Running Microwave from inverter

[Valmika]

I have a small microwave on board (900 watt)that I am trying to run with a Magnum 2000 watt inverter charger. It is a modified sine wave inverter. I have 2 4D batteries for the house bank roughly 400 amp hours. The batteries are AGM and fairly new, seem to be in good shape. The Microwave seems to have a hard time heating and makes a buzzing noise when using the inverter to power it. It does better when I run the main engine with my 120 amp alternator but still not great. Could this be a modified sine wave issue?

Any help would be appreciated.

It works but not like when plugged into shore power.

[CarstenWL]

... if it works better when the engine / alternator is running, it looks like a too high voltage drop on the house bank / input of the inverter.

Also, a 900W microwave will have around 1500W (check your manufacturer) input power / connection power.

The Magnum 2000 is specified at a continuous power output at 25°C of 2000 VA so you should be fine ...

I would look at losses at the inverter DC input.

Regards,

Carsten

[skipmac]

No hard evidence but I have seen several comments on previous threads that microwaves or at least some of them, strongly prefer true sine wave power.

[sailorboy1]

the only thing that would change with running it with the engine running is battery voltage , check/clean all your connections and cables

[wrwakefield]

Originally Posted by Mark Killam

I have a small microwave on board (900 watt)that I am trying to run with a Magnum 2000 watt inverter charger. It is a modified sine wave inverter. I have 2 4D batteries for the house bank roughly 400 amp hours. The batteries are AGM and fairly new, seem to be in good shape. The Microwave seems to have a hard time heating and makes a buzzing noise when using the inverter to power it. It does better when I run the main engine with my 120 amp alternator but still not great. Could this be a modified sine wave issue?

Any help would be appreciated.

It works but not like when plugged into shore power.

Mark,

Our boat came with a 700watt microwave oven [115VAC] and an adequately sized modified sine wave inverter. The microwave worked OK on the inverter, but sometimes made sounds like you describe, and sometimes the countdown clock would accelerate. [This on a 900AH 12VDC battery bank and very little voltage drop to inverter when the microwave was running on high... Also, the VAC meter showed a steady 110volts form the inverter during microwave use.]

It also took much longer [twice or more? subjective...] to heat things compared to when we were on shore power. [But was that occasionally due to an accelerated countdown timer giving that perception?]

Last year the charger portion of our older modified sine wave inverter started failing, so we replaced it with a new pure sine wave inverter/charger. The microwave now performs and behaves as it does on shore power.

To me this means our microwave had issues when using modified sine wave power. Not all microwaves may have these issues.

In case this is useful.

Cheers!

Bill

[Frankly]

If you have a microwave oven and it works with a modified sine wave inverter consider yourself lucky.

Over the years (almost 20) I have wrestled with this combination and some combinations will not work. The older models with the mechanical timers seemed to do better. Currently using a 800 watt (don’t remember the model) with a Freedom 10 (1000 watt MSW) and it does work. Replacing the Freedom 10 with a Freedom 20 (2000 watt MSW), the inverter shuts down immediately when we start the microwave (go figure).

A couple I have tried sound like they might explode. None of the microwaves provide normal cooking power or are very long lived operating from MSW inverters.

[lyleh]

I have the Magnum MSH 2000 inverter and use it to power a 900W microwave. It works fine - sounds to me like a voltage issue. If you have a AC voltmeter in the boat see what voltage the inverter is putting out when the microwave is running. Nominal 110 VAC voltage will change with a change in 12VDC voltage - the microwave puts a load of 100 amps in the DC system so depending on the size of your battery bank the DC voltage can drop considerably. Obviously when you are running the engine alternator it carries the load without the same voltage drop.

[StuM]

Good article here which did various tests:
https://amplepower.com/pwrnews/micro2/index.html

The following extract is very relevant. Note the last two sentences:

"An inverter is called to operate over a wide voltage range. When an alternator is running, the applied DC voltage may be 14.5 Volts. When the battery is not being charged, the DC voltage may vary from 12.8 Volts down to 10.2 Volts. The inverter attempts, and quite successfully, to maintain a constant RMS AC output over these extremes of DC input. Our experiment, then, is to apply different DC voltages to the inverter and measure the cooking power of the microwave. With a constant RMS voltage outpput, will the microwave operate the same for any DC voltage input? The graph shows the results. Cooking power, measured in Btu, is plotted against DC voltage applied to the inverter. What the graph shows is the fact that cooking power is directly related to the DC voltage applied to the inverter. In other words, the inverter cooks better at high DC voltages than at lower voltages, despite the fact that the AC output is a relatively constant RMS value. Is this curious?
Also shown on the graph is an efficiency percentage. Since the inverter maintained a relatively constant 120 VAC RMS output, independent of the input DC voltage, we took the Btu that the microwave produced on a 120 VAC sinewave and called that value 100%. The 120 VAC heating power was 62 Btu. To compute efficiency, we reduced the heating power provided by the inverter/microwave combination by 10%. The reason we did this is the fact that the inverter is electrically about 90% efficient. Its true efficiency, however, must take into account the inefficiencies that arise from the difference in waveshapes.
Notice that to produce 62 Btu of heating in 3 minutes requires that the inverter input be greater than 14 Volts! That is, to get the same amount of heating power as 120 VAC sinewave, the inverter must be run when charging, since batteries alone will not provide 14 Volts. Notice how inefficient the microwave is when the inverter is driven by 11.5-12 Volts, a rather typical range with heavy draw."

