Deriving a Neutral from a Transformer

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Little Bill

Little Bill

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Semi-Retired Electrician
I've hooked up a few transformers but usually just like the specs say. Also, either there is a neutral already there or I don't need one.
I see a post sometimes mentioning deriving a neutral where there is none.
Could someone explain to me how that's done?
Please do it so my simple mind can comprehend it!:blink::)
 
For me we usually hook up delta / wye transfomers 480 volt to 208Y/120.

These transformers have H1,H2,H3 as the primary side delta connections.

On the secondary side they will have X0, X1, X2, X3. XO will be the neutral or grounded conductor connection in addition to the load neutral being connected to XO there will be the GEC and the system bonding jumper ... or at least the system bonding jumper.

That is all I can really say, how that XO is connected to the other three coils is beyond me.
 
Little Bill said:
I've hooked up a few transformers but usually just like the specs say. Also, either there is a neutral already there or I don't need one.
I see a post sometimes mentioning deriving a neutral where there is none.
Could someone explain to me how that's done?
Please do it so my simple mind can comprehend it!:blink::)
Click to expand...
If you have 240 volts only and wish to have 120/240, you need to connect any 120/240 load and you establish a neutral point from that load. Best thing to use when loads you wish to feed are not anything definite is to connect another transformer that has a a 120 volt midpoint - there is your neutral, you have kind of made an autotransformer or voltage divider out of it. With three phase - similar concept - connect any three equal 120 volt loads in a wye configuration across the three 208 volt phases and you will read 120 volts from any phase to the common point of the wye.

Keep in mind you may not be able to ground this "neutral point", and probably shouldn't even if you can, but you can create 120 volts where it doesn't otherwise exist.
 
It helps me to remember there is no direct electrical connection between the H side and the X side, everything happens because of magic in the iron of the transformer.

Ignore three phase for the moment. The XO/neutral is nothing more than the midpoint of the windings of a single phase transformer and a lug that you can attach to.
 
kwired said:
If you have 240 volts only and wish to have 120/240, you need to connect any 120/240 load and you establish a neutral point from that load. Best thing to use when loads you wish to feed are not anything definite is to connect another transformer that has a a 120 volt midpoint - there is your neutral, you have kind of made an autotransformer or voltage divider out of it. With three phase - similar concept - connect any three equal 120 volt loads in a wye configuration across the three 208 volt phases and you will read 120 volts from any phase to the common point of the wye.

Keep in mind you may not be able to ground this "neutral point", and probably shouldn't even if you can, but you can create 120 volts where it doesn't otherwise exist.
Click to expand...

When you say "another transformer", are you saying two transformers are needed?
 
Little Bill said:
When you say "another transformer", are you saying two transformers are needed?
Click to expand...
No. I think he is saying you can find a neutral point in equipment besides transformers.

For instance, wire up two 120V heaters in series and apply 240V..from end to end you have
240V, from either end of the heaters to the center you will measure 120V.

Similar with motor windings, you tie leads 2&3 together when you are hooking them up 240V. The voltage across 1&2 or 3&4 is 120V.
 
kwired said:
If you have 240 volts only and wish to have 120/240, you need to connect any 120/240 load and you establish a neutral point from that load. Best thing to use when loads you wish to feed are not anything definite is to connect another transformer that has a a 120 volt midpoint - there is your neutral, you have kind of made an autotransformer or voltage divider out of it. With three phase - similar concept - connect any three equal 120 volt loads in a wye configuration across the three 208 volt phases and you will read 120 volts from any phase to the common point of the wye.

Keep in mind you may not be able to ground this "neutral point", and probably shouldn't even if you can, but you can create 120 volts where it doesn't otherwise exist.
Click to expand...

Are you trying to make things confusing? If so you have succeeded. I have no clue what you are trying to get across. :?
 
iwire said:
Are you trying to make things confusing? If so you have succeeded. I have no clue what you are trying to get across. :?
Click to expand...
Read what ActionDave posted, he was right on with what I was tying to get at. Sorry I didn't present it well enough.
 
There is also a method of developing a neutral from a delta system by use of "zig-zag" transformer connections. Only saw it once in 5 decades.. did not understand it then, don't now. Some of our sharper Forumites can perhaps explain it.
 
Little Bill said:
When you say "another transformer", are you saying two transformers are needed?
Click to expand...
Another transformer parallel to the first effectively is an autotransformer, but may be more stable voltage then connecting to a center tap of a resistive load. The size of that second transformer only need be based on the neutral load you are trying to supply and not necessarily sized to match the source transformer.

This may not be the ideal permanent install, but can come in handy as a temporary thing, say you lost underground neutral or something like that. You could connect about any 120/240 volt coil to temporarily derive a neutral point, it only needs enough capacity to handle the neutral load so if you are fairly balanced it doesn't need to be too large, and continue to operate the system until the failed underground can be repaired.
 
ActionDave said:
No. I think he is saying you can find a neutral point in equipment besides transformers.

For instance, wire up two 120V heaters in series and apply 240V..from end to end you have
240V, from either end of the heaters to the center you will measure 120V.

Similar with motor windings, you tie leads 2&3 together when you are hooking them up 240V. The voltage across 1&2 or 3&4 is 120V.
Click to expand...

