Simple transformer hookup question- 480V/3ph to 240/3ph & 240/1ph & 120/1ph 2

[FreddyZ]

I have the 6kVA Acme electric T-2A-53329-1S Transformer to install in a testing machine.

Like

http://www.grainger.com/product/ACME-ELECTRIC-Transformer-4WTZ2

Input is 480V 3ph. Three hots and Protective Earth ground.
Outputs will be 240V 3ph and of course readily available off that is 240V single phase and there is another tap for 120V 1ph.

I would add a pic of the wiring schematic but I don't see how to do that here.

My question is, in attaching to get 120V 1ph, it appears that we are tapping one end of a 240V winding and halfway down the winding, of course. Fine. BUT... it seems to me that neither of these wires will really be the usual "neutral"- that both wires will have a high voltage with respect to say the cabinet which is grounded. Therefor I worry that if a convenience outlet were installed for the machine operator, and wired to this 120V single phase tap.... both the outlet's terminals will be hot, and the only one with zero volts will be the ground terminal.

Or am I confused on this?

I have not had time yet to connect it to power and see if either end of the 120V tap has essentially N or Ground voltage.

If this can't be done because neither leg of the 120V tap is a viable Neutral, we can leave out the convenience outlet and just use the 120V 1ph for control and relay actuation within the enclosure. On the other hand if we need only use a 5-wire outlet and plug, in order to bring the building's Neutral in with the 3+1 power feed wires, that is a viable solution. Just a bit of guidance please.

 

[xnuke]

Sounds like a high-leg delta secondary. You need to ground the center tap connection. This will establish a grounded conductor that will then serve as the neutral, and your 120 V single phase loads can use one of the delta corners and the neutral.

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 for tips on how to make the best use of Eng-Tips.

 

[FreddyZ]

Here is the wiring schematic: illustration #25 at

http://www.galco.com/techdoc/acme/t-2a-53329-1s_wd.pdf

Hope this helps.

 

[xnuke]

Connect X4 to ground, then connect your neutral lines to X4. 120 V circuits will then be X1 (Line) - X4 (Neutral) and X2 (Line) - X4 (Neutral).

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 for tips on how to make the best use of Eng-Tips.

 

[squeeky]

Good link Freddy. 456v primary plus or minus 5%. The steps are common and known to me but the voltage is strange. (I'm in South Africa).
The secondary it has a centre tap which would allow you to get your 120v. As there is no connection to the earth, therefore there is no reference and the voltage to earth can then "float." Your X4 would be a common reference to two 120v voltages and there would be 240v between the two.

What is unclear to me is the current capability. 6kVA at 120v is 50A but if you are suppling it with three phases on the primary, Is this a true three phase low voltage supply or 6kVA at 240v three phase is 14.4A which may be your current limit even at 120v. If you had a load demand of say 25A you would have to split them up between the two "phases" to keep within the current limitations of the transformer. I'm open to suggestions.

 

[FreddyZ]

The label says that the 120V and/or 240V 1-phase use must be limited to 5% of the rated KVA
I get a limit therefore of 0.3kVA or 300VA or... around 2.5A for the 120V 1-phase draw, which should be well above our actual usage except perhaps rare brief intermittent spikes.

Our nominal "480V" 3-phase measures right about 490V line to line consistently, so that part appears to be o problem.

xnuke- thanks so much, I am no expert at voltages above 240, but have always been taught that for e.g. household wiring one must never ever connect Neutral to Protective Earth Ground- except the one junction in the building entry panel where typically all the N wires and Ground wires and the actual bare wire to the rod in "the ground" [the earth/ soil outside the building] all interconnect.

 

[xnuke]

The secondary of your transformer is a separately-derived system. X4 can be connected to the equipment ground bus, where the equipment grounding conductor that accompanies the power conductors terminates. You are not creating a multi-grounded neutral by doing this - the condition you said you've been taught (rightly) to avoid. You are creating a new neutral on a system that is electrically isolated from the transformer primary circuit (i.e., separately-derived). If your primary conductors contained a neutral and you grounded it at the transformer, that would be a multi-grounded neutral.

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 for tips on how to make the best use of Eng-Tips.

 

[mikekilroy]

The label says that the 120V and/or 240V 1-phase use must be limited to 5% of the rated KVA
I get a limit therefore of 0.3kVA or 300VA or... around 2.5A for the 120V 1-phase draw, which should be well above our actual usage except perhaps rare brief intermittent spikes.

I did not download your drwing, but unless they used #20 wire to bring out the 120 tap, their limit of 5% is nonsense. But it is there xfmr so you gotta go with it.

Just know that engineering wise there is nothing they can design so wrong to make the 120% tap good for only 5%. I am saying I would have NO CONCERN over pulling 5,10, 20 amps from that 120v connection, so don't be overly worried if you pull 2.7amps....

http://www.KilroyWasHere<dot>com

 

[davidbeach]

Very common limitation on three phase 240V delta transformers with a center tap. They are designed to supply 3-phase loads with only incidental single phase loads. If you want mostly single phase with a bit of 3-phase use a bank of 2 or 3 single phase transformers.

 

[FreddyZ]

Thanks, folks. xnuke, I think you summed it up nicely.

We are using the transformer to provide mainly 240-3phase or 240-1phase to run the Test Motor, a mere 3-4A draw probably. The motors may require either 240V-1phase, 240V-3phase, or 480V-3phase, depending on the model. The 120V will be used for incidental things like operating 3 or 4 relays to select the voltage/phase power to be applied to the device under test, and perhaps the RPM strobe, a computer, illumination, and such for operator convenience.

Knowing that the output of the transformer is a separately derived circuit and can therefore be connected as set forth above is a great help.