Getting 120V from a 240Y/139V XFMR
Getting 120V from a 240Y/139V XFMR
(OP)
Gentlemen, I have attached a photo of the XFMR in question. The primary of the XFMR is Nominal 550V Delta. Our plant voltage is roughly 580V on all 3 phases. The secondary of the XFMR is 240Y/139V. We are only using two legs of the Y on the secondary. All secondary loads are from phase to neutral (139V). All of our loads are rated at 120V. I am hesitant to put 139V on 120V devices. My question is this... If I were to disconnect the neutral point of the unused secondary winding, wouldn't the voltage from the other two legs to neutral be 120V? At that point, you would have 240V across the two used legs, with the neutral acting as a center tap making it 120V from each leg to neutral. Of course I could be missing something... Any advice would be appreciated!





RE: Getting 120V from a 240Y/139V XFMR
Each secondary winding is producing 139 Volts, regardless of being connected to any other windings.
Try a 120/240:120/240 Volt isolation transformer to generate 120 Volts.
The transformer must have sufficient capacity to carry the maximum possible unbalanced neutral current. No fudging, the maximum current if only the loads on one line are energized.
You may be able to parallel the primary and secondary windings if they are identical.
This will double the capacity. Check for circulating currents before running the windings in parallel.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Getting 120V from a 240Y/139V XFMR
RE: Getting 120V from a 240Y/139V XFMR
Look at your secondary diagram:
The secondary diagram is a vector representation of the secondary voltages. You can scale that sketch to any voltage that you wish and the angles and relationships remain the same. There will always be a root 3 (1.73) relationship between the line to line voltages and the line to neutral voltages, be it 120:208 Volts or 139:240 Volts.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Getting 120V from a 240Y/139V XFMR
RE: Getting 120V from a 240Y/139V XFMR
xnuke
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RE: Getting 120V from a 240Y/139V XFMR
xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Getting 120V from a 240Y/139V XFMR
Keith Cress
kcress - http://www.flaminsystems.com
RE: Getting 120V from a 240Y/139V XFMR
RE: Getting 120V from a 240Y/139V XFMR
Keith Cress
kcress - http://www.flaminsystems.com
RE: Getting 120V from a 240Y/139V XFMR
It is what it is.
Nowhere in the description of the phase angle relationships between phases is the ground mentioned or needed.
If you remove the ground connection you will still have the same phase angles, the same line to line voltages and the same line to wye-point neutral voltages.
You will have added an NEC code violation.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Getting 120V from a 240Y/139V XFMR
If the transformer is some distance away from its loads, the voltage drop along the feeder lines will probably bring the utilization voltage closer to 120 volts.
RE: Getting 120V from a 240Y/139V XFMR
Splitting hairs there is the buck connection and the auto-transformer connection.
To use the buck connection, you need a transformer rated 140:20 Volts.
To use the auto-transformer connection you need a transformer rated 120:24 Volts.
That will give you about 123.5 Volts.
If you go up one tap on the primary you will get a nominal 121 Volts.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Getting 120V from a 240Y/139V XFMR
Look-up the open delta transformer configuration as an example of why disconnecting a single coil doesn't change anything. It still passes 3-phase voltages even with one "leg" missing.
Your rubber band analogy would have worked if you were also modifying the primary of the transformer. If it was a Y-Y connected transformer and you disconnected the primary as you suggested, then the 2 corresponding secondary phases would have shifted to become the voltages you wanted.
If you could break apart the primary delta connection and put the primary coils between the same 2-phases then the secondaries would be in phase and could add directly. But, it'd still be 139V/278V so it wouldn't accomplish what you want.
RE: Getting 120V from a 240Y/139V XFMR
Even though you may be using the auto-transformer connection, the transformer must still be "buck-boost" rated.
The buck-boost rating has to do with the insulation value of the secondary winding, regardless of the connection.
Regardless of the connection, auto-transformer or buck, the secondary will still see 139 Volts to ground and must have suitable insulation.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Getting 120V from a 240Y/139V XFMR
RE: Getting 120V from a 240Y/139V XFMR
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