Before assembly of a machine that contains a three phase generator I check the resistance between the 3 phases of the generator with an ohmmeter. The generator is kept stationary and the temperature is somewhere around 20 degrees centigrade.

The measurement ensures that the generator has no internal wiring fault.

The success criterion is that the resistance between the different poles is equal and matches the resistance specified by the manufacturer (58 Ohm).

Now I have a generator that has a correct measurement between phase 1-2, but any measurement involving the 3rd phase shows me a varying resistance value.

The value jumps around (from 20 to 140 Ohm), converges to the correct resistance value after 20-30 seconds and then starts diverging again.

What could cause this odd behaviour?

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    \$\begingroup\$ higher or lower, 10% off, double, or factor of 10? What could cause this odd behaviour of the OP that he asks a question and then withholds obvious information? Perhaps he likes seeing people wasting their time on wrong guesses? \$\endgroup\$ – Neil_UK Jul 4 '17 at 16:03
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    \$\begingroup\$ @Neil_UK. If you require more information to answer the question just leave a comment asking that I clarify my question. Why the attitude? \$\endgroup\$ – Saaru Lindestøkke Jul 4 '17 at 16:06
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    \$\begingroup\$ If it was lower, might guess a short, if higher, might guess an open. Higher and lower, unlikely to have both. I'm sure it's not turning, so generating an EMF which could upset the ohmeter, otherwise you would have volunteered that information, so it's probably stationary. Temperature fluctuations, would expect to be very slow at magnets or coils, but I've have thermal expansion cause intermittents before, both in wires and in magnet attachment, the latter could cause generated EMFs. Try percusive investigation, tap it while measuring it and report back. \$\endgroup\$ – Neil_UK Jul 4 '17 at 16:17
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    \$\begingroup\$ "as specified by the manufacturer" - which implies that this part is manufactured elsewhere; And this one does not meet specs. Send it back, to be replaced with one that does meet specs, and let them worry about why. \$\endgroup\$ – Ecnerwal Jul 4 '17 at 16:17
  • \$\begingroup\$ It's likely you are using a digital multimeter for the test. Since it gives you "snapshot" readings, you now have the additional problem of interpreting what it's showing you. I'd at least try a different meter, and an analog one if I can find it. \$\endgroup\$ – gbarry Jul 4 '17 at 16:47

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