Microwave on PSW vs MSW Inverter- Results

[BFL13]

I have been using a MSW inverter to run the microwave for several years now, and decided to try a PSW inverter. I expected the microwave to run a little better and for the inverter to draw more amps from the battery. But how much?

The microwave is a 2001 Kenmore (LG) output 1100w, input label says 1510w, so expecting about 150 amps draw using the times 10 rule. Battery bank is 450AH of deep cycle AGMs and appropriate wiring. The MSW inverter has worked just fine with that running this microwave.
I did a straight swap with the same wiring.

So the results are:

MSW- 123 amps, PSW- 162 amps (so it must want more than that 1510w)

Cooking power difference at full power doing a cup of cold water for two minutes, then check temp of water.

MSW- 150 degrees F, PSW- 175 degrees F

New inverter is this one:

https://www.amazon.com/PowerMax-2000-Watt-Power-Inverter/dp/B07DQTW1SW/ref=sr_1_26?ie=UTF8&qid=15331...

(In the ad, Inverters-R-US can't spell "complement" for some reason 😞 )

The MSW I have been using is this one (except it was on sale for $200)

http://www.canadiantire.ca/en/pdp/motomaster-eliminator-power-inverter-with-bonus-cables-3000w-19999...

[BFL13]

The inverter specs I have seen, PSW and MSW, all have a low voltage alarm when it keeps running and then a shut down when it gets a bit lower. 11.0v alarm and 10.5v shut down or some have 10.5 alarm and 10 shut down. They will run again after voltage gets back up to eg 12v.

[MEXICOWANDERER]

MSW
The first inverter I encountered was a Tripp Lite 550. That was 38 years ago. Then Trace introduced it's "improved" modified sine wave which supposedly was a great improvement. 2-steps to the top of the still square-top waveform.

And then...

"Someone (I cannot remember whom)" supposedly introduced an inverter with 1-2-3rd step waveform. My 1995 Trace 4024 still wrinkles the waveform and the 2013 Samlex is good enough for whom it is for.

What I do not know is if the pure sign wave inverters will follow suit with the MSW units and revert to a pure square wave if a low input voltage threshold is breached. Rubbing elbows with Brontosaurs, ancient Trace inverters supposedly had the lowest spec square wave voltage threshold. The Samlex however seems impressively heavy (heatsinks and transformer) for it's output spec.

[BFL13]

Got the Kill-A-Watt going. We now have a match between the AC watts and the DC draw. Also I tried another MW in the stick house.

RV inverter results: 116.9v no load , 161 amps DC draw MW on. (MW rated 1510w in, 1100w out--old LG MW from 2001)

111.5v, 15.3a, 1613w (going up and down a bit),60Hz, 0.94PF, 1694VA

Stick house MW (fairly new LG rated 1450w input, 1000w out) 115.2v no load:

111.9v, 14.72a, 1458w, 60hz, 0.88PF, 1650VA

So it is also about the PF of the microwave and it looks like the old one now wants more input than when new? Anyway the inverter is working properly, which is the idea.

I tried a resistive load ( a kettle) and the DC draw was around 91 amps, same as when it was on the MSW inverter before. So that came out right for PSW/MSW vs resistive loads being the same. (The MW is inductive so big diff there)

I was not getting inverter overload with my MSW before with the MW, so I did not ever have to try that trick of running a resistive load at the same time as the inductive to get it to behave. That would not apply to the PSW inverter AFAIK.

[wa8yxm]

Note teh 10 to one or to be more precise 1/10th the wattage is a "Rule of thumb" and not always accurite. It is a "Guideline" only.. And it's my rule. Far as I know I made it popular because it is easier than doing the hard math

Wattage/inverter effiency (IE. 0.90) then divide by voltage. NOTE voltage AT THE *INVERTER INPUT. which will be lower than the battery.

That is one complex job because the inverter effiency changes with both load and input voltage.

[Ed_Gee]

GordonThree wrote:

Ed_Gee wrote:

GordonThree wrote:
Strange that it would have any impact on the RF output. how many times did you run each test, at least 10 I hope?

Not strange if one assumes the microwave oven would run more efficiently on a proper sine wave rather the the stepped square waves of the cheaper Inverter.

