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Wattage in a MWO (microwave oven)

  1. Sep 9, 2015 #1
    What is the main determining factor in determing the watts in a MWO? I'm not asking how it is measured I understand how to do that, I'm asking what the engineer uses to determine it. Is the output voltage of the transformer more responsible than anything else? It seems they all put out the same 3v to heat the filament, and most seem to all put out about 2000v on the high V lead so I am wondering if the magnetron is the main determining factor in wattage output.
     
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  3. Sep 9, 2015 #2

    berkeman

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    What's an MWO?
     
  4. Sep 9, 2015 #3
    microwave oven
     
  5. Sep 10, 2015 #4
    Most (all?) microwave ovens use a cavity magnetron to generate high levels of microvaves.

    Magnetrons vary in their design. Some have more cavities, stronger or weaker magnetic fields, etc. These variations will affect the input impedance so given a constant voltage, some will carry more current and hence more power.

    The 3 V heats the electrode reducing it's work function. Most tubes have something similar.

    BTW, I'm not an expert in this field. That's about the limit of my knowledge.
     
  6. Sep 10, 2015 #5
    Thanks Jeff. I was pretty sure the power was more dependent on the mag than the HV xfmr since they pretty much all put out 2KV (before the doubler cap/diode circuit gets a hold of it). .
     
  7. Sep 10, 2015 #6

    sophiecentaur

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    "Dependent on", yes but the Power is not supplied by the Magnet circuit. Magnetrons can be permanent magnet based and a PM is definitely not a Power Source. A 'good' magnetron, like a 'good' loudspeaker and 'good' dynamo will need a strong magnet but the input Power comes from elsewhere. In a Magnetron, the power will come from the High Volts and the current in the electron beam. So the HV transformer would need to have appropriately enough copper and iron (basically) to provide the beam current without the volts sagging.
     
  8. Sep 10, 2015 #7

    Baluncore

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    A magnetron is difficult to drive because it has a sharp drop in “plate resistance” as it enters the operating region. For that reason it only requires sufficient voltage to “strike”, then it needs a current regulator to limit the current.

    The transformers are designed with windings having higher resistance than is efficient, to partly limit the power. Notice that the HV transformer includes magnetic shunts inserted between the primary and secondary windings. Those shunts increase the inductance of the secondary and so tend to stabilise the current, even with the rectifier diodes and HV capacitor in the circuit.

    So the special purpose transformer limits the current, while the microwave tube limits the voltage, W = I * V. Together they fix the maximum power available. The duty-cycle of the transformer primary is then used to regulate average power on lower power settings.
     
  9. Sep 11, 2015 #8

    sophiecentaur

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    Yes - nowadays. But they used to have an on-off cycle of seconds, when solid state power control was expensive or not invented yet. My 'new microwave describes the control as 'Inverter Technology'. They sell like hot cakes with a name like that.
    Very interesting stuff about the transformer construction. Cheers.
     
  10. Sep 11, 2015 #9

    Baluncore

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    By "duty-cycle" I was refering to the old n second on, m seconds off, contactor system used in ovens that have a mains frequency EHT transformer.

    New microwave ovens have no mains frequency EHT transformer as they use a switch mode step up converter with output current feedback to regulate power. That is the more recent inverter technology. Did you not notice how much less it weighs than your old one?
     
  11. Sep 11, 2015 #10

    sophiecentaur

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    The new one is a combination oven and it's quite big. I think any weight difference was hidden to me.
    BTW, it also has this 'Chaos' defrost system which, so I'm told on PF, allows the defrost time to be shorter.
     
  12. Sep 13, 2015 #11
    I was an aviation electronics technician in the USCG 81 - 85 and apart from being SAR aircrew, we were also the guys that did the component level repair on all the black boxes including nav/com/ etc. Back then the rating was AT but I believe since then it has been combined with what we called the "one wires" who were AE's and now one rating does it all.

    My forte was the Coliins AN/APN-195 color radar (we called it a "color radar" because they were still relatively new at that time at least to the USCG). It used a klystron of course but with that knowledge I gained in the early 80's I later went to work for my dad's HVAC & appliance company when my enlistment was up and I was able to add microwave oven repair to his list of appliance repair, until in the 90s they started to become disposable. The problem was almost always an interlock/door switch or a component in the high voltage circuitry and that was usually the rectifier or magnetron, and sometimes a transformer.

    I've forgotten most of what I was taught in A school and frankly we were never given a deep understanding of electronics per se, just enough for us to be able to maintain the black boxes of the aircraft we flew in, but I still have a basic understanding. However, I haven't asked the question properly so I will try a different tact.

    MWO = Microwave Oven

    • MWO #1 is a 800 watt output oven
    • MWO #2 is a 1200 watt output oven
    • They both have the exact same high voltage transformers even down to the part number
    What are the likely basic design element differences in these ovens that allows MWO #2 to put out 400 more watts?
     
  13. Sep 13, 2015 #12

    Baluncore

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    What are the part numbers of the magnetrons fitted. The number of electrons emitted by the cathode will be dependent on the filament and anode cavity axial length.
     
  14. Sep 13, 2015 #13

    russ_watters

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    Right: a transformer just transforms voltage. Beyond that, it has no impact on power used(unless it dies trying to satisfy the load).
     
  15. Sep 13, 2015 #14

    Baluncore

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    Unless it has magnetic shunts inserted between the primary and secondary windings, like in a microwave oven transformer.

    http://www.electroncoil.com/ferroresonant_transformers.php
    Notice in the first figure; square section magnetic shunts inserted in the gap between the primary and secondary windings.
    See also;
    http://www.vias.org/eltransformers/lee_electronic_transformers_08_06.html
     
  16. Sep 15, 2015 #15
    My understanding is the shunt is designed to limit inrush current, not typical operating current.

    I could easily be wrong though. It's not clear to me what limits the current in a magnetron. I could easily envision electron packets being formed and the packet size/frequency varying with the tube geometry which would limit operating current, but I have no idea if this actually occurs. I can just as easily see how the a shunt transformer could limit the typical operating current.
     
  17. Sep 15, 2015 #16

    Baluncore

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    I have not studied the exact balance between regulation features, but I believe shunts operate every cycle, not just for short circuit surge. The resistance of the secondary is also deliberately high in oven transformers. Magnetron dimensions play an important part. The capacitor in the voltage doubling rectifier used is also a series reactance that must limit total charge per conduction cycle.

    I match scrap MOTs, then strip the shunts and secondaries, placing both primaries on the one core. That gives a very high power isolation transformer. It can also be wired as an autotransformer for 115V : 230V conversions. Without the shunts and secondaries they are very useful.
     
  18. Sep 15, 2015 #17

    davenn

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    that was a little ambiguous the same part # for just the transformer or both the transformer AND the magnetron ??

    if it just the transformer, then just a higher power/more efficient magnetron


    Dave
     
  19. Sep 21, 2015 #18
    I was giving an example based on real experience. When I repaired them often the customer would not want to spend the money on the part especially if the part was an expensive circuit board;. They would pay the troubleshooting charge and tell me to keep the oven. Of course I used the ovens for parts (never sold used parts as new FYI) and I remember there were times when I found that the high voltage xfmr in one oven was the exact same one in another oven but with different output.

    So I don't have a part number but this knowledge is based on real experience and is the nexus of my question; what determines the output of a microwave oven apart from the xfmr. I appreciate the members who have offered their best answer to that question.
     
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