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Old 05-16-2019, 01:47 PM   #1
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Inverter and Microwave

So got a pure sine 2000 W inverter and of all things it won't run the microwave! Here's my story:

Magnum CSW2012 just purchased and installed Outdoors RV trailer. Connected via 4/0 cables of 4' length to 2 x 6 volt 225AH golf cart batteries connected in series with 2' x 4/0 cable. Batteries are fully charged new as of late 2018. Cleaned posts before installing cables. Inverter is grounded to trailer chassis. Unit turns on fine and will run 500 Watt halogen light, 1,000 watt hair dryer, 1,000 watt electric blower all individually just fine. Inverter reports battery voltage of about 11.5-11.8V when running above loads and does not report any error codes or otherwise complain. However, when I try to run the trailer microwave (High Pointe EM925ACW 1.35KW) which draws around 1,350 watts on inverter display it would only start on first attempt and ran for 2 minutes via timer. Any subsequent attempts to start microwave cause it to beep, display to blank momentarily and microwave won't start. If you try this a couple times it finally trips the GFCI on the inverter. Microwave works fine on shore power. Pulling power to the microwave for a few seconds actually cleared the problem for a while but now even that doesn't work. What appears to be happening is when the microwave tries to kick on the voltage is dropped momentarily well below 100V which resets its memory and it ceases to try starting up. On advice of Magnum I tried our home microwave and it worked just fine even though it registered 1,750 watts on the Magnum!



Anybody have any suggestions beyond buying new microwaves until I find one that works? Maybe a start capacitor hack to reduce momentary voltage drop? Thanks in advance for any ideas/suggestions!

Robert
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Old 05-16-2019, 02:05 PM   #2
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Can you run the RV's MW from shore power and measure the wattage/amperage it is really using. It maybe acting up and drawing more power than you think. As MW age they do start to fail and one issue is they draw additional power.
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Old 05-16-2019, 02:17 PM   #3
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If your batteries drop below 12V under load as you report then they are suspect.
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Old 05-16-2019, 02:29 PM   #4
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3 points.

Have the batteries be kept charged while in storage or abused by letting them discharge and sit in a discharged state. A year of abuse will shorten battery life.

Quick test, get any other 12 volt battery and use jumper cables to supplement the pair you have.

Point 2. You say you used 4/0 cables. Are you sure they are 4/0 and not 4 gauge.
4 gauge would be marginal with a 2000 watt inverter.

Point 3. You say you have the inverter grounded to the chassis. Is that the only ground path to the inverter, or do you also have a negetive cable from the battery to the inverter ?

Any switchs or breakers in the circuit. They need to be rated for 200+ amps.
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Old 05-16-2019, 03:16 PM   #5
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I ran a 1000 watt inverter of a pair of Trojan T105 batteries in series with some 2/0 cable. My loads were a vacuum cleaner and a blender and the batteries held up fine. I can imaging a continuous microwave load could pull the voltage down on the batteries pretty fast The low voltage cutout on the inverter would stop the inverter until the voltage recovers.
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Old 05-16-2019, 03:25 PM   #6
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We had eight Trojan T-105 6v batteries and a Magnum pure sine MS2800 inverter in our last motorhome and it would run the Sharp 1874 microwave, but just barely. The microwave sounded like it was struggling. The load is just too much and the microwave sounded bad so we made it a point not to run it on the inverter.
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Old 05-16-2019, 04:28 PM   #7
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If you have a Xantrex inverter, you can set it to shut down at 12.1v, 11.4v or 10.5v. Make sure you set it to 10.5v. I have a 2000w Xantrex and my 1600w microwave runs fine.
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Old 05-16-2019, 11:41 PM   #8
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Additional info

Thanks for all the quick replies! So to provide some more info relative to your posts:


1. On shorepower I have a basic watt meter which reported right around 1,100 watts for this High Pointe Microwave. For the couple times I got it running on the inverter the Magnum status display reported 1,350 watts which is exactly what the specs say for the unit. So steady state I don't think it draws anything unexpected but during the first second of startup is the problem. This microwave is essentially new as of last year and was only used a couple times during the Fall.


