1 ph loading of 3 ph transformers.

[Gen Tech]

As I recall from past training I believe that a 3 ph Y connected transformer can be loaded to 58% with single phase loads if there are no 3 phase loads. I also remember that a Delta connected transformer can be loaded to 66% 1 ph load. I was recently in a conversation that a 3 ph transformer can only be loaded to 50% single ph regardless of connection. I have done some calculations which I believe confirms that the 58% & 66% loading are correct. Is there anyone with transformer expertise that can answer this for me?

 

[jim dungar]

It is easy to hear a discussion about transformers and not realize you are missing some key information.
In particular the numbers 57.7% and 86.6% and 33.3% all have to do with if you are starting with a connection of 3-windings and then loosing some, or never even have 3-windings to begin with. Then we run into problems with delta and wye connections.

A three phase transformer is effectively the sum of (3) single phase transformers, so each 'winding' can put out 1/3 of the total 3-phase rating, for example a 75kVA 208Y/120 is in reality (3) 25kVA 120V transformers.

Now lets say we loose one of the transformer windings entirely, this creates either an open-wye or an open-delta, at this point we no longer have 100% of the total 3-phase rating. For example, an open-delta connection will be 57.7% of the total 3-phase rating: so a 75kVA 3-phase rating becomes an effective 43kVA if a winding is lost.

But how about if we start with only (2) 25 kVA transformers, then we get 86.6% of the sum {43/(25+25)=.866} of the installed transformers.

So by playing with the math we can say:
25kVA + 25kVA + 25kVA = 75kvA rating = 100%
25kVA +25kVA = 43kVA rating = 57.7% of what we could have had
25kVA = 25kVA = 33.3% of what we could have had

This can be almost overwhelming, so it is best to where you are starting from and where you wan to go, step by step.

 

[templdl]

As I recall from past training I believe that a 3 ph Y connected transformer can be loaded to 58% with single phase loads if there are no 3 phase loads. I also remember that a Delta connected transformer can be loaded to 66% 1 ph load. I was recently in a conversation that a 3 ph transformer can only be loaded to 50% single ph regardless of connection. I have done some calculations which I believe confirms that the 58% & 66% loading are correct. Is there anyone with transformer expertise that can answer this for me?

As a former applications and sales engineer I became skeptical of those who ask questions with the understanding that all the facts aren't being presented. You just stated that the transformer is being loaded with 1ph loads with no 3ph loads present. It may be only assumed that you are unbalancing the transformer with 1ph loads because your statement was followed up by a percentage loss of capacity. However you can certainly most certainly load a 3ph transformer with 1ph loads with no penalty should those loads be balanced even though there are no 3ph loads at all.

 

[T.M.Haja Sahib]

Providing 120/240V supply with 3 single phase transformers

Providing 120/240V supply with 3 single phase transformers

Let there be three single phase transformers with 120V secondaries and with terminal markings(A1 A2),(B1 B2),(C1 C2). Connect them in Zig-Zag:A1-A2B1-B2C1-C1.With primaries connected to three phase supply,the secondaries in Zigzag connection may be used for feeding 120/240V single phase supply with neutral connected at A2B1,and A1 and C1 as hot legs. The total capacity of this 120/240V supply is equal to 2/3 or 67% of total capacity of three single phase transformers.

Is the above in your mind,Gen Tech?

 

[Smart $]

Let there be three single phase transformers with 120V secondaries and with terminal markings(A1 A2),(B1 B2),(C1 C2). Connect them in Zig-Zag:A1-A2B1-B2C1-C1.With primaries connected to three phase supply,the secondaries in Zigzag connection may be used for feeding 120/240V single phase supply with neutral connected at A2B1,and A1 and C1 as hot legs. The total capacity of this 120/240V supply is equal to 2/3 or 67% of total capacity of three single phase transformers.

Is the above in your mind,Gen Tech?

Your configuration does not provide a standard 240/120 3? 4W output. Let's say we stick to a common configuration here in the US...

We have three equally-rated transformers each with terminal markings H1, H2, X1, X2, and X3. H terminals being primary, X terminals being secondary where X2 specifically is a 50% voltage tap. Configure these for standard 240/120 3? 4W service.

Connect all H2 together and connect each H1 to one phase conductor of the primary 3? supply. Connect X1 of each transformer to X3 of another but no two X1 connected to the same X3. One X2 is grounded and is connected as N, and its respective X1 and X3 terminals connect to L1(A) and L3(C) of the service.

Now, if all loads are single phase and connected across any combination of L1, N, and L3, the max available power is two-thirds (66.7%) of the total kVA of all three transformers. The easiest explanation is that, as configured and as connected, you have two single phase power supplies acting in parallel and sharing the load current... the transformer with X2 connected is one, while the other two tranformers being connected in series between L1 and L3 are the second. With each transformer having the same impedance (z), you have the first power supply with an impedance of z and the second with double the impedance (2z). Total impedance is thus 2/3*z.

From there, note that when the X2-connected transformer is operating at max power, the other two are operating at half-power. Half of two-thirds plus all of one-third is equal to two-thirds or 66.7% total. Also note that at this operating condition, no 3? load can be added to the system, as it would overload the system. However single phase loads can be added if connected L1-L2 or L2-L3 without overloading the system.


Then we have to get into the practicality of operating a system as such... (rhetorical comment, no reply solicited)

 

[T.M.Haja Sahib]

Your configuration does not provide a standard 240/120 3? 4W output. Let's say we stick to a common configuration here in the US...

