Modified UPS as inverter


Sorry to bother again.
I am looking for a UPS or an inverter. I don't have real experience with these things, I only know how they work. I read Wind-Sun tech note on inverter choosing and tried surfing a few hours, but can't really find what I need to know.

This is how I want to use a UPS ( or an inverter ).
I need to run a chest freezer ( probably 150 - 200 watts unit ) 12 hours a day on a battery bank and the other 12 hours on 230v 50hz. The freezer is 230V 50hz model.

I asked a local UPS manufacturer if their design or unit can run a compressor based equipment. They said no. I will ask some other reputable imported brand like APC next Monday. Why I wanted an offline UPS is because, basically a UPS is an inverter with small amps charging capability and built in batteries..this is my understanding on it, I hope I am right.

I thought of using a UPS for its inverter section and its +-20 milli seconds switching transfer capability. I will disable its charging section, remove its battery and hook it up to the 24V domestic battery bank. The idea is to have 24/7 freezer running regardless of power source and without needing manual switching by a person. The vessel will be generator powered once the domestic battery does below a preset limit. This will be the standard drill for the crew to follow.

I talk to my friend who is in HVAC business. He adviced me that any refrigerant compressor, will not stand up to repeated short duration ( 1 second or more ) power outage. I need to wait for about 3 minutes to allow refrigerant pressure equilization before powering up again. Newer freezer has this "delay upon repowering" safety function, older one with simple thermostat doesn't.

I initially wanted to use Steca PF 166 12/24V DC freezer, but today it cost US$3300+ landed Jakarta for a 166 liter model. Shipping and 30% import duty is too much. Steca is Sundancer or something in the USA market. So DC freezer is off the list and 230v ones at US$400 or so seems a better deal. Besides a freezer life in boats will be shortlived, under 4-5 years to be optimum, usually corrosion death.
If the boat sees bad weather often, the life will be shorter. Small compressor does not have special oil pan to allow good oil suction during repeated angle change due to heeling and so on of a boat in bad weather. The bigger compressor for marine air-cond has that special oil pan, the same with marine engines. So I must choose sort of "disposable freezer".

Does my idea make sense, modfying a UPS for this purpose ?
Or perhaps the other way round, an inverter with 20 milli seconds switching transfer capability once it sense 230V is back on line ?

If inverter is the choice, what model and brand have you guys experience as robust and proven to take a 200watt freezer compressor kind of load repeatedly switched.

I never really tested the starting current surge needed by a 200 watt refrigerant compressor but have done study on squirel cage induction motor and 500-600% starting surge current when direct online starting without using a soft starter is true, that is the basic design of such motor. And it seems refrigerant compressor has the same charateristic, this is what I gather from what I been reading.

I am ready to have to live with the fact that I may need a bigger inverter to cope with starting current surge but loose efficiency because of only 200 watts needed.

Thanks a lot guys...



  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Modified UPS as inverter


    All but the most expensive, 'continuous converting' UPS device are made for occasional, short-term use only. If used as a full time inverter, they will fail quite rapidly.

    Your best bet here is to go with something like a Xantrex XW series, which can proved continuous 240 VAC pure sine wave which will keep the motor "happy" (AC induction motors do not like modified sine wave - shortens their life). It also has AC input capability which will automatically charge the inverter bank while powering any loads. But you would need a "European version" to provide the 50 Hz frequency. I don't know if Xantrex has these (others on this forum will know for sure!) Outback does have such "World" models:

    But they are somewhat "pricey" per Watt compared to Xantrex. Also, they are not They also make models specifically for marine environment, but I think they are all 60 Hz. They tend to require more external "support" hardware too (i.e. a Mate controller to program with).

    Be sure you know the actual wattage of the freezer, as the start-up current is several times the run current and the inverter must be able to handle both. For a freezer around 200 Watts this shouldn't be a problem though.

    Stand by for further answers! :D
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Modified UPS as inverter

    i agree on the short term usage for an ups so get yourself a sinewave inverter and note that the surge on a 200w freezer can hit 1kw or more. there are such good sinewave inverters out there for 230vac use as you will just have to research them as to price, availability, warranty, etc.
  • IyaIya Solar Expert Posts: 32
    Re: Modified UPS as inverter

    You are a bunch of sweet guys.........;)

    I just read thru the Manual for FX2024ET ( 230V at 50hz spec ) and just emailed Outback. There is something I am not clear about and hope they get back to me next Monday.

    AA. When the FX sense 230V from the generator is available, page 16 of the manual only stated that it can be program to delay the connection to warm up the generator. What I need to know is, how fast is the switching between an FX being an inverter to an FX allowing direct 230V ( bypass ) to the freezer ?

