Turns Ratio

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roy167

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Turns ratio of a transformer is Pri voltage/sec voltage or N1/N2. If we are not told which one is Pri and which one is secondary. Let's say you are just given it is 240/120 transformer. The transformer can be used as step or step down. How would you know what is turns ratio in this case?

If I take 240 primary then a= 2
If I take 120 Primary then a= 0.5
 
181215-2200 EST

roy167:

The turns ratio of a transformer is defined by the actual turns on the transformer.

If the number of turns on the primary side is 1000 and there are 100 turns on the secondary, then the ratio is 10 to 1, or 1 to 10 depending on how you want to use the ratio.

The direction of power flow does not change the ratio.

On a tightly coupled magnetic core transformer you can approximately determine the turns ratio by applying a known sine wave voltage to one coil, meaning you measure the value, and measuring the open circuit voltage of the other coil. The voltage ratio is quite close to the actual turns ratio.

.
 
roy167 said:
Turns ratio of a transformer is Pri voltage/sec voltage or N1/N2.
Click to expand...

gar said:
181215-2200 EST]The turns ratio of a transformer is defined by the actual turns on the transformer..
Click to expand...


Note gar's distinction is important in three phase transformers with different primary and secondary winding configurations (as in delta vs wye) because they change the voltage by a factor of 1.73.
 
191216-0844 EST

electrofelon:

In a three phase transformer it is still the turns ratio of a primary to its secondary that determines the relationship between the voltage across said primary winding to its secondary winding.

.
 
gar said:
181215-2200 EST

roy167:

The turns ratio of a transformer is defined by the actual turns on the transformer.

If the number of turns on the primary side is 1000 and there are 100 turns on the secondary, then the ratio is 10 to 1, or 1 to 10 depending on how you want to use the ratio.

The direction of power flow does not change the ratio.

On a tightly coupled magnetic core transformer you can approximately determine the turns ratio by applying a known sine wave voltage to one coil, meaning you measure the value, and measuring the open circuit voltage of the other coil. The voltage ratio is quite close to the actual turns ratio.

.
Click to expand...

If you are not given Primary/secondary information, you are not given primary turns, secondary turns. Only information you are given is voltage ratio for e.g 240/120

then how do you find what turns ratio is what my question was. In 240/120, any side could be primary depending upon whether it is used as step up or step down.

Turns ratio also equals E1/E2, so we should be able to find the ratio with given voltage, correct? N1/N2= V1/v2=E1/E2 in an ideal transformer.
 
181216-1005 EST

roy167:

If the primary voltage rating is 240 and the secondary is 120, then the primary has about 2 times the turns of the secondary. A rating of 240/120 is a ballpark rating not a precise definition of turns ratio. I can explain later if you need to know, but I have to leave now.

.
 
roy167 said:
If you are not given Primary/secondary information, you are not given primary turns, secondary turns. Only information you are given is voltage ratio for e.g 240/120

then how do you find what turns ratio is what my question was. In 240/120, any side could be primary depending upon whether it is used as step up or step down.

Turns ratio also equals E1/E2, so we should be able to find the ratio with given voltage, correct? N1/N2= V1/v2=E1/E2 in an ideal transformer.
Click to expand...
If voltage ratio is 240/120 then one side has twice as many turns as the other. Both sides are going to be rated same VA, therefore the lower voltage side will be wound with a larger conductor.

Not sure what else you may be asking about.
 
roy167 said:
If you are not given Primary/secondary information, you are not given primary turns, secondary turns. Only information you are given is voltage ratio for e.g 240/120

then how do you find what turns ratio is what my question was. In 240/120, any side could be primary depending upon whether it is used as step up or step down.

Turns ratio also equals E1/E2, so we should be able to find the ratio with given voltage, correct? N1/N2= V1/v2=E1/E2 in an ideal transformer.
Click to expand...
I don't understand why 2:1 doesn't answer your question. It sounds like your question is:

"If someone hand's you an unmarked transformer, and you're told only that one winding is rated at 120v and the other at 240v, how do you know which is which?"

