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Controlled Microwave Oven Input/Output Power

Discussion in 'General Electronics Chat' started by MrAl, Aug 11, 2017.

  1. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello there,

    I thought i would let you guys know that i finally got around to trying the input power variation to a microwave oven in order to lower the power setting. This is done instead of using the push buttons on the oven because in a conventional microwave oven the lower power levels are achieved by pulsing the magnetron instead of actually lowering it's power in a linear fashion. Panasonic uses an internal inverter to lower power in a linear fashion but i wanted to see if we could do it with a regular microwave oven too.

    It worked pretty well. I used a variac to lower the input voltage to the oven and thus attained lower power level settings that way, while the oven is set on the highest level. The normal input power is about 1100 watts to this small oven and i was able to lower it to 600 watts input, and it cooked the food nicely.
    The setting of the variac is tricky, but watching the current input or power input from the line helps with the adjustment setting.

    If anyone else wants to try this it would be interesting to hear your results.
    Comments welcome.

    UPDATE 08/17/2017:
    It worked down to 300 watts input power and still cooks to some degree. Turned it down to 200 watts input but did not check to see if it was still cooking because i dont need it that low. At 400 watts input i get about 200 watts cooking power (as measured with the boiling water test) so at 300 watts input maybe i get 100 to 150 watts cooking power.
     
    Last edited: Aug 17, 2017
  2. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    An obvious problem with that method is the heater of the magnetron, by lowering the heater voltage the magnetron may not start up at all.
     
  3. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    Well apparently it works ok as i have cooked several things with it now. Amazingly, a little while ago i set the input power down to 400 watts and still cooked a hamburger :)

    With my very old oven it would not cook once the line dropped to a certain low level. I guess it just takes so much to stop it, but above that it still works.
     
  4. dave

    Dave New Member

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  5. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    In which case your line voltage must drop a LOT!! :D
     
  6. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi Nigel,

    Yes, it's ridiculous sometimes. In the very hot weather it can go down as low as 80 volts, and drop suddenly from 110v to 80v and that causes the air conditioner compressor to turn off and it takes about three minutes for it to turn back on. It's a real pain in the azz.

    I noticed that the critical point for this oven i am experimenting with is around 90v. Once i hit that point the oven power drops suddenly. Even around 95vac i see a difference though.

    At some point i'll try to log it and graph it so i get a better idea how it is reacting to the lower voltage. I am not sure i will be able to test the output power that well though because it would take lot of cups of water and waiting for it to boil or whatever. Maybe on a couple settings though.
    I was very surprised though that with only 400 watts of input power (voltage maybe around 90vac but i'll check that again later) it still cooks a hamburger within a very reasonable amount of time. I can only guess that the output power is only something like 200 watts, but i guess if that is concentrated in the food volume (the meat) maybe that's enough. A 200 watt light bulb gets very very hot as all that power is concentrated on the surface of the bulb.
     
  7. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Nasty, those kinds of variations aren't allowed here, the actual voltage is still 240V in the UK, but the specification (in line with the EU standard) is 230V +10%/-6% - which nicely covers the normal 240V, but meets the EU 230V requirement. On the Continent it's the other way round, 220V in actuality, but specified as 230V -10%/+6%.

    So from the normal 240V we're only allowed variations of about +/-8% or about 19V - but it's pretty rare to see any noticeable variations these days, and even going back a fair few decades it was pretty rare, with excessive variations usually down to faults. I do remember one problem area (in the town where I work), as they built a great load more houses fed from an underground cable that wasn't designed for that amount of load - they cured it by digging the road up and fitting a more suitably sized cable.
     
  8. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi Nigel,

    You're lucky then :)
    We've had problems for many years now, and we also get some floods here in the main town so they turn off the electric, but we are on higher ground so they never used to turn ours on this block off, but then recently they started doing that too.
    I think we have problems with bureaucracies here too though. They interrupt the cable shows with 'tests of the emergency system' any time they feel like it.

