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Apparently broken circuits in a resin cast stator

  1. Aug 10, 2015 #1
    I've been building a 3 phase delta wired alternator. I recently cast the stator in fiberglass resin, and now i get no response from the terminals. Before casting, I used the multimeter's resistance setting to test the connections between each pair of the three terminals, and all registered a closed circuit. After casting the stator, no pair of terminals registered a closed circuit (I tried two different multimeters).

    Since the stator is wired in a delta configuration, each pair of terminals are connected to each other independently of the other one. There are no visible breaks in the wires (the resin is partly transparent), and I can't think of how the casting process could have broken the circuit in at least three different places.

    The wiring of the stator (simplified):


    A picture of the stator after casting:


    Does anyone have any ideas as to what might have gone wrong, or other tests I could do?
  2. jcsd
  3. Aug 10, 2015 #2


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    Rag bag of answers :

    Fibre glass resin when setting in thick sections can get very hot - may have done some damage

    Energise the coils and see if you get any magnetic field .

    Clean the leadout out wires - very easy to get a near invisible coating of resin on them .

    Test your meters .
  4. Aug 11, 2015 #3
    Thanks for the suggestions.
    My meter's test out fine on other circuits.
    I checked the terminals (they're cast together with the wires), and they conduct across themselves fine, so there's no resin on them.
    I tried connecting pairs of terminals to a battery and a load, but there was no connection. The rotor setup isn't quite complete yet, but soon I'll activating the alternator (at low speed) to see if it can power a lamp, and if I can get any measurable voltage at the terminals.

    As for the heat, Do you know what sort of damage it could have done? The solder melts at 187 degrees Celcius. I'm not sure about the copper wires themselves. But I felt the wooden mold while the resin was curing inside it; it felt warm, but if it had passed 187 degrees it would, if nothing else, have burned my fingers.
  5. Aug 11, 2015 #4


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    I doubt whether your relatively small casting got hot enough to actually melt anything .

    Most likely it was heating and cooling strains that pulled something apart in the wiring .

    Odd though that all three phases are dead .
  6. Aug 11, 2015 #5
    Yes, it's very odd. The heating and cooling strains would have had to pull at least 3 specific things apart in the wiring. And since I can still see the wires through the semi-transparent resin, it would have to pull it apart in a subtle enough way so as not to be visibly obvious.
  7. Aug 20, 2015 #6
    I suspect the connection at the three terminals, they are on the perimeter and would have cooled first and put enough stress on the connection to pop the wire and the break would not be visible to the naked eye, this would explain why all three terminals are affected. One test we ran on stators to test for open circuits is to apply a high voltage low ampere power to terminals, if there is a small break in the wire the high volts will jump the break. If there is a tool repair shop nearby they will have the tool to do this test.
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