Transformer circuit quistion

Discussion in 'Homework Help' started by TheSpArK505, Oct 18, 2014.

  1. TheSpArK505

    Thread Starter Member

    Sep 25, 2013
    92
    0
    Hi everyone .

    To answer a question u need to understand it 100%...and that's what I'm going for .
    I have a question about transformers...and it says:

    The transformer secondary is connected to an electrical load , and it delivers its rated current at rated voltage and 0.85 pf lagging , find primary voltage ?

    Can any one explain please : what does rated means , and is the power of the load is only due to secondary voltage and as nothing to do with the primary voltage ...and also the underlined part please ??:confused::confused:
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,361
    Can you please post the complete question and not just part of the question?
     
  3. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,788
    945
    transformers 'transform' the voltage, current from one side to the other.

    The power input = the power output theoretically.
    Real world we know there are losses.
    The 'power factor' is one of the corrections we apply to determine real world conditions
    In the real world power in is ALWAYS more than power out.

    Rated levels are given in a piece of equipments specs.

    12 V 25 VA transformer(watts)
    operates from 110 VAC

    So you plug the numerical values found there into your calculations.
     
  4. TheSpArK505

    Thread Starter Member

    Sep 25, 2013
    92
    0
    the question says that a 50 KVA , 2400/240 V, 60 Hz before mentioning any load .....Where does the 50 KVA come from ?
     
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    And did you miss the lecture that explained VA ?

    Also known as VARs or Volt-amps reactive.
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    VA is volt-amps which is just the product of the volts and amps that the transformer is rated for. It is the phasor (phase vector) sum of the real and reactive power.
     
  7. The Electrician

    AAC Fanatic!

    Oct 9, 2007
    2,281
    326
    Volt-amps (VA) are not the same as volt-amps reactive (VAR).
     
  8. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    You are quite right I misspoke in haste.

    The proper story is explained here very well.

    http://en.wikipedia.org/wiki/AC_power
     
  9. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,346
  10. TheSpArK505

    Thread Starter Member

    Sep 25, 2013
    92
    0
    Thanks guts......some of u miss understood me ...my question that there is no load.....where does the 50 KVA come from?
    also..does the KVA = Vp,rated*Ip,rated=Vs,rated*Is,rated
     
  11. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    The 50 KVA is the transformer maximum rating. That actual KVA depends upon the load. If there is no load the KVA is essentially zero.
     
  12. TheSpArK505

    Thread Starter Member

    Sep 25, 2013
    92
    0
    OK ...But is it true that KVA = (Vp*Ip)both rated=(Vs*Is)both rated
     
  13. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    Neglecting the small transformer losses, yes.
     
Loading...