This is a 60+ year old building that has many transformers. This one in particular is located in an ideal place to use for a new show room that will have several floor model stoves. There will be very little 120V load. Most of the new stoves need a neutral, because they have all kinds of electronics, like wi fi and such. Transformer is 75KVA 480V delta-240V delta, currently corner grounded. I didn't know you could replace a secondary winding. Tell more.
Generating a neutral for 240V corner-grounded system (so line to neutral is 120V)
- Thread starter donw
- Start date
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- Wisconsin
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- PE (Retired) - Power Systems
No, I was thinking about smoke coming out eventually.
There is no cost effective way to modify your existing dry type 75KVA transformer.
Why not?
leave 240V 3PH 3W system alone. Have it feed a 3-phase panelboard, if it doesn't already
Purchase a small standard single phase transformer and wire it 240V 2W in and 120/240 1PH 3W out. Install a small loadcenter to handle any 120V loads
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
181114-2401 EST
donw:
My comment on replacing a secondary was just a joke because it would not be practical.
But you might consider a 480 to 240 center tapped transformer because it seems like you will have considerable 240 load, and that load has 120 needs. The 120 and 240 sources need to be correlated, and it would not be wise to ground one side of the stove 240 and float what would normally be the center point of the 240. One does not know the internal design of a stove, know how it would react to its normally grounded lead being 120 V above ground, and also the other 240 line being 240 above ground rather than 120.
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donw:
My comment on replacing a secondary was just a joke because it would not be practical.
But you might consider a 480 to 240 center tapped transformer because it seems like you will have considerable 240 load, and that load has 120 needs. The 120 and 240 sources need to be correlated, and it would not be wise to ground one side of the stove 240 and float what would normally be the center point of the 240. One does not know the internal design of a stove, know how it would react to its normally grounded lead being 120 V above ground, and also the other 240 line being 240 above ground rather than 120.
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gar
Senior Member
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- Ann Arbor, Michigan
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- EE
181114-2458 EST
donw:
Do you really have a true common core three phase transformer, or is it really 2 or 3 single phase transformers?
If the three phase transformer is made up of 2 or 3 single phase transformers, then replace one of the transformers with one with a center tapped secondary, provided there was no necessary reason for corner grounding.
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donw:
Do you really have a true common core three phase transformer, or is it really 2 or 3 single phase transformers?
If the three phase transformer is made up of 2 or 3 single phase transformers, then replace one of the transformers with one with a center tapped secondary, provided there was no necessary reason for corner grounding.
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If you just want to demonstrate the operation of the stoves and not cook on them, the actual load is a lot less than 75 kVA. What you have now, the corner grounded delta, essentially one leg is the neutral, but the line to ground voltage is 240 V when the stove may need 120.
If you are looking to save money, you would probably be looking at reusing as much of the existing distribution wring after the transformer, which could be either 3 phase type with three hot busses to attach breakers, or single phase type with two hot busses to attach breakers.
I would maybe be looking to swap the transformer to something large enough to power the actual load (which I am guessing you will not be demonstrating more than 20 kW at any one time) and chosen to match the existing load side distribution equipment and wiring, either three phase type or single phase type.
Assuming the store is old and the old stoves were 240 V no neutral. If the store is going in new to repurpose an old space, you will want all new secondary distribution equipment to match the new loads along with the new transformer.
If you are looking to save money, you would probably be looking at reusing as much of the existing distribution wring after the transformer, which could be either 3 phase type with three hot busses to attach breakers, or single phase type with two hot busses to attach breakers.
I would maybe be looking to swap the transformer to something large enough to power the actual load (which I am guessing you will not be demonstrating more than 20 kW at any one time) and chosen to match the existing load side distribution equipment and wiring, either three phase type or single phase type.
Assuming the store is old and the old stoves were 240 V no neutral. If the store is going in new to repurpose an old space, you will want all new secondary distribution equipment to match the new loads along with the new transformer.
Dan, thanks for your input.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
181116-2041 EST
donw:
The circuit in your post #26 will work if you remove the corner ground to the delta, use what is called neutral busbar as neutral and ground it, erase the label where they say 120 V neutral (it is not). The drawing is a very poor way to illustrate what is being done in my opinion.
