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Marcus20VT Regular Member Joined: 02/09/2008 Location: WalesPosts: 49
Posted: 02:15am 01 Apr 2009
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Hi - been a while since I last visited.
And during this time, I have actually got my squit together and bought a generator to be used as a wind turbine in tandem with my small pv solar panel.
The question is: SWMBO wants a freezer in the shed, which at the moment is powered only by solar.
I'm guessing this (small)freezer would drawer about 500W as it is not particularly efficient (its graded a "C" - and I can't find the stamp on the back to tell me the exact loading!)
So - consequently I'm guessing that I would need an inverter that handles at least 500W as mine does. My inverter peaks at 600W and runs continuously at 350W. However I've seen other inverters which state that they do not support an inductive load. As mine is "soft start", I'm guessing that my inverter won't support an inductive load either - is that correct?
Also, for this 500W freezer power (max - probably) what sort of amp/hour battery would I need. I've got a 12v 50 a/h deep cycle at the mo, I would expect that I would need to double that to allow for losses, yes?
Thanks for your help, in advance.
GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127
Posted: 03:11am 01 Apr 2009
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Check the loading and power supply ratings. Very small battery. a problem waiting to surface here, even with twice the battery. You need to provide much more info before someone can help.
become more energy aware
Marcus20VT Regular Member Joined: 02/09/2008 Location: WalesPosts: 49
Posted: 04:44am 01 Apr 2009
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I've had a look for the data stamp which isn't there- annoyingly, so I can't give any more data on the freezer. Its a swan SR5060
I'm thinking that this sort of small freezer has an average annual consumption of 100 kwhr which gives an average consumption of 300W.
Obviously there will be peaks and troughs to this consumtion as the motor kicks in and out as its thermostatically controlled.
A motor load is an inductive load - even with a larger bank of batteries, would my inverter be able to handle this?
does anyone else operate fridges or freezers from an inverter?
thanks
brucedownunder2 Guru Joined: 14/09/2005 Location: AustraliaPosts: 1548
Posted: 11:59am 01 Apr 2009
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Don't know if I'm right here,but here goes anyhow.
I once spoke to a aircon tech that installed my a/c systems . He stated that "soft-start" was when the appliance 's motor never actually switched off ,it just slowed down . So, maybe your frig motor would be the same ,,IE, once you get it started ,the inverter just feeds low power to the motor in stand-by stage and then supplies more power when the motor' power demand (compressor on ) increases.
Check that out..
Bruce.Bushboy
oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686
Posted: 11:49pm 01 Apr 2009
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Marcus20VT
The induction motors in a fridge will need a surge rating of at the least 5 times the run power... so at least 1500w surge to get the thing to start. Soft start is only relevant to the startup of the inverter. It means that when you start up your inverter it gradually increases the pulse width (normally decreases the dead time control) from 0 to max... at some point before then the control error amps take over. Once running, the error amps are in full control, and no sign of soft anything should be apparent then.
Soft start in a motor is normally using a big choke switched in and then out once running, or a star/delta switch ... so that a 415v delta motor will look like a 700v motor wired in star... so starts up slower and softer, then switched to delta for full power once up and rotating. (it keeps the inrush currents lower)
Bruce.... april fool thing yes??
............oztulesVillage idiot...or... just another hack out of his depth
Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904
Posted: 04:32am 02 Apr 2009
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That sounds like an April fool joke to me too.
I just bought a Kipoint pure sine wave inverter to run a fridge amongst other things. The fridge has the works from an ancient one relocated into a very efficient chest cabinet for keeping drinks cold.
Anyway, the inverter has a 1200W continuous rating and a 3600W surge capacity. It has no problem at all running the fridge. It also has a green power setting which drops the standby current to about 90mA and it automatically switches to run once the fridge (or any other load) turns on.
Power draw from the 24V 105Ah battery bank with the fridge only running is: initially 10A, this drops to around 7A when the fridge compressor had run for a few seconds.
For a 12V battery bank it would be double that. The original poster should take note that his freezer would require about double that Amps again, according to the wattage quoted. Battery quoted is way too small IMO.
Klaus
domwild Guru Joined: 16/12/2005 Location: AustraliaPosts: 873
Posted: 06:48am 10 Apr 2009
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Tinker,
Not an April Fool's joke. My aircon is an inverter, whatever that means, but the compressor runs all the time and once the temp is reached, it just slows down via pulse-width modulation. Inverter may mean AC input to DC.
Taxation as a means of achieving prosperity is like a man standing inside a bucket trying to lift himself up.
Winston Churchill
oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686
Posted: 11:09am 10 Apr 2009
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Dom,
That's not soft start, thats just low power vfd setting. It still uses real (significant) power. It is a bit cheaper than the bang bang (full or nothing) thermostat control older non-inverter ones.... most times for a temperate climate.... or never in a hot climate with a not big enough unit.
The inverter aircons are just variable speed induction motor compressors. This allows you to throttle them back, and not let the hysteresis cause the room to get hotter ...then have to try hard to cool it back down.
In fact, if the load is high (its bloody hot outside), the older ones are cheaper to run full bore (no inverter losses for the same power out), but in the intermediate zone, it is cheaper to wear some inverter losses (sometimes as high as 1/3 cheaper than the bang bangs), than to let the room get too hot (thermostat comes on and we go full bore) to too cold (thermostat goes off and we wait for the thermal load to get overwarm again). I suspect that the three phase compressors are more efficient than their single phase counter parts, which helps make up for the solid state losses.
The added bonus, is the units have big active booster pfc units in the front end.... after the first rectifiers, but before the big storage capacitors, which gives them a unity power factor... which helps the utility, not you.
After this stage, it gets driven through a big H bridge converter which gives you three phase variable hz for the compressor. When you turn it on initially, it gives you .....tah dah. (insert fanfare here)... computer chip controlled ....Soft Start, ( so it doesn't blow up the H bridge, and because the H bridge just doesn't have the surge capacity to hard start a big 3ph compressor at rest to full speed)....after that... just variable speed 3 phase for your compressor...... interestingly, I guess that would be "matching the load".... which is as good as it gets.
............oztulesEdited by oztules 2009-04-11Village idiot...or... just another hack out of his depth
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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