Turns Ratio

Status
Not open for further replies.
Guys, this is made way more complicated than what I am asking. Leave aside whether turn ration is 2.01 or not. That is not the point of discussion.

If I am a given transformer which is 240/120(no other information is given); what is the turns ratio? If your answer is 2. why not 0.5, because 120 could be primary.

If your answer is 0.5 then why not 2 as 240 volts could be primary .

Just because 240 is listed first doesn't mean that is the primary. Not interested in knowing how the tests that are done to find turns ratio.

If I am solving a problem which requires turns ratio and only information given is voltage ratio then how do I find turns ratio, that's all I want to know.
 
roy167 said:
Guys, this is made way more complicated than what I am asking. ...
Click to expand...
Yeah that is what I thought too.

roy167 said:
... If I am a given transformer which is 240/120(no other information is given); what is the turns ratio? If your answer is 2. why not 0.5, because 120 could be primary.

If your answer is 0.5 then why not 2 as 240 volts could be primary . ....
Click to expand...

With the limited context you have given, it doesn't matter, 'Which direction to you want it to go?'

roy167 said:
... If I am solving a problem which requires turns ratio and only information given is voltage ratio then how do I find turns ratio, that's all I want to know.
Click to expand...
Okay, that is a different question than you have asked before.

So, again, from an engineering viewpoint:
You have a problem to solve
You know it requires a (the) turns ratio
That means you have a math model
And that means you know which direction the turns ratio is to fit the model.
​
Again I'm thinking you already knew all this. So, I am at a loss as to exactly what puzzle there is to solve.

Perhaps if you gave us a specific example of a problem that highlights the horns of this dastardly dilemma.
 
For US power transformers, the convention is that the reported voltage ratio corresponds exactly to the turns ratio, while for control transformers the voltage ratio is given as the measured ratio at full load with the intended primary being driven. For the latter, this will not correspond exactly to the turns ratio, and the voltage ratio measured by the same standard with the transformer driven "backwards" will be quite far from the simple inverse of the normal operating voltage ratio.
Apparently outside the US and maybe the rest of North America, the compensated voltage ratio (not directly corresponding to turns ratio) is commonly used even for power transformers.
 
181216-2154 EST

roy167:

You have presented insufficient information to determine what the turns ratio is.

Why do you want to know the turns ratio? How precise?

You have not clearly defined your question.

If you really want to know the turns ratio, and don't like my suggestions, then open the transformer and count turns.

You list yourself as an electrical engineer. Thus, you should already understand transformers.

.
 
roy167 said:
Guys, this is made way more complicated than what I am asking.
Click to expand...
On this forum?! Never! :blink:

If I am a given transformer which is 240/120(no other information is given); what is the turns ratio? If your answer is 2. why not 0.5, because 120 could be primary.
Click to expand...
I said 2:1 earlier because you said 240/120. Had you said 120/240, I would have said 1:2.

If your answer is 0.5 then why not 2 as 240 volts could be primary .
Click to expand...
In that case, it can be considered either or both. (Schrodinger's cat)

Just because 240 is listed first doesn't mean that is the primary. Not interested in knowing how the tests that are done to find turns ratio.
Click to expand...
To this exact statement, yes, the first number "is" the primary.

If you insist we don't test, then we can't answer your question.

If I am solving a problem which requires turns ratio and only information given is voltage ratio then how do I find turns ratio, that's all I want to know.
Click to expand...
The voltage ratio is the turns ratio, theoretically. Sorry you don't like that answer.


Now, to answer your question without your restrictions, you're asking how to determine whether the transformer was designed and built as a step-up or step-down unit. The only way to determine this on the fly is to apply a voltage to one winding and measure both in and out voltages.

Now you can label the H1 and H2, and L1 and L2 leads. Now, you can apply whichever voltage you have handy to the winding you've determined appropriate, and measure both voltages again. Because of the previously-mentioned loss-compensation, one will measure slightly high or lower than expected.

If the output voltage is slightly higher than the expected voltage, you have it connected as designed, and you're supplying power to the primary. If the output voltage is slightly lower than expected, you have it connected "backwards," and you're supplying power to the secondary.

Added: The actual turns ratio can pretty well be determined by the actual measured voltage ratio when unloaded. (The transformer, not you.)
 
meternerd said:
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...


When you get consistently unexpected results in electrical testing we always, check our test equipment, our test procedures and contact the manufacturer. Ideally, this test would have been performed with a TTR on a de-energized transformer.
 
roy167 said:
Guys, this is made way more complicated than what I am asking. Leave aside whether turn ration is 2.01 or not. That is not the point of discussion.

If I am a given transformer which is 240/120(no other information is given); what is the turns ratio? If your answer is 2. why not 0.5, because 120 could be primary.

If your answer is 0.5 then why not 2 as 240 volts could be primary.

Just because 240 is listed first doesn't mean that is the primary. Not interested in knowing how the tests that are done to find turns ratio.

If I am solving a problem which requires turns ratio and only information given is voltage ratio then how do I find turns ratio, that's all I want to know.
Click to expand...

Is the transformer designed as a step up or step down?

http://machineryequipmentonline.com/electric-equipment/transformersturns-ratio/

If a step down the ratio is greater than 1.

If a pure isolation transformer 1 to 1. Primary the same as the secondary.

