John T - RMS voltmeter and new Inverter Microwave

While we're sort of on the topic of RVs, DC to AC, AD to DC, etc. - I've got a comment and a question to anyone that knows.

The general topic is the difficulty of making microwave ovens work properly when run off an inverter. One problem is the Root-Mean-Square thing (RMS voltage). It is used to give voltage "equivalents" so a person can figure what will run what off an inverter. For example - a cheap modified wave inverter will often only read 95 volts at the AC output with a common voltmeter, but will read 120 volts AC when checked with an RMS capable voltmeter. RMS voltage works OK with resistance-element heat makers, e.g. toaster, electric ovens, etc. It does not work however with microwave ovens.

So, to make a long-story shorter. Most microwaves will barely work off a mod-wave inverter when that inverter has 14 volts at the DC input. That cannot happen when only running off batteries. Thus the reason why the microwaves in RVs often work MUCH better if you start the engine (or genny) and make battery voltage rise from 12.7 volts to 14 something volts. With a fully charged battery bank - at 12.7 (or around) volts - a microwave is lucky to work at 1/2 its rated output, if that. And often, battery voltage with such a draw will drop to 11.5 or 12 volts which can render the microwave near useless.

Now -about conventional microwaves. None have variable power. The way they work is - at full current draw but slamming "on" and "off" like machine guns. That's why a small 700 watt microwave often draws 14 amps of AC current in repeated surges. That "700 watts" only refers to cooking power - not AC power draw.

Now - Panasonic has a new tech microwave called the "Inverter Microwave." Note the word "inverter" here has nothing to do with the inverter we use to make AC power from a DC source. This new microwave actually has steady linear power that can be trimmed down at a lower power setting. I'm not sure how it all actually works but I just ordered one. I'm anxious to see how it performs.

"For example - a cheap modified wave inverter will often only read 95 volts at the AC output with a common voltmeter, but will read 120 volts AC when checked with an RMS capable voltmeter."

LJD,
Don't all AC voltmeters measure RMS voltage? The old analog meters even had peak to peak voltage scales and peak voltage too.

If your output voltage isn't a perfect sine wave then things could be a little gppfy.
George

gppfy = goofy

I've got over a dozen cheap digital volt meters here. None of them read RMS voltage. If a digital meter only costs $5 to $60 - I doubt it reads RMS. I burnt up my two-hundred-dollar something Fluke and went searching around for a "cheap" RMS meter. The cheapest I found at the time was my Extech "True RMS" model 470. That cost me $130 and it's an "el cheapo." Works nice though and has an over-the-air laser temp gauge that is kind of neat.

I just checked a few recent electronics catalogs. The cheapest RMS meter I can find is $80. Every other meter priced less does not have it.

If you now who sells one cheaper - post the info. I'd like to get another to keep somewhere else.

Cheapest I can find is the "Amprobe TRMS" model AM-530 . Sold as an "electrical contractor multimeter" for $80.

Digital Multimeter, Voltage (AC) 1000, Voltage (DC) 1000, 10 Amps AC, 10 Amps DC, Resistance (Ohms) 40M, Basic DC Accuracy 0.8 Percent+1d, 4000 Counts, True RMS Y, Auto Ranging Y, Safety Rating CAT III 600V, CAT II 1000, Temp. Range -40 to 1832 Degrees F, Capacitance 0.00 - 40.00nF / 400.0nF / 4.000micronF / 40.00micronF / 400.0micronF / 4000micronF, Frequency Counter 10Hz, 10MHz, Special Features 0 - 60MHz, Duty Cycle N, Audible Continuity Y, Diode Test Y, Peak Hold N, Backlit LCD Display

I never run any bigger loads such as a microwave or heaven forbid a rooftop AC off my small RV inverter, I only use it (when dry camping) for maybe TV or my laptop and charging cell phones etc.

Over the years Ive owned the good old reliable analog Simpson 260 and an RCA WV38A I believe it is meters along with some cheap digital which I never liked. ALL OF THEM read around 120 or so volts when I was checking things and thats the RMS value. I never owned any meters that read the peak to peak of the sine wave which is much higher !!!!!!!!!!

The cheap inverters produce like a stepped wave to mimmick a sine wave and I think the microwave ovens get all freaked out thinking the frequency isnt correct????????

