Issue with power inverter running a microwave oven.

[JKromberg]

I?m not real sure where to go for this problem but I thought I would start here.

I have an RV with a 900w microwave that actually pulls around 1300 watts (as expected). I put in a 2200-watt pure sine wave power inverter with ONLY the microwave plugged into it. Around half the time when I try to start the microwave, the inverter gets an over-current fault within a second or so and shuts down. The other half the time, it works wonderfully. When it fails, the microwave sounds like it is trying to start up but does not have the power. The turntable seems to be trying to start to turn. I tried to measure the power with a ?Kill-a-watt? meter. It blew out when the microwave seemed to fail with excessive current. This tells me that it was pulling more than 15 amps to cause the meter to blow out. 1300 watts should be less than 12 amps. It is on a 15-amp breaker. The breaker has never tripped. The microwave seems to work fine on city power and generator power. I replaced the microwave with a new, similar 900-watt unit. Same thing happens. Works great half the time and fails half the time.

My research implies a microwave does NOT have a startup surge such as an electric motor will have. The inverter vendor said that it does and I need to get an inverter over 3000 watts just to run a 900/1300-watt microwave. They claimed to have recreated the problem in their lab with an even smaller microwave. Recent experience implies the vendor is correct. There IS a startup surge.

Has anybody else tried to run a microwave on a power inverter? Experience with startup surges? Thoughts?

Thanks for your help and advice.

 

[Larry N.]

I've run a 700 watt MW for very short periods off the inverter, but I normally don't do that because it drains the batteries so much, even for just a minute or so. I expect battery condition has as much to do with problems when running the MW as the inverter has. That's a LOT of drain in a hurry. So if I really need it and can't plug in, I'll fire up the generator.

 

[Alaskansnowbirds]

What is your battery status when the MW won't start?

Sounds like the inverter isn't getting enough power to handle the load.

We used to run our MW off the inverter all the time to heat coffee etc. when we had our MH.

 

[Kevin Means]

When powering a high consumption appliance (like a microwave) off an inverter, it's important to ensure that your batteries have enough AH capacity to power a microwave, all the wiring connections are good and the wiring between the battery bank and the inverter are of adequate size. The inverter should also be installed as close to the batteries as is practical, but not in the same compartment unless you're using Li or AGM batteries.

The symptoms you've described sound like the appliance just isn't getting the power it needs. If your batteries are charged, and they have enough capacity to power a microwave, you may have to move the inverter closer to the battery bay, and/or install heavier gauge wires.

Kev

 

[John From Detroit]

Microwaves do have a start up surge. So do televisions and radios and anything uses DC power in operation... However it is not like a motor (Much shorter surge time) but it's there. No question about that...  Even your cell phone charger or computer power supply will have a start up SURGE.. Too small to measure on the phone charger (You likely can measure it on the computer's brick) but it's there.

 

[solarman]

When powering a high consumption appliance (like a microwave) off an inverter, it's important to ensure that your batteries have enough AH capacity to power a microwave, all the wiring connections are good and the wiring between the battery bank and the inverter are of adequate size. The inverter should also be installed as close to the batteries as is practical, but not in the same compartment unless you're using Li or AGM batteries.

The symptoms you've described sound like the appliance just isn't getting the power it needs. If your batteries are charged, and they have enough capacity to power a microwave, you may have to move the inverter closer to the battery bay, and/or install heavier gauge wires.

Kev

I would agree with Kevin here..

I fitted a 1500 Watt inverter in my previous TH,  with T105 batteries ( at 12 Volts ) and had no problem running a 900 W microwave
or the a/c for 30 minutes at a time..

you could well be suffering from low voltage issues due to failing batteries or incorrect cabling causing too much voltage drop.

what is the wire gauge and length from the battery terminals to the inverter ?

FYI: in my case I had less than two feet of 2 AWG from the batteries to the inverter.

 

[Gary RV_Wizard]

I suspect your problem is the battery bank rather than the inverter.  A 1300 watt load means an amp draw from the batteries of at least 108 @ 12v and even a large and new set of batteries will suffer a severe voltage drop with that much load.  As the voltage falls below 12, the amps go up even higher.  You probably have no more than two medium-size batteries in your Chateau, so they simply lack the amp capacity to operate the microwave reliably.  I've "been there, done that!".

You might try hooking in another battery, even a tamp set-up with jumper cables, and see if that helps the micro start & run. I'm betting it will. You could even use jumper cable to your car battery for a test run.

 

[sightseers]

What would happen if you took a start capacitor from an A/C unit and installed it at the microwave plug ?

