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Old 01-07-2012, 08:14 PM   #1
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microwave/invertor problem

I bought a whirlpool corner microwave. 0.5 cu. ft. to fit a small space. It is 750 watt output, 1200 input. I installed a Cobra 1575 Invertor. Used large cable to the batteries.

The light comes on, and the turntable turns, controls work, but it will not warm anything up.

If I take the microwave in the house and plug it in, it works fine.

I took another small used microwave, and plugged it into the invertor. That one works fine. Both are 750 watt output, but the other one is too large for the space I have.

I have never heard of a microwave not working with an inverter before. LOTS are being used that way.

Anyone have any idea what is going on?

Link to the microwave: Whirlpool 0.5 cu. ft. Countertop Microwave Oven Model WMC20005YD

Thanks for any help.
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Old 01-07-2012, 11:16 PM   #2
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I looked up your inverter online and discovered that its 120v ac power output is an "modified sin" wave and not a "pure sin" wave. The power curve of a modified isn't as clean as a pure one. Newer electronics and this does include microwaves need the cleaner source of power run properly. I had the same issue in my trailer. The inverter works good on simple devices like a toaster. But as soon as you hook up something like flat-screen TVs or newer microwaves. Hope this helps.
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Old 01-08-2012, 07:54 AM   #3
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Times two.

However stand by for "sticker shock" when you price a 2KW Pure Sine Wave inverter.
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Old 01-08-2012, 10:01 AM   #4
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How "large" a cable, and how far a run between the battery and the inverter? Also, how many amp-hours of battery?

About the only thing that my set-up won't do, as elaborate as it is, is extended (more than a minute or two) use of the microwave. My cables are only 5' long, 1/0 (#0) cable. My deficiency is not having enough battery. I currently only have a pair of 6v golfcart batteries, giving me 220Ah of capacity. I'm considering doubling this, with a second pair, before my next boondock trip.
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Old 01-08-2012, 02:00 PM   #5
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Quote:
Originally Posted by skgeorgy View Post
I looked up your inverter online and discovered that its 120v ac power output is an "modified sin" wave and not a "pure sin" wave. The power curve of a modified isn't as clean as a pure one. Newer electronics and this does include microwaves need the cleaner source of power run properly. I had the same issue in my trailer. The inverter works good on simple devices like a toaster. But as soon as you hook up something like flat-screen TVs or newer microwaves. Hope this helps.
This inverter DOES run another microwave. And LOTS of invertors, the cheap ones, run microwaves in over the road semis. I know, I have several. And know of lots that do.
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Old 01-08-2012, 02:33 PM   #6
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The issue with the modified sine wave is very true. The newer microwaves have become more electronically complicated. I'll try to write this without getting to technical. A circuit board ultimately operates on DC voltage. Even though you plug that "wall-wart" into the wall, there is a box on the cord, or the converter is built into the device, the circuit board is working on DC. Now, the inverter is taking a DC voltage from the battery, creating a modified sine wave, which is usually a square wave or close to it, thus changing the RMS (root mean square) voltage and it comes out as "dirty power". The devices converter (aka power supply) can't clean that dirty power and properly convert it back to DC voltage for use by the circuit board. I hope that makes sense?
In previous applications you stated, there are many questions that need to be answered before an explanation of why the microwave's worked in other similar situations. 1. how the microwave circuitry to made, and protection from low or dirty power. 2. Type of inverter used, pure sine wave or modified sine wave (in my work vehicle I will only have a pure sign wave in inverter (I am an electronics/ communications technician). 3. Peak/ surge power from the inverter (may 2000 watts, but that is 2000 watts peak (for a few micro seconds) but run may be 1250 watts. 4. I am not well versed in the outfitting of tractor trailer trucks, but I do know that there are lines of specialized equipment for home type appliances in the trucks. Why aren't they used in our RV's? Cost! Looking at the cost of some of these devices would drive the selling price of the camper up.

I don't think there are to many people that can truly answer the question you have asked without physically coming to your RV with an oscilloscope, a DVM, and maybe some other test equipment and test all parts of the system.

