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Resistance and Ohms

I’ve been told a bunch of different answers I want to completely understand. Checking resistance on motors whether single phase or 3 phase this happens to be single phase. I went across both leads GOT OL.( is it bad is it good) my mind says that’s bad. Went each lead to ground one lead read OL. The other read 17-20 MegaOhms to me motor is bad to my knowledge. What is short to ground? What is considered bad windings? Is OL ever ok? What do I look for when testing resistance on motors?

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 24,984

    OL on most meters (read it as "open leads") means simply that the resistance is greater than the maximum reading possible on the meter — which is usually a completely open circuit. Which is fine, if that's what you want. 17 to 20 megohms to ground is alright.

    To go back to a reading of "OL" across the leads to the motor (presumably the hot and neutral to a single phase motor). This is NOT good. The actual winding resistance, lead to lead, depends very much on the motor in question — a small motor may have 20 to 30 ohms winding resistance, but a big one may have only a fraction of an ohm. Resistance to ground should always be at least 10 megohms, and OL is better.

    There is a definite "gotcha" on the "OL" reading, however: it may be that you have poor contact (or none!) on one test probe or the other. Make sure you have a very good connection and, if possible, test it.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    In_New_England