    BB. When the FX becomes a charger, does the inverter section of it still working simultenously or its 230V output is purely a generator sourced alias transfer switching occured ? I am thinking that the FX may have programming features being like an on-line UPS kind of charateristic.

    I have read over and over again the manual but I can't find any info that can clearly answer the two questions. Funny, the export version did not state the charger power like the US model which is 55 amps. Asked that too to Outback, must make sure.:p
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Modified UPS as inverter

    The switch over can be delayed or it can be instantaneous. There is no noticeable lag in the switching. I have an FX 3524 and there is never so much as a 'blip' on the computer when it switches between generator source and batteries in either direction.

    When operating in 'charger' mode, AC loads are passed through to the AC source: the inverter acts only as a charger (there's not much sense in down-converting AC to DC to charge batteries and then up-converting it back to run AC loads when you can just run the loads directly from the AC source).

    From our host NAWS site:
    FX2024 has a 55A charger
    VFX3024 has an 85A charger

    NOTE: if you're going to run one of these off the boat's 24V system, you will probably need to upgrade the batteries and the charging capacity. 2000W @ 24V = 85A (approximately). That's potentially a lot more strain on the existing system.
  • BB.BB. Super Moderators, Administrators Posts: 30,735 admin
    Re: Modified UPS as inverter


    I am sorry--but I do not have any better answer than what you have seen from the manuals. Outback, or others will have to help with the details.

    The switch over is typically less than 16 milliseconds (0.016 seconds) and should be fine for a fridge/freezer.

    You have options of "all-in-one" systems (like the Xantrex inverter/chargers with internal automatic transfer switches)--Or you can build your own out of components (inverter, charger, external transfer switch).

    The problem with many of the all-in-one systems is that they many times have pretty tight tolerances on the input voltage and frequency for proper operation. The Xantrex XW Hybrid Inverter may be a better choice from that point of view--but is probably way over kill (in power and price) for your application.

    If you cannot find a nice Inverter/Charger/Transfer switch all in one package that meets your needs exactly... I would probably go with the "piece part" route and get a good quality inverter, a power factor corrected charge controller (with remote temperature sensor) and a separate auto transfer switch.

    Will be more wiring/work to design and install--but probably, in the long term, easier and cheaper to maintain. Any one item fails, and you repair/replace that piece--not the whole A-i-One.

    It may also give you options when you have failures so that you can work around them until you return to port (run the AC genset more or use the battery bank/DC gen power, etc.).

    And you can manually decide if you want to run the inverter in "transfer mode" or continuous conversion (i.e., on AC power then transfer to Inverter, or use the AC power to always charge the battery bank, and the Inverter to always power the freezer--does provide for better protection against power surges).

    Continuous conversion is usually more wasteful of energy (AC to DC to AC, vs AC only when on shore power) and may be overkill as a requirement for a simple freezer. If you had other loads (desktop computer, laptop, nav computer, audio/visual entertainment center) -- then continuous conversion may be a good alternative with dodgy shore power.

    I am not sure that you could run a "all in one" inverter/charger as continuous converter (heat issues) unless you found one designed for the purpose (more of a true computer UPS than a solar based RE type).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • IyaIya Solar Expert Posts: 32
    Re: Modified UPS as inverter


    Thanks for the info...needed that.

    For now the user for the 2000watt inverter is only the 200watt freezer and some small things which I shall limit to no more than another 300 watts intermittent. Next candidate will be the PC but this is only when needed. I shall wire inverter output to a few dedicated 230V receptacles only, not into the grid and will put 2A MCB for all other receptacles except the freezer.

    I know its a waste of a 2000 watts inverter power but the freezer is the main goal. The food for the entire crew while not on charter will be in that freezer and running generator 24 hours a day is too expensive on the fuel and generator life.
    Non charter days the generator used will be a small 3-4Kw diesel perhaps runned 8 hours a day.

    If that export model FX pumps out 55 charging amps and it is supposedly a robust inverter, the price is not too bad. Stand alone marine charger at 55 amps/24V can run US$700+ for Newmar. I am kind of unhappy with Newmar for the last two installations.

    When I know all the fine details of other 24V consumers and the inverter, I shall work out the battery bank size, 4 hours bulk charging rate at 20% loss and plus MUST DO 3 hours float and may buy another 30 amps battery charger to complement and serve as back up.

    My original intention wass to have a +-95amps charger but the FX seems like a good choice now because I need the inverter section of it and its good to try something else aside from Newmar.