Im not a very good hand nowadays on electronics (was decent back in 69 when I graduated Purdue lol), my later engineering years were spent in AC Power Distribution so im not much help on this stuff, sorry

John T

True RMS reading voltmeters measure the waveform a great number of times during each ½ cycle and then perform a mathematical calculation on the collective readings to find the RMS value while the cheaper meters assume the waveform is a perfect sinewave and the RMS value is always 0.707 times the peak value of the sinewave,

Wave forms such as pulse width modulated waveforms that are created by pulsing a DC voltage on and off will not read accurately unless you are using a true RMS reading meter. Most of the cheaper inverters use some type of pulse technology so the cheap meter will not give a “true” reading.

If you google Fluke instruments and go to the technical publications area on their site you can find all you ever wanted to know about “True RMS”.

The older microwaves used a phase back technique where the circuitry was designed to use “Zero crossing” information of the sinewave to determine at what point in the phase to fire the magnetron, at lower heats the magnetron was fired latter in the cycle.

With an applied pulse width modulated waveform you have a number of Zero crossings and the on and off applied voltage will allow the thyristor that fires the magnetron to recover or turn off. When the thyristor has a sinewave applied, once it has turned on it will “latch on” until the applied waveform approaches “Zero Crossing” at the end of the waveforms cycle. So the magnetron will be on for less time during a cycle if feed with the pulsing voltage.

Don’t know anything about the new Microwave it would be interesting to find out the technology they are using

LJD,
I'm curious, what would the wave from your inverter look like on an oscilloscope? You say it slams on and off, so is it a square wave? A Sine wave? What's its frequency?

George

I didn't say the power from the inverter "slams on and off." I said all conventional microwave ovens work that way. They cycle between full power - and no power -on and off like slow machine gun. Turning the power level down or up simple changes the frequency of the switching.

Inverter and their wave forms are just about all "stepped" in some way. The worst ones are usually called "modifed wave" inverters. The more expensive ones called "true sine wave" still have angled steps instead of curves on the waves - but work much beter on sensitive electronics.

In the early days of inverters some were really bad and called "square wave.". So, in a nut-shell from worst to best, and cheapest to most expensive - it's square wave, modified wave, and "true sine wave" when it comes to marketing terms. The only ones that are near identical to grid power are the very expensive inverters made to make grid-tie household power - like the two that run my house and barn. I have twin 6000 watt inverters and each one cost over $2000.

The new "inverter microwaves" use pulse-width-modulation to keep more-or-less steady heat instead of the magnetron going on for a git, going off for a bit, etc. From all the test reports I've read - it makes a big difference in the quality of heating and defrosting. Much more even and steady. But that aspect does not interest me. My interest is getting a microwave that is more "linear", so to speak. I.e., turn it down to 1/2 power and it actually only draws 1/2 power from the AC source. A conventional microwave draws full power at any power setting. Just does it "on and off" for different time intervals.

I just ordered one from Panasonic. I'm anxious to see if it actually makes a difference in amp draw when using on lower heat.

Conventional microwaves have a tough time running from inverters due to DC input voltage problems. A conventional 12 volt inverter will run most equipment with an input of 10.5- 16 volts. But - when hooked to a microwave- once the input drops below 13.5 volts the oven works poorly. Needless to say when running off a battery bank - voltage will always be below 13 volts unless something is charging the battery bank while being used.

LJD,
Thanks for the reply. Sorry for the confusion. I don't have $2000 for an inverter or an RV. Your reply explains why a cheap meter can't give you the reading you are after. As for me, the cheap EVOM's has always given me good (RMS) readings on 60 hz from the power company.

A simple half wave power supply is a diode in series with a capacitor. The capacitor charges up to the peak value of the wave. What would the dc capacitor voltage be from your inverter? If I recall, a micro-wave has just that, except it also has a transformer that produces a very high voltage. A special diode and cap is used to power up the magnetron. Short out that capacitor and it makes one big bang.

George

RMS means Root of the Means Squared. It is primarily used in the measurement of non sinusoidal waveforms where the effective value is the norm for pure sinusoidal waves. Both are intended to measure the equivalent heating effect of straight DC so that you have a comparison heating effect.

So an RMS meter takes a sampling of the instantaneous voltages and squares them, adds them together and then takes the square root of the product. Reason for this is that energy responds to the square of the voltage so if you square a large number vs a small number (voltage amplitude) you will have a much larger number but since it isn't continuous, it has to be combined with the other amplitudes.

At least that's the way I remember it and it has been awhile.

Mark