EEVblog Electronics Community Forum
General => General Technical Chat => Topic started by: DTJ on July 27, 2016, 01:58:18 am
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Our Panasonic microwave oven seemed to be down on power so I measured it by heating a litre of water and measuring before/after temps etc.
Its a 1100W inverter oven. I measured its power output as ~800W.
Is it down on power? The 1100W rating is actual load so efficiency wise I guess 800/1100 = 72% is ok?
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was 1100W written on the PSU? Devices often use PSUs with higher rating for safety and efficiency rating. What matters is the rating of your oven on the brochure in its cooking power. So 1100W could be a peak power rating.
Some components could be mechanical in the sense that they use a lot of power to start up but once they are running they use less power.
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The magnetron is pretty efficient, as vacuum tubes go; that sounds good to me. :)
Tim
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72% power transfer into the water seems high enough. There is a lot of heat lost in the transformer, the magnetron, and power electronics and blown out the vents.
Did you calculate the heat lost by evaporation?
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A bit of a daft post by me, as soon as I hit 'post' I realised it made sense that 70% was probably ok.
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Microwaves are essentially RF devices, so the power transfer is a function of the matching between the impedances of the magnetron and of the cavity with the "food" inside. The specified output power is typically the maximum power output of the magnetron assuming perfect matching with the load, which in practice is pretty much impossible to achieve.
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I believe the power advertised is actually the output power, not the input.
Did you measure the input power?
How old is the microwave? Magnetrons fade as they age.
Another possibility not all of the energy is being absorbed by the water. Some of it could be reflected back into the magnetron and dissipated there or the control circuit could be detecting a light load and reducing the power output.
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I believe the power advertised is actually the output power, not the input.
Did you measure the input power?
How old is the microwave? Magnetrons fade as they age.
Another possibility not all of the energy is being absorbed by the water. Some of it could be reflected back into the magnetron and dissipated there or the control circuit could be detecting a light load and reducing the power output.
I didn't bother measuring input pwr, I just did a quick check as my hot chocolate didn't seem as warm as usual.
After thinking about it the sums add up, after doing a bit of searching 70% efficiency seems typical.
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I believe the power advertised is actually the output power, not the input.
Did you measure the input power?
How old is the microwave? Magnetrons fade as they age.
Another possibility not all of the energy is being absorbed by the water. Some of it could be reflected back into the magnetron and dissipated there or the control circuit could be detecting a light load and reducing the power output.
I didn't bother measuring input pwr, I just did a quick check as my hot chocolate didn't seem as warm as usual.
After thinking about it the sums add up, after doing a bit of searching 70% efficiency seems typical.
Yes, 70% efficiency is about right, so if the advertised output power is 1100W, then the input power should be just over 1571W so no, I'd say it doesn't add up. The only way to be sure is to measure the input power and do so using a proper power meter, rather than just multiplying the current by the voltage.
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Magnetrons fade as they age.
What is the cause of this? I though that a magnetron basically just was a passive cavity device that did some rf-black magic with the energy. :-//
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Magnetrons fade as they age.
What is the cause of this? I though that a magnetron basically just was a passive cavity device that did some rf-black magic with the energy. :-//
Loss of emission on the filament perhaps
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I have noticed that Panasonic inverter microwave ovens gradually decrease their power level while running. They return to full power after
some cool down period or after doing a power on/off/on cycle. This behaviour appears to be programmed into the controller.
Measuring the mains current draw indicates the power drop.
One particular model takes this power drop behaviour to the extreme, http://www.productreview.com.au/p/panasonic-nn-sf550w.html (http://www.productreview.com.au/p/panasonic-nn-sf550w.html)
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What has surprised (maybe not) me is that our 26 year old Panasonic (Yes 26 and it has a digital display!), which was sold as a 800W unit produces the required results from packaged food when they recommend you heat for "five minutes (based on an 1100W microwave oven)"
We follow the same instructions and get the same results using our 800W oven (and that's with any Magnatron aging)
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What has surprised (maybe not) me is that our 26 year old Panasonic (Yes 26 and it has a digital display!), which was sold as a 800W unit produces the required results from packaged food when they recommend you heat for "five minutes (based on an 1100W microwave oven)"
We follow the same instructions and get the same results using our 800W oven (and that's with any Magnatron aging)
Your old Panasonic is probably labeled for output power, and the marketing types have the newer ovens rated by input power, because of course 1100W sounds 'better' than 800W to those who don't know any better. Around 70-75% sounds about right for efficiency.
-Pat
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In the UK I have only ever seen microwave ovens rated by output power, with 800 W being by far the most common and usually with an input rating about 1200 W.
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Advertised output power of a microwave oven is usually output power rating of the transformer.
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Magnetrons fade as they age.
What is the cause of this? I though that a magnetron basically just was a passive cavity device that did some rf-black magic with the energy. :-//
Like any thermionic vacuum tube, the cathode (filament) wears out and emission drops. Tubes get weak, CRTs and VFDs get dim, it's all the same mechanism. With the general demise of the CRT, the magnetron stands as the last ubiquitous domestic vacuum tube.
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The advertised power is typically the maximum output power assuming a matched load (RF is still RF). This means that if the food (load) is not well matched (like for example frozen food, dry food or simply very small quantities) the overall delivered power will be lower than advertised.
The input power is typically significantly higher: I once measured my old 800W microwave when heating up a nice big pot of water and the power draw at the plug was in the order of 1300W.
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Like any thermionic vacuum tube, the cathode (filament) wears out and emission drops. Tubes get weak, CRTs and VFDs get dim, it's all the same mechanism. With the general demise of the CRT, the magnetron stands as the last ubiquitous domestic vacuum tube.
As with semiconductors, purity is a big player in the performance of thermionic tubes. The very best quality were used in undersea telephone cables: they boasted several decades of uninterrupted, continuous, maintenance-free operation, in multiple cables! They were made with very high purity materials, exacting tolerances, and used lots of precious metals (gold).
The outward differences between industrial and commercial tubes are slight: usually some additional support rods for strength, or more robust materials in the most demanding types (like using hard borosilicate glass for the envelope, and ceramic wafers instead of mica sheets for support). But the big difference is what you can't see: tighter formulations and higher purity in the cathode and grid materials.
Occasionally, you'll get lucky with commercial tubes, which is why you sometimes find 12AX7s that have been in continuous service since 1960, or magnetrons as old that are still going strong. These examples are on the extreme side of the survival curve, and not at all representative of typical quality -- interpreting it as such would be a rather egregious case of survivor bias! :)
Tim