Autotransformer question

[bethy]

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]

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

 

[bethy]

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]

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.

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]

Optimus Prime, FTW, suckas!!!!

 

[Flyer_PE]

^Mechanical Interloper Alert !!!

 

[bethy]

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.

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.

I got it!

Thanks.

 

[BamaBino]

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?