If a step up the ratio would be less than 1.


http://machineryequipmentonline.com/electric-equipment/transformersturns-ratio/
 
W
LarryFine said:
On this forum?! Never! :blink:

Added: The actual turns ratio can pretty well be determined by the actual measured voltage ratio when unloaded. (The transformer, not you.)
Click to expand...
But I prefer to be loaded when I wind transformers... like baking a cake... it seems better when you are loaded... the fact it does not work right afterwards just gives you an excuse to get another bottle and try again
 
brian john said:
When you get consistently unexpected results in electrical testing we always, check our test equipment, our test procedures and contact the manufacturer. Ideally, this test would have been performed with a TTR on a de-energized transformer.
Click to expand...

Ah yes....the ideal is to use a digital TTR which would show ratio, polarity, excitation current, etc. But, this was back in the '70's and using a TTR to ratio a 500VA control transformer would be a very unpopular and expensive example of overkill. Back in that "era", TTR's were analog meters with selector switches for ratio and a needle to show readings....time consuming and not exactly user friendly for "mass production" testing. There were literally hundreds of transformers to test. In my humble opinion, it should have been a go/no-go test, but nuclear was always a showplace for quality control "nitpicking". We even had to test fuses by measuring resistance end to end with appropriate tolerances for acceptability. Those were the days......made good money, though.:D
 
roy167 said:
Guys, this is made way more complicated than what I am asking. Leave aside whether turn ration is 2.01 or not. That is not the point of discussion.

If I am a given transformer which is 240/120(no other information is given); what is the turns ratio? If your answer is 2. why not 0.5, because 120 could be primary.

If your answer is 0.5 then why not 2 as 240 volts could be primary .

Just because 240 is listed first doesn't mean that is the primary. Not interested in knowing how the tests that are done to find turns ratio.

If I am solving a problem which requires turns ratio and only information given is voltage ratio then how do I find turns ratio, that's all I want to know.
Click to expand...

As was stated....you can't tell actual turns ratio by voltage only, just an approximation. You need specialized equipment. You basically answered your own question....The discussion is to help others who may have similar questions. My opinion is that if it doesn't stray too far from the subject, it may be of value to more than just the OP. I seldom fail to learn something every time I read a thread.
 
Last edited:
181226-2451 EST

The question of how to accurately measure turns ratio of a transformer from electrical measurements has been bugging me.
So I have done some experiments that may give me a closer answer than what I obtained in post #12. Has anyone else been thinking about methods?

As a side note I have noticed a time delay effect when changing voltage. What this implies is the need for the core flux to reestablish a new equilibrium condition.

.
 
If an un-potted transformer I have slipped a small piece of magnet wire around the core and measured the voltage on all windings with a scope with 10% voltage applied to any of the regular windings. and look at voltage ratios.

Measure primary and secondary inductances unloaded. Measure primary and secondary inductances with the other winding shorted and apply correction factor for leakage inductance. Calculate sqrt of inductance ratio based on primary and secondary inductances corrected for leakage paths.

Combining the voltage ratios from the voltage ratios and inductance results and getting consistent integral numbers is one way of getting the correct answer.

One can also excite the primary with open secondary and measure core loss, then do current ratio at low current and correct for core loss current.
 
gar said:
181226-2451 EST

The question of how to accurately measure turns ratio of a transformer from electrical measurements has been bugging me.
So I have done some experiments that may give me a closer answer than what I obtained in post #12. Has anyone else been thinking about methods?

As a side note I have noticed a time delay effect when changing voltage. What this implies is the need for the core flux to reestablish a new equilibrium condition.

.
Click to expand...



https://megger.com/products/transformer-testing/turns-ratio-testing


https://electrical-engineering-portal.com/power-transformer-turns-ratio-test
 
Last edited:
181229-621 EST

Wire-Smith:

The Megger discussion mentions 0.5% accuracy, and apparently always excites the lowest voltage coil. I did not see where they described their theory of measurement. Since Megger always excites the lowest voltage coil this means their ratio is always above 1. Possibly an answer to the original post question.

I suspect the Megger approach is a simple open circuit voltage measurement with possibly some compensation for magnetizing current.

Your second reference did not seem to provide any useful information.


junkhound:

To what accuracy do you think you were able to predict actual turns ratio? And how might that compare with my method of using excitation in both directions, and averaging the two results?

.
 
gar said:
181229-621 EST

Wire-Smith:

The Megger discussion mentions 0.5% accuracy, and apparently always excites the lowest voltage coil. I did not see where they described their theory of measurement. Since Megger always excites the lowest voltage coil this means their ratio is always above 1. Possibly an answer to the original post question.

I suspect the Megger approach is a simple open circuit voltage measurement with possibly some compensation for magnetizing current.

Your second reference did not seem to provide any useful information.


.
Click to expand...

they just take high resolution measurements, the TTR's use a low voltage

https://www.netaworld.org/sites/default/files/public/neta-journals/NWwtr05JowettFeature.pdf
 
181230-1318 EST

Wire-Smith:

Unfortunately none of those references really discuss how they make the measurements. What shows up is low voltage, excite the coil with the fewest turns, and using some sort of balancing technique.

.
 
gar said:
181226-2451 EST


As a side note I have noticed a time delay effect when changing voltage. What this implies is the need for the core flux to reestablish a new equilibrium condition.

.
Click to expand...
In inductive/capacitive circuit there would be time delay when changing current/voltage. So perhaps it may be the resultant circuit being capacitive.
 
Status
Not open for further replies.
Top