Recently on DC drives I have found several tacho gen´s that give hi bearing fault indications. They also sound like a disaster waiting. Normally I don´t collect data on those tiny thingys but when the levels get so hi it transmit and give hi levels on the motor I look where it comes from. Thingy is basically a DC generator I think, normally a brand with Radio in the name. Is this just a statistical event or do you also see this more frequent? Olov

Hello Oli,
With dc scr drives or ac vfd drives often one bearing of the ac or dc motor is insulated to prevent circulating bearing current. Usually the nde bearing is selected for insulation thus protecting the driven machine bearings also in case the coupling is electrically conductive.
Now the tricky thing is that the tach bearings may serve as a parallel path short-circuiting the nde insulation provided the coupling to tach is conductive.
To protect their tiny device most incremental encoder suppliers come with non-conductive couplings as a standard, often aluminium oxide (= insulator) material. Some tach suppliers do not come with such a protective measure !
Arie Mol

Oli,

Are you talking about the kind that bolts on to the back end of the motor and is either mounted on a stub shaft or uses a little flexible coupling? Are you referring to bearings in the tach or the motor?

With the shaft mounted ones, I have seen the key slip and cause problems, but have never seen bearing defect frequencies. I've never looked for them either, though I'll give it a listen with the yellow handled analyzer, put it in the report and it is gone next time I'm back, so I usually don't know what was wrong with it.