# Autotransformer question



## bethy (Oct 18, 2010)

Hello, I am a new member. I have a question about the autotransformer ratio. When should we use the formula:

V1/V2 = N1/(N1+N2)

and: V1/V2 = N1/N2

Here is the problem:

1. In a edeal step-up autotransformer. Give N1= 200 and N2=400. If 100V is applied across the primary, what is the voltaage across the load?

2. A 280 turn ideal autotransformer has a secondary tapped at 14 turns. If the input voltage is 240V and the secondary load is a .006 ohm resistor, find the current in the section of the winding supplying the load.

Solution:

1. it uses the top formular to cal. V2.

2. It uses the second formular to cal. V2.

I am confuse. Hope someone can point to me the right key here.

Thanks.


----------



## patelpe (Oct 19, 2010)

I came with V2=300 V. Second formula doesn't apply to the Auto transformer.


----------



## bethy (Oct 20, 2010)

patelpe said:


> I came with V2=300 V. Second formula doesn't apply to the Auto transformer.



Oop! hit the wrong button. Couldn't see the send button. how do I remove the coppy email?

That's what I thought. But the solution use the second formula V1/V2 = N1/N2 to cal. V2 on problem #2. Then it this V2 to cal. the current load. I keep looking around with other textbook. but could't get the answer yet. Hopefully someone on board can explain it. Thanks.


----------



## DK PE (Oct 20, 2010)

bethy said:


> patelpe said:
> 
> 
> > I came with V2=300 V. Second formula doesn't apply to the Auto transformer.
> ...


Think the trick is to recognize the total turns is 280 turns and this has 240 V impressed on it. Then the secondary is just proportional... ie, 14/280 * the 240 on primary.

Draw a vertical coil with 280 turns with 240 V across coming from left side. Then on right side, the two conductors are are 1. at bottom, then 2. at 5% or so up the 280 turn coil. If that isn't clear, let me know and I'll scan something later.


----------



## Capt Worley PE (Oct 20, 2010)

Optimus Prime, FTW, suckas!!!!


----------



## Flyer_PE (Oct 20, 2010)

^Mechanical Interloper Alert !!!


----------



## bethy (Oct 20, 2010)

DK PE said:


> bethy said:
> 
> 
> > patelpe said:
> ...



I got it!

Thanks.


----------



## BamaBino (May 26, 2011)

bethy said:


> 2. A 280 turn ideal autotransformer has a secondary tapped at 14 turns. If the input voltage is 240V and the secondary load is a .006 ohm resistor, find the current in the section of the winding supplying the load.


The answer is 100 A, right?


----------

