Is 40 Ohms too much when checking the windings of a small single-phase motor? - Page 1
Forum section: electronics / beginners

author: Jay112
author_rank: Regular Contributor
timestamp: April 01, 2016, 09:18:28 pm
content: 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!

author: Jay112
author_rank: Regular Contributor
timestamp: April 01, 2016, 09:42:18 pm
content: 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?

author: Andy Watson
author_rank: Super Contributor
timestamp: April 01, 2016, 09:52:58 pm
content: 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.

author: Seekonk
author_rank: Super Contributor
timestamp: April 02, 2016, 03:47:02 am
content: 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.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: April 02, 2016, 04:43:04 am
content: Quote from: Jay112 on April 01, 2016, 09:42:18 pmCheck 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?

author: Jay112
author_rank: Regular Contributor
timestamp: April 02, 2016, 05:46:16 pm
content: 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.

author: Seekonk
author_rank: Super Contributor
timestamp: April 02, 2016, 06:24:41 pm
content: 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.