Inverter question.

Discussion in 'NZ Computing' started by NRENNIE, Dec 10, 2006.

  1. NRENNIE

    NRENNIE Guest

    Hi there,

    I'm wanting to power a small appliance by using an inverter connected up to
    a battery whenever we get a power cut.

    The device is rated at 200VA. Can anyone tell me what size inverter I will
    need to buy to saftely run it please.

    Does 200VA = 200 watts?


    Thanks,

    Nick.
    NRENNIE, Dec 10, 2006
    #1
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  2. NRENNIE

    Adam Guest

    NRENNIE wrote:

    > Hi there,
    >
    > I'm wanting to power a small appliance by using an inverter connected up
    > to a battery whenever we get a power cut.
    >
    > The device is rated at 200VA. Can anyone tell me what size inverter I
    > will need to buy to saftely run it please.
    >
    > Does 200VA = 200 watts?
    >
    >
    > Thanks,
    >
    > Nick.



    Basically, yes. But in practice, it can be problematic to assume that.

    If you can get a better rated inverter, say 250 300 350VA, then so much the
    better, for nominal 200VA.

    Some things like fridges etc, probably with motors in them, can require
    inverters with double their VA rating to get them going. Less of an issue
    with solid state stuff, where the inverter rating can come closer to the
    power rating of the device.

    Using a 200VA inverter for a device rated at 200W, might not be best
    practice. But might work fine. Try it.

    VA is used instead of Watts where power factor is an issue, like with
    significant inductance of coils. Watts is best applied to power in
    purely resistive loads. Watts derived from VA for complex loads.
    Adam, Dec 10, 2006
    #2
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  3. NRENNIE

    thingy Guest

    NRENNIE wrote:
    > Hi there,
    >
    > I'm wanting to power a small appliance by using an inverter connected up to
    > a battery whenever we get a power cut.
    >
    > The device is rated at 200VA. Can anyone tell me what size inverter I will
    > need to buy to saftely run it please.
    >
    > Does 200VA = 200 watts?
    >
    >
    > Thanks,
    >
    > Nick.
    >
    >


    Depends on the device, some can have a huge start load, something that
    could last as little as a few milli-seconds, but enough to blow the
    inverters rating...and probably the inverter.

    I have a 3kva unit with about 1kva of batteries....enough for some
    lights and a tv / pc, but nothing like cooking...if you wan that look at
    a log burner, you can put pots and kettles on top.

    Post what you want to try and run.....

    regards

    Thing
    thingy, Dec 10, 2006
    #3
  4. Re: Inverter question

    In message <elgc3j$qs5$>, NRENNIE wrote:

    > Does 200VA = 200 watts?


    I think "VA" means "peak volts multiplied by peak amps". Whereas the wattage
    rating would be average power over a complete duty cycle.

    In AC, if the voltage wave and current wave are exactly in phase, then you
    have what's called a "pure resistive load", or a power factor of 100%. In
    this case (assuming both waves are sinusoidal) the VA value would equal the
    wattage.

    But if they're exactly 90° out of phase, then you have a "pure capacitive"
    (if the current phase is ahead of--leads--the voltage) or a "pure
    inductive" (if the current phase is behind--lags--the voltage) load. The
    power factor is 0%, and the wattage is 0.

    If the phase is something in-between, then the factor varies accordingly.
    Lawrence D'Oliveiro, Dec 10, 2006
    #4
  5. NRENNIE

    NRENNIE Guest

    It's just a small ventilator. I've got sleep apnea and as we have frequent
    power cuts here on the Thames Coast it would be easier using an inverter
    rather than getting out the generator every time. Most power cuts aren't
    too long. The thing is basically just a little air pump.

    Might get a true sine wave inverter - don't want to muck up its electronics.

    Thanks for your replys!

    Nick

    "NRENNIE" <> wrote in message
    news:elgc3j$qs5$...
    > Hi there,
    >
    > I'm wanting to power a small appliance by using an inverter connected up
    > to a battery whenever we get a power cut.
    >
    > The device is rated at 200VA. Can anyone tell me what size inverter I
    > will need to buy to saftely run it please.
    >
    > Does 200VA = 200 watts?
    >
    >
    > Thanks,
    >
    > Nick.
    >
    NRENNIE, Dec 10, 2006
    #5
  6. NRENNIE

    David Guest

    thingy wrote:
    > NRENNIE wrote:
    >> Hi there,
    >>
    >> I'm wanting to power a small appliance by using an inverter connected
    >> up to a battery whenever we get a power cut.
    >>
    >> The device is rated at 200VA. Can anyone tell me what size inverter I
    >> will need to buy to saftely run it please.
    >>
    >> Does 200VA = 200 watts?
    >>
    >>
    >> Thanks,
    >>
    >> Nick.
    >>

