How to do simple transformer calculation to find secondary side voltage?

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adamaero

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Transformer is 4:1, primary is at 115 V. Find secondary.

Dividing by four did not result in any correct answer choices. There is no other info given.
 
Answer question asks, "Suppose primary fluctuates between 105 and 125 V on a 5/1 step down transformer. The maximum secondary peak voltage would be about how many volts?" (88, 18, 74, 35)

I would think to just take 1/5th. So I think I'm missing something.
 
adamaero said:
Answer question asks, "Suppose primary fluctuates between 105 and 125 V on a 5/1 step down transformer. The maximum secondary peak voltage would be about how many volts?" (88, 18, 74, 35)

I would think to just take 1/5th. So I think I'm missing something.
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My guess would be that the 105-125V is RMS. So stepping down the maximum from 125V on a 5/1 ratio would be 25V RMS. Then from RMS to the peak voltage would be 25*Sqrt(2) or 35V peak.

Doing the same steps for the original question also gave me the 40.6V.
 
tezuka said:
My guess would be that the 105-125V is RMS. So stepping down the maximum from 125V on a 5/1 ratio would be 25V RMS. Then from RMS to the peak voltage would be 25*Sqrt(2) or 35V peak.

Doing the same steps for the original question also gave me the 40.6V.
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Yeah, I see that, but giving the setup voltage in RMS and expecting the answer in peak without phrasing the question as such is kind of sucky, don't you think?
 
ggunn said:
Yeah, I see that, but giving the setup voltage in RMS and expecting the answer in peak without phrasing the question as such is kind of sucky, don't you think?
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Yes, but common in classes where they want us to THINK rather than regurgitate results from a single formula. I'm accustomed to being WARNED that questions use different units, and be sure to allow for that. A distance may be in miles, kM, inches, and nautical miles. Temperatures may be in a mix of F, C, K, and R. In the hydraulics classes, you cannot imagine how many pressure units they can come up with.

I think using what is between our ears is important.
 
GeorgeB said:
Yes, but common in classes where they want us to THINK rather than regurgitate results from a single formula. I'm accustomed to being WARNED that questions use different units, and be sure to allow for that. A distance may be in miles, kM, inches, and nautical miles. Temperatures may be in a mix of F, C, K, and R. In the hydraulics classes, you cannot imagine how many pressure units they can come up with.

I think using what is between our ears is important.
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Well ok, but using RMS V in one part of a problem and peak in another part when there is absolutely no reason or convention is just ridiculous.
 
Two sneaky things which might be stated elsewhere or implied in the questions:

What is the type of voltage measurement being made (RMS vs average vs peak vs ....)?

What is the coil arrangement (single phase, delta:wye, delta:delta....)?

The turns ratio in a common 480V:208V delta:wye transformer is 4:1. But the transformer voltage ratio is different from 4:1 because of the different coil connentions.

Put 120V RMS into a single phase transformer with a 1:1 turns ratio, and the peak output voltage is about 170V. Not because the voltage changed (it didn't) but because the peak of a 120V sine wave is 170V.

Jon
 
electrofelon said:
Okay, I reread the question, I guess I didn't read it very thoroughly the first time and I see they did say "peak". I didn't really notice that at first or assumed they meant peak as in the maximum of the fluctuations.
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In post #1 I don't see any mention of "peak".
 
LarryFine said:
Just pointing out where "peak" appeared.
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And I was just pointing out that the answer to the first question was apparently in peak V where the primary was in RMS V and the question did not ask for it in those terms. I guess one could look at the choices of answers and figure it out, but it's kind of sucky to do that to a student.
 
I had a full professor in engineering school who would do stuff like that. Also, on his exams, wrong answers subtracted from your score, so it was possible to get a negative grade on an exam (many students did), and with only three exams in the semester that would wreak havoc on your average. He was brought out of retirement to help weed students out of the program at the junior level. His was the only course I failed (I got a D), and when I retook it I made an A with virtually no study time, so I knew the material. That was 38 years ago and I am still angry about it. :D
 
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