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Idle current on a 500watt transformer ?

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by john1, Aug 23, 2003.

  1. john1

    john1 Active Member

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    Hi,

    could someone give me some idea what idle current to expect from a
    transformer on my 240 volt house mains ?

    I am thinking of a 500watt or maybe a bit bigger if the idle current is
    fairly low, maybe a 1000watt if the idle current is low.

    Is it possible to reduce the idle current by matching a capacitor to
    the transformer ?

    Cheers, John :)
     
  2. Sebi

    Sebi New Member

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    The idle current strongly depend from iron core quality and assembly.
    (one old trafo with M-core 80VA suck 75mA, the modern 75VA trafo with welded E-I core sucks 98mA!)
    An 500VA transformer can suck about 500...800mA.
    Yes, You can compensated with condenser the reactive part of current
    (when measured the current with multimeter, and apply a paralel cap.
    with primary winding, the current decreased,because it compensate
    the cos-fi.) But this method not better for You, because the motor-type
    kilowatt-hour-meter no measure the reactive part.(with compensation
    Your costs higher...)
     
  3. john1

    john1 Active Member

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    Hi Sebi,

    Thanks for the info.
    I am surprised that you say that a 500 W transformer can
    draw up to half an ampere with no load!

    I would be using the older type construction with removable
    laminations, and i would be packing them tightly with minimum
    air gaps, as i understand these reduce the overall efficiency.

    But i did not know that the lamination shapes had any effect.

    Do you mean that the 'E' & 'I' type would be more efficient
    than the 'T' & 'U' type, i have seen closed types that have
    the middle section made like a tongue, and you sort of slot
    them over and in, when in there are no joins around the edges.

    What about toroidal types, are they much better ?

    I can't see me wanting to leave a half an ampere running
    continually on my house mains.
    I did not realise the idle current would be that much.

    As for the capacitor correction,
    i actually thought that the inclusion of a cap
    would improve the current/voltage angle, and reduce the peak
    current demand.
    You say the KWH meter does not cover this ?

    I suppose their costing is on current only?
    Maybe their profits are more if they charge for 'out of phase'
    or unused current.

    Regards John :)
     
  4. dave

    Dave New Member

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  5. john1

    john1 Active Member

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    Hi,

    I'm also trying to find out about 'Ferroxcubes' without much
    success. Do you think they would have a lower idle current ?

    I'm thinking of feeding some of my lighting circuit with a
    slightly lower voltage because i am getting too many lamps
    burn out, the voltage is around 240, i intend to drop it to
    about 220. The lamps are not quite as bright at that, but not
    noticeably dimmer. I am hoping they will last longer.

    I was thinking of a transformer with '240 in, 220 out' but i
    am now wondering if i could just use a 20v winding, 240 in
    20 out, and just put the 20 volts in series with the supply.

    That would mean instead of looking for 500watts i would be
    looking for about 60 or so. And the idle current would be
    less. ... if i can do it like that??

    My brain hurts now.

    Cheers, John :)
     
  6. john1

    john1 Active Member

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    Hi,

    I'm not sure about the way the wattage works out with the
    'autotransformer' type of arrangement, if thats what it is...

    Any suggestions?

    John :)
     

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  7. john1

    john1 Active Member

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    Hi,

    This will probably be the arrangement that i try to do,
    i am guessing that the transformer would only handle
    the power difference from the 240 to the 220.

    That would mean that the transformer could possibly be
    handling about 40 per cent of the power transferred,
    thats just a guess. (maybe less ?)

    So how could i estimate the idle current ?

    John
     

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  8. Sebi

    Sebi New Member

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    Hi, John

    sorry for the late answer, i'm surprised if my computer clock and date go to 2002 jan. i can't see the new posts.
    And now understand completely what You need....
    You need only 240V minus 20V with 500VA capability.
    The current only about 2.2A.
    20V 2.2A only about 50VA, so You need a 240/20 (24)V 50VA regular
    transformer, the secondary serial in load with reverse phase.
     

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  9. john1

    john1 Active Member

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    Thanks Sebi,

    So that approach is a workable set-up then ?
    From what you say, a fifty watt transformer should cover it.
    I reckon thats about 50mA idle current, roughly.

