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RVing message boards => Boondocking => Topic started by: velcrostrip on June 15, 2017, 03:11:42 PM

Title: 600 watt inverter won't power 75 watt fridge
Post by: velcrostrip on June 15, 2017, 03:11:42 PM
I tried a test to see if  my 600 watt PSW inverter would power a 75 watt 1 amp fridge and it over powers the inverter.
Seems weird. Any ideas?
Title: Re: 600 watt inverter won't power 75 watt fridge
Post by: docj on June 15, 2017, 03:49:36 PM
it's quite possible that the startup current is more than the inverter can handle.  For my Samsung RF197 which draws ~100 watts on the average Samsung recommended an inverter that could provide an instantaneous current of 11A for starting purposes.  Because of that I've always recommended a 1000 watt inverter for powering most larger residential fridges.
Title: Re: 600 watt inverter won't power 75 watt fridge
Post by: judway on June 15, 2017, 04:10:43 PM
What size of cables do you have from the battery and the inverter? How long are the cables?
Title: Re: 600 watt inverter won't power 75 watt fridge
Post by: John From Detroit on June 15, 2017, 07:11:05 PM
Most people do not know just how big the start up surge is.. I don't have hardware that can measure spikes that fast.  But 5-10 times the running current is not uncommon.
Title: Re: 600 watt inverter won't power 75 watt fridge
Post by: judway on June 15, 2017, 07:44:29 PM
I have the Xantrex 1000 Watt sine wave inverter running my refrigerator. I have a 100 Amp meter in the 12 Volt line. When the refrigerator comes on the Ammeter is pegged for a short time indicating an input power of over 1200 watts. After the surge the power drops back to about 100 Watts (I don't remember the exact amount) while it is running. My input cables are 4/0 and about 10 feet long. The system works great.
Title: Re: 600 watt inverter won't power 75 watt fridge
Post by: supermanotorious on June 15, 2017, 07:47:00 PM
What about adding a large (think car audio) capacitor for startup? Who thinks that would work? I've been pondering this for awhile myself.

https://www.amazon.com/Rockford-RFC1-1-Farad-Capacitor/dp/B0012BWMKW
Title: Re: 600 watt inverter won't power 75 watt fridge
Post by: velcrostrip on June 16, 2017, 09:03:53 AM
What size of cables do you have from the battery and the inverter? How long are the cables?
12 gauge and 20' 2 6 volt
Title: Re: 600 watt inverter won't power 75 watt fridge
Post by: Gary RV_Wizard on June 16, 2017, 09:28:16 AM
What make/model inverter and what exact fault occurs?

Despite the start-up amp load, most 600W inverters should be sufficient for that, so I'm thinking a low voltage situation between battery and inverter is the likely cause. Poor wire connections or too small gauge.  If you are using 12 gauge to the inverter, that isn't enough. The inverter can easily draw up to 60A from the battery and 12 ga will  cause a severe voltage drop anywhere above 20A.  The inverter will probably quit if the voltage drops below about 11v for even an instant.  Change that wire out to 4 gauge and it will probably work.
Title: Re: 600 watt inverter won't power 75 watt fridge
Post by: judway on June 16, 2017, 09:51:23 AM
X2 for what Gary said. The wires are away too small. That size would be good for the 120 V lines. I wouldn't use anything smaller than 0 size. Cables are available from Harbor Freight and on eBay. They will not be cheap. I also use a pair of 6 V batteries.
Title: Re: 600 watt inverter won't power 75 watt fridge
Post by: judway on June 16, 2017, 10:01:32 AM
Go to eBay and search for "1/0 gauge battery cable".
Title: Re: 600 watt inverter won't power 75 watt fridge
Post by: Gary RV_Wizard on June 16, 2017, 11:29:10 AM
I agree with judway that 1/0 gauge is desirable. However, 4 gauge is good for 70A and chances are that a 600W inverter can't handle anything larger on its terminals. Maybe not even that big.
Title: Re: 600 watt inverter won't power 75 watt fridge
Post by: Len and Jo on June 22, 2017, 11:51:24 AM
I have a refrig that draws 60 watt (average draw) during the on cycle and I am using 1000 watt Xantrex inverter powering it.  I am using 4 ga. wire that is 4 feet long from the inverter to my house two golf cart batteries.