    >
    > Depends on the device, some can have a huge start load, something that
    > could last as little as a few milli-seconds, but enough to blow the
    > inverters rating...and probably the inverter.
    >
    > I have a 3kva unit with about 1kva of batteries....enough for some
    > lights and a tv / pc, but nothing like cooking...if you wan that look at
    > a log burner, you can put pots and kettles on top.
    >


    A small gas stove might be more convenient depending on how often you
    have power cuts (I wouldn't want my wood burner going in the middle of
    summer)

    > Post what you want to try and run.....
    >
    > regards
    >
    > Thing
    >
    David, Dec 10, 2006
    #6
  7. NRENNIE wrote:
    > It's just a small ventilator. I've got sleep apnea and as we have frequent
    > power cuts here on the Thames Coast it would be easier using an inverter
    > rather than getting out the generator every time. Most power cuts aren't
    > too long. The thing is basically just a little air pump.
    >
    > Might get a true sine wave inverter - don't want to muck up its electronics.
    >
    > Thanks for your replys!
    >
    > Nick


    Hi Nick,

    Your consideration of a sine wave inverter is wise, however 'electronics' s
    less likely to be upset than mains driven motors or transformers which are
    seriously incompatible with the vastly more common square wave inverter.

    A sine wave inverter should work fine with almost any load.

    I generally keep the load on an inverter to a maximum of half it's rating to
    allow for inrush currents, power factor, and to improve reliability.

    A UPS (Uninterruptable Power Supply) may suit you better as it'll give you no
    break power so you won't even need to get up for a short outage (or a long one
    if you hook up larger batteries). If you buy a used one (which are often
    available for peanuts) you'll almost certainly need to replace the batteries.

    Look for UPSes that hold their batteries bottom down for long battery life.

    Depending on the design of your appliance it may be feasible to hook a battery
    up to it directly.

    I have some reservations about using equipment not certified for the purpose in
    this life support application. Perhaps it'd be worth ringing up the Medical
    Electronics people at Auckland Hospital or the supplier of the ventilator for
    some advice.

    All the usual disclaimers apply.

    Mark


    > "NRENNIE" <> wrote in message
    > news:elgc3j$qs5$...
    >> Hi there,
    >>
    >> I'm wanting to power a small appliance by using an inverter connected up
    >> to a battery whenever we get a power cut.
    >>
    >> The device is rated at 200VA. Can anyone tell me what size inverter I
    >> will need to buy to saftely run it please.
    >>
    >> Does 200VA = 200 watts?


    This depends on the nature of the load. For a light bulb or a heater, yes. For
    motors, discharge lights, anything with a transformer or anything with a switch
    mode power supply then no.

    As stated elsewhere a non-resistive load changes the phase relationship between
    the applied voltage and the current drawn. As this difference increases the VA
    (Volts times Amps) increases confusingly exceeding the power drawn in Watts
    (Volts times Amps). The Power Factor is the ratio of VA to Watts.

    http://en.wikipedia.org/wiki/Power_factor

    >>
    >>
    >> Thanks,
    >>
    >> Nick.
    >>

    >
    >
    Mark Robinson, Dec 10, 2006
    #7
  8. NRENNIE

    thingy Guest

    Mark Robinson wrote:
    > NRENNIE wrote:
    >> It's just a small ventilator. I've got sleep apnea and as we have frequent
    >> power cuts here on the Thames Coast it would be easier using an inverter
    >> rather than getting out the generator every time. Most power cuts aren't
    >> too long. The thing is basically just a little air pump.
    >>
    >> Might get a true sine wave inverter - don't want to muck up its electronics.
    >>
    >> Thanks for your replys!
    >>
    >> Nick

    >
    > Hi Nick,
    >
    > Your consideration of a sine wave inverter is wise, however 'electronics' s
    > less likely to be upset than mains driven motors or transformers which are
    > seriously incompatible with the vastly more common square wave inverter.
    >
    > A sine wave inverter should work fine with almost any load.
    >
    > I generally keep the load on an inverter to a maximum of half it's rating to
    > allow for inrush currents, power factor, and to improve reliability.
    >
    > A UPS (Uninterruptable Power Supply) may suit you better as it'll give you no
    > break power so you won't even need to get up for a short outage (or a long one
    > if you hook up larger batteries). If you buy a used one (which are often
    > available for peanuts) you'll almost certainly need to replace the batteries.
    >
    > Look for UPSes that hold their batteries bottom down for long battery life.
    >
    > Depending on the design of your appliance it may be feasible to hook a battery
    > up to it directly.
    >
    > I have some reservations about using equipment not certified for the purpose in
    > this life support application. Perhaps it'd be worth ringing up the Medical
    > Electronics people at Auckland Hospital or the supplier of the ventilator for
    > some advice.
    >
    > All the usual disclaimers apply.
    >
    > Mark
    >
    >
    >> "NRENNIE" <> wrote in message
    >> news:elgc3j$qs5$...
    >>> Hi there,
    >>>
    >>> I'm wanting to power a small appliance by using an inverter connected up
    >>> to a battery whenever we get a power cut.
    >>>
    >>> The device is rated at 200VA. Can anyone tell me what size inverter I
    >>> will need to buy to saftely run it please.
    >>>
    >>> Does 200VA = 200 watts?