One answer is to apply 120v to either winding and measure the voltage across the other. You'll get either 60v or 240v.

If the question has to do with not getting exactly 1:2 or 2:1, the exact difference reflects the actual ratio, losses aside.

Please forgive me for misunderstanding or condescending if I have done either.
 
I am reading this as a terminology question.

If you describe a transformer as having a turns ratio of 2:1, that means that the primary has twice the turns as the secondary.

If you describe a transformer as having a turns ratio of 1:2, that means that the secondary has twice the turns as the primary.

If you are given a transformer with one coil having a voltage of 120 and the other a voltage of 240, and you don't know which is primary and which is secondary, then what is the turns ratio? 2:1 or 1:2.

My answer to what I think the original question is: It depends on how you are using the transformer, and and either 2:1 or 1:2 would be correct depending on which is being used as the primary.

-Jon
 
winnie said:
I am reading this as a terminology question.

If you describe a transformer as having a turns ratio of 2:1, that means that the primary has twice the turns as the secondary.

If you describe a transformer as having a turns ratio of 1:2, that means that the secondary has twice the turns as the primary.

If you are given a transformer with one coil having a voltage of 120 and the other a voltage of 240, and you don't know which is primary and which is secondary, then what is the turns ratio? 2:1 or 1:2.

My answer to what I think the original question is: It depends on how you are using the transformer, and and either 2:1 or 1:2 would be correct depending on which is being used as the primary.

-Jon
Click to expand...
And we learn in PV work that the primary is the side that supplies the initial excitation, regardless of which way the power ends up flowing most of the time.
 
I have read somewhere, turns ratio is always greater than one. If that be the case, then it wouldn't matter whether Primary/secondary is given, whether transformer will be used as step up, step down. I just don't have anything concrete that I want to rely on hence asking questions. If the industry standard is greater than 1 then this would solve this puzzle easily.
 
181216-1441 EST

roy167:

An experiment on a small 60 Hz transformer.

The transformer is a Signal 241-6-20. Secondary is center tapped, and looks quite balanced. Rated 115 V in 20 V 1.5 A out.
See https://www.mouser.jp/ds/2/643/241-1130952.pdf .

With 122.6 V in open circuit out was 24.29 V. Ratio = 5.048 .
With 24,91 V in open circuit out was 123,0 V. Ratio = 4,938 .

There will be some flux leakage. Thus, the 5.048 is somewhat larger than the actual ratio. In other words the output voltage is not as large as the actual ratio would calculate the value to be. Similarly the ratio when feeding the secondary is not as high as what the actual ratio would imply. A good bet for the actual ratio is the average of the two measured values or 4.993 .

Using the transformer rating of 115 and 20 one gets a ratio of 5.75 . Why the difference? Because the transformer is rated at full load. Voltage drop with load is caused by two factors --- leakage of flux lines that don't completely couple both windings, and I*R drop in both coils.

.
 
Nuclear mystery.....

Nuclear mystery.....

LarryFine said:
I don't understand why 2:1 doesn't answer your question. It sounds like your question is:

"If someone hand's you an unmarked transformer, and you're told only that one winding is rated at 120v and the other at 240v, how do you know which is which?"

One answer is to apply 120v to either winding and measure the voltage across the other. You'll get either 60v or 240v.

If the question has to do with not getting exactly 1:2 or 2:1, the exact difference reflects the actual ratio, losses aside.

Please forgive me for misunderstanding or condescending if I have done either.
Click to expand...