One other thing that came to mind. Check by putting a DVM (digital volt meter) with a HOLD function and connect it to the battery. Set it to monitor LOW. Turn on the microwave. After the microwave stops, turn it off and check the DVM reading. Repeat the same steps but connect the DVM at the connections to your inverter. Record the results. If you have a reading below 11.8VDC, than that very well may be your problem. If it happens at the battery, you need to address it there. If it happens at the inverter and not the battery, then you need to look at the wire is to small for the length run, don't have large enough connections. There should also be no splices in the cables from the battery to the inverter. Also, check your in line fuse on the cable from the battery to the inverter making sure the connections are clean and tight.
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Old 01-08-2012, 05:18 PM   #7
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Quote:
Originally Posted by mpierce View Post
I bought a whirlpool corner microwave. 0.5 cu. ft. to fit a small space. It is 750 watt output, 1200 input. I installed a Cobra 1575 Invertor. Used large cable to the batteries.
Trying to cook with battery power is tough.
I know someone posted that semis do this. Semis have HUGE alternators!!
Just basic math for power looks like this--
You say your MW needs 1200 watts to run.
If you want to draw 1200 watts out of a 12 volt battery (or 2- 6 volts) you
need 100 amps. That's HUGE. Even basic battery jumpers won't normally
handle that much. Your problem might be "modified sine wave" but I'd bet
even money it's just simply not enough juice to the inverter.
(Plus the inverter is going to waste some power so you probably need more
like 120 AMPS from the battery to get the job done.)

Consider a honda 2000 or some other 2k generator.
That's a lotta cash just to warm supper.

We use stove top percolator and LP oven and stove and get by pretty
good on those boondocking trips without the MW.

My 2
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Old 01-08-2012, 06:48 PM   #8
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Quote:
Originally Posted by mpierce View Post
I bought a whirlpool corner microwave. 0.5 cu. ft. to fit a small space. It is 750 watt output, 1200 input. I installed a Cobra 1575 Invertor. Used large cable to the batteries.

The light comes on, and the turntable turns, controls work, but it will not warm anything up.

If I take the microwave in the house and plug it in, it works fine.

I took another small used microwave, and plugged it into the invertor. That one works fine. Both are 750 watt output, but the other one is too large for the space I have.

I have never heard of a microwave not working with an inverter before. LOTS are being used that way.

Anyone have any idea what is going on?

Link to the microwave: Whirlpool 0.5 cu. ft. Countertop Microwave Oven Model WMC20005YD

Thanks for any help.
If the controls and display are working properly, that would indicate
that the microprocessor is executing it's code correctly. This is a
light - low current - load.

You indicate "no heating" which sounds like the magnetron - a heavy
load - is not firing. This might be due to the "non pure sine wave"
from the inverter or it might not be. If it was me, I'd gather a bit
more info. If you use a voltmeter (analog type would be best) to
check both the DC input to the inverter AT THE INPUT TERMINALS
and the AC output under load (when you press the START button).
You may find the voltage falling under the heavy load of trying to
fire the magnetron.......

If the DC is falling, check those cables. 1200 watts AC input is about
10 amps or about 100amps at 12VDC. Every 100th of an ohm resistance
in the 12VDC circuit would cause a 1 volt drop. Those large cables
need to be VERY large with very low resistance connectors.
And, of course, the battery needs to be able to supply that current
with minimal voltage drop; many inverters shut down when the input
is below about 10 volts..................

cheers,
johnd
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Old 01-09-2012, 09:30 AM   #9
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It powers another microwave of the same size just fine, so I think all the talk of cable sizing, batteries, etc. is moot.

It is ONLY this one specific microwave that does not work. I have LARGER microwaves in other semis that work fine.

I have 4 large 12 volt batteries and a 180 amp alternator in the truck, so that is not a problem. I am only talking about a 2-3 minute run to warm up lunch in the truck, not cooking a 18 lb turkey for two hours!

I believe it is specific to this mag on this microwave.

MP
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Old 01-09-2012, 10:05 AM   #10
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Quote:
Originally Posted by mpierce View Post
It powers another microwave of the same size just fine, so I think all the talk of cable sizing, batteries, etc. is moot.

It is ONLY this one specific microwave that does not work. I have LARGER microwaves in other semis that work fine.

I have 4 large 12 volt batteries and a 180 amp alternator in the truck, so that is not a problem. I am only talking about a 2-3 minute run to warm up lunch in the truck, not cooking a 18 lb turkey for two hours!

I believe it is specific to this mag on this microwave.

MP
You may well be correct - it may be time to find a replacement
that works for you.


......But it never hurts to check rather than assume......
As the pilots say: "In God, we trust. All else, we check."
[Right Lou??]

cheers,
johnd
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