You mean single phase transformer with untapped secondary rated for 120V only(to take care of homes with the lighting and receptacles only) is no longer manufactured in U.S?

 

[Smart $]

You mean single phase transformer with untapped secondary rated for 120V only(to take care of homes with the lighting and receptacles only) is no longer manufactured in U.S?

Manufactured and/or used, I do not know for certain. I surmise, drawing from observation, that usage is somewhere between extremely rare to nonexistent.

 

[T.M.Haja Sahib]

Manufactured and/or used, I do not know for certain. I surmise, drawing from observation, that usage is somewhere between extremely rare to nonexistent.

Rare,I would accept:A home owner can not avail purely 120V single phase supply now because 120/240V single phase supply can only be availed for new homes.But for existing homes with 120V single phase availed earlier,these single phase transformers with untapped secondaries of 120V rating might still be of use.......

 

[jim dungar]

You guys do notice that the OP did not include any voltages in his question don't you?

 

[T.M.Haja Sahib]

You guys do notice that the OP did not include any voltages in his question don't you?

Yes.His query is not very clear.That is why I put a question to him in post#4.

 

[david luchini]

Let there be three single phase transformers with 120V secondaries and with terminal markings(A1 A2),(B1 B2),(C1 C2). Connect them in Zig-Zag:A1-A2B1-B2C1-C1.With primaries connected to three phase supply,the secondaries in Zigzag connection may be used for feeding 120/240V single phase supply with neutral connected at A2B1,and A1 and C1 as hot legs. The total capacity of this 120/240V supply is equal to 2/3 or 67% of total capacity of three single phase transformers.

I'm not seeing a zig-zag in the secondary connection that you describe. You would need 6 windings in the secondary to create a zig-zag. I think you would get 120/208V from your connections using A1 & C1 as hot and A2B1 as a neutral.

 

[Smart $]

Rare,I would accept:A home owner can not avail purely 120V single phase supply now because 120/240V single phase supply can only be availed for new homes.But for existing homes with 120V single phase availed earlier,these single phase transformers with untapped secondaries of 120V rating might still be of use.......

There may be some existing... but none that I'm aware. Even the existing homes with 120V only services are fed from a 120/240 secondary (multiple home services off one transformer).

 

[Smart $]

You guys do notice that the OP did not include any voltages in his question don't you?

My only comment as to voltage was in reply to Sahib's

 

[T.M.Haja Sahib]

There may be some existing....

That is the point and basis for post #4.

 

[Smart $]

I'm not seeing a zig-zag in the secondary connection that you describe. You would need 6 windings in the secondary to create a zig-zag. I think you would get 120/208V from your connections using A1 & C1 as hot and A2B1 as a neutral.

Not zig-zag, but "A" transformer in phase with open-delta "B"-"C" connected transformers... think square root symbol.

 

[T.M.Haja Sahib]

I'm not seeing a zig-zag in the secondary connection that you describe. You would need 6 windings in the secondary to create a zig-zag. I think you would get 120/208V from your connections using A1 & C1 as hot and A2B1 as a neutral.

Sorry,I made typos in that post.But Smart $ brilliantly explained the idea in post#15......

 

[david luchini]

Sorry,I made typos in that post.But Smart $ brilliantly explained the idea in post#15......

Yes, I see the connection the way Smart$ described it. But you still get 120/208, not 120/240. And since you get 2/3 of the rating of the 3 transformers, why not just install 2 of them?

 

[templdl]

As I recall from past training I believe that a 3 ph Y connected transformer can be loaded to 58% with single phase loads if there are no 3 phase loads. I also remember that a Delta connected transformer can be loaded to 66% 1 ph load. I was recently in a conversation that a 3 ph transformer can only be loaded to 50% single ph regardless of connection. I have done some calculations which I believe confirms that the 58% & 66% loading are correct. Is there anyone with transformer expertise that can answer this for me?

After reading all of the rabbit trails I went back to the OP and considered that the poster had a 'conventional' 'Y' connected 'transformer' and questioned what capacity would be lost should he have only 1ph loads.
My reply was very simply, very little to no lost capacity should he divide the 1ph loads as evenly as practical between the three phases.
If his question was, how much capacity would be lost should all of the 1ph loads be limited to 1ph of the 3ph ?y? that would be another issue but that isn?t what he asked but may have elided to by the percentages that he included in his OP..
Is there much to do about nothing with these rabbit trails or have I misinterpreted the OP?

 

[wchild1520]

Recently installed a 3ph trans. .had to change it out for a single phase because all i had was single phase 120/240v load. Her is what the manual for the transformer states "The maximum single-phase 120V load cannot exceed 10% of the three phase kva rating. The load must also be balanced at 5% maximum between X1 and X4 and 5% between X2 and X4. The three-phase kva rating will be reduced by 15% when lighting tap is used to maximum capacity."

 

[jim dungar]

Recently installed a 3ph trans. .had to change it out for a single phase because all i had was single phase 120/240v load. Her is what the manual for the transformer states "The maximum single-phase 120V load cannot exceed 10% of the three phase kva rating. The load must also be balanced at 5% maximum between X1 and X4 and 5% between X2 and X4. The three-phase kva rating will be reduced by 15% when lighting tap is used to maximum capacity."

That is for a particular design of a delta transformer with a center tap. This 5% limit is assciated with a three-legged core and 3 equally sized windings. There is no inherent single phase limit when the delta is constructed out of individual single phase transformers (as they can be different sizes and there is no flux 'interference'), especially when the connection is an open-delta.