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Power in alternating current

  1. Feb 5, 2010 #1
    Hi, recently I've bought a printer with its input voltage of 110V. Well, my home provides 220V instead, so I started thinking about getting a transformer.

    The printer specs (HP P1005) says that:

    Required input voltage: 110 to 127 V (±10 percent), 60 Hz (±2 Hz), 7 amp
    Power consumption: Active: 315 watts Ready: 3 watts Sleep: 3 watts

    Does that mean that I need to get a transformer that supports P = 127*7 = 889 VA ? or I would need just a 315 W (VA?) transformer?
  2. jcsd
  3. Feb 5, 2010 #2


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    Probably you would get a 1000 watt transformer.

    But you should get realistic here.

    A 1000 watt transformer is probably going to cost you a lot of money. Maybe more than the printer is worth.
    If your supply is 220 volts it is probably 50 Hz and if it is, this is not within the +/-2 Hz in the specifications

    Also, the first toner cartridge is also going to cost you heaps. The black cartridge is worth about US$68.

    I would ditch this printer and get one locally for 220 volts and get one with alternative sources of ink cartridges.
    Last edited: Feb 6, 2010
  4. Feb 5, 2010 #3
    It's not so easy as it seems.
    Here in my country the printers are not as cheap as in the US due to import customs etc.

    and here the power is generated in 60 Hz for both 110/220 V.

    I did some research and it is cheaper to buy a transformer here than a new printer.

    So is it safe to buy a 300 W transformer instead of 1000 W one?

    is 1000 VA the same thing than 1000 W?
  5. Feb 6, 2010 #4


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    The transformer would have to handle about 900 volt-amps even though the printer was using 300 watts.

    You could probably save some money by getting an auto transformer, but even so, I would think this would be a heavy and expensive transformer.

    I didn't know there were countries with domestic 220 volts at 60 Hz. Where are you?
    I know the US has 220 volts, but this is normally hard wired to large appliances like electric furnaces and 110 volts is used for small appliances.
  6. Feb 6, 2010 #5


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    I thought that 220 volt was more the norm in most countries. 220 volts in the USA is also common in 'plug-in' type equipment.
  7. Feb 6, 2010 #6
    I live in Brazil. And yes, it is few countries that use 220V at 60 Hz, I just checked http://en.wikipedia.org/wiki/Mains_power_around_the_world" [Broken] and confirmed.

    I will buy then a 1000 VA transformer. It is relatively cheap here, so it's ok.

    Could you explain to me the relation between VA and W for alternating current?

    EDIT: I know the relation between apparent power and real/reactive power from an electric circuits course, but still don't know how it works in real world applications (and I don't have the specs of the reactive power to see whether the equations match or not).
    Last edited by a moderator: May 4, 2017
  8. Feb 6, 2010 #7


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    Power is used when the AC voltage and current occur at the same time. This is always the case with resistive loads but not with inductive or capacitive loads.

    Inductive loads are quite common and include motors and fluorescent lighting. They should be corrected by adding capacitance in parallel with them, but this may not always be done in small applications.

    Being a laser printer, your printer has a heating element in it which is why it is using a lot more power than an ink-jet printer. I am not sure why it would use so much reactive current though, and why it isn't corrected for a better power factor.

    However, the printer is drawing 7 amps so your transformer has to handle 7 amps, regardless of power factor. This is the same problem the power companies have. If there is a lot of reactive current, they still have to generate it and distribute it, but they don't get paid for it.

    Note you live in Brazil. Actually, I have a 110 volt immersion heater I bought there many years ago. They seem to have a complex electrical system.

    Hi A. S. N. , interesting that 220 volts is sometimes used in the US for plug-in devices. I visit there often and have never seen one in a house and nobody I asked had seen the type of socket that was used. I found pictures of two kinds of plugs on Internet.
    Yes, 220 - 240 volts is very common (and works well ) but it seems to usually be at 50 Hz. Brazil has 220 volts but at 60 Hz.
  9. Feb 6, 2010 #8


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    The term is 'in phase'.
    It does help to use the right words when possible because it helps to avoid confusion and to relate things to received knowledge.
  10. Feb 6, 2010 #9


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    The term is NOT "in phase".

    You can have power used when the voltage and current waveforms overlap even to a small extent. They are NOT in phase then.
  11. Feb 6, 2010 #10


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    OK then - to find the power consumed, you need to know the scalar product of the V and I vectors. Or the product of the in phase components of the V and I waveforms.

    "Power is used" is a very vague clause, in any case, and could be interpreted in many ways. You could say that Power is only the same as V times I when they two waveforms are in phase.
    Like I said, let's use conventional terms from the start and avoid any confusion.
  12. Feb 6, 2010 #11


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    Maybe YOU would say:

    the scalar product of the V and I vectors. Or the product of the in phase components of the V and I waveforms.

    But I wouldn't, because it doesn't adapt to the level of the question and I would not be teaching anything.

    "Power is used" is plain English and everyone understands what that means.

    More to the point though, do you know why a laser printer would have such a poor uncorrected power factor?
  13. Feb 6, 2010 #12
    Ok, but my question isn't clear yet. Let me put it this way:

    Given only the real power of the device (300W in this case) is there any way to predict the apparent power needed to the transformer? If not, does it mean that knowing the value of the real power isn't useful for anything, right?
  14. Feb 6, 2010 #13


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    The power factor of the load does affect the voltage regulation of the transformer, but you could assume this effect won't matter in this case. The transformer will be used in a very conservative way.

    If possible, talk to the people that make the transformer and show them the specifications of your printer. They may be able to measure the actual currents required. If the reactive currents are only occasional, they may suggest a smaller transformer.

    Such a transformer here in a nice box would cost about US$200 and weigh about 10 Kg. I could buy a nice inkjet printer for less than $70 and a Laser printer for about $100.
  15. Feb 6, 2010 #14
    Funny that this printer costs ~$200 here and the transformer I'm about to buy is ~$50 (built here in Brazil).
    Why a transformer like that would be so expensive there?
  16. Feb 6, 2010 #15


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    It is the cost of wages. The printers are made in China, but the transformer and case would be made here where wages are higher.

    Quite often it is cheaper to get something replaced new than to get the old faulty one fixed.

    At $50 you could easily get a transformer.

    The water heater I got in Rio was made in Brazil and it made many cups of coffee in hotel rooms. It was beautifully made and still works.
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