    Did another quick test on the microwave oven, and found with 400 watts input the cooking power is roughly 200 watts, which is only about 50 percent efficient, down from about 63 percent at full power. Still cooks at that power level though. The setting is very touchie though, changing the variac setting by a small degree rotation cuts power by 50 watts.
     
  9. gary350

    gary350 Well-Known Member

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    I read once many years ago the magnetron will not come on at low power, it needs to come on at full power then power it down to a lower power. Were you able to get the magnetron to come on at lower power?
     
  10. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello,

    Well i guess it did because it cooked at the power i set it at, even if set before stating the cooking process.

    I did not try extremely low levels though so maybe that has something to do with it. The lowest i tried so far was 300 watts input power and that comes out to about 150 watts of cooking power, although after starting i have turned it down to 200 watts input power but did not try to cook like that because 150 watts of cooking power is just about the lowest i need. I just wanted to see if the oven would stay running at 200 watts input power.

    So i have not really tried to find out the lowest possible setting without starting at a higher level or even after starting at a higher level, but i would bet that it would go pretty low and if starting at the 400 watt input power setting i could just turn it down.

    I have also read about people who knew about other types of tubes like those used in radar and television, and they said that the coating on the tube heater could be damaged in those types. I have read that the microwave oven coating is better though, and i think the main reason for failure might be because they run the heater down low while at the same time running the high voltage to the tube at the normal level. When i do it i automatically lower both at the same time because for one thing the only control i have right now is to lower the input voltage and thus total power getting to the oven. I can not lower the heater voltage all by itself, which i would not want to do anyway.
     
  11. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    To be fair, that's the last thing you want to do - what you really want is to keep the heater running at it's proper voltage.

    However, it's a pretty pointless effort, as you're probably best just pulsing it as microwave ovens usually do, this probably works better for defrosting than a continuous lower power.
     
  12. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    Yes, and other people have suggested keeping a constant voltage on the heater with a separate power source.

    Yes on the defrosting too, but i dont do that much defrosting mostly cooking at low power which seems to be better.
     
  13. schmitt trigger

    schmitt trigger Active Member

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    I would suggest that you perform a very simple, controlled and repeatable experiment.

    1- Take a graduated jar which can hold 1 liter of water (I'll be working in Metric as it is far easier to convert everything into watts).
    2- Measure the initial temperature in Celsius
    3- Put the water inside your oven, set it up for a nice round number, 100 seconds. Do this at nominal voltage, 120 volt. I assume that you will be measuring electrical power.
    4- Remove the water from the oven, stir it a little to homogenize the temperature, and measure the new temperature.
    5- Calculating the temperature delta between 4 and 2,
    6- and knowing that a liter of water can hold 1000 calories for each degree Celsius of temp rise, you can calculate how much energy the oven actually transferred to the water.

    Repeat 1 thru 6 but now at 95 volts.

    If you took input watt readings for both conditions, calculate its ratio. And then the calorie ratio for both conditions.

    You can then determine a relative figure of merit between both ratios.
     
  14. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    Yes that's basically how i do it except i use temperatures of 'room temperature' to 'boiling' and i always use 8 ounces of water.
    The formula then is:
    Watts=1254/minutes

    so if it takes 2.5 minutes to boil then the actual cooking power is about 500 watts. This is based on a temperature rise of about 76 degrees C.

    This is just an estimate but i've been doing it this way since the early 1990's so it's comparative too, comparative to the other oven's i've owned that is. The formula was created after converting to grams and Joules, etc.

    For an 80 deg C rise (8 oz water again) we would have:
    watts=1318/minutes
     
    Last edited: Aug 18, 2017
  15. schmitt trigger

    schmitt trigger Active Member

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    Mr Al:
    So you have already done all your measurements and calculations...Good! this is what a true engineer does.:)

    I always told my students: everyone may have an opinion, but hard data talks.
     
  16. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    Well yes, that's how i found out the efficiency drops from about 63 percent at full 1100 watts input power to about 50 percent at 400 watts input power.
    But i guess i could have done more if it didnt take so long to boil water :)

    Yes data is the key. That's how the first exoplanet was found too.
     

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