XFMR ?

[Redskinsdb21]

if we are given a xfmr with a 48-V-120/208V rating....what does thie mean?

Does it mean primary Voltage, line to line - 480 V and secondary voltage, ;ine to line = 208 Volts and secondary voltage, line to neutral = 120?

Thanks ahead of time

 

[Wildsoldier PE]

You are correct thats a wye configuration also from line to ground is 120

(null)

 

[Redskinsdb21]

let me try again, looks like i had some typos:



if we are given a xfmr with a 480-V-120/208V rating....what does thie mean?

Does it mean primary Voltage, line to line - 480 V and secondary voltage, line to line = 208 Volts and secondary voltage, line to neutral = 120?

Thanks ahead of time

 

[Redskinsdb21]

Wildsoldier:

You are correct thats a wye configuration also from line to ground is 120

How can you tell its a wye configuration? We can model delta configurations as if a neutral exiast also correct?

 

[Wildsoldier PE]

If its a 3 phase delta on secondary with a neutral the voltage will be 120/240V but....you have to be careful with this configuration...because it will have a high leg...when you measure from 1 phase to neutral instead of getting 120 you would get 208V.... the best way to understand this is with a diagram....let me see if i can draw something for you when i got a chance.

 

[Flyer_PE]

Wildsoldier:

You are correct thats a wye configuration also from line to ground is 120

How can you tell its a wye configuration? We can model delta configurations as if a neutral exiast also correct?

You can determine some things mathematically on for a delta system by treating it the same as a wye. In order to reduce the balanced three-phase system to a single-phase equivalent, the equations wind up being the same for either type of connection.

The tip-off that this is a wye configuration is the 208/120 volt rating. 120V*sqrt3 = 208V

 

[Redskinsdb21]

Im starting to understand I think, thanks for feedback!

So, looking at our example, 480-120/280....I now see how we can see secondary is a Wye...but how would we get 240 from 120 to know its a delta arrangement? Not sure hot to calculate it since on a delta Voltage phase to phase = voltage line to line?

Also, is there anyway to tell if the primary arrangement is a Wye or Delta connected load just from the 480 Volts given? Or does it mean its probably delta since it only gives line to line voltage?

 

[Wildsoldier PE]

You will never find a Wye system with 120/240V never.... that standard voltage is for delta configuration only. Looks like you have some problems with the configurations. Have you read 3-phase systems from your college books?...You really need to understand these basics things to pass this test....are you taking the test on april?...looks like you need to start from very low... (the basics)...When i was preparing i started from verryy verry low....no shame on saying it!

 

[Wildsoldier PE]

I attach a pdf with both configurations 480V Delta to 120/208V Wye and a 480V Delta to 120/240V Delta.

Note that there is a high leg from Line to Neutral from phase C to Neutral that will measure 208V.

This is one of the reason why utility companies doesn't provide any more that configuration. Is a risk for an electrician that doesn't know what he is doing. How ever some old commercial buildings have that kind of configurations and special consideration needs to be consider.

How do i get the 208V? Well I'm not going to say but you can get the voltage if you use the formula to get the voltages on Delta configuration and you use phase angles.

Hope that you can read the pdf....my handwriting is not that good.

 

[Wildsoldier PE]

Sorry here is the file.

scan0003.pdf

 

[Redskinsdb21]

I think I have a good start on studying for this stuff...I undertsand for Y V_ln = V_ll = V_phase and for delta V_ll = V_phase...the problem this comes from is this question:

The Xfmr = 480-120/208 volts.....calculate primary protection and primary conductor size using 75 degree terminals....S = 45000 VA

1.) I see that 120/280 volts is wye connected pretty easily and that 120 = V_ln = 120 and Vll = 208....im getting confused on were 480 comes from....when its given like this with a hyphen does that mean its the delta V_ll? Delta_V_ln?

2.) In delta, V_ll = V_Phase to Phase.....but in first step of sol'n they go: 45000/(45000 VA * 1.732) = 54 Amps.....why multiply by 1.732? I would have just went 45000/1.732?

 

[Redskinsdb21]

Thanks fro diagram by the way...i can read it just fine....just not sure why they multiply by 1.732 above when we already have V_ll....and when they give us a voltage of a Xfmr as say, 480, np /, does that essentially mean that side is a delta Xfmr?

 

[Redskinsdb21]

oops, no / (slash)

 

[Wildsoldier PE]

Remember this....this apply to all configurations no matter if its delta or wye.

In a 3 phase system.

Line current = (total power)/(line to line voltage * square(3) )

Thats why. Write that formula you are going to use it a lot.

 

[Redskinsdb21]

Great, thanks much wildsoldier.....

can we assume the 480 V side is delta connected?

 

[Wildsoldier PE]

The primary side usually is delta when you want the secondary side a wye....but don't worry too much about the primary side. If configuration needs to be used they usually say it in the problem.

Also to let you know these are magnitude voltages. Some problems you need to use magnitudes and angles and each configuration have different rules. I already forgot most of it haven't use it in a long time you start to forget.

For example phase sequence.

Lag and leading

What leads and what lags on delta and Wye configuration....what leads by 30 degrees?

thats to name a few....you need to know that very very well to succeed and i'm not kidding.

 

[Redskinsdb21]

Sorry here is the file.

WILDSOLDIER

THE 4-WIRE DELTA DELTA ALWAYS HAS THE HIGHLEG ASSOCIATED WITH IT, CORRECT? Or the side that has 4 wires that is? in your diagram you show two of the phases to neutral = 120 while one of the phases to neutral = 208.......phase c in your diagram.....you also show line to line voltages of a to c and a to be = 240....just want to confirm im reading it correctly...the line to line voltage of phase c to phase b also = 240 correct? Thanks again for diagram.

 

[Wildsoldier PE]

Yes you are correct...you are reading it well. And you are also correct there is always a high leg asociated with a 3 phase delta 120/240v, 4wire system. The high leg is one of the reasons why utility companies dont provide any more that system because of the dangers associated with it. If some one doesnt know that well the delta system can easily install 208V in a 120v recept.

 

[Redskinsdb21]

Yes you are correct...you are reading it well. And you are also correct there is always a high leg asociated with a 3 phase delta 120/240v, 4wire system. The high leg is one of the reasons why utility companies dont provide any more that system because of the dangers associated with it. If some one doesnt know that well the delta system can easily install 208V in a 120v recept.

What about Delta 3-wire system? It doesnt have a high leg correct? Also, it will be listed something like 120/208-240 for a y-delta correct....thus nothaving the 120/240...or similar ratio?

 

[Wildsoldier PE]

Ifi it is 3-wire it will be 240V 3phase only no neutral so it doesn have a high leg.......regarding the high leg we never use the terminology 120/208-240V they electrician if it is licensenced shall know that 120/240V 3 phase 4 wire is a Delta system and shall know that there is a high leg involved. The only way you can get 240V is in Delta. You cant get 240V in a Y system.