Drive a motor smoothly slow
Figure 1. A typical low-powered AC gearmotor.


  Using a gearbox is no option for me.


It's not clear why this is so but if it is due to space constraints you may not be aware that miniature mains voltage AC all-in-one gearmotors exist. They are very low power (a few watts) and available in a very wide range of shaft speeds.

The only downside I can see to this solution is that the output shaft is generally off-centre and this may be an issue in your project.

Crouzet is a name that comes to mind from the distant past and they are still in business. I'm sure you will find something suitable.