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Megger/ohm meter needed please!!!!

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Figster10, Jan 22, 2019.

  1. Figster10

    Figster10 Member

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    I have opened the inverter and removed the mg1 mg2 cables to test them for resistance. The only problem I am having is finding the meter in the area to do the testing. It is a megger or a ohm meter or an insulation tester that I need. Is there anyone in the Bay Area/Sacramento/Modesto Area that can rent it out to me??? They seem very expensive for a one time use. Please and thank you.
     
  2. Patrick Wong

    Patrick Wong DIY Enthusiast

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    Start by using a regular ohmmeter because if the ground fault is sufficiently bad, that is all you need. If you cannot find the fault with an ohmmeter, then use a megger which uses high voltage to perform the ohmmeter function.
     
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  3. Figster10

    Figster10 Member

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    I’ve tried using the ohm meter but I get “0.000” on mg1 and mg2 ports. I may be doing it wrong but I will try to see if I can find video or pics just to make sure I’m doing it right
     
  4. jerrymildred

    jerrymildred Senior Member

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    I've never done this with the MGs, but have with other electric motors. You should get zero measuring from lead to lead and infinite measuring from lead to car frame.
     
  5. Patrick Wong

    Patrick Wong DIY Enthusiast

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    As Jerry indicated, the relevant measurement is from body ground to each of the three terminals on the MG1 and MG2 cables. The spec with a megger is 10 million ohms or more.

    If you measure resistance from one terminal to another, the resistance should be close to zero as you are measuring the resistance of the stator winding (there are three windings in each MG corresponding to the three phase AC produced by the MG) and the stator wire gauge is thick so the resistance will be minimal.
     
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  6. Figster10

    Figster10 Member

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    I’m pretty car savvy with diagnosing and fixing things but this mg1 and mg2 testing has been confusing to me. I keep using a basic klein model mm600 to measure the leads but all I keep getting is zero on volts and Omega. I’m not sure if I’m doing it right hence why I’m starting to believe the YouTube videos that I need a megger to see if I’m getting the proper continuity.
     
  7. jerrymildred

    jerrymildred Senior Member

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    It's best to use a regular multimeter the first time to measure motor lead to ground. If that shows leakage then you don't need a megger because you already found a short to ground. If the regular multimeter shows infinite resistance to ground, then it's time to use the megger. But ONLY on the conductor to ground test, not leg to leg.

    I am curious, though, why you want to make these measurements.
     
  8. Figster10

    Figster10 Member

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    Codes poaa6-526-613

    Someone in here sent me the tree checklist and it instructed me to test the leads for the cables mg1 and mg2 to determine the isolation leak. 613 code points to tranny or inverter problem
     
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  9. Patrick Wong

    Patrick Wong DIY Enthusiast

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    Measure resistance (not voltage) from an MG terminal to ground. The measurement should be infinite resistance. If you get anything close to zero then stop, you’ve found the defective component.
     
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  10. jerrymildred

    jerrymildred Senior Member

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    I was afraid it was something like that. So, yeah, the only thing you need to test is the conductor to ground for each motor lead. Since you're showing good results on the Klein, the next step is definitely a megger. Not something one needs very often and they are expensive as I'm sure you know, so it makes sense to rent one. Hope you find one and get some definitive results.

    If you can't find a local tool rental place with them, perhaps something online like this: Rent Megger Electrical Test Equipment | Transcat

    Or this: Rent, Hire or Lease Megger Test Equipment rental - Rental of Megger Test Equipment - Livingston
     
  11. Figster10

    Figster10 Member

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    I love this forum. Thanks a bunch as this is the complete opposite of what Craigslist goons and the dealership have been telling me.

    Thanks a whole bunch
     
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  12. Figster10

    Figster10 Member

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    I’ll follow up later this week.
     
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  13. jerrymildred

    jerrymildred Senior Member

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    Thanks. We would for sure like to know how it turns out.
     
  14. ChapmanF

    ChapmanF Senior Member

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    I haven't looked at what the typical rental rates are....