2. On battery voltage under load, I was also concerned they dropped down into the 11.5-12 under load but the inverter only complained once when I ran a 1,000 watt hair dryer and a 1,000 watt electric blower together and only then did the inverter complain about high load since that's the 2kw limit. In addition, a large/full size home microwave ran just fine and reported 1,750 watts on Magnum display so again inverter and batteries seem to be able to perform as designed with these other loads even at higher steady state wattages.



3. 4/0 cables are definitely 4/0 and measure full 3/4" in diameter with insulation. Ground to chassis is separate from battery 4/0 negative connection. Fuse on positive feed is 300 amp as specified by Magnum. Batteries have been kept above 50% since new.


4. Magnum inverter is set to give alert at 11.2V and shut down at 10.5V. The 11.2V alert only occurred once during testing when I was running a full 2,000 watt load of hair dryer + 1,000 watt electric blower.



I monitored the AC voltage at the microwave on startup and did see a large momentary drop from 120V to something under 100V which is probably killing the microwave startup. It acts like a power reset occurs every time you kick it off. So it's probably either this specific unit or this model in general that doesn't behave well with this high frequency inverter. If I had to guess it seems like this specific MW unit may be the problem since even Magnum support didn't indicate frequent problems with microwaves.


Thanks again for all the input and experiences!
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Old 05-17-2019, 12:31 AM   #9
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I strongly suspect the problem is the sustained load capacity of the batteries, 2 6 volt golf cart batteries in series can only sustain about 1,200 or so watts of power draw without having excess voltage drop. The exact amount depends on the specific battery chemistry and design. Is there any way you could upgrade to a 4 battery system?
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Old 05-18-2019, 08:45 PM   #10
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I was wondering if only 2 golf cart batteries would be up to the task - I agree that may be the problem. Although I have been able to run a 1,750 watt full size home microwave without any problems so it would appear this smaller microwave may be behaving badly on initial startup. The combination of that and the 6 volt golf cart battery limitation is just not a winning combination. Oh well... Thanks everyone for the advice and experience.
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Old 05-18-2019, 08:50 PM   #11
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I tried adding a pair of 12 volt batteries in my Duramax via 4 gauge jumper cables and definitely generated a boost in DC voltage but still no go. The microwave simply seems to generate a very significant split second load at startup that the inverter doesn't respond to fast enough and so AC voltage has a very significant momentary drop which causes microwave to reset before getting going.
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Old 05-18-2019, 09:26 PM   #12
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Just curious if you are using the same outlet when you test with your house microwave or a different one? If different, try plugging in to the same one and visa-versa and if that the issue then remove the outlet and check the connection's since the typical RV outlets are cheap push in wires and it may be a faulty connection adding just enough extra resistance...
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Old 05-19-2019, 08:16 PM   #13
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Did one better and brought microwave out to front of trailer and plugged directly into inverter and still the microwave resets immediately. Also confirmed the inverter AC output voltage takes a serious hit down to 40-50V. Hard to say exactly with just a digital multimeter. You can see the effect with a halogen bulb plugged in simultaneously that dims considerably but only briefly while inverter recovers. No error codes or complaints from inverter. Strange thing is the hit is immediate and the microwave magnetron doesn't kick in until a couple seconds after startup so that shouldn't be causing the large momentary load. On shorepower using a watt meter that records minimum voltage it shows a drop from 124V to 107V on startup so it's no surprise I guess that the inverter drops much further. Don't have any equipment that could measure sub-second spikes in voltage or amps. Going to consider doing a microwave upgrade to different make/model since the inverter ran a full size MW just fine at 1,750 watts. At this point it doesn't seem that more batteries would make any difference.
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Old 05-19-2019, 08:45 PM   #14
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I would agree that it sounds like a faulty microwave. Generally 5% of normal voltage say 120 is maximum allowed on a circuit according to NEC. As an IT professional with 40 years of experience, I have frequently seen equipment problems develop with voltage drops exceeding 5%. It may not be all at once, but it tends to degrade the equipment over time if it does not cause an immediate problem. I have some decent Fluke equipment that will log a variety of variables on a sub-second basis over several days. So I able to pinpoint many strange issues with power supply systems. A drop to 50-60 volts is a serious hit.
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