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]

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.

 

[cherry2000]

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?

 

[cherry2000]

Here is the SLD....

 

[rbulsara]

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?

 

[cherry2000]

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

 

[rbulsara]

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

 

[rbulsara]

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!

 

[sibeen]

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.

 

[cherry2000]

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.

 

[rbulsara]

sibeen:

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