The transformer needs to be sized in some way for expected fault currents to ground.
You need to do a safety analysis, and what should happen when one or both of the breakers open.
This not a good solution.
A better solution is new single phase isolation transformer 480 V to a center tapped 240 secondary sized for maximum expected load.
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donw:
The circuit in your post #26 will work if you remove the corner ground to the delta, use what is called neutral busbar as neutral and ground it, erase the label where they say 120 V neutral (it is not). The drawing is a very poor way to illustrate what is being done in my opinion.
The transformer needs to be sized in some way for expected fault currents to ground.
You need to do a safety analysis, and what should happen when one or both of the breakers open.
This not a good solution.
A better solution is new single phase isolation transformer 480 V to a center tapped 240 secondary sized for maximum expected load.
.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Just adding my 2 cents since donw sent a PM asking me to comment on 'zig-zag' transformers.
Zig-zag transformers are autotransformer arrangements used to derive a neutral for a balanced 3 phase source. These are used, for example, to ground a previously ungrounded delta source. The combination of a delta source and a zig-zag derived neutral is very similar to a true wye source. If a zig-zag transformer is properly sized, it can be used to feed line-neutral loads.
As has already been noted, a zig-zag transformer could not possibly be used in this situation, because it would convert the 240V secondary into a 240/139V secondary source, and the OP requires a 120/240V single phase source.
The OP has also brought up the possibility of a single phase derived neutral. A 240V coil with a center tap can be used as the single phase 'grounding transformer', to derive a neutral. However it would require significant engineering to properly size this transformer. The cost of this engineering is probable significantly greater than the value of a 75kVA transformer. As gar says, the circuit in post 26 would work in theory; I simply don't think it is of practical benefit after you consider the required design engineering.
Post #2 incorrectly assumes a single phase transformer, but offers an interesting possibility.
This transformer happens to be conveniently located, no longer being used for 240V loads, and oversized for the new application. I believe that you could disconnect one of the legs, and use it as a single phase 960V to 480V single phase transformer. You could then use this as 480V to 240V single phase transformer with a center tapped secondary. The resulting capacity would only be 25kVA, but you could wire and protect it normally as a 25 kVA transformer without any sort of fancy wiring or calculations.
-Jon
Zig-zag transformers are autotransformer arrangements used to derive a neutral for a balanced 3 phase source. These are used, for example, to ground a previously ungrounded delta source. The combination of a delta source and a zig-zag derived neutral is very similar to a true wye source. If a zig-zag transformer is properly sized, it can be used to feed line-neutral loads.
As has already been noted, a zig-zag transformer could not possibly be used in this situation, because it would convert the 240V secondary into a 240/139V secondary source, and the OP requires a 120/240V single phase source.
The OP has also brought up the possibility of a single phase derived neutral. A 240V coil with a center tap can be used as the single phase 'grounding transformer', to derive a neutral. However it would require significant engineering to properly size this transformer. The cost of this engineering is probable significantly greater than the value of a 75kVA transformer. As gar says, the circuit in post 26 would work in theory; I simply don't think it is of practical benefit after you consider the required design engineering.
Post #2 incorrectly assumes a single phase transformer, but offers an interesting possibility.
This transformer happens to be conveniently located, no longer being used for 240V loads, and oversized for the new application. I believe that you could disconnect one of the legs, and use it as a single phase 960V to 480V single phase transformer. You could then use this as 480V to 240V single phase transformer with a center tapped secondary. The resulting capacity would only be 25kVA, but you could wire and protect it normally as a 25 kVA transformer without any sort of fancy wiring or calculations.
-Jon
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
The transformer, as built, is a 3 phase delta to delta transformer, with no secondary center tap.
However consider what happens to the output voltage if the unit is 'single phased' meaning that 480V is only supplied between H1 and H3, with H2 _not connected_.
You will get 240V from X1 to X3, by _both_ paths through the delta. X2 will be a 'center tap' for the voltage between X1 and X3. Ground X2 and it would look like normal 120/240V.
In terms of capacity, I would expect something between 25 and 50 kVA, and if you protect this transformer as if it were a 25kVA single phase unit, I think you would be fine.
