Transformer

Discussion in 'Homework Help' started by lemon, Feb 8, 2010.

  1. lemon

    Thread Starter Member

    Jan 28, 2010
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    2
    A 36Ω resistive load is connected to the secondary of a transformer. The terminal p.d. at the secondary is 24v. If the primary is connected to a 480v a.c. supply, calculate:

    a) the transformer turns ratio
    b) the current and power drawn by the load
    c) the current drawn from the supply

    Relevant equations
    1: V=IR
    2:Vp/Vs=Np/Ns=ip/is
    3: Np:Ns
    P=IV

    Solution:
    a) Is=Vs/R = 24/36=0.667A
    P=0.667Ax24V=16W

    b) Ip=current drawn from the supply
    Ip=Vp/R = 480V/36Ω=13.3A

    Not sure if I'm getting this ok
    Can somebody please check - thank you :)
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    No, you are not right about the primary's current.

    Use that equatio:

    Np/Ns=Is/Ip or

    Ip=Ps/Vp, since Pp=Ps (ideally)
     
  3. hgmjr

    Moderator

    Jan 28, 2005
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    What do you mean by p.d.?

    hgmjr
     
  4. mik3

    Senior Member

    Feb 4, 2008
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    I guess potential drop even it is not a correct term for this case.
     
  5. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    Yeah, I was thinking potential difference but I disdain having to guess what the initials mean when there are multiple possibilities.

    hgmjr
     
  6. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    Let's hope the OP will let a drop of lemon on p.d and make its taste better. :p
     
  7. lemon

    Thread Starter Member

    Jan 28, 2010
    125
    2
    sorry. Didn't think that would cause confusion.
    I meant potential difference.
    Should I define things before I use abbreviations?
     
  8. mik3

    Senior Member

    Feb 4, 2008
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    It should be better unless they are frequenctly used abbreviations.
     
  9. lemon

    Thread Starter Member

    Jan 28, 2010
    125
    2
    Is=24v/36Ω=0.67A

    Ip=Vp/Vs χ Is=0.67A χ 480v/24v = 13.3A

    How can I calculate the number of turns if I don't know either number of turns on one or two?

    Np/Ns=Is/Ip so Ns=Is/Ip x Np
    but no Np:confused:
     
  10. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    You do not need to know the actual number of turns but their ration which is Vp/Vs.

    The input and output power is the same ideally.

    Thus,

    Pin=Pout

    Vp*Ip=Vs*Is

    therefore,

    Ip=Vs*Is/Vp
     
  11. lemon

    Thread Starter Member

    Jan 28, 2010
    125
    2
    Hi mik3:

    So the transformer turns ratio is 20:1

    for question b) I want the current and power drawn by the load. So this is Is and Ps.
    Is =Vp*Ip/Vs
    But I don't have the primary current and not sure how to get it from a circuit like this. I would attempt V=IR but there is no primary resistor.:confused:
     
  12. mik3

    Senior Member

    Feb 4, 2008
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    Np/Ns=100/200=0.5

    Vs=Vp*Ns/Np=6*200/100=12V

    Is=Ps/Vs=24/12=2A

    Pp=Ps=Vp*Ip

    Thus, Ip=Pp/Vp=24/6=4A
     
  13. lemon

    Thread Starter Member

    Jan 28, 2010
    125
    2
    Where did the values for the number of turns come from?
    100 and 200?
     
  14. lemon

    Thread Starter Member

    Jan 28, 2010
    125
    2
    I think you may have taken those values by mistake from one of the other questions you are currently helping me with mik3. My other question on transformers has 100 and 200 turns
     
  15. mik3

    Senior Member

    Feb 4, 2008
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    You are right, I got confused. :rolleyes:

    The current drawn by the load is Is=Vs/R
     
  16. lemon

    Thread Starter Member

    Jan 28, 2010
    125
    2
    So summary is:

    a)Vp/Vs=Np/Ns=480v/24v=20:1

    b) Is=Vs/R = 24v/36Ω=0.67A
    P=IV, so 0.67A x 24v = 16.08W

    c) Ip=Vs*Is/Vp = 24v*0.67A/480v=0.0335A

    How's that? :D
     
  17. mik3

    Senior Member

    Feb 4, 2008
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    The answer the is correct because the input power (primary) equals the output power (secondary) and thus Ip has to be less than Is because Vp is greater than Vs.
     
  18. lemon

    Thread Starter Member

    Jan 28, 2010
    125
    2
    The answer the is correct because ???
    Does this mean the answers to the above questions are correct?
     
  19. mik3

    Senior Member

    Feb 4, 2008
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    Yes, the answers are correct.
     
  20. lemon

    Thread Starter Member

    Jan 28, 2010
    125
    2
    Wow! I can't tell you how good that feels. You've been a rock with all my questions today mik3. Thank you so much.
    Regards
     
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