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  1. #1
    dkmc is offline Diamond
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    Looking for a back-up solution for the Boiler and Tankless Hot water heater.
    Both have AC circulator pumps, vent blower, and microprocessor controllers.

    I have access to UPS backup units of sufficent KW size rating which are mostly square wave form. I was told they would run the electronics just fine but the motors would do better on real sine wave power. The motors run hot on square wave supposedly?

    Is this true? Not a good idea to use square wave power for induction motors??

    dan k

  2. #2
    macona's Avatar
    macona is offline Diamond
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    You will need at least modified sine to work well, which I believe most UPSs are.

    Better since you dont need instantaneous switchover is use a standard power inverter with batteries to switch over in the event of a power failure. I think pretty much all inverters are at least modified sine and many are pure sine now. And they are getting really cheap now.

    How much current are you pulling?

  3. #3
    dkmc is offline Diamond
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    It will be about 500-600 watts total MAX load, worst case.

    The idea was, with a UPS, I wouldn't get caught in the shower with 15 seconds till the water goes cold...and the lights are out to boot. That and with battery power, I could use a vehicle to charge the batteries and not have to run continuously. Maybe a small 3 or 5KW 120VAC generator is the better solution ??

    I can get a 1200 or 1400 watt UPS for probably $125-150....so that seemed like the low cost way to go. Most likely a Smart ups (APC). Not sure if they are mod sine wave or not.....

    dk

  4. #4
    jim rozen is offline Diamond
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    Check the starting surge for the circ pumps.

    Jim

  5. #5
    gnorbury is offline Hot Rolled
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    Presumably the tankless water heater is gas fired? If its electric, its not going to put out much heat with only 500W input. [img]redface.gif[/img]

    Based on the size of your estimated demand, I would recommend a sinewave inverter. They are kind to motors & electronics, and the price is reasonable below about 1KW. Usually available at your local RV or marine suppliers.

    Graham

  6. #6
    dkmc is offline Diamond
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    Jim......
    Already did that as opposed to just reading watt rating on the name plates.
    I plugged each into a Kill-o-watt meter and that value range is based on the highest wattage number I saw for each appliance on startup....with a bit to spare. The circ. pumps are small Grundfoss pumps, nothing with a capacitor start motor.


    Ah, here's an opportunity for a *Millacron Moment*

    Presumably the tankless water heater is gas fired? If its electric, its not going to put out much heat with only 500W input.
    gnorbury,

    You presume correctly.... :rolleyes:
    A 500 watt tankless -electric- hot water heater would be kinda useless now wouldn't it?

    Based on the size of your estimated demand, I would recommend a sinewave inverter. They are kind to motors & electronics, and the price is reasonable below about 1KW. Usually available at your local RV or marine suppliers.
    Will said sinewave inverter switch on automatically if the power fails to advert previously described cold water condition?
    If not, it ain't gonna solve the major reason to have backup......

    End of *Milacron Moment*

    I looked at the APC specs and it seems the network /server Smartups versions as well as the XL (extended run) models are all listed as -sine wave-.
    Sams club Bestups and smaller models are some sort of "stepped modified square wave"...??

    dk

  7. #7
    gnorbury is offline Hot Rolled
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    If you buy an inverter with a built-in transfer switch, it'll switch over in within a hundred or so milliseconds.

    As you've noted, computer UPS are basically just inverters with built-in transfer switches and far-too-small battery packs. Get one that will handle your desired load, and substitute a couple deep-cycle lead-acid batteries for the small gel cells they usually come with.

  8. #8
    dkmc is offline Diamond
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    Didn't realize there were auto-transfer inverters.
    Will check that out...thanks...

    Other problem with re-charging from a vehicle, most inverters AND UPS units are either 24 or 48 volts in this (1.5-2KW) size......


    dk

  9. #9
    macona's Avatar
    macona is offline Diamond
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    stepped or modified sine literally looks like this (more steps though) versus the nice smooth sine wave we all know and love.

    __
    _| |_
    | |
    ----------------
    |_ _|
    |__|

    Cheaper to produce and will run fan motors well enough. Electronics dont care since switching power supplies just convert the AC into DC right after it is filtered.

    You could also make a switchover circuit as well with a couple relays. Not immediate like a UPS but still a fraction of a second.

    Edit: Well that didnt look so good. Needs to be viewed in a monospace font like Times.

  10. #10
    HelicalCut is offline Stainless
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    Standard motors don't like square wave as others have mentioned, motors can be made to run off square wave but the sort I have seen have been PM brushless units with specially designed pole pieces. I have seen a packaging machine with single phase motors run off a 1.2Kw UPS sucessfully. Be carefull with a switchover circuit, if the delay is more than a small fraction of a cycle you could end up trying to pick up a running motor well out of phase and the surge current can be greater than starting a motor for a cycle or 2. That is why VFD's have a function to pick up a running motor. As Macona mentioned, cheaper fan motors (shaded pole) and even some expensive ones designed for speed control can run on square wave, they have so much leakage inductance that it filters out a lot of the harmonics in square wave.

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