Servo drives keeps hunting

good evening everyony,
even thoug I place the folowing question in the LinuxCNC forum  it may well be that this is not really a LinuxCNC problem.
For my new LuxTurn LTSI lathe I adopted  750W chinese AC servo Motor as spindle drive. The same motors I used for my LuxMill as X,Y,Z drives.
Up to today I did'nt have the belt to connect the spindle motor with the spindle and up to today everything seemed fine as far as the speed control was concerned. When I now tried to put the lathe in real operation the spnindle startet to change speeds erratically. It is very obvious that the controller canot maintain a stable speed. I am not talking about som few rpm more or less but the whole lathe is shaking and the current speed goes from a few hundred revs above and below the required speed. Even when I stop the spindle the chuck keeps rotating to a fro and it only stops when I  damp it by hand.
To me it looks like a severe overshoot in the spindle controller. Looking though the manual I don't seem to find the PID  control parameters. since I've this motor now 4 times I am surprised to get this behaviour. Cound this point to a poor factory setting that I have to correct?

Rainer

Hi Andy,
there was one useful video in Youtube that described exactly this phenomenon. The default value for the inertia was indeed just the rotor of the motor although the manual claimed that the default should be 2x rotor inertia. It pushed it up to 6 now and altered the system stiffness parameter slightly too and now it is already much better apart from a vibration that I sometimes manage to induce when I slow down from full speed but I guess that I'll manage to get rid off some time too

Rainer