Hi Chris!
When things went wrong, I didn't want to let that Marathon motor go.... so I toted it into our local electric motor shop/repair, and when I told the guy what had happened, he was familiar with the problem. He told me it was due to cheap, or contaminated wire used for the motor windings.
I don't claim to fully understand, but his explanation was that Marathon is a "bargain brand" motor, and as such, they tend to cut corners on components. He explained that the winding wire used is often unequal diameter/thickness over it's length, or of a very low grade copper with possible contaminates, either causing problems. He went on to tell me that if I stuck with Leeson or Baldor brand motors, he'd bet I'll never have another issue with VFDs..... and so far it's proven true. I don't know if that's the case with your motor, but it's kinda odd that we both have had VFD issue with the same brand motor.
He ended up throwing me a lowball offer for that Marathon motor, and I let him have it. He's always been really good to me when I need motor work, so why not.
