3-phase transformers

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hardworkingstiff

Senior Member
Location
Wilmington, NC
I sure hope I can ask my question correctly. Here goes.

Hypothetically, we have a 3-phase WYE transformer feeding some dwelling units. To make it easy, let?s say 3 units and the units are connected to the transformer ABN, ACN, BCN.

Let?s say something happens to the transformer and C phase is lost, burnt out, nothing coming through it. So you have A and B phases and N.

What happens to the voltages on A and B? Nothing? Something?

My question is doesn?t the transformer (after C coil failure) electrically become a single-phase center-tapped transformer? If so, then the voltage would need to change, right? Would the voltage then become 104/208?
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
A three phase transformer is just three single phase transformers sitting next to one another. One of them dying doesn't have an affect on the other two.

My gut says this is not true, although I cannot provide the engineering calculations to back it up.

I think if C coil is lost, the voltage secondary would be A-B=208V and AorB-N=104V.
 

iwire

Moderator
Staff member
Location
Massachusetts
I think if C coil is lost, the voltage secondary would be A-B=208V and AorB-N=104V.

Why do you feel that way? How is the loss of C going to effect A and B?

If you had a 3 phase multiwire branch circuit and shut off one phase would the voltage change between any two points excluding the shut off phase?
 
My gut says this is not true, although I cannot provide the engineering calculations to back it up.

I think if C coil is lost, the voltage secondary would be A-B=208V and AorB-N=104V.

What's not to get? Losing the C winding will have no effect on the other two phases in reference to each other or to the neutral. The voltage will remain the same in the other two phases. The third phase doesn't magically contribute to the other two in single phase applications. The transformer will still work as a single phase transformer provided there was no damage to the other two windings.

The primary input is not changing so the secondary output will not change. I think that is where you are confusing this. The input will still be 120? rotation so the square root calculations for the transformer will not change. If it was fed with a single phase primary then it may make a difference but that would affect both the phase to phase and phase to neutral voltage, not just the phase to neutral voltage as you are trying to state. The relation to the primary input here is not changing so the output will remain the same.

The WYE is open ended and if only two windings are being used, they don't care about what the third one is doing. It is business as usual.

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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I believe that we need more information to answer the question: just what sort of transformer is this, and what is meant by 'phase C is lost'.

If you have a wye secondary, and something happens to open the C leg on the _secondary_ side, then I absolutely agree, the voltages between A, B, and N will remain unchanged.

But what happens if you have a delta-wye transformer, and the 'C' coil goes open circuit on the primary side? I think that the A, B, and N voltages remain essentially unchanged.

However if you have a delta-wye transformer and the C primary supply phase opens (leaving the delta intact but disconnecting only one of the supply voltages), then the voltages of all the phases will change on the secondary side. I believe that one phase will remain 120V L to N, and the other two phases will drop to 60V L to N.

-Jon
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
I think I figured out where I'm off base. I was thinking about a 10-amp load on each of the phase coils for A and B. In one scenario the load is connected A-B in the other it is connected A-N, B-N. 10-amps flowing through each coil but the power consumption in the A-N, B-N configuration is more. It seems intuitive that coil C is providing some power to the N to make up the difference that the coils in A&B are not seeing.

I bet this sounds ignorant, so teach on please. / Thanks.
 

drbond24

Senior Member
I think I figured out where I'm off base. I was thinking about a 10-amp load on each of the phase coils for A and B. In one scenario the load is connected A-B in the other it is connected A-N, B-N. 10-amps flowing through each coil but the power consumption in the A-N, B-N configuration is more. It seems intuitive that coil C is providing some power to the N to make up the difference that the coils in A&B are not seeing.

I bet this sounds ignorant, so teach on please. / Thanks.

The phases don't provide power to the neutral, the power (current) flowing through the phases returns to its source through the neutral. With no current on phase C, there will be no current returning through the neutral for phase C, so the neutral load would be less. I think. I'm sure someone will put the smack down on me if that is not correct.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
With no current on phase C, there will be no current returning through the neutral for phase C, so the neutral load would be less. I think. I'm sure someone will put the smack down on me if that is not correct.
Ka-pow! Consider yourself smacked. :grin:

Start with a balance-loaded 3ph Y system, let's say at 10a; no neutral current. Now, subtract one amp from one phase; one amp neutral current. Subtract all 10a from that phase; 10a neutral current.
 

drbond24

Senior Member
Start with a balance-loaded 3ph Y system, let's say at 10a; no neutral current. Now, subtract one amp from one phase; one amp neutral current. Subtract all 10a from that phase; 10a neutral current.

Gotcha. That's what I get for not thinking my posts through enough before I let fly with it.

Ka-pow! Consider yourself smacked. :grin:

I'm gonna go put some ice on that.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Well it would become one single phase with center tap, no?
No. There would still be two separate phases, still 120 degrees apart (well, on one side of the V; on the other side, they'd be 240 degrees apart.)

That's why you still get 208v line-to-line instead of the phases suddenly "snapping to 180 degrees apart", which would have to happen to get 240v.

You can't combine more than one phase's windings on one core. A three-phase transformer is not one core; it's three cores mounted on a single frame.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
091207-2121 EST

The following assumes the output side of a Y source, transformer or generator.

A three phase Y source with hot lines A, B, C and a neutral N, and only line to neutral loads, and after a break in line C will be exactly the same for loads relating to A, B, and N as before the break. For loads connected C to N there will be no current or power after the break.

Still assume only line to neutral loads. If A, C, and N feed one panel, then loads A to N will have power, and C to N no power. Similarly for the other combinations.

Also note that after a break in C if there is no load on C, then C's voltage floats. If there is any load on C, then its voltage to N will be about 0.

If there are 208 loads from C to the other lines, then after the break there will be voltage on C relative to N of a value defined by these additional loads.

.
 
No. There would still be two separate phases, still 120 degrees apart (well, on one side of the V; on the other side, they'd be 240 degrees apart.)

That's why you still get 208v line-to-line instead of the phases suddenly "snapping to 180 degrees apart", which would have to happen to get 240v.

You can't combine more than one phase's windings on one core. A three-phase transformer is not one core; it's three cores mounted on a single frame.

I understand that but the phase relation is coming from the primary input, not the core orientation. That's why it won't snap to 180?. The input rotation is what is keeping it at 120? apart. That would have to change for the output voltage to change phase to phase. The winding ratio in relation to phase shift is determining the voltage output.

It is three transformers in one case or two left working in one case or a single phase transformer (what is left working ) with a center tap, however you want to view it. The voltage will be different than 120/208 with an input rotation of 180? on the two coils but electrically it is still just a single phase transformer with center tap if one phase opens.
 
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