    Would it be difficult to make some sort of sensor, to switch
    it off when no lights are on?
    I have been thinking about this for a while, if it could be
    turned off when the lights are off, it would save a bit on
    the idle current.

    Still 50mA may not be too bad, i will try to work it out for
    the price of a quarter (three month period) at ten pence per
    unit (Kilo-Watt Hour), maths not my strong point !!

    Cheers, John :)


    did you say you set back the date on your PC, and this
    stopped you seeing the new posts ? Are you on 98se ?
    I will have to watch out for that, i often alter the date
    to use CDs that have date-expired.
     
  10. Sebi

    Sebi New Member

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    Yes, if You switched of the primary of trafo, the secondary works
    as a piece of wire so the output is 240V. For current sensing - i think -
    the simply method a current-relais. Do it yourself from any small relais
    with new winding (coil). (wire diameter about 0.5mm, and enough turns for minimum load sensing...)

    P.S. I use XP with a defected motherboard. It have only a small failure:
    no sense the backup battery, and without mains power forget the setup.
     
  11. john1

    john1 Active Member

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    Hi Sebi,

    You're using 'XP' ... ?
    http://www.hevanet.com/peace/microsoft.htm

    The thing with current sensing is that the initial supply would be
    at the full voltage. The lamps usually blow at switch-on anyway.

    I was thinking maybe the current sensing could be on the lighting
    circuit at a very low voltage, until lighting is switched on.
    Then the transformer could be switched in, without the lamps ever
    getting the higher voltage, but i haven't figured a way yet !!

    I am still looking at this, and i haven't figured the cost yet
    either.

    10p/KW Hr for 3 months at @ 50 mA,
    thats if i leave it on.

    John :)
     
  12. Sebi

    Sebi New Member

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    Maybe simplyer if You apply NTC serial with lamp(s). PC-PSU also contain it for initial current reducing.
     
  13. john1

    john1 Active Member

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    Maybe ...

    What is it ... ?
     
  14. john1

    john1 Active Member

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    well, i have fixed up a small transformer, 20 watts, to the lamp
    in my kitchen. its giving me a 15 volt drop to about 220 volts.

    The lamp is not noticeably dimmer, the transformer uses about
    35mA idle current, but i have it go on with the light switch,
    so its not on all the time.

    If this lamp lasts an appreciably longer time, i shall extend the
    arrangement to more of my lighting circuits.

    And i should try to work out how much 35mA would cost for 3 months
    at about 10p per kW/H.

    I'll see how it goes, Cheers, John :)
     
  15. Roff

    Roff Well-Known Member

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    I get about 1 pound 80 pence.
     
  16. john1

    john1 Active Member

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    Thanks Ron,

    I havent got it down to an equation yet!
    By the way, ... could you tell me anything about NTC ?
    (from three posts back)
    I'm having a hard time pinning it down on google.

    John :)
     
  17. Sebi

    Sebi New Member

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    The NTC (Negative Temp.Coefficient) resistor can limiting the initial current.
     
  18. john1

    john1 Active Member

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    follow-up

    Hi,

    I've been replacing most of the lamps in my house with
    the 'low-energy' types.

    They work fine and i've only had one fail in a few years.

    John :)
     
  19. Klaus

    Klaus New Member

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    NTC = negative temperature coeficient.

    The device DECREASES its resistance as it gets hotter - resistors do the opposite. It looks like a small disk with two leads, a bit like a fat ceramic capacitor.

    Klaus
     
  20. Phasor

    Phasor Member

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    As far as I am aware, the vast majority of domestic installations are charged per kWh, not Ah (I don't know about your location, but certainly in Australia this is the case). Normal electricity meters DO take account of phase angle, and VAr's are not charged, only 'real' watts.

    Thus, if you have an idle current of 500mA, at a phase angle of, say, 80 degrees, you are only charged for 86mA (ie, cos 80 * 0.5A).
     
  21. john1

    john1 Active Member

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    Hi,

    Thank you all for your help and advice,
    i've decided to go with the 'long-life' lamps,
    instead of reducing the lighting supply voltage by a percentage.

    I still feel that a small voltage reduction would give a much
    longer life to filament lamps, but using these new lamps is so
    much easier.

    In case anyone is interested, i have found that a reduction in
    voltage which can barely be noticed in loss of brightness will
    give a significant extension to the life of the lamp.

    Best regards, John :)
     

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