USA Voltages

[raithrovers1]

Guys
I am a little embarassed to ask but I can't work this out at the moment (still new to the USA standard voltages!):
I have a transformer with 240Vac primary to 240Vac/120Vac secondary. I have an unbalanced load. If I measure 50A on L1 with an amp clamp and 10A on L2, what is the primary current if all loads are purely resistive and no losses(for ease)?
Sorry for such a rudimentary question but my head hurts today!

UPS Service Manager

http://www.nolanpower.com

 

[jraef]

Let's assume for the sake of argument that you have 50A at 120V on L1-N and 10A at 120V on L2-N. That means you have 6000VA + 120VA total load on the transformer right? Now because ALL of that power has to come from that primary winding, you have 6120/240 = 25.5A primary current.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

How can I say this gently?? Impeccable logic but math from grandma's house. 6000VA + 1200VA = 7200Va 7200 VA/240 = you get the idea.
I got 30 A in my head but took 5 tries with the calculator to verify. Same kind of day as yours Jeff. grin.

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

 

[raithrovers1]

Thanks guys.
What was confusing me was what part of the current was 240V and what was 120V. I now realize that if we take the 10A at 240V we get 2400VA and then the other 40A would have to be at 120V which gives 4800VA. Add them together and we still get 7200VA.
Strange how asking the question always gets it clearer in my head!

UPS Service Manager

http://www.nolanpower.com

 

[Skogsgurra]

I don't even understand the question. Is it a three-phase system? (OP mentions unbalanced load). Or is the primary connected to L1-L2 and nothing more (in which case current in and out should be the same). Or is it some kind of autotransformer? Or a transformer with a secondary (240 and 120 V) winding? In the latter case, current in and out of primary should be equal again.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[davidbeach]

Skogs, one 240V primary windings and two 120V secondary windings. Secondary windings in series with the center point grounded.

 

[jraef]

"6000VA + 1200VA..."

D'oh!

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[Skogsgurra]

Thanks davidbeach. I can sleep now.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.