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?
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
RE: Transformer Primary current
RE: Transformer Primary current
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
RE: Transformer Primary current
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
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
RE: Transformer Primary current
RE: Transformer Primary current
RE: Transformer Primary current
RE: Transformer Primary current
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
RE: Transformer Primary current
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
RE: Transformer Primary current
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
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.
RE: Transformer Primary current
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
RE: Transformer Primary current
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'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
RE: Transformer Primary current
RE: Transformer Primary current
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
RE: Transformer Primary current
RE: Transformer Primary current
By the way this is my last post in this thread!
RE: Transformer Primary current
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
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
I did not say piece of cr*ps are not made. One does not have to fall for it.