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# What is the turns ratio?

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Reputation ## What is the turns ratio?

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A wye-delta transformer has a primary voltage of 7200 V and a secondary voltage of 360 V. What is the turns ratio?

The test considered this answer wrong. The primary to secondary voltage ratio is 1 to 20. I'm sure the correct answer is 20:1, but grammatically speaking, the 1:20 answer is more correct. The following is from the noted study material:

Since there is a ratio of 1 to 4 between the turns in the primary and secondary circuits, there must be a ratio of 1 to 4 between the primary and secondary voltage and a ratio of 4 to 1 between the primary and secondary current. Please advise.  Reply With Quote

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Reputation   Originally Posted by franko26 Transformers
A wye-delta transformer has a primary voltage of 7200 V and a secondary voltage of 360 V. What is the turns ratio?

The test considered this answer wrong. The primary to secondary voltage ratio is 1 to 20. I'm sure the correct answer is 20:1, but grammatically speaking, the 1:20 answer is more correct. The following is from the noted study material:

Since there is a ratio of 1 to 4 between the turns in the primary and secondary circuits, there must be a ratio of 1 to 4 between the primary and secondary voltage and a ratio of 4 to 1 between the primary and secondary current. Please advise.
Is the 7200 line or phase voltage ?  Reply With Quote

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Reputation  Originally Posted by lester mcmanaway Is the 7200 line or phase voltage ?
I copied and pasted the question at the top of the comment, your guess is as good as mine.  Reply With Quote

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Reputation   Originally Posted by franko26 I copied and pasted the question at the top of the comment, your guess is as good as mine.
My guess would be line voltage then. So 7200/1.732 = 4157/360 roughly 11.55:1 would have been what I was thinking.  Reply With Quote

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Reputation  Originally Posted by lester mcmanaway My guess would be line voltage then. So 7200/1.732 = 4157/360 roughly 11.55:1 would have been what I was thinking.
i think the point i was trying to make is that the answer is 20:1, even though, grammatically, it should be 1:20 per the study guide that I included. I really hope the real test is not like this. Whats the point of having questions that will trick you, they really should be testing our knowledge. Though I think this website is excellent for getting your mind thinking in the right direction, some of the questions are questionable. There was another one: what is the fla for a single phase 115vac 1hp motor? the answer according to test guy is 16a, this is correct per nec table 430.28. remember though, the code book is only a guide. if you use the formula P=I xE or I=P/E, with P=to 746, the answer is 6.49 which is close to the 8a answer thy included. I dont believe they are going to let us use the nec on the test, so you have to figure it out mathematically.  Reply With Quote

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Reputation  Originally Posted by franko26 Transformers
A wye-delta transformer has a primary voltage of 7200 V and a secondary voltage of 360 V. What is the turns ratio?

The test considered this answer wrong. The primary to secondary voltage ratio is 1 to 20. I'm sure the correct answer is 20:1, but grammatically speaking, the 1:20 answer is more correct. The following is from the noted study material:

Since there is a ratio of 1 to 4 between the turns in the primary and secondary circuits, there must be a ratio of 1 to 4 between the primary and secondary voltage and a ratio of 4 to 1 between the primary and secondary current. Please advise.
Wouldn't 1:20 be a step-up transformer? The transformer in question is a step-down.

If the secondary winding has fewer turns than the primary, a lower voltage will be induced in the secondary. This type of transformer is called a step-down transformer.

A secondary coil with twice as many turns as the primary will be cut twice as many times by the magnetic flux, and twice the applied primary voltage will be induced in the secondary. This transformer is known as a step-up transformer.

My answer would be 11.55:1 assuming 7200 is the line voltage.  Reply With Quote

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Reputation ## Transformers Originally Posted by iwanttotest Wouldn't 1:20 be a step-up transformer? The transformer in question is a step-down.

If the secondary winding has fewer turns than the primary, a lower voltage will be induced in the secondary. This type of transformer is called a step-down transformer.

A secondary coil with twice as many turns as the primary will be cut twice as many times by the magnetic flux, and twice the applied primary voltage will be induced in the secondary. This transformer is known as a step-up transformer.

My answer would be 11.55:1 assuming 7200 is the line voltage.
Keep in mind, some transformers are both step up and step down, For example, a power plant uses grid power for back feed when it is not producing power. In this case, the Generator Step Up (GSU) transformer also acts as a step down transformer when the Generator Circuit Breaker (GCB) is open. When the generator is shut down (GCB open), the 230kv grid voltage is stepped down to 19kv through the GSU and the 19kv is stepped down again to 4160v with the auxiliary transformer to provide medium voltage back feed to power all plant loads while the turbine generators are shut down. I hope this was not too confusing and keep in mind, these are common voltages but all will vary depending on the location and plant configuration.  Reply With Quote

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Reputation  Originally Posted by 10beers Keep in mind, some transformers are both step up and step down, For example, a power plant uses grid power for back feed when it is not producing power. In this case, the Generator Step Up (GSU) transformer also acts as a step down transformer when the Generator Circuit Breaker (GCB) is open. When the generator is shut down (GCB open), the 230kv grid voltage is stepped down to 19kv through the GSU and the 19kv is stepped down again to 4160v with the auxiliary transformer to provide medium voltage back feed to power all plant loads while the turbine generators are shut down. I hope this was not too confusing and keep in mind, these are common voltages but all will vary depending on the location and plant configuration.
Great point and I would also add that the same could be said for just about any transformer. I've seen 480-208/120 transformers used backwards as step ups many times, it just depends on which side you energize.

As for the original question, it did specify that the primary voltage was 7200.  Reply With Quote

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Reputation  Originally Posted by franko26 i think the point i was trying to make is that the answer is 20:1, even though, grammatically, it should be 1:20 per the study guide that I included. I really hope the real test is not like this. Whats the point of having questions that will trick you, they really should be testing our knowledge. Though I think this website is excellent for getting your mind thinking in the right direction, some of the questions are questionable. There was another one: what is the fla for a single phase 115vac 1hp motor? the answer according to test guy is 16a, this is correct per nec table 430.28. remember though, the code book is only a guide. if you use the formula P=I xE or I=P/E, with P=to 746, the answer is 6.49 which is close to the 8a answer thy included. I dont believe they are going to let us use the nec on the test, so you have to figure it out mathematically.
I think, to answer this question you need to dived first voltage by the second one: 7200/360= 20 so the ratio is 20:1. If first one was 360 and the second one 7200, then the answer would be 1:20. so for this question 20:1 is the right answer.
regarding your fla question, you forgot to consider PF: P=I*E*PF.  Reply With Quote

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Reputation ## turns ratio Originally Posted by franko26 Transformers
A wye-delta transformer has a primary voltage of 7200 V and a secondary voltage of 360 V. What is the turns ratio?

The test considered this answer wrong. The primary to secondary voltage ratio is 1 to 20. I'm sure the correct answer is 20:1, but grammatically speaking, the 1:20 answer is more correct. The following is from the noted study material:

Since there is a ratio of 1 to 4 between the turns in the primary and secondary circuits, there must be a ratio of 1 to 4 between the primary and secondary voltage and a ratio of 4 to 1 between the primary and secondary current. Please advise.  Reply With Quote 