The important part of the microwave runs off DC, stepped ac or otherwise would get sorted by the boost transformer and hv rectifier.

The boost transformer and HV rectifier were not designed to be powered by stepped AC. Efficiency is lost in the conversion.

[BFL13]

road-runner wrote:
I think you'll find that the AC power draw and cooking power on MSW will vary with the battery voltage. The cooking power of a conventional microwave is totally dependent on the peak voltage of the AC waveform. MSW peak is always less than sine wave peak, but is also directly proportional to the DC input voltage to the inverter.

I was not able to observe that with my MSW, having only the Trimetric showing DC on the battery bank. With the inverter, you do get "amps creep" where it pulls more amps as battery voltage goes down, as it tries to maintain the watts. The above suggests that as that is happening, you are getting less cooking power even with the higher amps draw? Whatever, it never seemed to matter. Now with the PSW, that is all history for me! 🙂

These AGMs have 0.3 higher voltage going in than my Flooded had, so that must have helped with the MSW running the same microwave, but no way to tell really.

[BFL13]

I meant the "divide by ten rule", but spelt "divide" wrong. The inverter has a sort of watt meter using 10 bars for 2000w, and was showing 8 bars so that would be 1600w. Or anywhere over 1400 and under 1600? Don't know yet how those bars work. Kill-A-Watt will say.

[Dutch_12078]

SidecarFlip wrote:

SidecarFlip wrote:
Your math is fishy.....

times 10, not 100. If your micro is pulling 150 amps, has the be the largest microwave I've ever heard of.

How about 1500 watts equals 15 amps or there about, not 150+ amps.

The 150 amps would be the 12-volt draw. The 120-volt 15 amps x 10...

[MEXICOWANDERER]

Impedance of an inductor definitely will affect draw via various "sinewave" configurations. My gripe with MSW was the time needed for long term defrosting, or near twice the amount of time needed to heat a monster mug of coffee.

Good verification BFL13 it puts to rest assumptions.

[SidecarFlip]

SidecarFlip wrote:
Your math is fishy.....

times 10, not 100. If your micro is pulling 150 amps, has the be the largest microwave I've ever heard of.

How about 1500 watts equals 15 amps or there about, not 150+ amps.

On inverters, never had a MSW unit. Always a PSW, not that much more really. 1500 watt Aims remote switching for me on twin group 31 flooded cell deep cycle Interstates.

No problem though, I knew what you meant anyway.

[road-runner]

I think you'll find that the AC power draw and cooking power on MSW will vary with the battery voltage. The cooking power of a conventional microwave is totally dependent on the peak voltage of the AC waveform. MSW peak is always less than sine wave peak, but is also directly proportional to the DC input voltage to the inverter.

[BFL13]

GordonThree wrote:
Thanks for the observation. One set of measurements shouldn't be considered an indication of anything. That's just my observation.

Congrats on the gear upgrade, hopefully the install went smooth.

Yes it did. The MSW one has twin pos and neg (nut and bolt) inputs with the idea of using four three-foot #4s to the battery bank.

The new one has a single pos and neg for input, but with stubs that have nut and bolt connections for ring lugs. I used the same twin pos and negs with one on top and one under the stub held with nut and bolt. So the wire ring lugs are not stacked, but each touches the stub.

Easy swap!

[BFL13]

pianotuna wrote:
Hi BFL13,

Thanks for taking the time to do this test.

Did you note the "under load" voltage output from the two inverters? (i.e. AC voltage).

No. I can get that with the Kill-a-Watt on the PSW but the MSW is not installed anymore. I can jury rig something maybe. I think the Kill-A-Watt will measure the MSW one for RMS, not sure. My multi-meter won't--reads around 90v ISTR.

[BFL13]

I will put my Kill-A-Watt on the job tomorrow. I am puzzled about the 162 amps seen on the Trimetric for what should be a 1510w input.

DC voltage was 12.0 loaded with that 162, so input was 1944w. If the 162 was really for 1620w (times 10) then that would be 1620/1944 = 83% efficiency, which as about what the inverter spec is.

No idea if the 2001 microwave is pulling more in at its old age or what. The Kill-A-Watt will say. It won't fry at under 1800w.

I will also compare a resistive load- electric kettle. Should be the same as with MSW for that.