    These kinds of tools have been expensive historically, because pretty much everybody has a use for an ordinary multimeter, but the market for things like meggers for the most part has been the much smaller number of people needing them for work, and work pays for 'em.

    But two things kind of change that nowadays ....

    1. There are now cheap Chinese versions of flat-out everything.

    [​IMG]

    I don't know anything about the quality of that unit, but at least one PriusChat user has reported buying one and using it successfully. Strictly featurewise—ignoring quality, for all I know, it poops out after a week—it's got some features, like more than two test voltages, and a guard terminal, that you normally wouldn't see at the very low end, so, hey.

    2. Sometimes older instruments turn up on ebay.

    [​IMG]

    For me, I'll often go that way ... partly because I halfway suspect an older Megger or AEMC or Fluke might be better put together than a brand-new $50 Chinese no-name, and partly because I just kind of like old stuff.

    My latest ebay adoption lives down at the other end of the resistance spectrum:

    4300.jpg

    Here it is measuring an inch of copper strap at 277.6 micro-ohms. It tickles me that the company that made it is still in business, and their oldest currently-employed technician doesn't even remember this model, and it still works. :)

    Interesting physics things happen at both the high and low extreme regions of resistance.

    In a nutshell, a normal multimeter is good for measuring resistance of things you would normally think of measuring resistance of. Meggers are for the weird job of measuring resistance of stuff you don't even think of as conductive (like insulation), and micro-ohmmeters for the weird job of measuring resistance of stuff you don't even think of as resistive (like wire, or connections).

    In both of those weird regions, questions come up that you might not think of before, like (for a megger) how does cable capacitance affect me? or what is dielectric absorption? or what do I do with this third terminal? Or (for a micro-ohmmeter) how does cable inductance affect me? or what is thermal EMF? or what do I do with these four terminals?

    There are some worthwhile reads that cover those two strange regions. For the very high resistance, megger applications, there's A Stitch in Time... from the Megger people, and for the extreme low resistances, A Guide to Low Resistance Measurement from Cropico (the Megger people have a similar guide with a similar title).

    Both of those will tell you a lot more about how to use the instruments than you're likely to get just from the manual (especially from the one-page "manual" that comes with the Chinese no-name) ... they're also helpful for knowing what to look for in a list of used ones on ebay.

    In any case, be sure to read enough to understand the safety implications. Even in a world where meggers can be fifty bucks, a thousand volts is still a thousand volts.

    (Like, in one of the used listings on ebay, I read the description saying one of the test leads was damaged and repaired with "transparent tape". For an instrument that puts out a kilovolt, I think I'd pass on that one.)
     
    #14 ChapmanF, Jan 23, 2019
    Last edited by a moderator: Jan 24, 2019
  15. Figster10

    Figster10 Member

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    I went with the cheap $50 Chinese one just for the forum’s sake to help others that come across this problem down the road. I plan on buying the semi-expensive one I just want more time to shop around for one. For now I just want to wait for amazon to ship mine by Friday and see what happens. Consider this a 50$ test run for the next guy I guess.
     
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  16. ChapmanF

    ChapmanF Senior Member

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    Looks like you'll be at least the second PCer to try that one out.

    While you're waiting for delivery is a good opportunity to read through Stitch....
     
  17. Figster10

    Figster10 Member

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    Any word how it went for the first guy?
     
  18. ChapmanF

    ChapmanF Senior Member

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  19. Figster10

    Figster10 Member

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    I will follow up with results and pictures hopefully this weekend.
     
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  20. Figster10

    Figster10 Member

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    “Alright alright alright...”

    Amazon came through and delivered part one day earlier than originally scheduled.

    I used the ever so easy to use Chinese 50$ megger knock off thing and it gave me some interesting readings.

    With inverter open and mg1 and mg2 cables removed and dangling,of course.

    Mg1 gave me reading 800+ and climbing fast.

    Mg2 Gave me start of maybe 200 and dropping all the way down to 003 and sometimes 002 holding not even moving at all.
    *picture shows “004” megohms but after I kept testing it stayed “002-003”

    I have been hearing it needs at least 10 megaohms and anything close to “000” is bad.
     

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