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redsil
Apprentece
USA
6 Posts |
 Posted - May 09 2007 : 3:19:40 PM
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| I have a 700/1800 watt inverter. I want to run a small (office size) refrigerator on it while I'm driving. The fridge is listed at 115v., 60 cy., 1.1 amp. Why won't it work? A friend suggested it had something to do with the capacitor start motor. The fridge works fine on 115v. A/C. |
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cyclopsitis
Nobel Prize Winner
Canada
732 Posts |
Posted - May 10 2007 : 01:00:18 AM
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I've never tried to run a small fridge off of an inverter before but it could be because of the cap. The motor used in most compressor does not have enough torque when starting to power the pump. This would cause the start windings burn out and the motor is toast. In a normal operating split phase motor a start winding centrifugal switch disconnects the start winding when the motor reaches about 70% of its full speed. A cap start motor uses the cap to put the start winding's and run winding's currents at 90degrees (or there about). It is at this point that a split phase motor has the most amount of torque. Most inverters don't produce sinusoidal AC but a square wave or in some higher end models they have a modified square wave. That could be the problem.
If not that then it could be caused by a voltage drop but then we are getting out of my area of half-assed expert-ness... I don't really have any solutions though... I'll think about this one for a bit or maybe someone on here knows more about these inverters then I do. There is a huge discussion on inverters in the "power" section of this forum site. If no one else replies here try your question there, someone might be of more help then I.
K |
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Aaron Cake
Administrator
Canada
6718 Posts |
Posted - May 10 2007 : 09:02:59 AM
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| If the inverter is a true sinewave inverter, it should work fine. 1.1A at 120V is only 132W. |
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redsil
Apprentece
USA
6 Posts |
Posted - May 10 2007 : 11:01:33 AM
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| The 700/1800 watt inverter I am using is a modified sine wave. I would think that if I am only requiring 1.1 amps, that a 700 watt inverter would be plenty hefty enough, even if a hard start is required. The fridge works fine in 115v. A/C. Any other ideas? |
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cyclopsitis
Nobel Prize Winner
Canada
732 Posts |
Posted - May 11 2007 : 01:38:28 AM
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With what Aaron said it might be the fact that its not a true sine wave. The amp rating on a motor is for its running current however, the starting current of motor can be anywhere from 5 to 7 times the name plate rating. So quickly you can see your 1.1A current can become 8A. At a rough estimate of the power EXI 120X8 = 960W. Even 960W is being light, it could be more because of the inductive reactance of the motor and capacitor. I would try the frige on a similar size inverter but a "pure" sine wave one. If it works on one like that then your problem is the modified sine wave. Hope that helped.
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mrenjan
Apprentece
India
19 Posts |
Posted - May 11 2007 : 08:17:48 AM
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| The AH capacity of Battery too matters... hope the capacity is above 120 AH or so |
Renjan |
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Aaron Cake
Administrator
Canada
6718 Posts |
Posted - May 12 2007 : 10:52:15 AM
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| "Modified" sinewave is just a way of saying that it's a slightly less dirty square wave. Not a good thing for inductin motors... |
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redsil
Apprentece
USA
6 Posts |
Posted - May 12 2007 : 2:24:27 PM
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| Thanks for all the explanations and suggestions. Anybody got a true sine-wave inverter of 700-1000 WATTS for sale? |
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Aaron Cake
Administrator
Canada
6718 Posts |
Posted - May 13 2007 : 10:11:41 AM
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Probably easier and cheaper to get a 12V fridge.  |
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ruralmcguyver
New Member
USA
4 Posts |
Posted - Jun 24 2008 : 2:56:29 PM
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I use a 1200/3200 watt modified sine wave inverter for ALL my power at home. I got this size because I was lucky enough to test a couple to see if they would start my fridge. The fridge is rated at 4.2amps. The inverters I tried were an 800/1600 watt, and a 1500/3600 watt. The 800/1600 watt one wouldn't start the fridge. The larger one had no problem what-so-ever. You may only need a larger inverter, and don't need to spend the extra $$$ for a true-sine. Newer ones even have a stand-by, and instant-on feature that draws much less power. Also, look for ones that draw less than 1amp under no load. A lot of cheaper ones are real power hogs even with no load! Good luck!
added: remember, you don't convert 12volts to 110 for free. in other words, 110 (or 120) is 10 times 12 volts! So, if you want 1 amp of 110volts, depending on the efficiency of the inverter, you may need to draw as much as 10 amps from your 12 volt source.
Watts=Volts x Amps
so if your 110 volt fridge is drawing 8 amps at startup, that's 880 watts (I can't go back & see your figures), but that's more than your inverter! For a modified sine wave inverter to start most fridges reliably, they should be at least double the wattage.
Mine draws 4.2 amps times 110 volts, that's 462 watts. So double that is 924, that's why an 800 watt inverter wouldn't start my fridge. |
ruralmcguyver@yahoo.com |
Edited by - ruralmcguyver on Jun 26 2008 09:25:53 AM |
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ruralmcguyver
New Member
USA
4 Posts |
Posted - Jun 26 2008 : 09:35:46 AM
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Sorry, I went back to look at your numbers. 115v and 1.1amps is only 126 watts. It shouldn't have any problem starting that!
Are you using an extension cord?
I used one on mine for some time with no problem, then in 1 day it started having problems starting the fridge. After a whole day of taking everything apart, cleaning (it needed that anyway), and testing, it turned out to be the heavy-duty extension cord!
Have you tried: unplug the fridge, and turn any switch or temperature control all the way off. Then, turn on the inverter. Then plug in the fridge. Now turn on the fridge / raise the temperature control.
Let us know what happens. |
ruralmcguyver@yahoo.com |
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Inverter
