I've been wanting to buy one of those Chinese spindles for quite some time now. I'd like to use one to replace the spindle on my Taig.

One thing that is holding me back is I'm unsure of the minimum speed I can make the spindle spin. The auction descriptions usually say 6000 or 8000 up to 24000. Since the VFDs can output 3 phase power between 0 and 400 Hz I know 400 Hz = 24,000 RPM. I am guessing 100Hz = 6000 RPM. What happens at frequencies lower than 100 Hz?

Through reading the VFD thread it looks like the VFD's default frequency is 50Hz. People who have run their air-cooled spindles at that frequency have burned them up as the cooling fan doesn't come on until 100Hz or greater. Is there anything stopping the water cooled spindles from working at 50 Hz?

If 6000 RPM is the minimum speed does that mean these spindles are mostly unsuitable for drilling? If fly cutting also out of the question? How about milling steel or using larger end mills?

Thanks!