 

[JKromberg]

Thanks for the input.
I have two 6 month old, high end 12v AGM, 1300 cranking amps batteries so they should be good. I have 4 foot, 2 gauge wires from the batteries to the inverter.
The last time it failed, I had stopped after driving 4+ hours. The batteries should have had a good charge. It worked several times and then started failing. Did not work again until I ran the generator. Before that, I had been boondocking for a week with no problems. Solar was keeping the battery charged.
This morning, it seems to be working wonderfully. Started the MW about a dozen times heating a cup of water to boiling. Let it sit and tried again multiple times. Again, it worked wonderfully.
Based on the comments above, I will take a closer look at DC voltage when it fails. Since it would take a bit to jump the batteries on an unplanned failure, I might try starting the engine to see if that helps with a battery boost. If convenient, I will try the jumping in more batteries idea. I might also look at battery voltage after charging from the engine vs charging from solar.
At this time, I don't have any trips planned. Might have to find someplace to go just to test out the ideas.

Any thoughts on the capacitor idea? Would that go on the 12v battery side or the 120v inverter output? From what I know of cap's, it makes more sense on the 12v side. Supply a bit of a surge to get the MW started.

Thanks again for the suggestions.

 

[solarman]

What would happen if you took a start capacitor from an A/C unit and installed it at the microwave plug ?

Smoke. fire, real bad hair day for you.. !!

 

[Lou Schneider]

What would happen if you took a start capacitor from an A/C unit and installed it at the microwave plug ?

Like Solarman said, a real bad hair day.  The starting capacitor is in series with the A/C motor winding, if you install it at the microwave plug you'll create a dead short.

However, on the DC side a supercapacitor like they use to stabilize the voltage on those boom-boom car stereos may help with the microwave's starting surge.  I've been thinking about using one for a similar problem, getting over the starting surge of a refrigerator compressor.  The usual failure mode in a home refrigerator installation is the batteries discharge to a point where they still have enough power to run the compressor, but not enough to get over the starting surge without the DC voltage dropping below the inverter shutoff point.  The charge stored in a supercapacitor may help with that.

They make supercapacitors that can store enough power to start a car engine.  Just be aware they'll look like a dead short for a few seconds until they build up a charge.  And the same if their output is accidentally shorted while they are charged.

 

[sightseers]

Why would it explode ?  The cap will be charged with the same 120 volt power the inverter is supplying at the plug for the microwave.?

We used big old caps on the a/c power supply side of parking lot lights because of the voltage drop upon start-up.  Most modern caps have a bleed off resister so they will not stay charged when the power is shut off.

 

[Gary RV_Wizard]

I have two 6 month old, high end 12v AGM, 1300 cranking amps batteries so they should be good.

Neither AGM nor CCA rating are very relevant here.  What matters is how many amps can be continuously delivered without significant voltage drop.  A high CCA helps a bit with the first 2-3 second surge, but if the voltage falls to 7v-8v the amp load increases by about 50%.  either the low voltage or the high amps will cause the inverter to fault - it's mostly a matter of which condition it detects first.  What you need is a lot of Amp-Hour capacity, something north of 300AH.  400+ preferred.  Unless those two batteries are size 4D or 8D, you are probably well short of that.

With a combined 1300 cranking amps, I'm guessing you have two Group 27 or maybe Group 31 batteries. That would typically yield a combined AH in the range of 160-220.

 

[JKromberg]

Two type 27 batteries for a combined ~180 AH.
This morning, it was working. At the batteries, 12.8v unloaded. Down to 11.8 under MW load.

 

[Kevin Means]

If you're reading the voltage while the microwave is running, you're not seeing an accurate indication of the battery's true state of charge (SOC.) It's normal for lead/acid batteries to indicate a lower voltage while under a load - especially a heavy load, like a microwave. The heavier the load, the greater the inaccuracy. Once the load is removed, the voltage indication will more accurately reflect the battery's true SOC after a few minutes.

Voltage is a relatively inaccurate method of determining SOC. A much more accurate method is to install a battery monitor, like a Victron, or Trimetric RV2030. They indicate not only a battery's voltage, but how many amps are going in, or out of the batteries at any given time, and they display (in percentage) the remaining battery capacity. In my opinion, they are a must-have if you do much boondocking.

Kev

 

[Gary RV_Wizard]

11.8 indicates a serious load drawing amps (but you knew that).  If at rest, 11.8v would represent about 30% SOC, but with an active load we can only guess. When the micro shuts off, it will surely bounce back up a bit, but while the micro is running, the batteries act like they are near dead.

 

[solarman]

Why would it explode ?  The cap will be charged with the same 120 volt power the inverter is supplying at the plug for the microwave.?

We used big old caps on the a/c power supply side of parking lot lights because of the voltage drop upon start-up.  Most modern caps have a bleed off resister so they will not stay charged when the power is shut off.

unlikely to explode unless the working voltage is too low...

it really won't do anything for you as the cause of your problem is with your batteries.