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Transformer Primary current

Transformer Primary current

Transformer Primary current

(OP)
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?
 

RE: Transformer Primary current

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

RE: Transformer Primary current

(OP)
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.

RE: Transformer Primary current

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

RE: Transformer Primary current

(OP)
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.

RE: Transformer Primary current

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.

RE: Transformer Primary current

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

RE: Transformer Primary current

(OP)
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?

RE: Transformer Primary current

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

RE: Transformer Primary current

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.

RE: Transformer Primary current

Get the nameplate info not what vendor says.

RE: Transformer Primary current

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.

 

RE: Transformer Primary current

Quote (7anoter4):

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.

RE: Transformer Primary current

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

RE: Transformer Primary current

Never seen one.

RE: Transformer Primary current

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

RE: Transformer Primary current

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.blush
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.

 

RE: Transformer Primary current

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 :)

RE: Transformer Primary current

perhaps along with snake oil give away!

RE: Transformer Primary current

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

RE: Transformer Primary current

I would agree with waross about the convention except that the left hand is labelled 2u and 2v.  The 2 and the lower case is IEC standard terminology for secondary.  The 1 and upper case on the right is standard for primary.  Also, because of the IEC terminology, 120/240 V is an unlikely nominal voltage.

I've never heard of applying more than one phase to a common core; I suspect waross is correct and it would test the blown fuse indicators.
 

RE: Transformer Primary current

(OP)
As seen in the attached SLD, the Bypass Xmer is a 3ph - 1ph Xmer.Sibeen is correct in saying that the reason behind the 3ph Primary was to decrease the burden, since some of the ratings on 1ph are pretty high.(20kVA upto 60kVA). Looking at the photo, Terminal X090 expects a three phase power supply. Now back to my query..if such a three phase power supply is connected across L1,L2,L3, what will be the line currents?

RE: Transformer Primary current

I don't care how it works, do not fall for it.

You can easily have 3 ph input to 3ph-4w output UPS at whatever voltage (208/120V in this case) and feed single phase loads.

What is that non-sense about lessening burden? On what?

RE: Transformer Primary current

(OP)
Looking at the SLD, the line currents reported are : L1=L3=20A; L2=40A. The Xmet T090 is 20kVA. It appears that the each winding is 10kVA rated and with 600V applied, the line currents would be 10000/600=16.67A; now the central phase if tied together results in 1.732 x 16.67A =28.87A. If this same Xmer would have been single phase, the line current would have been 20000/120=166.67A. We do not have provision for supplying such a high current

RE: Transformer Primary current

why do you have to have single phase output? You can feed single phase loads from 3 phase output.

RE: Transformer Primary current

Get a 3 phase UPS, 600V input, 600V-3phase output, feed a 600V: 208Y/120V transformer and feed your 120V or 208V single phase loads. Whatever you have sounds like a clap-trap! At least I cannot understand why someone produce that!

By the way this is my last post in this thread!





 

RE: Transformer Primary current

rbulsara, whether you have ever seen a UPS in this configuration is besides the point. A quick google search shows a multitude of manufacturers that produce models for this application.

Off the top of my head I can think of a few reasons for using a 3 in/1 out UPS.

1. Load balancing. By using a 3 phase in the load is balalanced across the 3 phases. Probably advantageous, especially if a smallish on-site generator is used as emergency supply.

2. Harmonic mitigation. Especially for the older style, thyristor rectifiers, a 3 phase input will have a lower THDi and there will be no triplen harmonics in the 3 phase input model.

3. Sites using an existing single phase distribution system where upgrading switchboards, wiring etc would outweigh the cost of a 3 phase output UPS.

cherry2000, I must admit the transformer has me a tad perplexed and I haven't seen its like before. I'd just use the manufacturers reccomendation about the size of circuit breaker to be used on the bypass supply, but as a guess I'd say 63 amps.

RE: Transformer Primary current

(OP)
Sibeen..in my 16 yrs career, this is maybe the second/third time that I have come across a 3ph-1ph Xmer. It is a standard solution being offered by this well known Swiss UPS manufacturer. I admit that ideally we should have selected a 3ph UPS (as correctly suggested by rbulsara) for the higher ratings with a 3ph 4 wire output.

We are planning to use a 50A CB based on 1.25 x 40A (L2 line current).In some other Projects, I have encountered the Bypass feed as a single phase feed, 600V line to line. Automatically, the CB rating increases, since the current increases by 1.73 times. This definitely adds to unbalance on the MCC bus, but the degree of unbalance is not noticeable with other larger loads being present. Thanks to all of you for your valuable contributions.

RE: Transformer Primary current

sibeen:

I did not say piece of cr*ps are not made. One does not have to fall for it.

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