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Topic # 183700 27-Oct-2015 11:43
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I'm considering getting an inverter so I can run some low power fans in my greenhouse from a 12V battery. The battery would be charged from a solar panel, and I'd have some kind of thermostat. I'd also consider this for emergency use, say keeping my fridge/freezer running. How about a microwave? I have telecommunications on a dedicated DC UPS.

You can get pure sine wave, modified sine wave, and unspecified. Examples:
 - Modified sine wave $170 for 300W
 - Unspecified $70 for 500W
 - Pure sine wave $380 for 350W.

Can fans, a fridge, microwave, run off all or any of those? Obviously pure sine wave is best, but I'm looking for "good enough".

Update - I'd like to refocus the question. Could a cheap inverter like the "unspecified" one above power a couple of desk fans? They're typically low current.




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  Reply # 1414525 27-Oct-2015 11:45
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Small fans maybe.

Fridge or microwave, no show. You'd probably need 1.5kW minimum for those.




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  Reply # 1414532 27-Oct-2015 11:51
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microwave, highly doubt it as most are rated at 800+ watts and thats only the output power, the input power is going to be higher due to losses, and the need to power the motor and light etc

fridge/freezer, you are probably looking at 150-400w, it will draw the most power when it turns on for the cooling cycle

a fan would be fine as they are only about 75-100w, but you could get a 12 fan that would be more efficent

better off to get a generator for those kitchen appliances

 
 
 
 


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  Reply # 1414542 27-Oct-2015 11:59
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Stick with 12V fans so you don't have losses with the conversion to 230v.  A thermostat doesn't care about voltage (as long as you keep within its rating, so 230v will switch 12v just fine).  You would have plenty of selection at a car wreckers yard - radiator or aircon fan for example which seem to have a very long life and are certainly robust enough.  I'm thinking if you run 2x 12v fans in series they get 6v each and should run slowly.

Echoing the previous poster, no show on the fridge due to the starting 'inrush' current.  Have a bit of a read here:
https://www.google.co.nz/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=refrigerator%20inrush%20current

I think even low power microwaves use 1000w or more, plus any inrush current.  Invest in s gas cooker or buy a spare BBQ gas bottle and make sure one is always full.  :)




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  Reply # 1414547 27-Oct-2015 12:07
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how much flow do you want from a fan? as a radiator fan is 1250CFM for a 10in fan at 12v, and draws 4 amps.


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  Reply # 1414550 27-Oct-2015 12:09
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  Reply # 1414551 27-Oct-2015 12:09
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Electric motors draw significantly more power on startup (up to 6x). The inverter needs to be able to supply that.




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  Reply # 1414552 27-Oct-2015 12:11
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andrewNZ: Electric motors draw significantly more power on startup (up to 6x). The inverter needs to be able to supply that.


depends on if they have soft start capabilities

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  Reply # 1414575 27-Oct-2015 12:31
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i had one in a mobile library, running off separate marine batteries and it ran laptops , lights even printers but plug a jug into it and bang.  it cost thousands but it couldnt run a jug.



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  Reply # 1414591 27-Oct-2015 12:42
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Thanks for all the thoughts and info. I considered 12V first, it's sometimes a bit tricky to find what I'm after. Maybe what I want is impractical. We already have gas cooking

The main thing I want is a small oscillating fan or two, to move heat around the greenhouse, and because a bit of a breeze encourages plants to grow stronger rather than higher - I'm told. You can get 12V fans made for in cars but they're tiny - my greenhouse isn't huge, 4m long, 2.5m wide, 2.5m high, but I'm thinking something like small desk fan size up to household fan size. Anyone know where I can get them in 12V?

I was thinking about exhaust fans, because it does get pretty hot in there. In summer you're meant to change the air every minute or so, which requires an impractically large fan. Let's say we want to change 10% of the air a minute, 2.5 cubic meters per minute, which is around 85 cubic feet per minute or 1 cubic foot per second. If my calculations are right that $40 fan from trademe would do that, but it's still 1/5 of the recommended change volume. Still, probably better than nothing :)

A radiator fan would be fine as an extrator, but I'd have to get one, mount it somehow, cut holes, etc. Sounds like more hassle than I want to go to!




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  Reply # 1414617 27-Oct-2015 13:06
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It is not (too) hard to find high-powered 12 vdc motors. If you enjoy improvising, you could always try running a 240v fan of the right size from such a motor, via a belt if not directly.
 




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  Reply # 1414623 27-Oct-2015 13:14
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Induction motors hate square wave inverters. Modified sine wave is just marketing BS for a square wave with reduced duty cycle to have the same peak as sine wave.

They will run hotter and noisier than on a sine wave.

Inverter microwaves are sweet as on an inverter or noisy power. They don't have a large transformer in them, and they don't just cycle on and off so you can use the lower power ranges on a small inverter and just take longer to cook things.

There are quite a lot of 12v cieling fans on aliexpress. Perhaps you could get several and then just cycle them on and off to simulate oscilation?




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  Reply # 1414626 27-Oct-2015 13:17
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http://www.trademe.co.nz/motors/car-parts-accessories/other-accessories/auction-971876416.htm

your greenhouse is the same size as mine, i just open the roof vents and a couple of the ground vents and it creates its own airflow

where are you finding this information on air changes? im pretty sure none of the commercial growers that use green houses change air that frequently. ive never done it except as mentioned above and i havent seen temps ovr 40 in there and what was with the vents shut. in the peak of summer the vents are open all the time

i think you are focusing on the wrong type of things for your plants

also what defines a stronger plant? what benefits do they provide?



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  Reply # 1414629 27-Oct-2015 13:20
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Rikkitic: It is not (too) hard to find high-powered 12 vdc motors. If you enjoy improvising, you could always try running a 240v fan of the right size from such a motor, via a belt if not directly.
 


I want something easy, clean, and compact. It's not a big greenhouse so any big spinning fans would be really intrusive. One of those inline type fans would be the kind of thing I'd look at, but probably a bigger one.




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  Reply # 1414631 27-Oct-2015 13:22
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timmmay:
I want something easy, clean, and compact. It's not a big greenhouse so any big spinning fans would be really intrusive. One of those inline type fans would be the kind of thing I'd look at, but probably a bigger one.


Look for bilge blowers perhaps? No idea how they would go for continuous use tho.




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  Reply # 1414632 27-Oct-2015 13:23
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Jase2985: http://www.trademe.co.nz/motors/car-parts-accessories/other-accessories/auction-971876416.htm

your greenhouse is the same size as mine, i just open the roof vents and a couple of the ground vents and it creates its own airflow

where are you finding this information on air changes? im pretty sure none of the commercial growers that use green houses change air that frequently. ive never done it except as mentioned above and i havent seen temps ovr 40 in there and what was with the vents shut. in the peak of summer the vents are open all the time

i think you are focusing on the wrong type of things for your plants

also what defines a stronger plant? what benefits do they provide?


I have a book on greenhouse design, also read it online. It has no roof vents, it's a lean to style greenhouse and wasn't practical. I have three self opening windows and doors at both ends that I can open, but in summer it still gets well up into the high thirties or even the forties. I was thinking a fan might help change the air a bit more quickly.

Regarding stronger plants, what I was told was if there's more air movement the plants don't grow as tall. That's a problem, I get lots of really tall plants that aren't very strong, which is annoying if I start indoors and then move outdoors. Plus my tomatoes get just massive, I have to constantly trim them.




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