Shima knitting machine and YASKAWA main motor

Hello, We have one strange problem (and some my partner too :( ) with YASKAWA AC servo motor model: USASEM-18SVB22. This motors , some years work good and after (usually) 5-7 they start be "afraid" surrounding temperature.This problem is rare at winter and additional cooling of main motor helps only autumn and springtime. In summer motor works somtimes 10 min, somtimes 10 hours, it depends on air temperature. After motor fail, LED of main AC servo motor controller show error: "C." - it meens: "Overrun prevention". Manual explain: "Wrong wiring of motor circuit or PG signal line." We tested cable and tried to change - the problem is not here. We thing , that problem is optical encoder. If we remove cover from this encoder motor often work good long time, and if we try to cover it, usually after 30-60 sec. shima fail. Shima distributor only change all motor and it cost about 1600$. We can it tolerate one time but when it is disease ..... So, maybe know this problem and can help for this. Optical encoder have not many part but start blindly to replace all this is complicated.

Thanks for any help!

Sigis

----- I am sorry for my poor English :(

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supersega
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