This use should probably be reviewed by someone else, but I think it would let you reuse the existing hardware with minimal changes.
-Jon
However consider what happens to the output voltage if the unit is 'single phased' meaning that 480V is only supplied between H1 and H3, with H2 _not connected_.
You will get 240V from X1 to X3, by _both_ paths through the delta. X2 will be a 'center tap' for the voltage between X1 and X3. Ground X2 and it would look like normal 120/240V.
In terms of capacity, I would expect something between 25 and 50 kVA, and if you protect this transformer as if it were a 25kVA single phase unit, I think you would be fine.
This use should probably be reviewed by someone else, but I think it would let you reuse the existing hardware with minimal changes.
-Jon
ramsy
NoFixNoPay Electric
- Location
- LA basin, CA
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- Service Electrician 2020 NEC
This xfmr internal diagram --post #4-- shows 3-phase coils tied to each other. If converting to single phase, center-tap Neutral, how do we separate those coils to avoid bolted fault line to neutral, or line-1 to line-3?
View attachment 21616
View attachment 21616
Good point.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
There is a coil between X1 and X3. It is energized to 240V with 480V on the primary between H1 and H3.
However thinking on this further, I think that the fact that you would essentially have two 240V:120V transformers in series might give a very high impedance neutral point.
-Jon
Great insight Jon. I like it and would try it. I am guessing kVA would be at 50 of the 75 and the line side OCPD should be good as is. Probably would need load side panelboard to fit commom breakers if that is not there now.
The guy who can understand it and reconnect it is an artist.
Could not get drawing to host on the forum. trying Imgur.
https://imgur.com/a/kt4gALF
Last edited:
One thing that may happen, since there are two iron cores in use, there will be two different flux densities. Voltage from primary to secondary will be determined solely by the winding turns ratio, 2 to 1.
But on the primary side, it's possible voltage will not divide evenly between the two windings if the secondary load currents are not identical. The core fluxes may be unequal. The primary could end up as a voltage divider and the error carried over to the secondary voltage.
I would want to test it and see what it does. Hook it up and load one secondary with the other open circuited. Check it for voltage regulation. The secondary with the load may just drop voltage and raise the voltage on the other. I would want to see what it does.
But on the primary side, it's possible voltage will not divide evenly between the two windings if the secondary load currents are not identical. The core fluxes may be unequal. The primary could end up as a voltage divider and the error carried over to the secondary voltage.
I would want to test it and see what it does. Hook it up and load one secondary with the other open circuited. Check it for voltage regulation. The secondary with the load may just drop voltage and raise the voltage on the other. I would want to see what it does.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
181118-2228 EST
I don't know whether it is has been absolutely determined if this is a common core transformer or three single phase transformers.
I have never really studied common core transformers, but I think logic suggests at least two things.
(1) both primary and secondary windings on the center leg should be opened electrically from the delta connection. Otherwise the magnetic circuit is not balanced.
(2) The two primary phasings should be such that the flux is going around the outer part of the core in phase. I don't believe the center magnetic leg will be a shunt because the two flux sources, if entering the center leg, would cancel each other in the center leg. If phased the other way the fluxes in the center magnetic leg would add. I believe this would create greater leakage flux and poorer coupling between the primaries and secondaries. One wants a low impedance magnetic path in a transformer to minimize leakage flux. Increased flux leakage between primary and secondary coils increases internal AC impedance (inductance).
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I don't know whether it is has been absolutely determined if this is a common core transformer or three single phase transformers.
I have never really studied common core transformers, but I think logic suggests at least two things.
(1) both primary and secondary windings on the center leg should be opened electrically from the delta connection. Otherwise the magnetic circuit is not balanced.
(2) The two primary phasings should be such that the flux is going around the outer part of the core in phase. I don't believe the center magnetic leg will be a shunt because the two flux sources, if entering the center leg, would cancel each other in the center leg. If phased the other way the fluxes in the center magnetic leg would add. I believe this would create greater leakage flux and poorer coupling between the primaries and secondaries. One wants a low impedance magnetic path in a transformer to minimize leakage flux. Increased flux leakage between primary and secondary coils increases internal AC impedance (inductance).
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