I DO NOT want to hijack or confuse the posters, but I worked as a nuclear plant startup tech for years. On one job, we were to test each 480/120V step down control transformer for ratio on MCC's. The procedure was to apply 120V to the "secondary" 120V side and measure voltage on the "primary" 480V side. Expected voltage was 480V (1:4 ratio). The limit for acceptability was +/- 10%. We consistently got 12% to 15% less than expected. After rejecting dozens of transformers, we tried applying 480V to the high side and measuring the low side. They were almost exactly 120V (4:1). I still have no idea why. There was no load on the winding, so we thought maybe that was why, but just reversing the source voltage solved the problem and saved the customer a lot of money.
 
meternerd said:
They were almost exactly 120V (4:1). I still have no idea why. There was no load on the winding, . . .
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Because, even unloaded, a transformer is not 100% efficient. They're manufactured to compensate for the energy lost in magnetizing the core and in IR losses by adding a few turns to the intended secondary.
 
gar said:
With 122.6 V in open circuit out was 24.29 V. Ratio = 5.048 .
With 24,91 V in open circuit out was 123,0 V. Ratio = 4,938 .
Click to expand...
Generally the turns ration doesn't exactly match the input/output voltage ratio.
To get the rated output on load voltage usually requires a few more turns to compensate for transformer regulation.
 
roy167 said:
I have read somewhere, turns ratio is always greater than one. If that be the case, then it wouldn't matter whether Primary/secondary is given, whether transformer will be used as step up, step down. I just don't have anything concrete that I want to rely on hence asking questions. If the industry standard is greater than 1 then this would solve this puzzle easily.
Click to expand...

From an engineering standpoint:
I don't recall ever seeing a turns ratio on a nameplate. It will always be a voltage ratio. I don't recall any IEEE papers regulating the format for turns ratio. A math model using the turns ratio will be clear which direction it is. A transformer turns ratio test form will be clear which direction it is.

Generally if the terminology is ambiguous, one adds words to accurately define.

Yes, a turns ratio is given as a number greater than one. Examples:
480D/480Y One might say the TTR is 1.73205080757:1 rather than TTR = .57735026919:1 Or one would say, "It is a 480D/480Y, feed it either direction you want."

14400:120 PT One might say TTR is 120:1. Rarely (like never) TTR is .08333333333:1 Likely one would say, "It is 14.4 to 120." and let the listener figure out they meant 14.4kV.

And I'm thinking you already knew all this. So, I am at a loss as to exactly what puzzle there is to solve.

the worm
 
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meternerd said:
Nuclear mystery.....
I DO NOT want to hijack or confuse the posters, but I worked as a nuclear plant startup tech for years. On one job, we were to test each 480/120V step down control transformer for ratio on MCC's. The procedure was to apply 120V to the "secondary" 120V side and measure voltage on the "primary" 480V side. Expected voltage was 480V (1:4 ratio). The limit for acceptability was +/- 10%. We consistently got 12% to 15% less than expected. After rejecting dozens of transformers, we tried applying 480V to the high side and measuring the low side. They were almost exactly 120V (4:1). I still have no idea why. There was no load on the winding, so we thought maybe that was why, but just reversing the source voltage solved the problem and saved the customer a lot of money.
Click to expand...

I think you got the "Mystery" part right - cause that doesn't match up with any known physics. No, I'm not casting doubts on your technical ability.

Unless there was some weird fluxing in the core :jawdrop: Careful how you pronounce that.

I worked in a Nuc shipyard for a while - Surface ship and Submarine refueling overhauls. It was disturbing seeing the equipment we threw out because it was too tight on the spec. Even more disturbing was the thought that every part on a nuc sub was furnished by the low bidder.

Attributed to John Glenn
‘I felt exactly how you would feel if you were getting ready to launch and knew you were sitting on top of 2 million parts — all built by the lowest bidder on a government contract.’ ​
 
retirede said:
This is all interesting, but unless you’re designing or manufacturing transformers, why would you care about the turns ratio?
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Check it during commissioning

Check it during troubleshooting:
Maybe overheating
Maybe dissolved gas issues
 
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