The system is marginal but works.  I figure (hard to measure the start up amps, the time is in milliseconds) refrig draws 8-9 amps on startup.  Going from 120 volts AC to 12 volts DC (10x the current) with a 90% efficient inverter means that the house batteries have to supply a serge current of: ( 9 x 10 / 0.9  ) of 100 amps for those few milliseconds it takes to get the compressor motor turning!! 

For my wiring that would be about 0.2 volt drop in the wiring from the batteries to the inverter.  BUT with that 100 amp draw the battery voltage also drops significantly for a short time,  about 1.3 volts.  When my house batteries are full at 13 volts that means the inverter at start up sees (13.0-1.5-0.2) 11.5 volts.  After running my refrig for about a day and half the drain on my house batteries drops their voltage down to about 12.5 volts.  From then on every time the refrig starts up we hear a very short millisecond "beep" because the Xantrex has a low voltage alarm hard wired in that is set for 11.0 volts. 

If I had 20 feet of 12 ga. wire my line loss would be about 3.2 volts.....so on start up my inverter would only see the house batteries at (13.0-1.5-3.2) 8.3 volts!!!  So at a minimum you will need larger 1/0 wiring, and larger inverter that can handle the serge current.  I would recommend finding an inverter that does NOT have a low voltage alarm or cut off.  Or at least one that those can be deactivated.  If you also have 4 house batteries instead of the 2 I have then the serge battery voltage load drop would probably be halved.

Title: Re: 600 watt inverter won't power 75 watt fridge
Post by: Clay L on June 22, 2017, 12:35:43 PM
judway is right.
Here are the results using 1/0. You can get away with smaller wire - 0 or 01 but 1/0 gives you quite a bit of leeway.
See here (http://smg.photobucket.com/user/ClayL/media/Screen%20shots/wire%20size%20A_zpsxjr1r0lq.jpg.html)
Title: Re: 600 watt inverter won't power 75 watt fridge
Post by: Gary RV_Wizard on June 23, 2017, 10:50:57 AM
Part of the problem here is the sensitivity of the inverter's built-in self-protection circuitry. Some respond to low voltage or high amps extremely quickly and may shut down with just a few milliseconds of high load. There is some good reason for that - the inverter output voltage and frequency is highly dependent on the DC input voltage. If the battery voltage sags badly for an instant, the 120v & 60 cycle output may fall outside the acceptable range. Since 60 cycle alternating current peaks in voltage every 16.67 ms, the time window is short unless the inverter circuitry does some averaging  to assess power over a somewhat longer time period. Still, it is going to be a very short time as best. Therefore extra battery capacity (AH) and short, large gauge, cables are key to keeping the inverter happy during start-up loads.
Title: Re: 600 watt inverter won't power 75 watt fridge
Post by: Len and Jo on June 24, 2017, 10:52:59 AM
Thanks for the insight Gary.
Title: Re: 600 watt inverter won't power 75 watt fridge
Post by: robertusa123 on June 25, 2017, 07:03:26 PM
Your inverter cant handle the starting load  you need minum 750-1500 inverter..... also alot of motors dont like inverter power.
Title: Re: 600 watt inverter won't power 75 watt fridge
Post by: gatorcq on June 26, 2017, 02:27:28 PM
For every 1 amp AC requires 10 Amp DC. Rule of thumb.
Longer the cable run, less voltage, means higher current to overcome the drop in voltage.
Title: Re: 600 watt inverter won't power 75 watt fridge
Post by: velcrostrip on June 26, 2017, 09:21:23 PM
Golly. I love this group. Wish there was a voting system to vote for the best answer but it seems to be the consensus the connections are too small. I've been very busy but I guess I'll try 2 gauge wires as that's the largest I'll be able to connect. As soon as I have time.
Title: Re: 600 watt inverter won't power 75 watt fridge
Post by: velcrostrip on July 05, 2017, 01:21:12 PM
Ok. Final update. The 2 gauge wires worked well. Voltage went down to 12.9 this morning and I expect it will go back up to 14.5 with the sun.
There is an FYI I'd like share. I have a remote switch for the inverter, which is a regular 15 amp residential wall switch. I was only able to use 6 gauge wire to the switch (4 feet) but it didn't seem to affect the performance.
Thanks for all your help.