutral)as their load supply. I understand that the output 120VAC to neutral will not be in phase with the input 120VAC to neutral. Why won't the 208VAc in and the 208VAc out?

Sibeen. I followed you until you said draw a line from the neutral to mid point of one of the legs. Why would I go to the midpoint of one of the legs? Sorry if I am missing something obvious!

UPS Service Manager

http://www.nolanpower.com

 

[davidbeach]

A delta-wye isolation transformer will create a phase shift that will create a voltage between A in and A out.

 

[raithrovers1]

Sorry! One more detail. The UPS is 208 single phase in 208 single phase out. No delta wye transformer in the UPS.

UPS Service Manager

http://www.nolanpower.com

 

[DRWeig]

Diagrams help.

Click this link to a quick sketch I made. Is this close to what you're telling us?

If so, can you see where the phase shift originates?

If not, please enlighten.

Best to you,

Goober Dave

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[ScottyUK]

Post a sketch of this thing showing all the voltages, phases, transformer connections, switches, etc. I'm confused by the series of posts above.

 

[raithrovers1]

I have attached a sketch I made today. I am at another site today. The phase difference is between the A phase input to the UPS and the A phase output of the UPS after the output transformer. These terminals go to the bypass switch contacts. A Fluke was connected between these terminals on AC and a difference of 60VAC was measured. Please also not that this was measured while the UPS was in static bypass, i.e. the inverter was not supplying load.

UPS Service Manager

http://www.nolanpower.com

 

[davidbeach]

Just look at the what the transformer windings are drawn; there's a 30 degree phase shift between phase-neutral on the wye winding at the left and the phase-center tap of the transformer to the right. The output is in phase with the phase-phase voltage input, not with the phase-neutral voltage.

 

[stevenal]

Might help to remove the UPS from the diagram. When bypassed, the UPS is effectively not there. No possible phase shift on a single phase transformer, so the 120 on the right is in phase with the 208. The only oddball is the 120 on the left, which is connected phase to neutral for a 30 degree shift from your phase to phase 208.

 

[ScottyUK]

And if you calculate 120 sin30 you account for the 60V you are measuring.

 

[sibeen]

Sibeen. I followed you until you said draw a line from the neutral to mid point of one of the legs. Why would I go to the midpoint of one of the legs? Sorry if I am missing something obvious!

Sorry, my bad. A lazy post where I truncated the ending.

All I was trying to show was how the 30 degree phase difference came in between the delta and the wye.

By drawing a line from the neutral point to the centre of one of the delta legs handwaving shows approximately the voltage difference between the two. By going to the centre point of the delta leg you are actually going to 104 volts and not 120 volts, so you can adjust the line from the centre (neutral) to a point 120 units along the delta line. Do that and the difference will be about 62 units (volts).

 

[tesladog58]

once again some one does not understand the voltages of a 120/208 3 phase system, and has them mixed up as a 120/240 single phase system, the same people that believe you can have 120/240 equipment work on a 120/208 and than ask the utility why the voltage is low and not 240 but at best 216. please look at your transformer wiring diagrams.

 

[raithrovers1]

davidbeach: "The output is in phase with the phase-phase voltage input, not with the phase-neutral voltage. ".
I understand the phase neutral relationship as I have previously mentioned. Everyone has stated that the 208 will be in phase. If this is the case if I attached a scope to the input 208VAC A to B and the Output A to B I would see 208 on both traces and 0 degrees phase shift. This means that the A phase is rising and falling at the same time on both the input and the output. This should mean that if I connect a voltmeter between A phase input and A phase output it should be the same? This is where I get lost. This is where I find the 60V difference. Once again I am truly sorry if I am missing something obvious.

tesladog58:I am originaly from a country that does not have 120/208. Sometimes new things can be slightly confusing. I thought I made it clear in my first post that I did understand the difference between 208VAC and 120/240 as I stated "I understand that the 120VAC will be out of phase because they are artificially created by winding taps on the secondary of the transformer rather than the A phase and B phase being 120 degrees apart and the neutral being created by a star(or as the USA calls it wye) center point."
Also, it has been twenty years since I had to do vector analysis. I am a little rusty!
I am sorry you felt the need to reply as you did.

UPS Service Manager

http://www.nolanpower.com

 

[waross]

Forget the UPS there is no phase shift there.
As for vectors, your sketch is adequate.
Look at your supply transformer. It is labelled A, N, B You can easily see the phase shift between A-B and A-N
The supply to the UPS is in phase with A-B.
The output of the UPS is in phase with A-B. The 120 Volt tap of the output transformer is in phase with A-B.
You are taking a bypass voltage from A-N. The phase shift between A-B and A-N is shown in your sketch.
The phase shift originates from the supply of the 120 Volts, not in the UPS.
If you use a 208 Volt to 208 Volt bypass the phase shift will go away.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[sibeen]

Try this.

 

[ScottyUK]

sibeen,

Looks very like a rough sketch on my desk - I was going to scan it later but your document shows things nicely.