×
Follow Us
Page 1 of 2 12 LastLast
Results 1 to 10 of 16

What is the turns ratio?

Go to latest post
Page 1 of 2 12 LastLast
    #1
  1. Join Date
    Apr 2015
    Posts
    3
    Reputation

    What is the turns ratio?

    Transformers
    A wye-delta transformer has a primary voltage of 7200 V and a secondary voltage of 360 V. What is the turns ratio?
    Your Answer: 1:20

    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.

  2. #2
  3. Join Date
    Apr 2015
    Location
    OH/MI
    Posts
    31
    Reputation
    Quote Originally Posted by franko26 View Post
    Transformers
    A wye-delta transformer has a primary voltage of 7200 V and a secondary voltage of 360 V. What is the turns ratio?
    Your Answer: 1:20

    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 ?

  4. #3
  5. Join Date
    Apr 2015
    Posts
    3
    Reputation
    Quote Originally Posted by lester mcmanaway View Post
    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.

  6. #4
  7. Join Date
    Apr 2015
    Location
    OH/MI
    Posts
    31
    Reputation
    Quote Originally Posted by franko26 View Post
    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.

  8. #5
  9. Join Date
    Apr 2015
    Posts
    3
    Reputation
    Quote Originally Posted by lester mcmanaway View Post
    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.

  10. #6
  11. Join Date
    Jan 2017
    Posts
    5
    Reputation
    Quote Originally Posted by franko26 View Post
    Transformers
    A wye-delta transformer has a primary voltage of 7200 V and a secondary voltage of 360 V. What is the turns ratio?
    Your Answer: 1:20

    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.

  12. #7
  13. Join Date
    Aug 2017
    Posts
    1
    Reputation

    Transformers

    Quote Originally Posted by iwanttotest View Post
    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.

  14. #8
  15. Join Date
    Jan 2017
    Posts
    5
    Reputation
    Quote Originally Posted by 10beers View Post
    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.

  16. #9
  17. Join Date
    Oct 2017
    Posts
    2
    Reputation
    Quote Originally Posted by franko26 View Post
    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.

  18. #10
  19. Join Date
    Jan 2018
    Posts
    2
    Reputation

    turns ratio

    Quote Originally Posted by franko26 View Post
    Transformers
    A wye-delta transformer has a primary voltage of 7200 V and a secondary voltage of 360 V. What is the turns ratio?
    Your Answer: 1:20

    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.

Page 1 of 2 12 LastLast
Subscribe

Login or register to leave a reply!




Related Topics

  1. Saturation test of a 1500/5 multi-ratio CT
    By Tseppish in forum Electrical Testing Talk
    Replies: 6
    Last Post: July 21, 2017, 02:56 PM

Related Content

Tags for this Thread

Follow us


Explore TestGuy


NETA Certification Training


NICET Electrical Power Testing


Help and Support