xfmr bank

[paul]

this question gave a 1 sentence scenario about a 300VA transformer bank consisting of 3 100VA transformers. Each transformer was 480V in and 120/208 out. The question was 3 parts:
1. find the amperage of the primary side if running 100%
2. find the amperage of the secondary if running 100%
3. if 1 XFMR is removed, what will be the new VA of the system?

1 & 2 I got correct, just by using:
300VA= 480v * A* 1.73 and..
300VA = 208V *A* 1.73

3 I got wrong. I figured 300-100=200VA. The correct answer involved something with 1.73. Where does the squareroot of 3 come into play for the third question? I knew to put it in the first 2, but can't see why it is needed for the 3rd?

[Homer]

The scenario that you have is an Open Delta Primary with a Wye Secondary.

Whenever you have an Open Delta arrangement of transformers you are reducing the VA or kVA rating of the bank by a factor of 1/1.73 or 0.577 .

Because this is no longer a complete 3-phase arrangement, each transformer in the bank can't supply it's full rating.

So in your example, instead of the answer being 200VA, it would be 173.1VA (300 x 0.577).

Instead of each 100VA transformer contributing 100VA to the total, it only contributed 86.55VA (total of 173.1VA) or 86.55% of its rating.

This can be proven mathematically using the rules for 3-phase power and transformers. If it still isn't clear we can go through this as an exercise.

Here's a reference link (http://www.marelco.com/faq.htm#1d).

Homer

PS Those 100VA transformers would be mighty tiny.

[Wgoodrich]

Wow I am IMPRESSED with both of you ! bow_1

[Ohm1]

bow_1 sniper_1 LOL! Had to utilize the new smilies!

[mdshunk]

I cut a slot in the top of my transformer to make a transformer bank. Now that the transformer bank is full of coins, I'm wondering if I should put it in a transformer vault? Any suggestions?

[Homer]

Whether a bank or a vault, be careful sticking your hand in to make your withdrawals.

Homer

[Ohm1]

Put some current on the transform before trying to retrieve your money. I say 1000v. It helps you retrieve it faster! :D