Yea, with the heaters you have 120 --120 as long as you don't connect any 120 loads. The voltage regulation is really bad.

With a transformer, the regulation is going to be much better, as long as you don't overload the transformer.

Sorry, kwire, wrote before I read every thing.
 
Voltage is always defined between two points.

If you consider a reasonably balanced electrical system, you can measure the voltage and phase angle between all of the supply terminals. The neutral is simply a supply terminal that all the other supply terminals are approximately evenly distributed around. (So in a 208/120V wye system, each of the 'phase' terminals is 120V relative to the neutral terminal, and the phase angles of A-N-B, B-N-C and C-N-A are all about 120 degrees).

Again, the neutral is simply a system terminal at a particular voltage relative to all of the other system terminals.

You can 'derive' a neutral using any of the same techniques that you use to derive some other voltage not available in your supply system. A 'derived' neutral is an terminal from some device (such as a transformer) which is at approximately the neutral voltage for the system, and is not 'isolated' from the system. (Using a delta-wye transformer creates a new neutral for a separately derived system.)

Take for example a single phase system where you just have the 240V 'hot' lines, but no neutral. You could connect a 240V to 120V transformer to those supply lines. If you now connect _one_ of the secondary terminals to one of the supply lines, the _other_ secondary terminal will be at either 240V or 0V relative to ground, depending on phasing. If the not yet connected secondary terminal is at approximately 0V, then it is arguably a 'derived neutral'.

A more efficient way of doing this is with a proper autotransformer. This will be a single 240V coil with a center tap. The turns ratio of the full coil to the half coil (center tap to terminal) is 240:120, and the center tap will be at approximately 0V, and will be a good derived neutral.

For deriving a neutral in a three phase system, you need to be careful. Simply connecting a bank of three coils in a wye configuration to a delta system will not derive a very good neutral. This is because the 'neutral point' will have very high impedance relative to the system terminals. Each phase of this 'why' is standing alone, not coupled to the other phases, and thus the impedance of this neutral point is set by the impedance of the inductors making up the wye. This is in many ways like a 'neutral' point derived with resistors; good for measurements but not good to carry any current.

A 'proper' way to derive such a neutral is either to use a wye/delta transformer, with the wye connected to the system and the neutral of the wye used as your derived neutral, with the delta left floating. Any attempt to pull current through this neutral will cause current to circulate in the delta. Another 'proper' way to derive a three phase neutral is with a 'zig-zag' transformer. The 'zig-zag' transformer works because each 'leg' of the 'zig-zag-why' is split between two cores, and thus each phase of the 'zig-zag' is coupled to the other phases.

-Jon
 
Thanks, but I'm having trouble following what is described here. I tried to draw it out but the directions are a little "fuzzy" to me.

Take for example a single phase system where you just have the 240V 'hot' lines, but no neutral. You could connect a 240V to 120V transformer to those supply lines. If you now connect _one_ of the secondary terminals to one of the supply lines, the _other_ secondary terminal will be at either 240V or 0V relative to ground, depending on phasing. If the not yet connected secondary terminal is at approximately 0V, then it is arguably a 'derived neutral'.
Click to expand...

I'm not understanding if you're suppose to get 120V to neutral or 240V to neutral.
I'm not understanding if you connect the 240V hot lines to the primary side
I'm not.......forget all that
:blink::?:)

Maybe someone could draw this for me because I have more questions than there are steps involved, if that's possible!:huh:

I understand measuring to a midpoint, just not seeing the setup for what is being described above.
 
Last edited:
Little Bill said:
I've hooked up a few transformers but usually just like the specs say. Also, either there is a neutral already there or I don't need one.
I see a post sometimes mentioning deriving a neutral where there is none.
Could someone explain to me how that's done?
Please do it so my simple mind can comprehend it!:blink::)
Click to expand...

if the secondary winding isn't tapped for a neutral, well, going in with
a piece of aluminum bus and a tig welder may affect the ul listing a bit.

you need a place to land X0 somewhere, and unless you are doing something
like building two or three separate single phase xfmr's into an array, there
isn't any way to do it that i'm aware of. there are only two places that neutral
can be attached to the secondary winding... where all the windings connect in
a wye secondary, and the centertap of the A-C winding in a delta.

sometimes i've had to run a xfmr backwards to get 480 for a load in a 208
or 240 volt occupancy, but i've never needed a neutral for something like that.

i dimly remember the zigzag thing from apprenticeship. that's the last time i've
seen one.... right in there with quadrophonic stereo systems.

a three phase delta secondary is supposed to have one phase grounded.
the first time you do it, it feels kinda odd tying B phase to ground, and turning it on.
 
ronaldrc said:
Here is a diagram I posted a while back.

http://i235.photobucket.com/albums/ee286/ronaldrc/Create-120240-with-a-neutral_zps13a6202f.jpg
Click to expand...

Thanks, that is exatly what I was attempting to describe earlier. You are using the transformer as an auto transformer.

You could do the same thing with resistance loads but the transformer will have better voltage regulation, a resistive voltage divider would need to have load fairly balanced or you may not get desired output voltages, but could work well if carefully selected for a fixed load.
 
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