Hi hudmut,
Like you, I'm carrying out a project that involves running a 4-pole 1350rpm motor at twice (or more) its rated speed. Being similarly concerned whether this was safe, I looked at the manufacturers' spec sheets for my motor and similar types. The specs included the bearing types employed and I found that for a given motor frame size/spindle diameter, the manufacturers were using exactly the same bearings for both their 4-pole and 2-pole motors. Makes sense really 4-pole machines command a higher price anyway so it would be perverse penny-pinching to provide them with lower-rated bearings than their cheaper 2800rpm counterparts. So, the odds are your 1350rpm 4-pole motor should be fine running at 100Hz at least, though you might like to check the motor spec sheets, bearings in particular, to be on the safe side.
Taking the idea further, you might also check whether your motor is specified for both 50/60Hz operation. If it is, then its bearings must be rated for the higher running speeds provided by the North American 60Hz mains frequency. The speed of a 2-pole motor running at the American frequency is commonly stated to be around 3450rpm. On the other hand, your target is 1350rpm x 125/50Hz = 3375rpm. So, if the bearings are the same as used for the manufacturer's 2-pole machines, and the unit is specified for 60Hz operation, you should be OK for 125Hz operation as well.
The above remarks only relate to the motor's capabilities as JimM says, you'll need to check the VFD's duty-cycle at what is probably the high end of its operating range. You should also bear in mind that the power/torque delivery of older/cheaper VFDs is often substantially reduced at the high/low fequency ends of their operating range. That may need to be factored into your feed-rate/tooth number calculations as well.
Joe
