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Microwave Oven Drawing Slightly Over Rated Current?
I was looking at a GE 1500W microwave oven, and the nameplate current was 14.0A. Upon testing with a good true RMS ammeter, I found that the oven was drawing 16.5A. The complaint was an “electrical smell” from the oven when operating for an extended period. Typically, overcurrent is a failure in the interlock switch and a dead-short condition blows the fuse. This was not an interlock failure. Is it a shorted turn in the transformer? A bad magnetron? (I had a spare diode and swapped it; no change.) The oven was 14+ years old and decided to advise “end-of-life,” but still wondering about this overcurrent condition….
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4 Replies
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Magnetrons as they age do have load problems. The transformer is made to limit the current and provide the proper voltage to the magnetron. Generally the mag gets grease in it that causes a bleed of current and overheating of the windings. Or even a internal arc. Is this machine 115Volt or 240Volt? Most commercial 115V need a 20 amp plug and supply. Most domestic units are 500 watt to 800 today. But I’ve worked on some commercial 10,000 watt units. absolutely scary and high maintenance.
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I have never worked on a microwave oven but had 10 years working on the microwave powered UV lamp system. The systems I worked on are dual magnetron and were 1500W to 3000W per magnetron. These things run 24/7 so every few months the magnetrons would fail. When they are at end of life the current that they draw is no longer constant but rather up and down. They called it moding. Do you see the current flunctuating? When I checked for moding I had to actually measure the high voltage DC that goes into the magnetron. Not the AC current on the primary side of the transformer.
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Never found DC out of the transformer. That would require a rectifier after the transformer. A Magnetron is no more that a electro magnet with a multi-cylinder and a cathode. Also a high voltage capacitor and a diode. The electro-magnet shapes the wave around the tubes from the cathode. Internal of the magnetron is a oscillator that makes the correct frequency (2.45ghz) for the wave length (12.2 cm).
Now when you see a current change, it can be reflected waves coming back to the magnetron from not enough load. Or a bad stirrer. This causes excess heat that destroys the insulation of the magnetron. Has a very distinct smell that will linger in the unit. Also when they arc, they make huge amounts of ozone and nitrogen oxides. In certain conditions they can heat air to the point it makes a plasma that will sustain a arc. A trick you may not have heard of is to take thermo printing paper and cover the bottom of the chamber. A short run will show you the coverage.
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To power the magnetron you need a transformer to up the voltage to about 1000 to 2000V then rectify it to make DC power. You also have to power the filament with 12 or 6V. On the equipment that I work on they make a shunt resistor to allow measuring the DC current going into the magnetron.
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