EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Jay112 on April 01, 2016, 09:18:28 pm
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I'm trying to troubleshoot a convection fan motor in an oven. I checked and made sure it's receiving 120v. When testing between the 2 wires coming out of the motor, the resistance reading is 40 Ohms.
Do you think this number is too high? I saw some websites suggesting that the resistance readings should be in the single digits, and that if they're any higher it usually indicates a problem.
Thanks!
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Also I just read that checking the resistance between the terminal wires and the motor's casing should give a very high Ohms reading. I keep getting 0 Ohms when checking between the black or white wires and the motor's casing. Is it possible that the small motor on convection ovens might not have their casings grounded?
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40 ohms sounds about right for the DC resistance of a small motor - you would have to compare to a known good motor to be sure. The resistance between windings and case should be very high to infinite - if by "0 Ohms" you mean no reading/off scale then it's probably ok.
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Did you remove the wires to the motor? You may be reading the transformer. If the microwave is still plugged in, house wiring is connecting ground and neutral.
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Also I just read that checking the resistance between the terminal wires and the motor's casing should give a very high Ohms reading. I keep getting 0 Ohms when checking between the black or white wires and the motor's casing. Is it possible that the small motor on convection ovens might not have their casings grounded?
Check resistances with the oven disconnected from the house wiring. ie. Remove plug. Otherwise neutral may be connected to the chassis through the house wiring.
You didn't mention microwave, is it a microwave or conventional oven?
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Thanks for the advice, everyone.
@Seekonk: Yes, I removed the wires from the motor, and that's when I took the readings.
@HackedFridgeMagnet: I'm sorry I forgot to mention. It's a conventional oven.
I managed to remove the convection fan motor just now, and I ordered a replacement. The motor is 120v, and when I give it external power nothing happens. The strange thing is that I couldn't find any problems when testing the motor with a multimeter. Is it possible for a motor to fail but everything to look okay on a multimeter? My biggest guess is that I'm probably reading something incorrectly. :)
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These small motors see a lot of heat over time and the scintered bearings dry out of oil. Even a small amount of drag can stop them from running. In some applications the shafts can be polished up with some very fine emery paper and they can be re-oiled. That can work with some test equipment that has easy access. They are almost guaranteed to fail a year later. Replacement is your best option for the oven, but try re-oiling and polishing this education.