Transformer Primary current 1

[cherry2000]

Consider the Single Phase transformer in the attachment. The rating is 40kVA, and the Primary voltage is 600V.

The question : What is the primary line current of this Xmer? Notice that terminals 55 & 56 tied to terminal L2.Also, what shall be the Circuit breaker size for a feeder to this Xmer?

 

[jghrist]

What is the load? Is the 600V from L1 to L3?

 

[cherry2000]

The power supply on the primary side is 3 phase, 600V across L1-L2-L3. Is this a case of open delta Xmer? The secondary is single phase 120VAC.

 

[waross]

The full load primary current will be 40000VA/600V = 66.67A
Check your local code book for the proper protection sizing.
Not open delta, just a single phase load connected line to line. (I'm taking your word for this, there is not enough information on the drawing to tell for sure.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cherry2000]

Looking at the wiring diagram, it appears to me that two primary windings,on the same magnetic core are tied together at terminal L2. The Transformer gets three phase, 600V supply (across L1-L2-L3) and the output is single phase. One can solve this problem probably by knowing the primary connection and the resultant vectorial difference in the line currents. Definitely Line L2 for the external supply will carry more current than the L1 & L3 lines. I still think that this is primary windings are equivalent to a open delta whereas the secondary is a single phase winding.

 

[rbulsara]

Why would a single phase transformer be fed by a three phase source?

Something is not right with the picture! I would suspect that nothing is connected to L2 and 600V should be connected between L1 and L3 as jghrist suspects.

What does the nameplate say? If there is one.

 

[waross]

I think that the primary is on the left. The connections on the right are for a center tapped supply (120/240V?).
The secondary is feeding a three wire load (-FO73, -F074 and -FO75, 120/240V).
The terminals at L1, L2 and L3 are fed from four fuses with blown fuse indicators.
This shows both windings on the same core. Applying power from two phases to two windings on the same core will certainly test the blown fuse indicators.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cherry2000]

This is definitely a three phase to single phase Xmer..or thats what the vendor claims. The Xmer is serving as a Bypass Transformer for a single phase UPS. The single line diagram says : 3ph --1ph. Unfortunately, the Xmer is located in such a way in the UPS Bypass cabinet, that reading the nameplate was virtually impossible. The BOM is not very helpful, and does not give the make/model of the Xmer. It is an IEC Xmer since the UPS is by a swiss vendor. Also, the primary is definitely on the right. The single phase secondary serves as the Bypass output supply. One query..if the 600V is not connected at L2, with only connection to L1 & L3, will this Xmer function?

 

[rbulsara]

Again why do you need a three phase source/transformer to bypass a single phase UPS?

 

[rbulsara]

I would also have thought same as waross that 600V may be on the left, had you not insisted that the primary is on the right.

 

[rbulsara]

Get the nameplate info not what vendor says.

 

[7anoter4]

Three winding transformer apparent power will be:
S=Ia*Va+Ib*Vb+Ic*Vc where :
Ia current in "a" transformer winding[typical]
Va=voltage across winding "a"[typical]
But if they are only 2 windings S=Ia*Va+Ib*Vb
I(L1)=Ia I(L3)=Ib I(L2)=-(Ia+Ib).
If module(IL1)=I then module(IL3)=I and IL1+IL3=-IL2 module(IL2)=I
As Ia*Va=Ib*Vb then |IL1|=S/2/VL1_2=|IL2|=|IL3|
|IL1|=40/2/0.6=33.33 A.
If there are 3 windings then [star connected] I=40/sqrt(3)/0.6= 38.49 A .But secondary voltage will be 0 as VL1+VL2+VL3=0. So Iprimary =0 also and S=0 too.

 

[davidbeach]

Ia*Va+Ib*Vb+Ic*Vc

Only if the '*' denotes conjugation and the multiplication is implied.. Power requires the use of the conjugate of the current.

 

[sibeen]

I'd suspect that the UPS used is a 3 phase input - 1 phase output unit. The 3 input phases, to the rectifier, fed via fuses 73, 74 + 75 in the diagram - perhaps, or maybe the fuses are for monitoing the incoming voltage.


To 'save' having to run an oversized single phase supply to the bypass they derive a single phase supply from the incoming 3 phases

 

[rbulsara]

Never seen one.

 

[waross]

This three phase single phase and the salesman said reminds me of a generator one of my customers purchased.
He needed a 50 KVA single phase diesel genset. I priced a set for him. He did some checking on his own and a salesman was able to beat my price. He sold him a three phase 50 KVA set.
The salesman swore that the set would put out 50 KVA, single phase. I asked him to multiply the size of the main breaker times the voltage and explain to me how that would be 50 KVA. No problem, when they reconnected the set from three phase to single phase, they forgot to change the breaker. I'll have our service department send a replacement breaker. When I contacted the service department, they explained to me that the 50 KVA three phase set would only produce 35 KVA when used on single phase.
Well my customer needed a 50 KVA single phase set and the salesman swore that that was a 50 KVA single phase set.
In the end, my customer got a sizable refund and I had to figure out how to curtail the loading so as not to overload the set. (kW only drops from 40 kW to 35kW so by watching the power factor we got the job done)
Anyhow, what was that salesman saying again about the three phase connection to a single phase transformer???

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

Thank you davidbeach! I treated S as a scalar and not as complex number.
If one is busy and tired he doesn't has to be as superficial as I was.
I am so ashamed! So the current is indeed 40/.6 =66.7A as waross said.
I wander what may be the purpose of this redundant winding–only to waste a good copper.
Three phases could be converted to single phase only if the rectifier is for three phases and the inverter is single phase, I think.

 

[sibeen]

The three phase in - single phase out UPSs weren't common, but they were sold and still are. Of course the bypass (reserve) supply to the UPS had to be sized to supply the load and so this feed was larger than the three phase input to the rectifier.

I am surmising that the transformer shown in the opening post is to overcome this by deriving a single phase supply from the three phase input.

Of course I could also be wrong

 

[rbulsara]

perhaps along with snake oil give away!

 

[waross]

Whatever you do, DON'T buy a single phase generator from this vendor.
I think that this comes under the heading of;
"It's fed from the three phase panel so it must be three phase." NO, NO, NO.
The thought that a single phase load must be three phase because it's fed from a three phase panel is common among people who don't have enough electrical training.
It is wrong.
If the transformer is a three phase transformer, that is the wrong drawing.
Common convention would be the 600 volts in on the left and 120/240 out on the right. That is why there are two windings.
That is why the fuses are arranged as they are.
Engineers and trouble shooters tend to believe what they are told and work with that as fact. However, those of us who have seen a lot of transformers and a lot of salesmen and vendors have to shake our heads on this one.


Bill
--------------------
"Why not the best?"
Jimmy Carter