  • IyaIya Solar Expert Posts: 32
    Re: Modified UPS as inverter

    OK BB, I shall do more thinking and reading on the inverter thing. 16 milli seconds speed of the switching transfer is OK for the freezer. My HVAC friend said 1 second or more is bad.

    I saw the XW Hybrid...dang its so big even the smallest model.

    Yep, battery temp sensor is a must and is there any way to test it to know that it is accurate by using a DVM ? Its a resistance thing no ?

    One Newmar I installed, use the BTS but I think the sensor was a bad batch and made the charger sort of dumb.

    One aircond I had worked on, also got the same temp sensor problem and was 5 degrees *C , but I can tell because there is a display to read the values.

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Modified UPS as inverter

    The voltage reading on the FX can be adjusted (through the Mate programmer) to match an accurate DVM reading at the batteries. If there is a difference due to wiring length this can be compensated for. The BTS will change the reading when it's plugged in (unless it happens to be at the exact calibration temp). Unless it's actually defective you only need to 'align' it once, as the differentiation is relative.
  • mike95490mike95490 Solar Expert Posts: 8,873 ✭✭✭✭✭
    Re: Modified UPS as inverter

    For running ONLY a small freezer, the Morningstar Sure sine may be able to start the compressor, it has a 10 min 600W surge, and 300W running.

    Or consider the Honda EU-1000 inverter generator. (I don't know if you can get a 240V model) It has 700W cont rating.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    gen: ,

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Modified UPS as inverter
    mike90045 wrote: »
    For running ONLY a small freezer, the Morningstar Sure sine may be able to start the compressor, it has a 10 min 600W surge, and 300W running.

    Or consider the Honda EU-1000 inverter generator. (I don't know if you can get a 240V model) It has 700W cont rating.

    Um, not on a boat. :p

    And the reason for the Outback is the need for "European" power: 230 V @ 50Hz :D

    If it weren't for that, this project could be a lot cheaper, eh? :p
  • BB.BB. Super Moderators, Administrators Posts: 30,735 admin
    Re: Modified UPS as inverter

    Has anyone ever seen a nice, quiet, fuel efficient diesel genset at 2kW capacity or less (marine or land)?

    The problem with most gensets is that they are two large for the load for efficient operation (typical gensets need to run at about 50% load for efficiency and, for diesels, proper combustion).

    Iya, your idea to use your "larger" diesel genset to charge the battery bank should be a real fuel/noise saver.

    Design the battery charger+battery bank (plus other loads) to run at 50% (or more) of generator capacity, and then run the rest of your loads (freezer, lights, crew "goodies", etc.) off of the battery bank+inverter.

    Depending on the ratio of generator capacity to load (and how much battery bank you can justify)--you may end up with a really nice solution for everyone (including paying passengers that would enjoy quiet time at anchor).

    This "Hybrid" system has been used in Africa (run the genset in the evening when the loads are heavy+charge battery bank, and run the inverter all other times for lighting and other small loads).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: Modified UPS as inverter

    well the BEST quietest marine diesel generators are made by Northern Lights in Alaska.. but dont know if they have one as small as 2kw I know they have a 4.5kw unit.. very fuell efficient and quiet
    Have used many of the larger ones.. they go forever or even longer than that.:cool:
  • IyaIya Solar Expert Posts: 32
    Re: Modified UPS as inverter

    Sorry guys, I forgot to explain the generator set up.

    Two main generators will be marine units with wet exhaust and sea water cooled heat exchanger type, a 24 kw and 20 kw placed in the engine room. This are the "business" generator during a charter. Vessel will have decently high electrical load during a charter, airr conds, water maker and at least a 3.5kw diving air compressor.

    The 3-4Kw I mentioned is an air cooled non marine ones, could be real waterlesss air cooled or with a radiator, which will be placed somewhere behind the helmstation rear open air deck. All generator will be diesel because main fuel source will be diesel. This one need not be too quiet since no guest will be on board when this generator on duty. Small generator can't be too efficient, that is very true.
  • RCinFLARCinFLA Solar Expert Posts: 1,302 ✭✭✭
    Re: Modified UPS as inverter

    The small diesel generators are often 3600 rpm engines. They probably don't last too long, at least in comparision to 1800 rpm engines.
  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: Modified UPS as inverter

    I agree if you can get one of the more heavy duty 1800 rpm ones thay last a lot longer.. not just because they rev slower but generally they made stronger.. but they are heavier.
  • Ralph DayRalph Day Solar Expert Posts: 920 ✭✭✭✭
    Re: Modified UPS as inverter

    Ralph's rule of thumb...if you have an off-grid setup and can move your generator without help, it's too small :p

    (modified from: If you can move it without a forklift it's too small)

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