    >
    > This depends on the nature of the load. For a light bulb or a heater, yes. For
    > motors, discharge lights, anything with a transformer or anything with a switch
    > mode power supply then no.
    >
    > As stated elsewhere a non-resistive load changes the phase relationship between
    > the applied voltage and the current drawn. As this difference increases the VA
    > (Volts times Amps) increases confusingly exceeding the power drawn in Watts
    > (Volts times Amps). The Power Factor is the ratio of VA to Watts.
    >
    > http://en.wikipedia.org/wiki/Power_factor
    >
    >>>
    >>> Thanks,
    >>>
    >>> Nick.
    >>>

    >>


    A PC UPS sounds the best bet IMHO. You can usually add more batteries,
    even basic car ones and get a lot more on line time, but the inverters
    electronics will be the limiting factor...

    I had a chat with a mate of mine a while back on getting more uptime off
    my UPS> We decided to have a trip curcuit so that when the mains came
    back the extra car batteries were disconnected, these could then be
    re-charged using manual car chargers the next day. I have not done it
    yet though....

    regards

    Thing
    thingy, Dec 11, 2006
    #8
  9. NRENNIE

    Don Hills Guest

    In article <>,
    thingy <> wrote:
    >
    >A PC UPS sounds the best bet IMHO. You can usually add more batteries,
    >even basic car ones and get a lot more on line time, but the inverters
    >electronics will be the limiting factor...
    >
    >I had a chat with a mate of mine a while back on getting more uptime off
    >my UPS> We decided to have a trip curcuit so that when the mains came
    >back the extra car batteries were disconnected, these could then be
    >re-charged using manual car chargers the next day. I have not done it
    >yet though....


    Most small UPSes, those without any visible means of heat dissipation such
    as external fins or a fan, cannot have their run time extended by attaching
    bigger batteries. Instead of a proper heatsink, they just have a block of
    aluminium. It is sized so that the battery goes flat before it overheats.
    If you add more battery capacity, the UPS will overheat and either shut down
    (if it's well designed) or melt down (if it's not.)

    One caveat with using a small PC UPS to run the apnoea pump is that its
    runtime may not be long enough to cover a typical power outage. They're
    intended to give you a few minutes to ride out short outages, and/or to let
    you finish what you're doing and shut down.

    Also, most of them beep loudly while the mains power is off. You can usually
    cancel the beep by pressing a button, but you're awake by then anyway.
    Regarding the reliability of a PC UPS in this "life critical" application,
    having the beeper can be seen as a safety device. It'll wake you up so you
    can choose to stay awake or go back to sleep after confirming the pump is
    working OK on the UPS.

    To size the UPS, you'd need to find out the approximate power consumption of
    the pump. I doubt it'll be more than 25 to 50 watts. Then you need to work
    out how long a given UPS will run it, for example if a UPS is speced to run
    a 200W load for 10 minutes, it'll run a 50 watt load for at least 40
    minutes. You then buy a UPS that will run the pump for the desired length of
    time. If you don't want to be woken up by it you'll need to install it in
    another room or cut the leads to the beeper. Lastly, you need to check that
    the momentary delay between the UPS detecting the outage and switching to
    its internal inverter is not long enough to cause problems with the pump.
    Without the UPS, turn the pump off and on again at the wall switch as
    quickly as you can. If it can handle this momentary outage without a
    problem, then it should work OK on a UPS.

    Yet another option, if you're in the market for a new pump, would be to buy
    a 12 volt CPAP pump such as those from http://www.cpapdownunder.com.au/
    They draw about 1.5 amps (2.3 amps with heated hose). You could run it with
    a 12 volt 18 amp-hour sealed lead-acid battery and a smart charger such as
    the Jaycar MB3602. (Smart chargers can be left connected continuously, they
    measure the battery charge and switch off when the battery is charged.) The
    specified charger is only 1.25 amp rating, so the battery would run down
    slightly overnight, but you'd get an all-night run even if the mains went
    off just after you fell asleep and it would be fully charged again by
    bedtime.

    --
    Don Hills (dmhills at attglobaldotnet) Wellington, New Zealand
    "New interface closely resembles Presentation Manager,
    preparing you for the wonders of OS/2!"
    -- Advertisement on the box for Microsoft Windows 2.11 for 286
    Don Hills, Dec 11, 2006
    #9
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