[belizesailor]

Originally Posted by StuM

Good article here which did various tests:
https://amplepower.com/pwrnews/micro2/index.html

The following extract is very relevant. Note the last two sentences:

"An inverter is called to operate over a wide voltage range. When an alternator is running, the applied DC voltage may be 14.5 Volts. When the battery is not being charged, the DC voltage may vary from 12.8 Volts down to 10.2 Volts. The inverter attempts, and quite successfully, to maintain a constant RMS AC output over these extremes of DC input. Our experiment, then, is to apply different DC voltages to the inverter and measure the cooking power of the microwave. With a constant RMS voltage outpput, will the microwave operate the same for any DC voltage input? The graph shows the results. Cooking power, measured in Btu, is plotted against DC voltage applied to the inverter. What the graph shows is the fact that cooking power is directly related to the DC voltage applied to the inverter. In other words, the inverter cooks better at high DC voltages than at lower voltages, despite the fact that the AC output is a relatively constant RMS value. Is this curious?
Also shown on the graph is an efficiency percentage. Since the inverter maintained a relatively constant 120 VAC RMS output, independent of the input DC voltage, we took the Btu that the microwave produced on a 120 VAC sinewave and called that value 100%. The 120 VAC heating power was 62 Btu. To compute efficiency, we reduced the heating power provided by the inverter/microwave combination by 10%. The reason we did this is the fact that the inverter is electrically about 90% efficient. Its true efficiency, however, must take into account the inefficiencies that arise from the difference in waveshapes.
Notice that to produce 62 Btu of heating in 3 minutes requires that the inverter input be greater than 14 Volts! That is, to get the same amount of heating power as 120 VAC sinewave, the inverter must be run when charging, since batteries alone will not provide 14 Volts. Notice how inefficient the microwave is when the inverter is driven by 11.5-12 Volts, a rather typical range with heavy draw."

Good stuff. I usually fire up an engine if we plan to use the microwave a lot, so mask this issue. For just a normal 30 sec to 1 minute run though, never has been a practical issue.

[moseriw]

Just a small comment: I had a modfied 1500W and simply threw it out and replaced it with a 1600W true sine wave.

Sorry but modified is always a problem. I don't think this is your problem but what I want to say is don'T use the cheaper modified sine wave inverters at all!

[sailersteve]

I've watched this guy's YouTube videos ( knurlgnar24 )and have learned a whole lot about inverters . He tests all kinds of inverters and such . The thicker and shorter the cables to the inverter the better ! And the best part he answers questions !
I'm pretty sure he's checked out your Magnum 2000 on a video you'll just have to look .
If the microwave is digital it will need to be a Pure Sine Wave Inverter , if it's the old turn the timer type microwave a modified inverter it should be fine !

[W3GAC]

I have 2000w modified inverter and older 1000w microwave that runs on switchable 8D AGM... no issues! But increased wire gauge +2 up the recommended chart. On a smaller 28 O'Day I use a modified 1000w sinewave inverter and a very small $69 Walmart special microwave (single rotating mechanical timer dial... hate having to set clock on other models before/ to use)... no issues and seems to make hot soup, coffee/ tea water, reheat BBQ chicken or pasta as fast as the bigger models. Agree that the issue might be the small but unfortunately critical small voltage drop under the big amp load. Might try setting cook power setting down some (if your model has that feature). Might work better overall at 1/2-3/4 power setting and less voltage drop. Good luck. As I've written many times, I rank having / using a microwave aboard as one of the best quality of life enhancements on our cruising (and weekend) sailboats. Worth getting it working right either by bigger wiring or different microwave model (pretty inexpensive to swap out) and as I/ others have learned, sometimes cheaper/ simpler/ less power models works better!


Sent from my iPhone using Cruisers Sailing Forum

[davefrediam]

Hi you really don't want to be running a microwave from a "modified" Sine Wave inverter. I initially installed a Sterling 2500 watt "modified sine wave inverter charger but found loads of stuff would not run properly on it even though you hear all over the place it's fine for most things. My dehumidifier smoked and gave off a terrible burning smell and my microwave was noisy as hell! I sold it on ebay and got a full sine wave one. Even electronics that would work are most likely having there life severely shortened by running them of a "modified" sinewave inverter.

[Nicholson58]

Originally Posted by CarstenWL

... if it works better when the engine / alternator is running, it looks like a too high voltage drop on the house bank / input of the inverter.

Also, a 900W microwave will have around 1500W (check your manufacturer) input power / connection power.

The Magnum 2000 is specified at a continuous power output at 25°C of 2000 VA so you should be fine ...

I would look at losses at the inverter DC input.

Regards,

Carsten

I think this is spot on. Check the DC wiring. Look also for poor connections. Wires may be marginal in size. We run a small Microwave from our Xantrex 2.5 KW inverter with no problems. The PO had all DC upsized due to a known undersize problem from the manufacturer.

[svfinlandia]

Last year the charger portion of our older modified sine wave inverter started failing, so we replaced it with a new pure sine wave inverter/charger. The microwave now performs and behaves as it does on shore power.
To me this means our microwave had issues when using modified sine wave power. Not all microwaves may have these issues.
Bill

Bill
Greetings from another Nauticater (NC36).
Just wanted to say that I am surprised that you didn't fry your microwave with a non-sine wave inverter. The spikes from a square wave type inverter are really hard on most electronics, and esp. a microwave.

Al, S/V Finlandia